Nace mr0175 certified user my reading 5 tm0177

Understanding NACE MR0175-Carbon Steel Written Exam Reading on NACE TM0177

Reading 5 (TM0177)

Fion Zhang/ Charlie Chong 27th Oct 2017

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Oil And Gas Production Industry

Fion Zhang/ Charlie Chong

Oil And Gas Production Industry

Fion Zhang/ Charlie Chong

Oil And Gas Production Industry

Fion Zhang/ Charlie Chong

Oil And Gas Production Industry

Fion Zhang/ Charlie Chong

Oil And Gas Production Industry

Fion Zhang/ Charlie Chong

Fion Zhang/ Charlie Chong

Fion Zhang/ Charlie Chong

Fion Zhang/ Charlie Chong

Fion Zhang/ Charlie Chong

过五关斩六将

NACE MR0175-Carbon Steel Written Exam NACE-MR0175-Carbon Steel -001 Exam Preparation Guide May 2017

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NACE MR0175-Carbon Steel Written Exam NACE-MR0175-Carbon Steel -001 Exam Preparation Guide May 2017

Introduction The MR0175-Carbon Steel written exam is designed to assess whether a candidate has the requisite knowledge and skills that a minimally qualified MR0175 Certified User- Carbon Steel must possess. The exam comprises 50 multiple-choice questions that are based on the MR0175 Standard (Parts 1 and 2).

multiple-choice Fion Zhang/ Charlie Chong

https://www.naceinstitute.org/uploadedFiles/Certification/Specialty_Program/MR0175-Carbon-Steel-EPG.pdf

EXAM BOK Suggested Study Material  NACE MR0175/ISO 15156 Standard (20171015-OK)  EFC Publication 17  NACE TM0177  NACE TM0198 NACE TM0316 Books  Introductory Handbook for NACE MR0175

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ANSI/NACE TM0177-2016 Item No. 21212 Standard Test Method Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments

Keywords:  SSC  SCC

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Foreword This standard addresses the testing of metals for resistance to cracking failure (SSC, SCC) under the combined action of tensile stress and corrosion in aqueous environments containing hydrogen sulfide (H2S). This phenomenon is generally termed:  sulfide stress cracking (SSC) when operating at room temperature and  stress corrosion cracking (SCC) when operating at higher temperatures.

In recognition of the variation with temperature and with different materials this phenomenon is herein called environmental cracking (EC). For the purposes of this standard, EC includes only SSC, SCC, and hydrogen stress cracking (HSC).

The primary purpose of this standard is to facilitate conformity in testing so that data from different sources can be compared on a common basis. Consequently, this standard aids the evaluation and selection of all types of metals and alloys, regardless of their form or application, for service in H2S environments. This standard contains methods for testing metals using tensile, bent-beam, C-ring, and double-cantileverbeam (DCB) test specimens. Certain ASTM(1) standard test methods have been listed as references for supplementary tests, creating a comprehensive test method standard. In addition, the four-point bent-beam test method is also referenced as a supplementary test.1,2 This standard is intended for use by laboratory and materials personnel to facilitate conformity in testing.

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Double-cantilever-beam (DCB) Test Specimens

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Four-point Bent-beam Test

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SSC of metals exposed to oilfield environments containing H2S was recognized as a materials failure problem by 1952. Laboratory data and field experience have demonstrated that even extremely low concentrations of H2S may be sufficient to lead to SSC failure of susceptible materials. In some cases, H2S can act synergistically with chlorides to produce corrosion and cracking (SSC and other mode) failures. However, laboratory and operating experiences have also indicated to materials engineers the optimum selection and specification of materials having minimum susceptibility to SSC. This standard covers test methods for SSC (at room temperature) and SCC (at elevated temperature), but other failure modes (e.g., hydrogen blistering, hydrogen-induced cracking [HIC], chloride stress corrosion cracking [SCC], pitting corrosion, and mass-loss corrosion) must also be considered when selecting materials for use in sour (H2S -containing) environments.

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The need for better understanding of the variables involved in EC of metals in oilfield environments and better correlation of data has become apparent for several reasons.

New design requirements by the oil and gas production industries call for higherstrength materials that, in general, are more susceptible to EC than lower-strength alloys. These design requirements have resulted in extensive development programs to obtain more resistant alloys and/or better heat treatments. At the same time, users in the petroleum refining and synthetic fuels industries are pushing present materials much closer to their mechanical limits.

Room-temperature (SSC) failures in some alloys generally are believed to result from hydrogen embrittlement (HE). When hydrogen is cathodically evolved on the surface of a metal (as by corrosion or cathodic charging), the presence of H2S (and other compounds, such as those containing cyanides and arsenic) tends to cause hydrogen atoms to enter the metal rather than to form hydrogen molecules that cannot enter the metal. In the metal, hydrogen atoms diffuse to regions of high triaxial tensile stress or to some microstructural configurations where they become trapped and decrease the ductility of the metal. Although there are several kinds of cracking damage that can occur in metals, delayed brittle fracture of metals resulting from the combined action of corrosion in an aqueous sulfide environment and tensile stresses (failure may occur at stresses far below the yield stress) is the phenomenon known as SSC.

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In some cases, however, failure may be the result of localized anodic corrosion processes that may or may not involve hydrogen. In such instances, failure is the result of anodic stress corrosion cracking (SCC). Such failures have historically been termed SSC even though their cause may not be hydrogen.

This standard was originally published in 1977 by NACE International Task Group T-1F-9, a component of Unit Committee T-1F, “Metallurgy of Oilfield Equipment.” The standard was revised in 1986, 1990, and 1996 by Task Group T-1F-9. It was revised in 2005 and 2016 by Task Group (TG) 085, “Sulfide Corrosion Cracking: Metallic Materials Testing Techniques.” TG 085 is administered by Specific Technology Group (STG) 32, “Oil and Gas Production—Metallurgy,” and is sponsored by STG 62, “Corrosion Monitoring and Measurement—Science and Engineering Applications.” The standard is issued by NACE under the auspices of STG 32.

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In NACE standards, the terms shall, must, should, and may are used in accordance with the definitions of these terms in the NACE Publications Style Manual. The terms shall and must are used to state a requirement, and are considered mandatory. The term should is used to state something good and is recommended, but is not considered mandatory. The term may is used to state something considered optional.

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Section 1: General 1.1 This standard covers the testing of metals subjected to tensile stresses for resistance to cracking failure in low-pH aqueous environments containing H2S. Carbon and low-alloy steels are commonly tested for EC resistance at room temperature where SSC susceptibility is typically high. For other types of alloys, the correlation of EC susceptibility with temperature is more complicated.

1.2 This standard describes reagents, test specimens, and equipment to use; discusses base material and test specimen properties; and specifies the test procedures to follow. This standard describes four test methods: Method A—Standard Tensile Test Method B—Standard Bent-Beam Test Method C—Standard C-Ring Test Method D—Standard Double-Cantilever-Beam (DCB) Test Sections 1 through 7 of this standard give general comments that apply to all four test methods. Sections 8 through 11 indicate the test method to follow for each type of test specimen. General guidelines to help to determine the suitability of each test method are given at the beginning of each test method description (Sections 8 through 11).

Reporting of the test results is also discussed.

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1.3 Metals can be tested for resistance to EC at temperatures and pressures that are either ambient (atmospheric) or elevated.

1.3.1 For testing at ambient conditions, the test procedures can be summarized as follows: Stressed test specimens are immersed in acidified aqueous environments containing H2S. Applied loads at convenient increments can be used to obtain EC data.

1.3.2 For testing at temperatures higher than 27°C (80°F), at either atmospheric or elevated pressure, Section 7 describes an alternative test technique. All methods (A, B, C, and D) are adaptable to this technique.

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1.4 This standard may be used for release or acceptance testing to ensure that the product meets a certain minimum level of EC resistance as prescribed in API (2) Specification 5CT,3 ISO (3) 11960,4 or as prescribed by the user or purchaser. This standard may also provide a quantitative measure of the product’s EC resistance for research or informational purposes. This rating may be based on:  Method A—The highest no-failure uniaxial tensile stress in 720 hours.  Method B—The statistically based critical stress factor (Sc) for a 50% probability of failure in 720 hours.  Method C—The highest no-failure circumferential stress in 720 hours.  Method D—The average KISSC (threshold stress intensity factor for SSC) for valid tests of replicate test specimens.

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KISCC

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1.5 Safety Precautions: H2S is an extremely toxic gas that must be handled with care. (See Appendix A [Non-mandatory]).

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Section 2: Environmental Cracking Testing Variability 2.1 Interpretation of stress corrosion test results is a difficult task. The test methods contained in this standard are severe, with accelerated tests making the evaluation of the data extremely difficult. In testing the reproducibility of the test methods among different laboratories, several undesirable side effects (frequent with many accelerated tests) that must be noted include: 2.1.1 The test environment may cause failure by HIC and hydrogen blistering.

This is especially true for lower-strength steels not usually subject to SSC. HIC may be detected by visual and metallographic observations. Blistering is normally visible on the test specimen surface. (For further information regarding this phenomenon, see NACE Standard TM0284).5 2.1.2 The test environment may corrode some alloys that normally do not corrode in actual field service and thereby induce EC failures in alloys that ordinarily do not fail by EC. This problem is especially acute with the martensitic and precipitation-hardened stainless steels.

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ANSI/NACE MR0175/ISO 15156 Part:1 3.12 hydrogen-induced cracking HIC planar cracking that occurs in carbon and low alloy steels when atomic hydrogen diffuses into the steel and then combines to form molecular hydrogen at trap sites Note 1 to entry: Cracking results from the pressurization of trap sites by hydrogen. No externally applied stress is required for the formation of hydrogen-induced cracks. Trap sites capable of causing HIC are commonly found in steels with high impurity levels that have a high density of planar inclusions and/or regions of anomalous microstructure (3.15)(e.g. banding) produced by segregation of impurity and alloying elements in the steel. This form of hydrogen-induced cracking is not related to welding.

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ANSI/NACE MR0175/ISO 15156 Part:1 3.23 sulfide stress cracking SSC cracking of metal involving corrosion and tensile stress (residual and/or applied) in the presence of water and H2S Note 1 to entry: SSC is a form of hydrogen stress cracking (HSC) (3.13) and involves the embrittlement of the metal by atomic hydrogen that is produced by acid corrosion on the metal surface. Hydrogen uptake is promoted in the presence of sulfides. The atomic hydrogen can diffuse into the metal, reduce ductility, and increase susceptibility to cracking. High-strength metallic materials and hard weld zones are prone to SSC.

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2.2 Furthermore, other aspects to be considered in the selection of test method(s) include: 2.2.1 Material anisotropy affecting mechanical properties and EC susceptibility can be an important parameter. The fracture path in the test specimen should match what is anticipated in the actual component.

2.2.2 Galvanic effects between dissimilar metals can either accelerate or suppress cracking susceptibility. Examples of this behavior are accelerated EC in some nickel-based corrosion-resistant alloys (CRAs) and reduced EC in some duplex stainless steels when these materials are coupled to electrochemically less-noble materials such as carbon and low-alloy steels.

2.2.3 Test temperature affects cracking susceptibility. Test temperatures above 24 째C (75 째F) can reduce SSC severity in steels, whereas test temperatures below 24 째C (75 째F) can increase SSC severity.

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2.2.4 Different test methods may not necessarily provide the same rankings of like materials.

2.2.5 Material inhomogeneity, such as weldments and segregation, can affect test results. This is particularly true when results from tests that evaluate a large volume of material (tensile test) versus a small volume of material (bentbeam test) are compared.

2.2.6 Maximum no-failure stresses for a specified exposure period should be considered apparent threshold stresses. Longer exposure times or larger numbers of test specimens may result in lower threshold values.

2.2.7 EC test results can show statistical variability. Replicate testing may be needed to obtain a representative value characterizing resistance to EC. 2.2.8 Some test specimens are better suited than others for measuring EC resistance in localized areas (e.g., near surfaces or other features, and in weld zones).

2.2.9 Some types of EC tests require considerably more time than others for determination of EC resistance.

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Section 3: Reagents 3.1 Reagent Purity 3.1.1 The test gases, sodium chloride (NaCl), acetic acid (CH3COOH), sodium acetate (CH3COONa), and solvents shall be reagent grade or chemically pure (99.5% minimum purity) chemicals. (See Appendix B [Nonmandatory]).

3.1.2 The test water shall be distilled or deionized and of quality equal to or greater than ASTM Type IV (ASTM D11936). Tap water shall not be used.

3.2 Inert gas shall be used for removal of oxygen. Inert gas (nitrogen, argon, or other suitable nonreactive gases) shall be pre-purified (99.998% or greater).

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Section 4: Test Specimens and Material Properties 4.1 The location and orientation of the test specimens to be removed from the product should be carefully considered so that test results provide the most meaningful indication of the performance to be expected from that product when placed in service. All test specimens in a set should be taken from metallurgically equivalent positions (i.e., all test specimens should have the same orientation with similar or nearly the same microstructure and mechanical properties).

4.2 When specified, tensile testing in accordance with standard test methods such as ASTM A3707 shall be used to determine base material properties.

Two or more test specimens shall be pulled, and the individual test results shall be averaged to determine the yield and ultimate strengths, percent elongation, and percent reduction in area for the material. Machining a tensile test specimen from material adjacent to and in the same position and orientation as the EC test specimen to be tested can minimize material property variations that normally occur from test specimen to test specimen.

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4.3 A number of fundamental material properties correlate with EC susceptibility.

Consequently, when specified, all pertinent data on chemical composition, mechanical properties, heat treatment, and mechanical histories (such as percent cold reduction or prestrain) shall be determined and reported with the tensile test data. Each different heat treatment and microstructure of a material of a fixed chemical composition shall be tested as though it were a different material.

4.4 When specified, hardness measurements shall be performed in accordance with ASTM E18 (Rockwell)8 or ASTM E384 (Vickers)9 before or after exposure to the test environment. These measurements shall not be made on the stressed evaluation portion of the test specimen. For test Methods A (subsize) and B, hardness shall be measured (1) on the test specimen, or (2) on an adjacent hardness test specimen that was sampled from a similar thickness/cross-section location as the material being tested. For test methods A (standard size), C, and D, hardness shall be measured on the test specimen. At least 3 indentations shall be performed on each specimen.

Hardness test results shall be reported.

NOTE: When hardness testing on convex cylindrical surfaces, the results may not accurately indicate the true hardness. Corrections applied to the measured hardness shall be in accordance with the selected ASTM standard and reported.

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Section 5: Test Vessels and Fixtures 5.1 The size, shape, and entry ports of the test vessel shall be determined by the actual test specimens and test fixtures used to stress the test specimens.

5.2 Test vessels shall be capable of being purged to remove oxygen before beginning the test and of keeping air out during the test. Using a small outlet trap on the H2S effluent line to maintain 250 Pa (0.036 psi) (1.0 inch in H2O) back pressure on the test vessel may be used to prevent oxygen entry through small leaks or by diffusion up the vent line. (See Appendix B section titled “Reasons for Exclusion of Oxygen�).

5.3 Test vessels shall be sized to maintain the test solution volume within the specified limits relative to the test specimen surface area to standardize the drift of pH with time. (See each test method for specified limits).

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5.4 Test vessels shall be constructed from materials that are suitable for the test environment. While some plastic test vessels and solution storage vessels give satisfactory service, others may cause varying test results from the time they are new until after they have been in continuous use. If plastic vessels are used, then the vessels shall be pre-conditioned with a documented procedure that has been validated. Glass test vessels have not exhibited this tendency and should not require pre-conditioning.

5.5 Test specimens shall be electrically isolated from test vessels and test fixtures made from dissimilar metals if the dissimilar metal is in contact with the test environment.

5.6 Rigid electrical insulating materials not exhibiting relaxation or flow under load should be selected for loading or deflecting the test specimen.

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5.7 Galvanic Coupling 5.7.1 It may be necessary to evaluate the effects of galvanic coupling on EC resistance, such as in the case of coupling stainless alloys or CRAs to steel (see Paragraph 2.2.2).

5.7.1.1 To evaluate this, galvanic couples of iron or steel having a surface area between 0.5 and 1 times the exposed area of the test specimen should be bolted securely to the test specimen.

5.7.2 Particles of iron sulfide are electrically conductive. If deposited on insulating materials, they can provide electrical connection between materials and affect the results of the tests.

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Section 6: Test Solutions 6.1 General 6.1.1 All reagents added to the test solutions shall be measured to ±1.0% of the quantities specified for the specific test solution.

6.1.2 The test solution shall be maintained at 24 ± 3°C (75 ± 5°F) for Methods A, B, and C; and 24 ±1.7 °C (75 ± 3.0°F) for Method D. Also, the test temperature range shall be specified in accordance with testing at elevated temperature (see Section 7). Any variations beyond this range shall be reported.

6.1.3 The test solution used for each test method shall be reported.

6.1.4 If required, the concentration of H2S in the test solution may be measured by iodometric titration or by other suitable methods. Accepted iodometric titration and reduced concentration of H2S procedures are detailed in Appendix C (Nonmandatory).

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6.1.5 Solutions and gas compositions other than those detailed below may also be used for fit-for-purpose testing and shall be reported.

6.1.6 The initial pH (before the introduction of the test gas) and final pH (at the end of the test) shall be measured and reported.

6.1.7 Unless expressly permitted in this standard, pH shall not be intentionally adjusted during the test.

6.1.8 Gas mixtures shall be certified pre-mixed or verified by suitable means, provided the pure component gases are individually certified.

6.1.9 The start of the test shall be measured from the time that saturation is achieved.

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6.2 Test Solution A 6.2.1 Test Solution A is an acidified H2S-saturated aqueous brine solution.

6.2.2 Test Solution A shall consist of 5.0 wt% sodium chloride and 0.5 wt% glacial acetic acid dissolved in distilled or deionized water (e.g., 50.0 g of NaCl and 5.0 g [4.8 mL] of CH3COOH dissolved in 945 g of distilled or deionized water).

6.2.3 Test solution pH before contact with a test specimen and before H2S saturation shall be between 2.6 and 2.8. Adjustment of test solutions chemistry to adjust pH is not allowed. If test solution pH is out-of-range then the solution shall be discarded. Each laboratory shall have a demonstrated and documented procedure for purging to verify that the pH has not exceeded 3.0 for Test Solution A after purging. During the test, pH may increase but shall not exceed 4.0. If the pH exceeds 4.0, the test is invalid. If the test-solutionvolume to test-specimen-surface-area ratio is maintained and steps are taken to exclude oxygen from the test vessel as specified in this standard, the pH will not exceed this value.

6.2.4 Test Solution A shall be used in Methods A, C, and D unless the use of Test Solution B, C or D is specified by the user or purchaser.

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6.3 Test Solution B 6.3.1 Test Solution B is an acidified and buffered H2S-saturated aqueous brine solution.

6.3.2 Test Solution B shall consist of 5.0 wt% sodium chloride, 2.5 wt% glacial acetic acid, and 0.41 wt% sodium acetate dissolved in distilled or deionized water (e.g., 50.0 g of NaCl, 25.0 g [23.8 mL] of CH3COOH, and 4.1 g of CH3COONa dissolved in 921 g of distilled or deionized water).

6.3.3 Test solution pH before contact with a test specimen and before H2S saturation shall be between 3.4 and 3.6. Adjustment of test solution chemistry to adjust pH is not allowed. If test solution pH is out- of-range, then the solution shall be discarded. During the test, pH may increase but shall not exceed 4.0. If the pH exceeds 4.0, the test is invalid. If the test-solutionvolume to test-specimen-surface-area ratio is maintained and steps are taken to exclude oxygen from the test vessel as specified in this standard, the pH will not exceed this value.

6.3.4 Test Solution B may be used with carbon and low-alloy steels.

6.3.5 Test Solution B may be used in Methods A, C, and D.

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6.4 Test Solution C 6.4.1 Test Solution C is a buffered aqueous brine solution with a chloride content, H2S partial pressure, and pH specified by the user or purchaser to simulate the intended service environment.

6.4.2 Test Solution C shall consist of distilled or deionized water containing 0.4 g/L sodium acetate (5 mM CH3COONa) and chloride (added as NaCl) at the same concentration as the intended service environment.

6.4.3 Hydrochloric acid (HCl) or sodium hydroxide (NaOH) shall be added to achieve the specified pH. The test solution pH shall be measured at the start of the test and at the end of the test. The test solution pH shall be maintained within 0.2 pH units of the specified pH.

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6.4.4 Test gas shall consist of a mixture of H2S and carbon dioxide (CO2), with H2S content sufficient to produce the specified H2S partial pressure of the intended service environment. The test gas shall be continuously bubbled through the test solution. The gas bubbling rate shall be optimized to maintain saturation of the test solution.

6.4.5 Test Solution C may be used with martensitic stainless steels.

6.4.6 Test Solution C may be used in Methods A, C, and D. NOTE: The combination of a lower acetate concentration (0.4 g/L) and acidification with HCl rather than acetic acid leads to a significantly reduced concentration of both acetic acid and acetate ion in Test Solution C when compared with Test Solution B. Although this may be adequate for maintaining the bulk solution pH constant when testing corrosion-resistant alloys, the ability to resist changes of pH at the metal-solution interface is reduced. In cathodic regions, there is less acetic acid to counterbalance the tendency for an increased pH. In localized anodic regions, there is less acetate to restrain lowering of the pH due to metal ion hydrolysis. In the latter case, the lower acetate concentration associated with Test Solution C would give a more conservative result, in the sense that cracking could be more likely.

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6.5 Test Solution D 6.5.1 Test Solution D is a buffered aqueous brine solution with a chloride content, H2S partial pressure, and pH specified for mild sour conditions.

6.5.2 Test Solution D shall consist of 5 wt% sodium chloride and 0.40 wt% sodium acetate dissolved in distilled or deionized water (e.g., 50.0 g of NaCl and 4.0 g of CH3COONa dissolved in 946 g of distilled or deionized water).

6.5.3 Hydrochloric acid (HCl) or sodium hydroxide (NaOH) shall be used to adjust the initial pH. The solution pH before contact with a test specimen and before H2S saturation shall be between 3.8 and 4.0. During the test, pH may increase, but shall not exceed 4.6. If the pH exceeds 4.6, the test is invalid. If the test-solution-volume to test-specimen-surface-area ratio is maintained and steps are taken to exclude oxygen from the test vessel as specified in this standard, the pH will not exceed this value.

6.5.4 The test gas shall consist of 7.0 Âą 0.3 mol.% H2S with balance of nitrogen. After initial saturation, the gas should be bubbled continuously at a rate that maintains saturation.

6.5.5 Test Solution D may be used with high-strength steels such as API 5CT Grade C110.

6.5.6 Test Solution D may be used in Methods A, C, and D.

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6.6 The test solution required for use in Method B is specified in Paragraph 9.3.

6.7 Deaeration Requirements and Procedures 6.7.1 Deaeration Requirements The method of solution deaeration and solution transfer used for testing shall have the dissolved oxygen levels in the test solution (immediately prior to the introduction of the H2S or H2S mixed gas) of less than 50 ppb for low-alloy steels up to the strength level of 552 MPa (80 ksi) and 10 ppb for low-alloy steels above the strength level of 552 MPa (80 ksi) and corrosion resistant alloys. Each laboratory shall have a demonstrated and documented procedure to achieve the required dissolved oxygen levels.

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6.7.2 Deaeration Procedures 6.7.2.1 The test solution shall be previously deaerated in a sealed vessel that is purged with inert gas at a rate of at least 100 mL/min per liter of test solution for at least one hour. The test vessel containing the test specimen(s) shall be purged separately with inert gas at a rate of at least 100 mL/min per liter of test solution for at least one hour. The deaerated test solution shall be transferred into the pre-purged test vessel, and it shall be further deaerated at a rate of at least 100 mL/min per liter of test solution for at least one hour or until the required dissolved oxygen level is reached.

6.7.2.2 Other methods of deaeration and transfer may be used if they have been demonstrated to achieve the required dissolved oxygen levels.

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6.7.3 Oxygen contamination is evident by a cloudy (opaque) appearance of the test solution when the H2S gas enters the test vessel. An opaque appearance of the test solution upon H2S entry shall disqualify the test. The test specimen shall be removed and cleaned, and the test solution make-up, transfer, and deaeration procedure repeated.

Note: To achieve the required oxygen levels, consideration needs to be given to the following factors: type of tubing used, ingress of oxygen into the test cell during the test, etc.

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Section 7: Testing at Elevated Temperature/Pressure 7.1 The dominant cracking mechanisms for most classes of materials in the presence of H2S vary with temperature. ď ą Ferritic steels and ferritic and martensitic stainless steels crack primarily by a hydrogen (i.e., cathodic) mechanism and have maximum susceptibility near room temperature. ď ą For austenitic stainless steels, as temperature increases, cracking susceptibility increases due to the major contribution from anodic processes. ď ą Duplex stainless steels exhibit mixed behavior, with maximum susceptibility to cracking in a mid-range of temperatures.

To facilitate testing in simulated service conditions or to predict worst-case conditions, and to facilitate testing with H2S partial pressure exceeding 100 kPa (absolute) (14.5 psia), the following modified techniques are available.

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To facilitate testing in simulated service conditions or to predict worst-case conditions, and to facilitate testing with H2S partial pressure exceeding 100 kPa (absolute) (14.5 psia), the following modified techniques are available.

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http://www.connection-mag.com/?p=2728

7.2 Testing at elevated temperatures and pressures involves additional safety considerations compared to room temperature and atmospheric pressure testing. While some general guidance is given herein, it may not address all aspects and should be supplemented to comply with local safety requirements. Because H2S may be consumed during the test, gas replenishment and continuous gas bubbling techniques are described. The H2S loss rate and its effect on the corrosiveness of the test environment are functions of several factors, including the corrosion rate of the test material and the partial pressure of H2S in the test environment. Guidance is given on measures that experience has shown to be appropriate for maintaining the required H2S partial pressure, but in all cases, it shall be demonstrated, by measuring H2S concentration in either the test solution or gas phase, that the required test conditions have been maintained. This information must be reported with the test data.

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7.3 Test Equipment The test equipment shall consist of a test vessel and accessory equipment rated to withstand corrosion and pressure commensurate with the test conditions and with an appropriate safety margin.

1. 7.3.1 The test vessel shall be equipped with a thermocouple well or other means of measuring the temperature of the test solution, inlet and outlet ports for gas, a dip tube on the inlet port, and a pressuremeasuring device.

2. 7.3.2 If continuous gas bubbling is to be used, a condenser on the outlet port may be used to limit loss of test solution. This has been found to be useful at temperatures greater than 50 °C (120 °F) and/or when the volume of the test solution is less than 200 mL. 3. 7.3.3 A bursting (rupture) disc or pressure-relief valve should be used for safety reasons.

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Test Equipment

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Test Equipment

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http://cortest.com/wp-content/uploads/pdfs/PROOF_RINGS.pdf

Test Equipment

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http://cortest.com/wp-content/uploads/pdfs/PROOF_RINGS.pdf

Test Equipment

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http://cortest.com/wp-content/uploads/pdfs/PROOF_RINGS.pdf

Test Equipment

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http://cortest.com/wp-content/uploads/pdfs/PROOF_RINGS.pdf

Test Equipment

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http://cortest.com/wp-content/uploads/pdfs/PROOF_RINGS.pdf

Test Equipment

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http://cortest.com/wp-content/uploads/pdfs/PROOF_RINGS.pdf

Test Equipment

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http://cortest.com/wp-content/uploads/pdfs/PROOF_RINGS.pdf

Test Equipment

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http://cortest.com/wp-content/uploads/pdfs/PROOF_RINGS.pdf

Test Equipment

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http://cortest.com/wp-content/uploads/pdfs/PROOF_RINGS.pdf

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Test Equipment

http://cortest.com/wp-content/uploads/pdfs/PROOF_RINGS.pdf

7.3.4 The pressure-measuring device shall have an accuracy of Âą1% of the maximum system pressure. If the pressure is measured by a gauge, the maximum system pressure shall be greater than 20% and less than 80% of gauge full scale. Schematic arrangements of test equipment used for the various test methods are shown in Figures 1 and 2.

7.3.5 Elastomeric seal materials, if used, must resist H2S at the temperature of use as verified by independent measurement.

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Figure 1: Schematic Arrangement of Test Equipment for Method A— NACE Standard Tensile Test

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Figure 2: Schematic Arrangement of Test Equipment for Method B— NACE Standard Bent- Beam Test, Method C—NACE Standard C-Ring Test, and Method D—NACE Standard Double- Cantilever-Beam Test

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Schematic Arrangement of Test Equipment for Method A—NACE Standard Tensile Test

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7.4 Test Solution The test solution used in the test may be selected as required by the test specification. The test solution usually consists of brine (NaCl) at concentrations up to saturation. Buffered acidification is permitted, analogous to room-temperature methods. Also, the test solution may contain elemental sulfur.

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7.5 Test Gas The test gas is usually a mixture of two or more of the following: H2S, CO2, and inert gas such as N2 or Ar. At low H2S partial pressures, tests in inert gas without CO2 require careful interpretation because of corrosion product solubility effects. The test gas mixture should be contained in a standard gas bottle equipped with a suitable pressure regulator (usually stainless steel) capable of gas delivery to the total test pressure required. A commercially supplied gas mixture with composition determined by analysis should be used.

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7.6 Test Stresses The test stresses may be derived from the actual yield strength at ambient temperature or from the actual yield strength at the planned test temperature or as otherwise required by the test specification.

7.7 Test Procedure Test procedures shall be identical to those specified for room-temperature tests unless amended as follows:

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7.7.1 The test solution and test specimen(s) shall be placed in the test vessel, then the test vessel shall be sealed and leak tested. Test vessels are usually tested for leaks with inert gas at 1.5 times the maximum test pressure.

7.7.2 The expansion of test solution on heating can fill the test vessel and risk explosion. The volume of test solution should be less than 75% of the total volume of the test vessel. Moreover, a greater safety margin (smaller percentage of total volume) should be used at temperatures exceeding 225 °C (435 °F).

7.7.3 The test solution shall be deaerated by bubbling inert gas through the gas inlet tube into the test solution for a minimum period of 1 h/L of test solution.

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7.7.4 The H2S partial pressure, ppH2S, in the test environment shall be determined by one of the following two methods: 7.7.4.1 Test vessel heated before test gas admitted 7.7.4.1.1 The test vessel shall be heated with valves closed to test temperature and stabilized. System pressure (the vapor pressure of the test solution), P1, shall be measured.

7.7.4.1.2 Test gas shall be admitted to the vessel until the test pressure, PT, is reached.

7.7.4.1.3 The H2S partial pressure, ppH2S (PH S), in the test environment is approximated by Equation (1): 2

ppH2S = (PT – P1) (XH S) (1) 2

where: PT = total absolute test pressure; P1 = vapor pressure above the test solution; and XH S= mole fraction of H2S in the test gas. (?)

2

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7.7.4.2 Test gas admitted before test vessel heated.

Test gas may be admitted to the test vessel before heating if a proven means of calculating ppH2S can be demonstrated.

7.7.4.3 The test method used shall be reported with the test data.

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7.7.5 Test gas shall be replenished as needed to maintain the required test conditions (primarily H2S partial pressure) as outlined in Paragraph 7.2.

Continuous test gas bubbling at 0.5 to 1.0 mL/min or periodic test gas replenishment once or twice weekly has been found necessary when testing: ď ą CRAs at H2S partial pressures below 2 kPa (absolute) (0.3 psia) or ď ą Carbon and alloy steels at H2S partial pressures below 100 kPa (absolute) (14.5 psia).

Test solution loss and ingress of oxygen during test gas replenishment shall be avoided.

Question?: Why lenient for carbon steel

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mechanisms.

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7.7.6 The test duration shall be as specified for the applicable test method (A, B, C, or D). The test temperature for Methods A, B, and C shall be maintained within ±3 °C (±5 °F) of the specified test temperature and recorded manually on a daily basis or at shorter intervals by data recorder. For Method D, test temperature shall be maintained within ±1.7 °C (±3.0 °F). Pressure shall be monitored and recorded daily. If test pressure falls by more than 40 kPa (6 psi) below the required test pressure, the test gas must be replenished.

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7.7.7 At the test completion, the test vessel should be purged with inert gas while cooling to ambient temperature before opening. The load should be relaxed before cooling, if possible, when using equipment with external loading.

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Section 8: Method A—NACE Standard Tensile Test 8.1 Method A, the NACE standard tensile test, provides for evaluating metals for EC resistance under uniaxial tensile loading. It offers a simple unnotched test specimen with a well-defined stress state. EC susceptibility with Method A is usually determined by time-to-failure. Tensile test specimens loaded to a particular stress level give a failure/no-failure test result. When multiple test specimens are tested at varying stress levels, an apparent threshold stress for EC can be obtained.

8.1.1 This section sets forth the procedure for testing at room temperature and atmospheric pressure. Special considerations for testing at elevated temperature and pressure are set forth in Section 7.

Tensile test specimens loaded to a particular stress level give a failure/no-failure test result.

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8.2 Test Specimen 8.2.1 The size and shape of the material available for testing often restricts selection of test specimens. The orientation of the test specimen can affect the results and should be noted.

8.2.2 The gauge section of the standard tensile test specimen (see Figure 3[a]) shall be 6.35 ± 0.13 mm (0.250 ± 0.005 in) in diameter by 25.4 mm (1.00 in) long (see ASTM A370). A subsize tensile test specimen with gauge section of 3.81 ± 0.05 mm (0.150 ± 0.002 in) in diameter by 25.4 mm (1.00 in) long is acceptable. After machining, tensile test specimens should be stored in a lowhumidity area, in a desiccator, or in uninhibited oil until ready for testing.

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8.2.3 The radius of curvature at the ends of the gauge section shall be at least 15 mm (0.60 in) to minimize stress concentrations and fillet failures.

Additional methods that have been found helpful in reducing fillet failures are to: (1) Eliminate undercutting of fillet radii in machined test specimens; and (2) Machine the test specimen gauge section with a slight (0.05 to 0.13 mm [0.002 to 0.005 in]) taper that produces a minimum cross-section in the middle of the gauge section.

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8.2.4 The ends of the test specimen must be long enough to accommodate seals for the test vessel and to make connections to the stressing fixture. (See Figure 3[b]).

8.2.5 The test specimen must be machined or ground carefully to avoid overheating and cold working in the gauge section. In machining operations, the final two passes should remove no more than a total of 0.05 mm (0.002 in) of material. Grinding is also acceptable if the grinding process does not harden or temper the material.

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Figure 3: Tensile Test Specimens (a) Dimensions of the Tensile Test Specimens

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Figure 3: Tensile Test Specimens (b) Tensile Test Specimen in an Environmental Chamber

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Tensile Test Specimen in an Environmental Chamber – Constant Load

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Tensile Test Specimen in an Environmental Chamber- Sustained Load

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http://subodhlabs.com/labServices.html

8.2.6 For all materials, the average surface roughness of the gauge section shall be 0.25 Îźm (10 Îźin) or finer, as defined by Ra value in ISO 4287.15 Final surface finish may be obtained by mechanical polishing or electropolishing if the roughness requirement is met. The finishing processes shall be reported with the test data. When electropolishing, bath conditions must be such that the test specimen does not absorb hydrogen during the procedure. When agreed with the end user, electropolishing shall only be used with low-alloy steels having a maximum of 1.5 weight percent chromium level. Each laboratory shall have a demonstrated and documented procedure for electropolishing and validating that electropolishing has comparable results to surface ground/mechanically polished specimens (per test material, grade and test condition).

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8.2.7 When a standard tensile test specimen cannot be obtained from the material because of its size or shape, a subsize tensile test specimen may be used. However, subsize tensile test specimens can produce shorter failure times than those observed for standard tensile test specimens. The report of test data using subsize tensile test specimens shall clearly state the use of subsize test specimens. If an alternate specimen configuration (one not specified in this document) is used, the dimensions shall be clearly stated in the test report.

8.2.8 Test Specimen Identification 8.2.8.1 Stamping or vibratory stenciling may be used on the ends of the test specimen, but shall not be used in the gauge section.

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8.2.9 Test Specimen Cleaning 8.2.9.1 Before testing, test specimens shall be degreased with solvent and rinsed with acetone.

8.2.9.2 The gauge section of the test specimen shall not be handled or contaminated after cleaning.

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8.3 Test Solutions—see Section 6.

8.4 Test Equipment 8.4.1 Many types of stress fixtures and test vessels used for stress corrosion testing are acceptable for Method A. Consequently, the following discussion emphasizes the test equipment characteristics required for selecting suitable items and procedures.

8.4.2 Tensile tests should be performed with constant-load or sustained-load (proof-ring or spring-loaded) devices (see ASTM G49).

8.4.2.1 All loading devices shall be calibrated to ensure accurate application of load to the test specimen. The error for loads within the calibration range of the loading device shall not exceed 1.0% of the calibration load.

8.4.2.2 The loading device shall be constructed and maintained to minimize bending and torsional loads.

Note: Unwanted bending stress increases the maximum total stress and may lead to an over-conservative test, specimen failure, and rejection of a material which otherwise would pass the Method A test if bending was absent or minimized.

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8.4.3 When susceptible materials are tested using sustained-load devices, it is possible for cracks to initiate and propagate only partially, not fully, through the test specimen (see Paragraph 8.7). Consequently, susceptibility determination from sustained-load test results requires the visual examination of the test specimens for the presence of part-through cracks. The determination may be difficult if the cracks are small and sparse or if obscured by corrosion deposits.

However, testing with constant-load devices ensures that susceptible materials will separate completely. This result clearly identifies the material as susceptible and does not rely on finding part-through cracks.

Question: When susceptible materials are tested using sustained-load devices, it is possible for cracks to initiate and propagate only partially, not fully, through the test specimen? Why? Load relaxation?

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8.4.4 Dead-weight testers capable of maintaining constant pressure on a hydraulic cell may be used for constant-load testing (see Figure 4).

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Figure 4: Constant-Load (Dead-Weight) Device

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8.4.5 Sustained-load tests may be conducted with spring-loaded devices and proof rings when relaxation in the fixtures or test specimen results in only a small percentage decrease in the applied load (see Figure 5).

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Figure 5: Sustained-Load Devices (a) Proof ring

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Figure 5: Sustained-Load Devices (b) Spring-loaded

8.4.5.1 If using proof rings, the following procedures are required: 8.4.5.1.1 Before calibration, proof rings shall be preconditioned by stressing at least 10 times to 110% of the maximum load rating of the proof ring.

8.4.5.1.2 The load on the tensile test specimen shall lie within the load range of the proof ring. Accordingly, proof rings shall be selected so that the applied load produces a ring deflection of more than 0.6% of the ring diameter, but not less than 0.51 mm (0.020 in). If it is less than 0.51 mm (0.020 in) or less than 0.6% of the ring diameter, the calibration deflection, calibration load, and test load must be specified.

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8.4.5.2 A substantial decrease in the proof ring deflection may signify: (a) The initiation and propagation of cracks in the test specimen; (b) Yielding of the test specimen; or (c) Relaxation of stress.

The proof ring deflection should be measured during the test or at the test completion.

8.4.5.3 The deflection should be monitored when the applied stress is within 10% of the material yield strength.

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The deflection should be monitored when the applied stress is within 10% of the material yield strength.

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The deflection should be monitored when the applied stress is within 10% of the material yield strength.

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https://www.youtube.com/watch?v=pcs11NmlTLU

8.4.6 The test specimen must be electrically isolated from any other metals in contact with the test solution.

8.4.6.1 The seals around the test specimen must be electrically isolating and airtight, but should allow movement of the test specimen with negligible friction.

8.4.6.2 In cases in which the complete test fixture is immersed in a test solution, the stressing fixture may be made of the same material, or if it is made of a different material, it must be electrically isolated from the test specimen. The stressing fixture may be coated with a nonconductive impermeable coating, if desired.

8.4.7 The test vessel shall be sized to maintain a test solution volume of 30 Âą 10 mL/cm2 of test specimen surface area.

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8.5 Stress Calculations Loads for stressing tensile test specimens shall be determined from Equation (2): P=SxA

(2)

where: P = load; S = applied stress; and A = actual cross-sectional area of the gauge section.

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8.6 Testing Sequence 8.6.1 The minimum gauge diameter of the tensile test specimen shall be measured, and the tensile test specimen load shall be calculated for the desired stress level.

8.6.2 The tensile test specimen shall be cleaned and placed in the test vessel, and the test vessel shall be sealed to prevent air leaks into the vessel during the test.

8.6.3 The load may be applied before or after the test vessel is purged with inert gas. Tensile test specimens may be stressed at convenient increments of the yield strength or load.

8.6.4 The load should be carefully applied to avoid exceeding the desired value. If the desired load is exceeded, the test shall be run at the new load or discarded.

8.6.5 Deaeration requirements are given in Paragraph 6.7.

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8.6.6 The test solution shall then be saturated with H2S at a minimum flow rate per a documented procedure that has been validated to obtain 2,300 mg/L minimum H2S concentration or other minimum value H2S concentration that is proportional to less than 1 bar H2S partial pressures (at the extent of the test specimen locations). When Solution D is used, the minimum H2S concentration shall be 160 mg/L. Saturation shall occur within an hour of contact with specimens in test vessels up to 1 L (see Note 1). For larger test vessels, saturation can require more than two hours. Analysis shall be done using iodometric titration (See Appendix C or other suitable methods).

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The validation of the documented procedure shall be performed:  after saturation at the start of test,  after 24 hours,  weekly and  at the end of the test.

A continuous flow of H2S through the test vessel and outlet trap shall be maintained with a positive pressure of H2S throughout the test that prevents air from entering the test vessel.

NOTE 1: One method found to give saturation within one hour in a proof ring test (approximately 1/3 L) is to purge at 100 mL per minute for 60 minutes.

NOTE 2: Laboratories at high elevations may find it necessary to compensate for lower atmospheric pressure in order to achieve the required saturation levels.

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8.6.7 The termination of the test shall be at tensile test specimen failure or after 720 hours, whichever occurs first.

8.6.8 When needed, additional tensile test specimens shall be tested to closely define the no-failure stress.

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8.7 Failure Detection Following exposure, the surfaces of the gauge section of the nonfailed tensile test specimens shall be cleaned and inspected for evidence of cracking.

Those tensile test specimens containing cracks shall be noted.

8.7.1 For all materials, failure is either: (a) Complete separation of the tensile test specimen; or (b) Visual observation of cracks on the gauge section of the tensile test specimen at 10X after completing the 720 hour test duration. Investigative techniques employing metallography, scanning electron microscopy, or mechanical testing may be used to determine whether cracks on the gauge section are evidence of EC. If it is verified that the cracks are not EC, then the tensile test specimen passes the test.

8.7.2 Time-to-failure may be recorded using electrical timers and microswitches.

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8.8 Reporting of Test Results 8.8.1 Time-to-failure and no-failure data or the visual observation of surface cracks at the end of the test shall be reported for each stress level (see Table 1).

8.8.2 If known, the chemical composition, heat treatment, mechanical properties, other information specified above, and data taken shall be reported.

8.8.3 Table 1 shows the recommended format for reporting the data. Data may also be presented on semilog graph paper (see Figure 6).

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Table 1

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Table 1

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Table 1

(A) Test method must be fully described if not in accordance with TM0177.

(B) Melt practice: open-hearth (OH), basic oxygen furnace (BOF), electric furnace (EF), argon-oxygen decarburization (AOD).

(C) E.g., cold work, plating, nitriding, prestrain.

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Figure 6: Applied Stress vs. Log (Time-to-Failure)

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Section 9: Method B—NACE Standard Bent-Beam Test 9.1 Method B, the NACE Standard Bent-Beam Test, provides for testing carbon and low-alloy steels subjected to tensile stress to evaluate resistance to cracking failure in low-pH aqueous environments containing H2S. It evaluates EC susceptibility of these materials in the presence of a stress concentration. The compact size of the bent-beam test specimen facilitates testing small, localized areas and thin materials. Bent-beam test specimens loaded to a particular deflection give a failure/no-failure test result. When testing multiple test specimens at varying deflections, a statistically based critical stress factor (Sc) for a 50% probability of failure can be obtained. NaCl is not added to the test solution for this test method. Laboratory test data for carbon and low-alloy steels have been found to correlate with field data.

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9.1.1 This section sets forth the procedure for bent-beam testing at room temperature and atmospheric pressure. Special considerations for testing at elevated temperature and pressure are set forth in Section 7.

9.1.2 Method B can be summarized as follows: 9.1.2.1 This method involves deflecting each test specimen in a series by applying a different bending stress. The stressed test specimens then are exposed to the test environment, and the failure (or no-failure) by cracking is determined. From these data obtained by testing multiple test specimens at varying deflections, a statistically based Sc for a 50% probability of failure is calculated to indicate the material’s resistance to SSC. 9.1.2.2 This method constitutes a constant-deflection test of high test specimen compliance. The computed stress is called a pseudo-stress because it does not reflect: (a) Actual stress or stress distribution in the test specimen; (b) Deviation from elasticity associated with plastic deformation; or (c) Decrease in stress in the test specimen as a crack or cracks grow.

Consequently, this method is not suitable for determination of threshold stress.

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9.2 Test Specimen 9.2.1 The dimensions of the standard bent-beam test specimen shall be 4.57 ± 0.13 mm (0.180 ± 0.0050 in) wide, 1.52 ± 0.13 mm (0.060 ± 0.0050 in) thick, and 67.3 ± 1.3 mm (2.65 ± 0.050 in) long (see Figure 7). After machining, test specimens shall be stored in a low-humidity area, in a desiccator, or in uninhibited oil until ready for testing.

9.2.2 Generally, 12 to 16 test specimens should be taken from a given sample to determine susceptibility of the material.

9.2.2.1 The orientation and location of the test specimen with respect to the original material must be reported with the test results.

9.2.3 The test specimens should be milled to an approximate size and then surface ground to final dimensions. The last two passes on either side shall be restricted to removal of 0.013 mm (0.00050 in) per pass (care must be taken to prevent overheating). The final surface roughness shall be 0.81 μm (32 μin) or finer.

9.2.4 As shown in Figure 7, two 0.71 mm (0.028 in) diameter holes (No. 70 drill bit) shall be drilled at the midlength of the test specimen, centered 1.58 mm (0.0620 in) from each side edge. Holes shall be drilled before machining the final surface.

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Figure 7: Dimensional Drawing of the Standard Bent-Beam Test Specimen

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9.2.5 Test Specimen Identification 9.2.5.1 The test specimens may be stamped or vibratory stenciled in a region within 13 mm (0.50 in) of either end on the compression side.

9.2.6 Test Specimen Cleaning 9.2.6.1 Surfaces and edges of the test specimen shall be ground by hand on 240 grit emery paper with scratches parallel to the test specimen axis.

9.2.6.2 The test specimens shall be degreased with solvent and rinsed with acetone.

9.2.6.3 The stressed section of the test specimen shall not be handled or contaminated after cleaning.

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9.3 Test Solution 9.3.1 The test solution shall consist of 0.5 wt% glacial acetic acid dissolved in distilled or deionized water (e.g., 5.0 g [4.8 mL] of CH3COOH dissolved in 995 g of distilled or deionized water). NaCl shall not be added to the test solution.

9.3.2 Use of Test Solutions A, B, C, and D with this test method has not been standardized.

9.4 Test Equipment 9.4.1 Many types of stress fixtures and test vessels used for stress corrosion testing are acceptable for Method B. Consequently, the following discussion emphasizes the test equipment characteristics required for selecting suitable items and procedures.

9.4.2 Tests shall be performed using constant-deflection fixtures that employ three-point bending of the test specimen. (See Figure 8).

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Figure 8: Typical Stressing Fixture for Bent-Beam Test Specimen (a) Dimensional Drawing

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Figure 8: Typical Stressing Fixture for Bent-Beam Test Specimen (b) Photograph

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Stressing Fixture for Bent-Beam Test Specimen http://www.sidertest.it/scc-test-stress-corrosion-cracking-nace-tm-0177/?lang=en

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Stressing Fixture for Bent-Beam Test Autoclave http://www.sidertest.it/scc-test-stress-corrosion-cracking-nace-tm-0177/?lang=en

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Stressing Fixture for Bent-Beam Test Autoclave http://www.sidertest.it/scc-test-stress-corrosion-cracking-nace-tm-0177/?lang=en

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9.4 Test Equipment 9.4.1 Many types of stress fixtures and test vessels used for stress corrosion testing are acceptable for Method B. Consequently, the following discussion emphasizes the test equipment characteristics required for selecting suitable items and procedures.

9.4.2 Tests shall be performed using constant-deflection fixtures that employ three-point bending of the test specimen. (See Figure 8).

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9.4.3 Test fixtures immersed in a test solution should resist general corrosion (UNS(4) S31600 is commonly used). Fixture elements contacting the test specimen must be electrically isolated from it.

9.4.4 Deflection gauges shall be graduated in 0.0025 mm (0.00010 in) divisions.

9.4.4.1 Test specimen deflection should be determined by separate gauges or by gauges incorporated in a loading fixture. In designing a deflection gauge to suit individual circumstances, the deflection at midlength of the test specimen should be measured.

9.4.5 Test Vessel 9.4.5.1 The test vessel shall be sized to maintain a test solution volume of 30 Âą 10 mL/cm2 of test specimen surface area. Maximum volume of the test vessel should be 10 L. 9.4.5.2 The test vessel shall be valved at both inlet and outlet to prevent contamination of the test solution by oxygen.

9.4.5.3 A fritted glass bubbler should be used to introduce the inert gas and H2S below the array of test specimens. The bubbles should not impinge on the test specimens.

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9.5 Deflection Calculations 9.5.1 An estimated outer fiber pseudo-stress (S) for the material shall be used in beam deflection calculations. For carbon and low-alloy steels, S values are typically in the range of 69 MPa (104 psi) at 22 to 24 HRC. As hardness increases, S generally decreases.

9.5.2 The selected range of estimated S values shall be used as pseudostresses to calculate the deflections of the test specimens.

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9.5.3 The test specimen deflection shall be calculated for each of the pseudostress values using Equation (3):

where: D = deflection; S = nominal outer fiber pseudo-stress;

l

= distance between centerlines of end supports; E = elastic modulus; and t = thickness of test specimen.

The formula assumes elastic conditions and ignores the stress concentration effect of the holes and the test specimen plasticity at high stress levels.

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9.6 Testing Sequence 9.6.1 The test specimen dimensions shall be measured, and deflections shall be calculated for desired pseudo-stress levels.

9.6.2 Test specimens shall be stressed in fixtures by deflecting them to the nearest 0.0025 mm (0.00010 in) with a dial or digital gauge and fixture. The deflection should be carefully applied to avoid exceeding the desired value. If the desired deflection is exceeded, the test shall be run at the higher deflection or discarded.

9.6.3 The stressed test specimens shall be cleaned and placed into the test vessel.

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9.6.4 Deaeration requirements are given in Paragraph 6.7.

9.6.5 The test solution shall then be saturated with H2S at a rate of at least 100 mL/min for at least 20 min/L of test solution. The H2S in the test vessel shall be replenished periodically by bubbling H2S for a duration of 20 to 30 min at a rate of at least 100 mL/min/L of test solution three times per week for the duration of the test. The days for the replenishment should be the first, third, and fifth day of each work week.

9.6.6 The test shall be terminated after 720 hours or when all test specimens have failed, whichever occurs first.

9.6.7 Additional test specimens and iterative testing may be necessary to define the Sc closely.

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9.7 Failure Detection 9.7.1 Crack presence shall be determined visually with the aid of a low-power binocular microscope. If the test specimen contains only one or a few cracks, the shape of the test specimen may have changed considerably, predominantly by kinking; this feature helps to identify cracked test specimens.

However, if many cracks are present, a shape change may not be apparent.

Because corrosion products may obscure cracks, a careful examination shall be made. Mechanical cleaning or metallographic sectioning of the test specimen may be necessary in these instances to detect cracks.

9.7.2 Failure is cracking of the test specimen. Consequently, following exposure, the surface of the test specimens should be cleaned and visually inspected at 10X for evidence of cracking following a 20 degree bending by hand. Test specimens found to contain cracks shall be

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9.8 Reporting of Test Results 9.8.1 Failure/no-failure data and nominal outer fiber pseudo-stress (S) values shall be reported. Time-to-failure data are optional.

9.8.2 The Sc shall be calculated using Equation (4) for S values expressed in MPa, or Equation (5) for S values expressed in psi: where: S = nominal outer fiber pseudo-stress (in MPa) used to calculate the beam's deflection; T = the test result (i.e., +1 for passing and -1 for failure); and n = the total number of test specimens tested.

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When Equation (4) is used, all pseudo-stress data that are more than ±210 MPa from the initial calculated value Sc x 68.95 MPa shall be discarded, and a new Sc value shall be recalculated. The recalculated Sc value eliminates low and high bias data.

where: S = nominal outer fiber pseudo-stress (in psi) used to calculate the beam's deflection; T = the test result (i.e., +1 for passing and –1 for failure); and n = the total number of test specimens tested.

When using Equation (5), all pseudo-stress data that are more than ±3.0 x 104 psi from the initial calculated value Sc x 104 psi shall be discarded, and a new Sc value shall be recalculated. The recalculated Sc value eliminates low and high bias data.

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9.8.3 The calculated Sc value for each material tested shall be reported. If Sc is recalculated, the recalculated Sc value shall be reported, not the initial calculated Sc value.

9.8.4 If known, the chemical compositions, heat treatment, mechanical properties, and other data taken shall be reported.

9.8.5 Table 2 shows the recommended format for reporting the data.

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Section 10: Method C—NACE Standard C-Ring Test 10.1 Method C, the NACE Standard C-Ring Test, provides for evaluating the EC resistance of metals under conditions of circumferential loading. It is particularly suitable for making transverse tests of tubing and bar. EC susceptibility with the C-ring test specimen is usually determined by time-tocracking during the test. C-ring test specimens, when deflected to a particular outer fiber stress level, give a failure/no-failure result. When testing multiple Cring test specimens at varying stress levels, an apparent threshold stress for EC can be obtained.

10.1.1 This section sets forth the procedure for C-ring testing at room temperature and atmospheric pressure. Special considerations for testing at elevated temperature and pressure are set forth in Section 7.

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Mitigation of Sulfide Stress Cracking in Down Hole P110 Components via Low Plasticity Burnishing Introduction of stable, high magnitude compressive residual stresses into less expensive carbon steel alloys alleviates the tensile stresses and mitigates SSC while also improving fatigue strength. This could allow the potential of using less expensive alloys in sour environments. Low plasticity burnishing (LPB) is highly effective when applied to metallic components using a proven reproducible process of producing deep, high magnitude compressive residual stresses in complex geometric components without altering the geometry, design or chemistry.

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C-ring specimen Stress on the specimens was monitored continuously using strain gage rosettes placed on the inner diameter opposite the exposed location of maximum applied tension.

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http://www.lambdatechs.com/documents/280.pdf

Strain Gage

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http://www.lambdatechs.com/documents/280.pdf

Comparison of LPB treated and un-treated C-ring specimens after testing. The untreated specimen failed in 10 hours at 45% of SMYS. The LPB specimens ran-out at 45%, 80%, 85%, and 90% SMYS with no cracking.

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Full size coupling blank pressurized test apparatus and setup

The full sized coupling blanks were internally pressurized hydraulically to impart the desired amount of applied hoop stress.

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http://www.lambdatechs.com/documents/280.pdf

LPB processed coupling blank and test fixture after run-out at 85% SMYS. FDI developer showing no signs of crack initiation.

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http://www.lambdatechs.com/documents/280.pdf

Dye penetrant inspection of failed un-treated coupling blank revealing thru wall axial SSC.

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10.2 Test Specimen 10.2.1 An unnotched C-ring test specimen in accordance with ASTM G3818 shall be used. Sizes for C-rings may vary over a wide range, but C-rings with an outside diameter (OD) of less than about 15.9 mm (0.625 in) should not be used because of increased difficulties in machining and decreased precision in stressing. A typical C-ring test specimen is shown in Figure 9.

10.2.2 The circumferential stress may vary across the width of the C-ring; the variation extent depends on the width-to-thickness (w/t) and diameter-tothickness (d/t) ratios of the C-ring. The w/t ratio shall be between 2 and 10, and the d/t ratio shall be between 10 and 100.

10.2.3 The material used in the bolting fixtures shall be of the same material as that of the C-ring test specimen or be electrically isolated from the C-ring test specimen to minimize any galvanic effects, unless specific galvanic effects are desired.

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10.2.4 Machining should be done in stages: the final two passes should remove a total of no more than 0.05 mm (0.002 in) of metal, and the final cut should leave the principal surface with a finish of 0.81 Îźm (32 Îźin) or finer.

After machining, test specimens shall be stored in a low-humidity area, in a desiccator, or in uninhibited oil until ready for testing.

10.2.4.1 A high-quality machined surface is normally used for corrosion test purposes. However, the as-fabricated surface of a tube or bar also may be evaluated by C-ring test specimens. Using any finishing process other than machining shall be reported with the test data.

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10.2.5 Test Specimen Identification The C-ring test specimen end segments may be stamped or vibratory stenciled.

10.2.6 Test Specimen Cleaning 10.2.6.1 Before testing, C-ring test specimens shall be degreased with solvent and rinsed with acetone.

10.2.6.2 The C-ring test specimen shall not be contaminated after cleaning.

Test specimens should be handled with new disposable gloves. Powdered gloves shall be cleaned of powder prior to handling test specimens.

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31 NACE International Figure 9: Dimensional Drawing of the C-Ring Test Specimen

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10.3 Test Solutions—see Section 6.

10.4 Test Equipment 10.4.1 The test equipment necessary for stressing C-ring test specimens shall include calipers or equivalent equipment capable of measuring to the nearest 0.025 mm (0.0010 in), wrenches sized to the bolting fixtures used, and a clamping device. C-ring test specimens shall be clamped during stressing by the bolting fixtures or the tips of the C-ring. No clamping shall take place in the central test section of the C-ring.

10.4.2 The C-ring test specimen shall be so supported that nothing except the test solution contacts the stressed area.

10.4.2.1 The supporting fixture shall be constructed of material compatible with the test solution.

10.4.2.2 Galvanic effects between the C-ring test specimens, supporting fixtures, and test vessel shall be avoided. For example, an isolating bushing or washer may be used to isolate the C-ring electrically from the supporting fixtures.

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10.4.3 Test Vessel 10.4.3.1 The test vessel should be sized to maintain a test solution volume of 30 Âą 10 mL/cm2 of test specimen surface area.

10.4.3.2 A fritted glass bubbler should be used to introduce the inert gas and H2S below the array of C-ring test specimens. The bubbles should not impinge on the C-ring test specimens.

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10.5 Deflection Calculations 10.5.1 The deflection necessary to obtain the desired stress on the C-ring test specimen shall be calculated using Equation (6):

D = deflection of C-ring test specimen across bolt holes; d = C-ring test specimen outer diameter; t = C-ring test specimen thickness; S = desired outer fiber stress; and E = elastic modulus.

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10.5.1.1 Deflections calculated by Equation (6) should be limited to stresses below the material elastic limit. For many CRAs, the elastic limit is well below the 0.2% offset proof (yield) stress. Deflection values beyond the elastic limit can be calculated from information obtained from the stress-strain curve of the material and the strain-deflection characteristics of the specific C-ring geometry being used.

10.5.1.2 Equation (6) may be used for carbon and low-alloy steels to calculate the deflection necessary to stress the test specimen to 100% of the 0.2% offset yield strength (SY) by substituting SY + E (0.002) for S in the original equation. This relationship is not valid for all alloy systems and should be checked before use.

10.5.1.3 No equation exists to calculate the deflection needed to stress C-ring test specimens to values between the material’s elastic limit and its 0.2% offset proof (yield) stress.

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10.5.2 The deflection can be determined directly by using electrical resistance strain gauges applied to the C-ring test specimen.

10.5.2.1 Each C-ring shall be strain-gauged on the outside diameter at a point 90° opposite the axis of the C-ring bolt. The bolt shall be tightened to the appropriate strain by monitoring the strain gauge output, then the strain gauge and glue residue shall be removed. The C-ring shall then be recleaned using the same procedures given in Paragraph 10.2.6.

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10.6 Testing Sequence 10.6.1 The C-ring test specimen dimensions shall be measured, and the corresponding C-ring deflections shall be calculated.

10.6.2 C-ring test specimens shall be stressed by tightening bolting fixtures to calculated deflections measured to the nearest 0.025 mm (0.0010 in).

Deflections shall be measured at the center line of the bolting fixture. These measurements may be taken at the outer diameter, inner diameter, or midwall with care to maintain consistency in the points of measurement. If the desired deflection is exceeded, the test shall be run at the higher deflection or discarded.

10.6.3 The C-ring test specimens shall be cleaned and placed into the test vessel.

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10.6.4 Deaeration requirements are given in Paragraph 6.7.

10.6.5 The test solution shall then be saturated with H2S at a minimum flow rate per a documented procedure that has been validated to obtain 2,300 mg/L minimum H2S concentration or other minimum value H2S concentration that is proportional to less than 1 bar H2S partial pressures (at the extent of the test specimen locations). When Solution D is used, the minimum H2S concentration shall be 160 mg/L. Saturation shall occur within an hour of contact with specimens. For larger test vessels, saturation can require more than two hours. Analysis shall be done using iodometric titration (See Appendix C or other suitable methods). The validation of the documented procedure shall be performed after saturation at the start of test, after 24 hours, weekly, and at the end of the test. A continuous flow of H2S through the test vessel and outlet trap shall be maintained with a positive pressure of H2S throughout the test that prevents air from entering the test vessel.

NOTE: Laboratories at high elevations may find it necessary to compensate for lower atmospheric pressure in order to achieve the required saturation levels.

10.6.6 The termination of the test shall be at all C-ring test specimens’ failure or after 720 hours, whichever occurs first.

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10.7 Failure Detection 10.7.1 Highly stressed C-rings of alloys that are appreciably susceptible to EC tend to fracture through the entire thickness or to crack in a way that is conspicuous. However, with more-EC-resistant alloys, cracking frequently begins slowly and is difficult to detect. Small cracks may initiate at multiple sites and be obscured by corrosion products. It is preferable to report the first crack, if detected at 10X magnification, as the criterion of failure. An alternative method of exposing cracking in C-rings after exposure is to stress the C-ring beyond the tested stress level. Cracks resulting from EC can be differentiated from mechanically induced cracks by the corroded nature of the crack surface.

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10.8 Reporting of Results 10.8.1 Failure/no-failure data shall be reported from each stress level for each specimen. If time-to-failure data are recorded, they shall be reported.

10.8.2 If known, the chemical composition, heat treatment, mechanical properties, and other data taken shall be reported.

10.8.3 Table 3 shows the recommended format for reporting the data.

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Section 11: Method D—NACE Standard Double Cantilever Beam Test 11.1 Method D, the NACE Standard DCB Test, provides for measuring the resistance of metallic materials to propagation of EC, expressed in terms of a critical stress intensity factor, KISSC for SSC and KIEC for the more general case of EC, using a crack-arrest type of fracture mechanics test. Method D does not depend on the uncertainty of pitting and/or crack initiation, because a crack is always initiated in a valid test. For SSC testing of carbon and lowalloy steels, this method requires little time. Method D gives a direct numerical rating of crack propagation resistance and does not depend on evaluation of failure/no-failure results.19 The subject of fracture mechanics testing for evaluation of EC resistance is currently under consideration by NACE TG 085 and Work Group (WG) 085c, and ASTM Committees E8.06.02 and G1.06.04.

The user of this standard should maintain contact with these groups and their technical activities for knowledge of current state-of-the-art testing techniques.

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11.1.1 KISSC is not an intrinsic material property, but depends on the environmental exposure conditions and method of testing. Nevertheless, the values obtained by carefully adopting this standard can be used for comparative purposes.

11.1.2 This section sets forth the procedure for DCB testing at room temperature and atmospheric pressure and enables computation of KISSC. When the special considerations set forth in Section 7 for testing at elevated temperature and pressure are observed, the computed stress intensity factor should be written as KIEC. The equations needed to compute KIEC are the same as those set forth in Paragraph 11.6 for KISSC. However, the following descriptions of material behavior under SSC conditions may not be accurate for the more general conditions of EC. ………………………………

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KISCC

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ANSI/NACE TM0177-2016 Item No. 21212

Standard Test Method Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments This NACE International standard represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone, whether he or she has adopted the standard or not, from manufacturing, marketing, purchasing, or using products, processes, or procedures not in conformance with this standard. Nothing contained in this NACE International standard is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or protecting anyone against liability for infringement of Letters Patent. This standard represents minimum requirements and should in no way be interpreted as a restriction on the use of better procedures or materials. Neither is this standard intended to apply in all cases relating to the subject. Unpredictable circumstances may negate the usefulness of this standard in specific instances. NACE International assumes no responsibility for the interpretation or use of this standard by other parties and accepts responsibility for only those official NACE International interpretations issued by NACE International in accordance with its governing procedures and policies which preclude the issuance of interpretations by individual volunteers.

Users of this NACE International standard are responsible for reviewing appropriate health, safety, environmental, and regulatory documents and for determining their applicability in relation to this standard prior to its use. This NACE International standard may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or referred to within this standard. Users of this NACE International standard are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of this standard.

CAUTIONARY NOTICE: NACE International standards are subject to periodic review, and may be revised or withdrawn at any time in accordance with NACE technical committee procedures. NACE International requires that action be taken to reaffirm, revise, or withdraw this standard no later than five years from the date of initial publication and subsequently from the date of each reaffirmation or revision. The user is cautioned to obtain the latest edition. Purchasers of NACE International standards may receive current information on all standards and other NACE International publications by contacting the NACE International FirstService Department, 15835 Park Ten Place, Houston, Texas 77084-5145 (telephone +1 281-228-6200).

Approved 2016-04-18 Revised 2016-03-29 Revised 2005-12-03 NACE International 15835 Park Ten Place Houston, Texas 77084-5145 +1 281-228-6200 ISBN 1-57590-036-X Š 2016 NACE International

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ANSI/NACE TM0177-2016 ____________________________________________________________________________ Foreword This standard addresses the testing of metals for resistance to cracking failure under the combined action of tensile stress and corrosion in aqueous environments containing hydrogen sulfide ( H2S). This phenomenon is generally termed sulfide stress cracking (SSC) when operating at room temperature and stress corrosion cracking (SCC) when operating at higher temperatures. In recognition of the variation with temperature and with different materials this phenomenon is herein called environmental cracking (EC).

For the purposes of this standard, EC includes only SSC, SCC, and hydrogen stress cracking (HSC).

The primary purpose of this standard is to facilitate conformity in testing so that data from different sources can be compared on a common basis. Consequently, this standard aids the evaluation and selection of all types of metals and alloys, regardless of their form or application, for service in H2S environments. This standard contains methods for testing metals using tensile, bent-beam, C-ring, and double-cantilever-beam (DCB) test specimens. Certain ASTM(1) standard test methods have been listed as references for supplementary tests, creating a comprehensive test method standard. In addition, the four-point bent-beam test method is also referenced as a supplementary test.1,2 This standard is intended for use by laboratory and materials personnel to facilitate conformity in testing.

SSC of metals exposed to oilfield environments containing H2S was recognized as a materials failure problem by 1952. Laboratory data and field experience have demonstrated that even extremely low concentrations of H2S may be sufficient to lead to SSC failure of susceptible materials. In some cases, H2S can act synergistically with chlorides to produce corrosion and cracking (SSC and other mode) failures.

However, laboratory and operating experiences have also indicated to materials engineers the optimum selection and specification of materials having minimum susceptibility to SSC. This standard covers test methods for SSC (at room temperature) and SCC (at elevated temperature), but other failure modes (e.g., hydrogen blistering, hydrogen-induced cracking [HIC], chloride stress corrosion cracking [SCC], pitting corrosion, and mass-loss corrosion) must also be considered when selecting materials for use in sour (H2S containing) environments.

The need for better understanding of the variables involved in EC of metals in oilfield environments and better correlation of data has become apparent for several reasons. New design requirements by the oil and gas production industries call for higher-strength materials that, in general, are more susceptible to EC than lower-strength alloys. These design requirements have resulted in extensive development programs to obtain more resistant alloys and/or better heat treatments. At the same time, users in the petroleum refining and synthetic fuels industries are pushing present materials much closer to their mechanical limits.

Room-temperature (SSC) failures in some alloys generally are believed to result from hydrogen embrittlement (HE). When hydrogen is cathodically evolved on the surface of a metal (as by corrosion or cathodic charging), the presence of H2S (and other compounds, such as those containing cyanides and arsenic) tends to cause hydrogen atoms to enter the metal rather than to form hydrogen molecules that cannot enter the metal. In the metal, hydrogen atoms diffuse to regions of high triaxial tensile stress or to some microstructural configurations where they become trapped and decrease the ductility of the metal. Although there are several kinds of cracking damage that can occur in metals, delayed brittle fracture of metals resulting from the combined action of corrosion in an aqueous sulfide environment and tensile stresses (failure may occur at stresses far below the yield stress) is the phenomenon known as SSC. In some cases, however, failure may be the result of localized anodic corrosion processes that may or may not involve hydrogen. In such instances, failure is the result of anodic stress corrosion cracking (SCC). Such failures have historically been termed SSC even though their cause may not be hydrogen.

This standard was originally published in 1977 by NACE International Task Group T-1F-9, a component of Unit Committee T-1F, “Metallurgy of Oilfield Equipment.” The standard was revised in 1986, 1990, and 1996

by Task Group T-1F-9. It was revised in 2005 and 2016 by Task Group (TG) 085, “Sulfide Corrosion Cracking: Metallic Materials Testing Techniques.” TG 085 is administered by Specific Technology Group (STG) 32, “Oil and Gas Production—Metallurgy,” and is sponsored by STG 62, “Corrosion Monitoring and

(1)

ASTM International (ASTM), 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959.

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Measurement—Science and Engineering Applications.� The standard is issued by NACE under the auspices of STG 32.

In NACE standards, the terms shall, must, should, and may are used in accordance with the definitions of these terms in the NACE Publications Style Manual. The terms shall and must are used to state a requirement, and are considered mandatory. The term should is used to state something good and is recommended, but is not considered mandatory. The term may is used to state something considered optional.

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NACE International Standard Test Method Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments Contents 1. General ....................................................................................................................................... 1 2. Environmental Cracking Testing Variability ................................................................................. 2 3. Reagents ..................................................................................................................................... 3 4. Test Specimens and Material Properties..................................................................................... 3 5. Test Vessels and Fixtures ........................................................................................................... 4 6. Test Solutions ............................................................................................................................. 4 7. Testing at Elevated Temperature/Pressure ................................................................................. 7 8. Method A—NACE Standard Tensile Test ................................................................................. 11 9. Method B—NACE Standard Bent-Beam Test ........................................................................... 11 10. Method C—NACE Standard C-Ring Test.................................................................................. 21 11. Method D—NACE Standard Double Cantilever Beam Test ...................................................... 30 References ...................................................................................................................................... 50 APPENDIX Appendix A Safety Considerations in Handling H2S Toxicity (Nonmandatory) ................................ 52 Appendix B Explanatory Notes on Environmental Cracking Test Method (Nonmandatory) ........... 53 Appendix C Determination of H2S Concentration in Test Solution by Iodometric Titration (Nonmandatory) .............................................................................................................................. 54 Appendix D Recommendations for Determining Mechanical Quality Assurance of Test Results for Method D (DCB Test) 28 (Nonmandatory) ................................................................... 57 Appendix E Recommended Method for Determining KIapplied and KLIMIT for the Method D (DCB) Test 29 (Nonmandatory) ................................................................................................. 59 FIGURES Figure 1: Flow Chart of Significant Factors Influencing the Selection of an NDE Inspection Method .............................................................................................................................................. 8 Figure 2: Schematic Arrangement of Test Equipment for Method B—NACE Standard Bent-Beam Test, Method C—NACE Standard C-Ring Test, and Method D—NACE Standard Double-Cantilever-Beam Test ............................................................................................ 9 Figure 3: Tensile Test Specimens ................................................................................................... 12 Figure 4: Constant-Load (Dead-Weight) Device .............................................................................. 14 Figure 5: Sustained-Load Devices................................................................................................... 15 Figure 6: Applied Stress vs. Log (Time-to-Failure) ......................................................................... 21 Figure 7: Dimensional Drawing of the Standard Bent-Beam Test Specimen .................................. 23 Figure 8: Typical Stressing Fixture for Bent-Beam Test Specimen) ............................................... 25 Figure 9: Dimensional Drawing of the C-Ring Test Specimen ......................................................... 31 Figure 10: Location of Hardness Impressions on DCB Specimen ................................................... 38 Figure 11(a): Dimensional Drawing of the DCB Specimen .............................................................. 39 TABLES Table 1 NACE Uniform Material Testing Report Form (Part 1): Testing in Accordance with NACE Standard TM0177(A) Method A—NACE Standard Tensile Test ..................................................... 19 Table 2 NACE Uniform Material Testing Report Form (Part 1): Testing in Accordance with NACE Standard TM0177(A) Method B—NACE Standard Bent-Beam Test ............................................... 28 Table 3 NACE Uniform Material Testing Report Form (Part 1): Testing in Accordance with NACE Standard TM0177(A) Method C—NACE Standard C-Ring Test...................................................... 35

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Table 4 Arm Displacements for API and Other Grade Oilfield Tubular Steels in Solution A and 100% H2S ........................................................................................................................................ 41 Table 5 Arm Displacement for API 5CT Grade C110 in Solution D and 7% H2S ......................................... 42 Table 6 Suggested Arm Displacements for Selected Alloys and Strength Levels in Solution A and 100% H2S ........................................................................................................................................ 42 Table 7 NACE Uniform Material Testing Report Form (Part 1): Testing in Accordance with NACE Standard TM0177(A) Method D—NACE Standard DCB Test ......................................................... 48

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ANSI/NACE TM0177-2016

________________________________________________________________ Section 1: General 1.1 This standard covers the testing of metals subjected to tensile stresses for resistance to cracking failure in low-pH aqueous environments containing H2S. Carbon and low-alloy steels are commonly tested for EC resistance at room temperature where SSC susceptibility is typically high.

For other types of alloys, the correlation of EC susceptibility with temperature is more complicated.

1.2 This standard describes reagents, test specimens, and equipment to use; discusses base material and test specimen properties; and specifies the test procedures to follow. This standard describes four test methods: Method A—Standard Tensile Test Method B—Standard Bent-Beam Test Method C—Standard C-Ring Test Method D—Standard Double-Cantilever-Beam (DCB) Test Sections 1 through 7 of this standard give general comments that apply to all four test methods.

Sections 8 through 11 indicate the test method to follow for each type of test specimen. General guidelines to help to determine the suitability of each test method are given at the beginning of each test method description (Sections 8 through 11). Reporting of the test results is also discussed.

1.3 Metals can be tested for resistance to EC at temperatures and pressures that are either ambient (atmospheric) or elevated.

1.3.1 For testing at ambient conditions, the test procedures can be summarized as follows: Stressed test specimens are immersed in acidified aqueous environments containing H2S. Applied loads at convenient increments can be used to obtain EC data.

1.3.2 For testing at temperatures higher than 27 °C (80 °F), at either atmospheric or elevated pressure, Section 7 describes an alternative test technique. All methods (A, B, C, and D) are adaptable to this technique.

1.4 This standard may be used for release or acceptance testing to ensure that the product meets a certain minimum level of EC resistance as prescribed in API(2) Specification 5CT,3 ISO(3) 11960,4 or as prescribed by the user or purchaser. This standard may also provide a quantitative measure of the product’s EC resistance for research or informational purposes. This rating may be based on: Method A—The highest no-failure uniaxial tensile stress in 720 hours.

Method B—The statistically based critical stress factor (Sc) for a 50% probability of failure in 720 hours.

Method C—The highest no-failure circumferential stress in 720 hours.

Method D—The average KISSC (threshold stress intensity factor for SSC) for valid tests of replicate test specimens.

1.5 Safety Precautions: H2S is an extremely toxic gas that must be handled with care. (See Appendix A [Nonmandatory]).

(2) American

Petroleum Institute (API), 1220 L St. NW, Washington, DC 20005-4070.

International Organization for Standardization (ISO), Chemin de Blandonnet 8. Case Postale 401, 1214 Vermier, Geneva, Switzerland.

(3)

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ANSI/NACE TM0177-2016

________________________________________________________________ Section 2: Environmental Cracking Testing Variability 2.1 Interpretation of stress corrosion test results is a difficult task. The test methods contained in this standard are severe, with accelerated tests making the evaluation of the data extremely difficult.

In testing the reproducibility of the test methods among different laboratories, several undesirable side effects (frequent with many accelerated tests) that must be noted include: 2.1.1 The test environment may cause failure by HIC and hydrogen blistering. This is especially true for lower-strength steels not usually subject to SSC. HIC may be detected by visual and metallographic observations. Blistering is normally visible on the test specimen surface. (For further information regarding this phenomenon, see NACE Standard TM0284).5 2.1.2 The test environment may corrode some alloys that normally do not corrode in actual field service and thereby induce EC failures in alloys that ordinarily do not fail by EC. This problem is especially acute with the martensitic and precipitation-hardened stainless steels.

2.2 Furthermore, other aspects to be considered in the selection of test method(s) include: 2.2.1 Material anisotropy affecting mechanical properties and EC susceptibility can be an important parameter. The fracture path in the test specimen should match what is anticipated in the actual component.

2.2.2 Galvanic effects between dissimilar metals can either accelerate or suppress cracking susceptibility. Examples of this behavior are accelerated EC in some nickel-based corrosionresistant alloys (CRAs) and reduced EC in some duplex stainless steels when these materials are coupled to electrochemically less-noble materials such as carbon and low-alloy steels.

2.2.3 Test temperature affects cracking susceptibility. Test temperatures above 24 째C (75 째F) can reduce SSC severity in steels, whereas test temperatures below 24 째C (75 째F) can increase SSC severity.

2.2.4 Different test methods may not necessarily provide the same rankings of like materials.

2.2.5 Material inhomogeneity, such as weldments and segregation, can affect test results. This is particularly true when results from tests that evaluate a large volume of material (tensile test) versus a small volume of material (bent-beam test) are compared.

2.2.6 Maximum no-failure stresses for a specified exposure period should be considered apparent threshold stresses. Longer exposure times or larger numbers of test specimens may result in lower threshold values.

2.2.7 EC test results can show statistical variability. Replicate testing may be needed to obtain a representative value characterizing resistance to EC. 2.2.8 Some test specimens are better suited than others for measuring EC resistance in localized areas (e.g., near surfaces or other features, and in weld zones).

2.2.9 Some types of EC tests require considerably more time than others for determination of EC resistance.

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ANSI/NACE TM0177-2016

________________________________________________________________ Section 3: Reagents 3.1 Reagent Purity 3.1.1 The test gases, sodium chloride (NaCl), acetic acid (CH3COOH), sodium acetate (CH3COONa), and solvents shall be reagent grade or chemically pure (99.5% minimum purity) chemicals. (See Appendix B [Nonmandatory]).

3.1.2 The test water shall be distilled or deionized and of quality equal to or greater than ASTM Type IV (ASTM D11936). Tap water shall not be used.

3.2 Inert gas shall be used for removal of oxygen. Inert gas (nitrogen, argon, or other suitable nonreactive gases) shall be pre-purified (99.998% or greater).

________________________________________________________________ Section 4: Test Specimens and Material Properties 4.1 The location and orientation of the test specimens to be removed from the product should be carefully considered so that test results provide the most meaningful indication of the performance to be expected from that product when placed in service. All test specimens in a set should be taken from metallurgically equivalent positions (i.e., all test specimens should have the same orientation with similar or nearly the same microstructure and mechanical properties).

4.2 When specified, tensile testing in accordance with standard test methods such as ASTM A3707 shall be used to determine base material properties. Two or more test specimens shall be pulled, and the individual test results shall be averaged to determine the yield and ultimate strengths, percent elongation, and percent reduction in area for the material. Machining a tensile test specimen from material adjacent to and in the same position and orientation as the EC test specimen to be tested can minimize material property variations that normally occur from test specimen to test specimen.

4.3 A number of fundamental material properties correlate with EC susceptibility. Consequently, when specified, all pertinent data on chemical composition, mechanical properties, heat treatment, and mechanical histories (such as percent cold reduction or prestrain) shall be determined and reported with the tensile test data. Each different heat treatment and microstructure of a material of a fixed chemical composition shall be tested as though it were a different material.

4.4 When specified, hardness measurements shall be performed in accordance with ASTM E18 (Rockwell)8 or ASTM E384 (Vickers)9 before or after exposure to the test environment. These measurements shall not be made on the stressed evaluation portion of the test specimen. For test Methods A (subsize) and B, hardness shall be measured (1) on the test specimen, or (2) on an adjacent hardness test specimen that was sampled from a similar thickness/cross-section location as the material being tested. For test methods A (standard size), C, and D, hardness shall be measured on the test specimen. At least 3 indentations shall be performed on each specimen.

Hardness test results shall be reported.

NOTE: When hardness testing on convex cylindrical surfaces, the results may not accurately indicate the true hardness. Corrections applied to the measured hardness shall be in accordance with the selected ASTM standard and reported.

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ANSI/NACE TM0177-2016

________________________________________________________________ Section 5: Test Vessels and Fixtures 5.1 The size, shape, and entry ports of the test vessel shall be determined by the actual test specimens and test fixtures used to stress the test specimens.

5.2 Test vessels shall be capable of being purged to remove oxygen before beginning the test and of keeping air out during the test. Using a small outlet trap on the H2S effluent line to maintain 250 Pa (0.036 psi) (1.0 inch in H2O) back pressure on the test vessel may be used to prevent oxygen entry through small leaks or by diffusion up the vent line. (See Appendix B section titled “Reasons for Exclusion of Oxygen”).

5.3 Test vessels shall be sized to maintain the test solution volume within the specified limits relative to the test specimen surface area to standardize the drift of pH with time. (See each test method for specified limits).

5.4 Test vessels shall be constructed from materials that are suitable for the test environment.

While some plastic test vessels and solution storage vessels give satisfactory service, others may cause varying test results from the time they are new until after they have been in continuous use.

If plastic vessels are used, then the vessels shall be pre-conditioned with a documented procedure that has been validated. Glass test vessels have not exhibited this tendency and should not require pre-conditioning.

5.5 Test specimens shall be electrically isolated from test vessels and test fixtures made from dissimilar metals if the dissimilar metal is in contact with the test environment.

5.6 Rigid electrical insulating materials not exhibiting relaxation or flow under load should be selected for loading or deflecting the test specimen.

5.7 Galvanic Coupling 5.7.1 It may be necessary to evaluate the effects of galvanic coupling on EC resistance, such as in the case of coupling stainless alloys or CRAs to steel (see Paragraph 2.2.2).

5.7.1.1 To evaluate this, galvanic couples of iron or steel having a surface area between 0.5 and 1 times the exposed area of the test specimen should be bolted securely to the test specimen.

5.7.2 Particles of iron sulfide are electrically conductive. If deposited on insulating materials, they can provide electrical connection between materials and affect the results of the tests. _____________________________________________________________________________ Section 6: Test Solutions 6.1 General 6.1.1 All reagents added to the test solutions shall be measured to ±1.0% of the quantities specified for the specific test solution.

6.1.2 The test solution shall be maintained at 24 ± 3 °C (75 ± 5 °F) for Methods A, B, and C; and 24 ± 1.7 °C (75 ± 3.0 °F) for Method D. Also, the test temperature range shall be specified in accordance with testing at elevated temperature (see Section 7). Any variations beyond this range shall be reported.

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ANSI/NACE TM0177-2016

6.1.3 The test solution used for each test method shall be reported.

6.1.4 If required, the concentration of H2S in the test solution may be measured by iodometric titration or by other suitable methods. Accepted iodometric titration and reduced concentration of H2S procedures are detailed in Appendix C (Nonmandatory).

6.1.5 Solutions and gas compositions other than those detailed below may also be used for fitfor-purpose testing and shall be reported.

6.1.6 The initial pH (before the introduction of the test gas) and final pH (at the end of the test) shall be measured and reported.

6.1.7 Unless expressly permitted in this standard, pH shall not be intentionally adjusted during the test.

6.1.8 Gas mixtures shall be certified pre-mixed or verified by suitable means, provided the pure component gases are individually certified.

6.1.9 The start of the test shall be measured from the time that saturation is achieved.

6.2 Test Solution A 6.2.1 Test Solution A is an acidified H2S-saturated aqueous brine solution.

6.2.2 Test Solution A shall consist of 5.0 wt% sodium chloride and 0.5 wt% glacial acetic acid dissolved in distilled or deionized water (e.g., 50.0 g of NaCl and 5.0 g [4.8 mL] of CH3COOH dissolved in 945 g of distilled or deionized water).

6.2.3 Test solution pH before contact with a test specimen and before H 2S saturation shall be between 2.6 and 2.8. Adjustment of test solutions chemistry to adjust pH is not allowed. If test solution pH is out-of-range then the solution shall be discarded. Each laboratory shall have a demonstrated and documented procedure for purging to verify that the pH has not exceeded 3.0 for Test Solution A after purging. During the test, pH may increase but shall not exceed 4.0. If the pH exceeds 4.0, the test is invalid. If the test-solution-volume to testspecimen-surface-area ratio is maintained and steps are taken to exclude oxygen from the test vessel as specified in this standard, the pH will not exceed this value.

6.2.4 Test Solution A shall be used in Methods A, C, and D unless the use of Test Solution B, C or D is specified by the user or purchaser.

6.3 Test Solution B 6.3.1 Test Solution B is an acidified and buffered H2S-saturated aqueous brine solution.

6.3.2 Test Solution B shall consist of 5.0 wt% sodium chloride, 2.5 wt% glacial acetic acid, and 0.41 wt% sodium acetate dissolved in distilled or deionized water (e.g., 50.0 g of NaCl, 25.0 g [23.8 mL] of CH3COOH, and 4.1 g of CH3COONa dissolved in 921 g of distilled or deionized water).

6.3.3 Test solution pH before contact with a test specimen and before H2S saturation shall be between 3.4 and 3.6. Adjustment of test solution chemistry to adjust pH is not allowed. If test solution pH is out- of-range, then the solution shall be discarded. During the test, pH may increase but shall not exceed 4.0. If the pH exceeds 4.0, the test is invalid. If the test-solution5

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ANSI/NACE TM0177-2016 volume to test-specimen-surface-area ratio is maintained and steps are taken to exclude oxygen from the test vessel as specified in this standard, the pH will not exceed this value.

6.3.4 Test Solution B may be used with carbon and low-alloy steels.

6.3.5 Test Solution B may be used in Methods A, C, and D. 6.4 Test Solution C 6.4.1 Test Solution C is a buffered aqueous brine solution with a chloride content, H2S partial pressure, and pH specified by the user or purchaser to simulate the intended service environment.

6.4.2 Test Solution C shall consist of distilled or deionized water containing 0.4 g/L sodium acetate (5 mM CH3COONa) and chloride (added as NaCl) at the same concentration as the intended service environment.

6.4.3 Hydrochloric acid (HCl) or sodium hydroxide (NaOH) shall be added to achieve the specified pH. The test solution pH shall be measured at the start of the test and at the end of the test. The test solution pH shall be maintained within 0.2 pH units of the specified pH. 6.4.4 Test gas shall consist of a mixture of H2S and carbon dioxide (CO2), with H2S content sufficient to produce the specified H2S partial pressure of the intended service environment.

The test gas shall be continuously bubbled through the test solution. The gas bubbling rate shall be optimized to maintain saturation of the test solution.

6.4.5 Test Solution C may be used with martensitic stainless steels.

6.4.6 Test Solution C may be used in Methods A, C, and D. NOTE: The combination of a lower acetate concentration (0.4 g/L) and acidification with HCl rather than acetic acid leads to a significantly reduced concentration of both acetic acid and acetate ion in Test Solution C when compared with Test Solution B. Although this may be adequate for maintaining the bulk solution pH constant when testing corrosion-resistant alloys, the ability to resist changes of pH at the metal-solution interface is reduced. In cathodic regions, there is less acetic acid to counterbalance the tendency for an increased pH. In localized anodic regions, there is less acetate to restrain lowering of the pH due to metal ion hydrolysis. In the latter case, the lower acetate concentration associated with Test Solution C would give a more conservative result, in the sense that cracking could be more likely.

6.5 Test Solution D 6.5.1 Test Solution D is a buffered aqueous brine solution with a chloride content, H 2S partial pressure, and pH specified for mild sour conditions.

6.5.2 Test Solution D shall consist of 5 wt% sodium chloride and 0.40 wt% sodium acetate dissolved in distilled or deionized water (e.g., 50.0 g of NaCl and 4.0 g of CH3COONa dissolved in 946 g of distilled or deionized water).

6.5.3 Hydrochloric acid (HCl) or sodium hydroxide (NaOH) shall be used to adjust the initial pH. The solution pH before contact with a test specimen and before H 2S saturation shall be between 3.8 and 4.0. During the test, pH may increase, but shall not exceed 4.6. If the pH exceeds 4.6, the test is invalid. If the test-solution-volume to test-specimen-surface-area ratio

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ANSI/NACE TM0177-2016 is maintained and steps are taken to exclude oxygen from the test vessel as specified in this standard, the pH will not exceed this value.

6.5.4 The test gas shall consist of 7.0 Âą 0.3 mol.% H2S with balance of nitrogen. After initial saturation, the gas should be bubbled continuously at a rate that maintains saturation.

6.5.5 Test Solution D may be used with high-strength steels such as API 5CT Grade C110.

6.5.6 Test Solution D may be used in Methods A, C, and D. 6.6 The test solution required for use in Method B is specified in Paragraph 9.3.

6.7 Deaeration Requirements and Procedures 6.7.1 Deaeration Requirements The method of solution deaeration and solution transfer used for testing shall have the dissolved oxygen levels in the test solution (immediately prior to the introduction of the H2S or H2S mixed gas) of less than 50 ppb for low-alloy steels up to the strength level of 552 MPa (80 ksi) and 10 ppb for low-alloy steels above the strength level of 552 MPa (80 ksi) and corrosion resistant alloys. Each laboratory shall have a demonstrated and documented procedure to achieve the required dissolved oxygen levels.

6.7.2 Deaeration Procedures 6.7.2.1 The test solution shall be previously deaerated in a sealed vessel that is purged with inert gas at a rate of at least 100 mL/min per liter of test solution for at least one hour.

The test vessel containing the test specimen(s) shall be purged separately with inert gas at a rate of at least 100 mL/min per liter of test solution for at least one hour. The deaerated test solution shall be transferred into the pre-purged test vessel, and it shall be further deaerated at a rate of at least 100 mL/min per liter of test solution for at least one hour or until the required dissolved oxygen level is reached.

6.7.2.2 Other methods of deaeration and transfer may be used if they have been demonstrated to achieve the required dissolved oxygen levels.

6.7.3 Oxygen contamination is evident by a cloudy (opaque) appearance of the test solution when the H2S gas enters the test vessel. An opaque appearance of the test solution upon H2S entry shall disqualify the test. The test specimen shall be removed and cleaned, and the test solution make-up, transfer, and deaeration procedure repeated.

Note: To achieve the required oxygen levels, consideration needs to be given to the following factors: type of tubing used, ingress of oxygen into the test cell during the test, etc. _____________________________________________________________________________ Section 7: Testing at Elevated Temperature/Pressure 7.1 The dominant cracking mechanisms for most classes of materials in the presence of H 2S vary with temperature. Ferritic steels and ferritic and martensitic stainless steels crack primarily by a hydrogen (i.e., cathodic) mechanism and have maximum susceptibility near room temperature. For austenitic stainless steels, as temperature increases, cracking susceptibility increases due to the major contribution from anodic processes. Duplex stainless steels exhibit mixed behavior, with maximum susceptibility to cracking in a mid-range of temperatures. To facilitate testing in simulated service conditions or to predict worst-case conditions, and to facilitate testing with H2S partial pressure exceeding 100 kPa (absolute) (14.5 psia), the following modified techniques are available.

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7.2 Testing at elevated temperatures and pressures involves additional safety considerations compared to room temperature and atmospheric pressure testing. While some general guidance is given herein, it may not address all aspects and should be supplemented to comply with local safety requirements. Because H2S may be consumed during the test, gas replenishment and continuous gas bubbling techniques are described. The H2S loss rate and its effect on the corrosiveness of the test environment are functions of several factors, including the corrosion rate of the test material and the partial pressure of H2S in the test environment. Guidance is given on measures that experience has shown to be appropriate for maintaining the required H2S partial pressure, but in all cases, it shall be demonstrated, by measuring H2S concentration in either the test solution or gas phase, that the required test conditions have been maintained. This information must be reported with the test data.

7.3 Test Equipment The test equipment shall consist of a test vessel and accessory equipment rated to withstand corrosion and pressure commensurate with the test conditions and with an appropriate safety margin.

7.3.1 The test vessel shall be equipped with a thermocouple well or other means of measuring the temperature of the test solution, inlet and outlet ports for gas, a dip tube on the inlet port, and a pressure-measuring device.

7.3.2 If continuous gas bubbling is to be used, a condenser on the outlet port may be used to limit loss of test solution. This has been found to be useful at temperatures greater than 50 °C (120 °F) and/or when the volume of the test solution is less than 200 mL. 7.3.3 A bursting (rupture) disc or pressure-relief valve should be used for safety reasons.

7.3.4 The pressure-measuring device shall have an accuracy of ±1% of the maximum system pressure. If the pressure is measured by a gauge, the maximum system pressure shall be greater than 20% and less than 80% of gauge full scale. Schematic arrangements of test equipment used for the various test methods are shown in Figures 1 and 2.

7.3.5 Elastomeric seal materials, if used, must resist H2S at the temperature of use as verified by independent measurement.

P Gas out

Pressure gauge

Gas in

Condenser

R Regulator

Test solution

Pressure vessel

Figure 1: Schematic Arrangement of Test Equipment for Method A—NACE Standard Tensile Test

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ANSI/NACE TM0177-2016

7.4 Test Solution The test solution used in the test may be selected as required by the test specification. The test solution usually consists of brine (NaCl) at concentrations up to saturation. Buffered acidification is permitted, analogous to room-temperature methods. Also, the test solution may contain elemental sulfur.10,11,12 7.5 Test Gas The test gas is usually a mixture of two or more of the following: H2S, CO2, and inert gas such as N2 or Ar. At low H2S partial pressures, tests in inert gas without CO2 require careful interpretation because of corrosion product solubility effects.13 The test gas mixture should be contained in a standard gas bottle equipped with a suitable pressure regulator (usually stainless steel) capable of gas delivery to the total test pressure required. A commercially supplied gas mixture with composition determined by analysis should be used.

P Gas out

Pressure gauge

Gas in

Condenser

R Regulator

Test solution

Pressure vessel

Figure 2: Schematic Arrangement of Test Equipment for Method B—NACE Standard BentBeam Test, Method C—NACE Standard C-Ring Test, and Method D—NACE Standard DoubleCantilever-Beam Test 7.6 Test Stresses The test stresses may be derived from the actual yield strength at ambient temperature or from the actual yield strength at the planned test temperature or as otherwise required by the test specification.

7.7 Test Procedure Test procedures shall be identical to those specified for room-temperature tests unless amended as follows:

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ANSI/NACE TM0177-2016 7.7.1 The test solution and test specimen(s) shall be placed in the test vessel, then the test vessel shall be sealed and leak tested. Test vessels are usually tested for leaks with inert gas at 1.5 times the maximum test pressure.

7.7.2 The expansion of test solution on heating can fill the test vessel and risk explosion. The volume of test solution should be less than 75% of the total volume of the test vessel.

Moreover, a greater safety margin (smaller percentage of total volume) should be used at temperatures exceeding 225 °C (435 °F).

7.7.3 The test solution shall be deaerated by bubbling inert gas through the gas inlet tube into the test solution for a minimum period of 1 h/L of test solution.

7.7.4 The H2S partial pressure, ppH2S, in the test environment shall be determined by one of the following two methods: 7.7.4.1 Test vessel heated before test gas admitted 7.7.4.1.1 The test vessel shall be heated with valves closed to test temperature and stabilized. System pressure (the vapor pressure of the test solution), P1, shall be measured.

7.7.4.1.2 Test gas shall be admitted to the vessel until the test pressure, PT, is reached.

7.7.4.1.3 The H2S partial pressure, ppH2S, in the test environment is approximated by Equation (1): ppH2S = (PT – P1) (XH2S)

(1)

where: PT = total absolute test pressure; P1 = vapor pressure above the test solution; and XH2S= mole fraction of H2S in the test gas.

7.7.4.2 Test gas admitted before test vessel heated.

Test gas may be admitted to the test vessel before heating if a proven means of calculating ppH2S can be demonstrated.

7.7.4.3 The test method used shall be reported with the test data.

7.7.5 Test gas shall be replenished as needed to maintain the required test conditions (primarily H2S partial pressure) as outlined in Paragraph 7.2. Continuous test gas bubbling at 0.5 to 1.0 mL/min or periodic test gas replenishment once or twice weekly has been found necessary when testing CRAs at H2S partial pressures below 2 kPa (absolute) (0.3 psia) or carbon and alloy steels at H2S partial pressures below 100 kPa (absolute) (14.5 psia). Test solution loss and ingress of oxygen during test gas replenishment shall be avoided.

7.7.6 The test duration shall be as specified for the applicable test method (A, B, C, or D). The test temperature for Methods A, B, and C shall be maintained within ±3 °C (±5 °F) of the specified test temperature and recorded manually on a daily basis or at shorter intervals by data recorder. For Method D, test temperature shall be maintained within ±1.7 °C (±3.0 °F).

Pressure shall be monitored and recorded daily. If test pressure falls by more than 40 kPa (6 psi) below the required test pressure, the test gas must be replenished.

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ANSI/NACE TM0177-2016 7.7.7 At the test completion, the test vessel should be purged with inert gas while cooling to ambient temperature before opening. The load should be relaxed before cooling, if possible, when using equipment with external loading.

____________________________________________________________________________ Section 8: Method A—NACE Standard Tensile Test 8.1 Method A, the NACE standard tensile test, provides for evaluating metals for EC resistance under uniaxial tensile loading. It offers a simple unnotched test specimen with a well-defined stress state. EC susceptibility with Method A is usually determined by time-to-failure. Tensile test specimens loaded to a particular stress level give a failure/no-failure test result. When multiple test specimens are tested at varying stress levels, an apparent threshold stress for EC can be obtained.14 8.1.1 This section sets forth the procedure for testing at room temperature and atmospheric pressure. Special considerations for testing at elevated temperature and pressure are set forth in Section 7.

8.2 Test Specimen 8.2.1 The size and shape of the material available for testing often restricts selection of test specimens. The orientation of the test specimen can affect the results and should be noted.

8.2.2 The gauge section of the standard tensile test specimen (see Figure 3[a]) shall be 6.35 ± 0.13 mm (0.250 ± 0.005 in) in diameter by 25.4 mm (1.00 in) long (see ASTM A370). A subsize tensile test specimen with gauge section of 3.81 ± 0.05 mm (0.150 ± 0.002 in) in diameter by 25.4 mm (1.00 in) long is acceptable. After machining, tensile test specimens should be stored in a low-humidity area, in a desiccator, or in uninhibited oil until ready for testing.

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ANSI/NACE TM0177-2016

D G Dimension

D G R (min.)

R

Standard tensile test specimen 6.35 ± 0.13 mm (0.250 ± 0.005 in) 25.4 mm (1.00 in) 15 mm (0.60 in)

Subsize tensile test specimen 3.81 ± 0.05 mm (0.150 ± 0.002 in) 25.4 mm (1.00 in) 15 mm (0.60 in)

(a) Dimensions of the Tensile Test Specimens Gas Out

FORCE

Gas In



Test Solution

Tensile Test Specimen

 FORCE

(b) Tensile Test Specimen in an Environmental Chamber Figure 3: Tensile Test Specimens 8.2.3 The radius of curvature at the ends of the gauge section shall be at least 15 mm (0.60 in) to minimize stress concentrations and fillet failures.

Additional methods that have been found helpful in reducing fillet failures are to: (1) Eliminate undercutting of fillet radii in machined test specimens; and (2) Machine the test specimen gauge section with a slight (0.05 to 0.13 mm [0.002 to 0.005 in]) taper that produces a minimum cross-section in the middle of the gauge section.

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ANSI/NACE TM0177-2016

8.2.4 The ends of the test specimen must be long enough to accommodate seals for the test vessel and to make connections to the stressing fixture. (See Figure 3[b]).

8.2.5 The test specimen must be machined or ground carefully to avoid overheating and cold working in the gauge section. In machining operations, the final two passes should remove no more than a total of 0.05 mm (0.002 in) of material. Grinding is also acceptable if the grinding process does not harden or temper the material.

8.2.6 For all materials, the average surface roughness of the gauge section shall be 0.25 Âľm (10 Âľin) or finer, as defined by Ra value in ISO 4287.15 Final surface finish may be obtained by mechanical polishing or electropolishing if the roughness requirement is met. The finishing processes shall be reported with the test data. When electropolishing, bath conditions must be such that the test specimen does not absorb hydrogen during the procedure. When agreed with the end user, electropolishing shall only be used with low-alloy steels having a maximum of 1.5 weight percent chromium level. Each laboratory shall have a demonstrated and documented procedure for electropolishing and validating that electropolishing has comparable results to surface ground/mechanically polished specimens (per test material, grade and test condition).

8.2.7 When a standard tensile test specimen cannot be obtained from the material because of its size or shape, a subsize tensile test specimen may be used. However, subsize tensile test specimens can produce shorter failure times than those observed for standard tensile test specimens. The report of test data using subsize tensile test specimens shall clearly state the use of subsize test specimens. If an alternate specimen configuration (one not specified in this document) is used, the dimensions shall be clearly stated in the test report.

8.2.8 Test Specimen Identification 8.2.8.1 Stamping or vibratory stenciling may be used on the ends of the test specimen, but shall not be used in the gauge section.

8.2.9 Test Specimen Cleaning 8.2.9.1 Before testing, test specimens shall be degreased with solvent and rinsed with acetone.

8.2.9.2 The gauge section of the test specimen shall not be handled or contaminated after cleaning.

8.3 Test Solutions—see Section 6.

8.4 Test Equipment 8.4.1 Many types of stress fixtures and test vessels used for stress corrosion testing are acceptable for Method A. Consequently, the following discussion emphasizes the test equipment characteristics required for selecting suitable items and procedures.

8.4.2 Tensile tests should be performed with constant-load or sustained-load (proof-ring or spring-loaded) devices (see ASTM G49).16 8.4.2.1 All loading devices shall be calibrated to ensure accurate application of load to the test specimen. The error for loads within the calibration range of the loading device shall not exceed 1.0% of the calibration load.

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ANSI/NACE TM0177-2016 8.4.2.2 The loading device shall be constructed and maintained to minimize bending and torsional loads.

Note: Unwanted bending stress increases the maximum total stress and may lead to an over-conservative test, specimen failure, and rejection of a material which otherwise would pass the Method A test if bending was absent or minimized.

8.4.3 When susceptible materials are tested using sustained-load devices, it is possible for cracks to initiate and propagate only partially, not fully, through the test specimen (see Paragraph 8.7). Consequently, susceptibility determination from sustained-load test results requires the visual examination of the test specimens for the presence of part-through cracks.

The determination may be difficult if the cracks are small and sparse or if obscured by corrosion deposits. However, testing with constant-load devices ensures that susceptible materials will separate completely. This result clearly identifies the material as susceptible and does not rely on finding part-through cracks.

8.4.4 Dead-weight testers capable of maintaining constant pressure on a hydraulic cell may be used for constant-load testing (see Figure 4).

Figure 4: Constant-Load (Dead-Weight) Device 8.4.5 Sustained-load tests may be conducted with spring-loaded devices and proof rings when relaxation in the fixtures or test specimen results in only a small percentage decrease in the applied load (see Figure 5).

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(a) Proof ring

(b) Spring-loaded Figure 5: Sustained-Load Devices 8.4.5.1 If using proof rings, the following procedures are required:

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ANSI/NACE TM0177-2016 8.4.5.1.1 Before calibration, proof rings shall be preconditioned by stressing at least 10 times to 110% of the maximum load rating of the proof ring.

8.4.5.1.2 The load on the tensile test specimen shall lie within the load range of the proof ring. Accordingly, proof rings shall be selected so that the applied load produces a ring deflection of more than 0.6% of the ring diameter, but not less than 0.51 mm (0.020 in). If it is less than 0.51 mm (0.020 in) or less than 0.6% of the ring diameter, the calibration deflection, calibration load, and test load must be specified.

8.4.5.2 A substantial decrease in the proof ring deflection may signify: (a) The initiation and propagation of cracks in the test specimen; (b) Yielding of the test specimen; or (c) Relaxation of stress.

The proof ring deflection should be measured during the test or at the test completion.

8.4.5.3 The deflection should be monitored when the applied stress is within 10% of the material yield strength.

8.4.6 The test specimen must be electrically isolated from any other metals in contact with the test solution.

8.4.6.1 The seals around the test specimen must be electrically isolating and airtight, but should allow movement of the test specimen with negligible friction.

8.4.6.2 In cases in which the complete test fixture is immersed in a test solution, the stressing fixture may be made of the same material, or if it is made of a different material, it must be electrically isolated from the test specimen. The stressing fixture may be coated with a nonconductive impermeable coating, if desired.

8.4.7 The test vessel shall be sized to maintain a test solution volume of 30 Âą 10 mL/cm2 of test specimen surface area.

8.5 Stress Calculations Loads for stressing tensile test specimens shall be determined from Equation (2): P=SxA

(2)

where: P = load; S = applied stress; and A = actual cross-sectional area of the gauge section.

8.6 Testing Sequence 8.6.1 The minimum gauge diameter of the tensile test specimen shall be measured, and the tensile test specimen load shall be calculated for the desired stress level.

8.6.2 The tensile test specimen shall be cleaned and placed in the test vessel, and the test vessel shall be sealed to prevent air leaks into the vessel during the test.

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ANSI/NACE TM0177-2016

8.6.3 The load may be applied before or after the test vessel is purged with inert gas. Tensile test specimens may be stressed at convenient increments of the yield strength or load.

8.6.4 The load should be carefully applied to avoid exceeding the desired value. If the desired load is exceeded, the test shall be run at the new load or discarded.

8.6.5 Deaeration requirements are given in Paragraph 6.7.

8.6.6 The test solution shall then be saturated with H2S at a minimum flow rate per a documented procedure that has been validated to obtain 2,300 mg/L minimum H2S concentration or other minimum value H2S concentration that is proportional to less than 1 bar H2S partial pressures (at the extent of the test specimen locations). When Solution D is used, the minimum H2S concentration shall be 160 mg/L. Saturation shall occur within an hour of contact with specimens in test vessels up to 1 L (see Note 1). For larger test vessels, saturation can require more than two hours. Analysis shall be done using iodometric titration (See Appendix C or other suitable methods). The validation of the documented procedure shall be performed after saturation at the start of test, after 24 hours, weekly and at the end of the test. A continuous flow of H2S through the test vessel and outlet trap shall be maintained with a positive pressure of H2S throughout the test that prevents air from entering the test vessel.

NOTE 1: One method found to give saturation within one hour in a proof ring test (approximately 1/3 L) is to purge at 100 mL per minute for 60 minutes.

NOTE 2: Laboratories at high elevations may find it necessary to compensate for lower atmospheric pressure in order to achieve the required saturation levels.

8.6.7 The termination of the test shall be at tensile test specimen failure or after 720 hours, whichever occurs first.

8.6.8 When needed, additional tensile test specimens shall be tested to closely define the nofailure stress.

8.7 Failure Detection Following exposure, the surfaces of the gauge section of the nonfailed tensile test specimens shall be cleaned and inspected for evidence of cracking. Those tensile test specimens containing cracks shall be noted.

8.7.1 For all materials, failure is either: (a) Complete separation of the tensile test specimen; or (b) Visual observation of cracks on the gauge section of the tensile test specimen at 10X after completing the 720 hour test duration.

Investigative techniques employing metallography, scanning electron microscopy, or mechanical testing may be used to determine whether cracks on the gauge section are evidence of EC. If it is verified that the cracks are not EC, then the tensile test specimen passes the test.

8.7.2 Time-to-failure may be recorded using electrical timers and microswitches.

8.8 Reporting of Test Results

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ANSI/NACE TM0177-2016 8.8.1 Time-to-failure and no-failure data or the visual observation of surface cracks at the end of the test shall be reported for each stress level (see Table 1).

8.8.2 If known, the chemical composition, heat treatment, mechanical properties, other information specified above, and data taken shall be reported.

8.8.3 Table 1 shows the recommended format for reporting the data. presented on semilog graph paper (see Figure 6).

Data may also be

Table 1 NACE International

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ANSI/NACE TM0177-2016 NACE Uniform Material Testing Report Form (Part 1): Testing in Accordance with NACE Standard TM0177(A) Method A—NACE Standard Tensile Test Submitting Company Submitted by Alloy Designation

Telephone No.

Submittal Date Testing Lab General Material Type

He at Nu mb er /I den t if i cat ion

Che mi st r y C Mn Si P S Ni Cr Mo V Al Ti Nb N Cu Other Material Processing History Melt Practice (e.g., OH, BOF, EF, AOD)(B) Product Form Heat Treatment (Specify time, temperature, and cooling mode for each cycle in process.)

Other Mechanical, Thermal, Chemical, or Coating Treatment(C) (A) Test method must be fully described if not in accordance with TM0177.

(B) Melt practice: open-hearth (OH), basic oxygen furnace (BOF), electric furnace (EF), argon-oxygen decarburization (AOD).

(C) E.g., cold work, plating, nitriding, prestrain.

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ANSI/NACE TM0177-2016 Table 1 (continued) NACE Uniform Material Testing Report Form (Part 2): Testing in Accordance with NACE Standard TM0177 Method A—NACE Standard Tensile Test Lab Data for Material: Tested per NACE Standard TM0177(A) Test Specimen Geometry: Standard Nonstandard Nominal Diameter Constant Load— Dead Weight Hydraulic Other Sustained Load— Proof Ring Spring Other Post-Test Proof Ring Deflection Measurement Finishing Process

Gauge Length

Finish

Start

Hardness ( )

Reduction in Area (%)

Elongation (%) Ultimate Tensile Strength ( ) Yield Strength(D) ( )

Orientation (C) Location(B)

Chemistry: Test Solution A Test Solution B Test Solution C (define)__________ Test Solution D Other Test Solution Outlet Trap to Exclude Oxygen Temperature Maintained 24° ± 3 °C (75° ± 5 °F) Temperature Maintained ±3 °C (±5 °F) For elevated temperature tests: Test vessel heated before test gas Test gas admitted before test vessel heated Test Specimen Properties Test Stress (% of Yield Test Applied Heat Remarks Strength) Solution Treatment (Including pH(E) Surface Condition and Material H2S Level) Identification

Time-to-Failure (Hours) NF = No Failure at 720 hours

(A)

Test method must be fully described if not in accordance with NACE Standard TM0177.

Location of test specimen may be: tubulars—outside diameter (OD), midwall (MW), or inside diameter (ID); solids—surface (S), quarter-thickness (QT), midradius (MR), center (C), or edge (E).

(C) Orientation may be longitudinal (L) or transverse (T).

(D) Open parentheses must be filled with metric or English units, as appropriate to the data. Yield strength is assumed to be 0.2% offset unless otherwise noted. Consideration may be given to the use of the service temperature when determining the value of the yield strength used in the calculations of the test stresses to be applied.

(E) Enter pH for test conducted on nonfailed tensile test specimen at highest stress if summarizing data .

(B)

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ANSI/NACE TM0177-2016

100 (MPa) (103 psi)

600

Applied Stress

80 500 60

400

80 70 60 50 40

300

40 30

Material _________________ Chemistry _______________

200 20 100

20

Physical Data ____________

Strength PercentPercent of Actualof(orYield Minimum Specified) Yield Strength

No-failure Data

100

No-Failure Data

700

90

Other ___________________

10

0

0 1

10

100

1,000 720

Log (Time-to-Failure [Hours]) Figure 6: Applied Stress vs. Log (Time-to-Failure) _____________________________________________________________________________ Section 9: Method B—NACE Standard Bent-Beam Test 9.1 Method B, the NACE Standard Bent-Beam Test, provides for testing carbon and low-alloy steels subjected to tensile stress to evaluate resistance to cracking failure in low-pH aqueous environments containing H2S. It evaluates EC susceptibility of these materials in the presence of a stress concentration. The compact size of the bent-beam test specimen facilitates testing small, localized areas and thin materials. Bent-beam test specimens loaded to a particular deflection give a failure/no-failure test result. When testing multiple test specimens at varying deflections, a statistically based critical stress factor (Sc) for a 50% probability of failure can be obtained. NaCl is not added to the test solution for this test method. Laboratory test data for carbon and low-alloy steels have been found to correlate with field data.17 9.1.1 This section sets forth the procedure for bent-beam testing at room temperature and atmospheric pressure. Special considerations for testing at elevated temperature and pressure are set forth in Section 7.

9.1.2 Method B can be summarized as follows: 9.1.2.1 This method involves deflecting each test specimen in a series by applying a different bending stress. The stressed test specimens then are exposed to the test environment, and the failure (or no-failure) by cracking is determined. From these data 21

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ANSI/NACE TM0177-2016 obtained by testing multiple test specimens at varying deflections, a statistically based Sc for a 50% probability of failure is calculated to indicate the material’s resistance to SSC. 9.1.2.2 This method constitutes a constant-deflection test of high test specimen compliance. The computed stress is called a pseudo-stress because it does not reflect: (a) Actual stress or stress distribution in the test specimen; (b) Deviation from elasticity associated with plastic deformation; or (c) Decrease in stress in the test specimen as a crack or cracks grow.

Consequently, this method is not suitable for determination of threshold stress.

9.2 Test Specimen 9.2.1 The dimensions of the standard bent-beam test specimen shall be 4.57 ± 0.13 mm (0.180 ± 0.0050 in) wide, 1.52 ± 0.13 mm (0.060 ± 0.0050 in) thick, and 67.3 ± 1.3 mm (2.65 ± 0.050 in) long (see Figure 7). After machining, test specimens shall be stored in a low-humidity area, in a desiccator, or in uninhibited oil until ready for testing.

9.2.2 Generally, 12 to 16 test specimens should be taken from a given sample to determine susceptibility of the material.

9.2.2.1 The orientation and location of the test specimen with respect to the original material must be reported with the test results.

9.2.3 The test specimens should be milled to an approximate size and then surface ground to final dimensions. The last two passes on either side shall be restricted to removal of 0.013 mm (0.00050 in) per pass (care must be taken to prevent overheating). The final surface roughness shall be 0.81 µm (32 µin) or finer.

9.2.4 As shown in Figure 7, two 0.71 mm (0.028 in) diameter holes (No. 70 drill bit) shall be drilled at the midlength of the test specimen, centered 1.58 mm (0.0620 in) from each side edge. Holes shall be drilled before machining the final surface.

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ANSI/NACE TM0177-2016 Drill diameter D through 2 holes W

H

C L

H t

L

Dimension L t W H D

Size (mm) 67.3 ± 1.3 1.52 ± 0.13 4.57 ± 0.13 1.58 ± 0.05 0.71 ± 0.0013

(in) 2.65 ± 0.050 0.060 ± 0.0050 0.180 ± 0.0050 0.062 ± 0.002 0.028 ± 0.0005 (No. 70 Drill)

Figure 7: Dimensional Drawing of the Standard Bent-Beam Test Specimen 9.2.5 Test Specimen Identification 9.2.5.1 The test specimens may be stamped or vibratory stenciled in a region within 13 mm (0.50 in) of either end on the compression side.

9.2.6 Test Specimen Cleaning 9.2.6.1 Surfaces and edges of the test specimen shall be ground by hand on 240 grit emery paper with scratches parallel to the test specimen axis.

9.2.6.2 The test specimens shall be degreased with solvent and rinsed with acetone.

9.2.6.3 The stressed section of the test specimen shall not be handled or contaminated after cleaning.

9.3 Test Solution 9.3.1 The test solution shall consist of 0.5 wt% glacial acetic acid dissolved in distilled or deionized water (e.g., 5.0 g [4.8 mL] of CH3COOH dissolved in 995 g of distilled or deionized water). NaCl shall not be added to the test solution.

9.3.2 Use of Test Solutions A, B, C, and D with this test method has not been standardized.

9.4 Test Equipment 9.4.1 Many types of stress fixtures and test vessels used for stress corrosion testing are acceptable for Method B. Consequently, the following discussion emphasizes the test equipment characteristics required for selecting suitable items and procedures.

9.4.2 Tests shall be performed using constant-deflection fixtures that employ three-point bending of the test specimen. (See Figure 8).

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ANSI/NACE TM0177-2016

(in) 31.75 ± 0.13

1.250 ± 0.050

Dimension

(A)

Size

(mm) A 15.9 B 31.75 ± 1.27 C 95.3 E 25.4 F(A) M12 x 1.25 G(A) 25.4 J 19.1 K(A) M6 x 40 long L ≥ 66.70 M 1.6 N(A) 6.35 P 6.35 R 3.3 Equivalent commercial sizes.

(in) 0.625 1.250 ± 0.050 3.75 1 0.5-20 NF thread 1 0.75 #10-32 NF x 1.5 long ≥ 2.625 0.063 0.25 0.25 0.13

(a) Dimensional Drawing

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ANSI/NACE TM0177-2016

(b) Photograph Figure 8: Typical Stressing Fixture for Bent-Beam Test Specimen 9.4.3 Test fixtures immersed in a test solution should resist general corrosion (UNS(4) S31600 is commonly used). Fixture elements contacting the test specimen must be electrically isolated from it.

9.4.4 Deflection gauges shall be graduated in 0.0025 mm (0.00010 in) divisions.

9.4.4.1 Test specimen deflection should be determined by separate gauges or by gauges incorporated in a loading fixture. In designing a deflection gauge to suit individual circumstances, the deflection at midlength of the test specimen should be measured.

9.4.5 Test Vessel 9.4.5.1 The test vessel shall be sized to maintain a test solution volume of 30 Âą 10 mL/cm2 of test specimen surface area. Maximum volume of the test vessel should be 10 L. 9.4.5.2 The test vessel shall be valved at both inlet and outlet to prevent contamination of the test solution by oxygen.

9.4.5.3 A fritted glass bubbler should be used to introduce the inert gas and H2S below the array of test specimens. The bubbles should not impinge on the test specimens.

9.5 Deflection Calculations 9.5.1 An estimated outer fiber pseudo-stress (S) for the material shall be used in beam deflection calculations. For carbon and low-alloy steels, S values are typically in the range of 69 MPa (104 psi) at 22 to 24 HRC. As hardness increases, S generally decreases.

9.5.2 The selected range of estimated S values shall be used as pseudo-stresses to calculate the deflections of the test specimens.

(4) Unified

Numbering System for Metals and Alloys (UNS). UNS numbers are listed in Metals & Alloys in the Unified Numbering System (latest revision). (Warrendale, PA: SAE International and West Conshohocken, PA: ASTM International).

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ANSI/NACE TM0177-2016 9.5.3 The test specimen deflection shall be calculated for each of the pseudo-stress values using Equation (3): D=

Sď Ź

2

(3)

6Et

where: D = deflection; S = nominal outer fiber pseudo-stress;

ď Ź

= distance between centerlines of end supports; E = elastic modulus; and t = thickness of test specimen.

The formula assumes elastic conditions and ignores the stress concentration effect of the holes and the test specimen plasticity at high stress levels.

9.6 Testing Sequence 9.6.1 The test specimen dimensions shall be measured, and deflections shall be calculated for desired pseudo-stress levels.

9.6.2 Test specimens shall be stressed in fixtures by deflecting them to the nearest 0.0025 mm (0.00010 in) with a dial or digital gauge and fixture. The deflection should be carefully applied to avoid exceeding the desired value. If the desired deflection is exceeded, the test shall be run at the higher deflection or discarded.

9.6.3 The stressed test specimens shall be cleaned and placed into the test vessel.

9.6.4 Deaeration requirements are given in Paragraph 6.7.

9.6.5 The test solution shall then be saturated with H2S at a rate of at least 100 mL/min for at least 20 min/L of test solution. The H2S in the test vessel shall be replenished periodically by bubbling H2S for a duration of 20 to 30 min at a rate of at least 100 mL/min/L of test solution three times per week for the duration of the test. The days for the replenishment should be the first, third, and fifth day of each work week.

9.6.6 The test shall be terminated after 720 hours or when all test specimens have failed, whichever occurs first.

9.6.7 Additional test specimens and iterative testing may be necessary to define the Sc closely.

9.7 Failure Detection 9.7.1 Crack presence shall be determined visually with the aid of a low-power binocular microscope. If the test specimen contains only one or a few cracks, the shape of the test specimen may have changed considerably, predominantly by kinking; this feature helps to identify cracked test specimens. However, if many cracks are present, a shape change may not be apparent. Because corrosion products may obscure cracks, a careful examination shall be made. Mechanical cleaning or metallographic sectioning of the test specimen may be necessary in these instances to detect cracks.

9.7.2 Failure is cracking of the test specimen. Consequently, following exposure, the surface of the test specimens should be cleaned and visually inspected at 10X for evidence of cracking following a 20 degree bending by hand. Test specimens found to contain cracks shall be considered failed.

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ANSI/NACE TM0177-2016 9.8 Reporting of Test Results 9.8.1 Failure/no-failure data and nominal outer fiber pseudo-stress (S) values shall be reported.

Time-to-failure data are optional.

9.8.2 The Sc shall be calculated using Equation (4) for S values expressed in MPa, or Equation (5) for S values expressed in psi:

S SC 

68.95 MPa

 2 T

(4)

n

where: S = nominal outer fiber pseudo-stress (in MPa) used to calculate the beam's deflection; T = the test result (i.e., +1 for passing and -1 for failure); and n = the total number of test specimens tested.

When Equation (4) is used, all pseudo-stress data that are more than ±210 MPa from the initial calculated value Sc x 68.95 MPa shall be discarded, and a new Sc value shall be recalculated.

The recalculated Sc value eliminates low and high bias data.

S

SC

 2 T 4 10 psi  n

(5)

where: S = nominal outer fiber pseudo-stress (in psi) used to calculate the beam's deflection; T = the test result (i.e., +1 for passing and –1 for failure); and n = the total number of test specimens tested.

When using Equation (5), all pseudo-stress data that are more than ±3.0 x 104 psi from the initial calculated value Sc x 104 psi shall be discarded, and a new Sc value shall be recalculated.

The recalculated Sc value eliminates low and high bias data.

9.8.3 The calculated Sc value for each material tested shall be reported. If Sc is recalculated, the recalculated Sc value shall be reported, not the initial calculated Sc value.

9.8.4 If known, the chemical compositions, heat treatment, mechanical properties, and other data taken shall be reported.

9.8.5 Table 2 shows the recommended format for reporting the data.

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ANSI/NACE TM0177-2016 ____________________________________________________________________________ Table 2 NACE Uniform Material Testing Report Form (Part 1): Testing in Accordance with NACE Standard TM0177(A) Method B—NACE Standard Bent-Beam Test Submitting Company Submitted by Alloy Designation

Telephone No.

Submittal Date Testing Lab General Material Type

He at Nu mb er /I den tif i cat ion

Che mi st r y C Mn Si P S Ni Cr Mo V Al Ti Nb N Cu Other Material Processing History Melt Practice (e.g., OH, BOF, EF, AOD)(B) Product Form Heat Treatment (Specify time, temperature, and cooling mode for each cycle in process).

Other Mechanical, Thermal, Chemical, or Coating Treatment(C) (A)

Test method must be fully described if not in accordance with TM0177.

Melt practice: open-hearth (OH), basic oxygen furnace (BOF), electric furnace (EF), argon-oxygen decarburization (AOD).

(C) E.g., cold work, plating, nitriding, prestrain.

(B)

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ANSI/NACE TM0177-2016

Table 2 (continued) NACE Uniform Material Testing Report Form (Part 2): Testing in Accordance with NACE Standard TM0177 Method B—NACE Standard Bent-Beam Test Lab Data for Material: Tested per NACE Standard TM0177(A) Test Specimen Geometry:

Chemistry:

Standard Nonstandard Statistical Sc Method Applied

Nominal Size

0.5 wt% glacial acetic acid in distilled or deionized water

Outlet Trap to Exclude Oxygen

Temperature Maintained

Psuedo-Stress (S) Value (

)

Sc Value

±3 °C (±5 °F)

Test Solution pH(E)

Finish

Start

Hardness ( )

Reduction in Area (%)

Elongation (%)

Ultimate Tensile Strength ( )

Yield Strength(D) ( )

Orientation(C)

Location(B)

Material Identification

Other Test Solution

Temperature Maintained 24° ± 3 °C (75 ° ± 5 °F)

Test Specimen Properties

Length

Applied Remarks Heat (Includin Treatme g nt Surface Conditio n and H2S Level)

Time-to-Failure (Hours) NF = No Failure at 720 hours

(A) Test

method must be fully described if not in accordance with NACE Standard TM0177.

Location of test specimen may be: tubulars—outside diameter (OD), midwall (MW), or inside diameter (ID); solids—surface (S), quarter-thickness (QT), midradius (MR), center (C), or edge (E).

(C) Orientation may be longitudinal (L) or transverse (T).

(D) Open parentheses must be filled with metric or English units, as appropriate to the data. Yield strength is assumed to be 0.2% offset unless otherwise noted.

(E) Enter pH for test conducted on nonfailed bent-beam test specimen at highest stress if summarizing data .

(B)

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ANSI/NACE TM0177-2016 _____________________________________________________________________________ Section 10: Method C—NACE Standard C-Ring Test 10.1 Method C, the NACE Standard C-Ring Test, provides for evaluating the EC resistance of metals under conditions of circumferential loading. It is particularly suitable for making transverse tests of tubing and bar. EC susceptibility with the C-ring test specimen is usually determined by time-to-cracking during the test. C-ring test specimens, when deflected to a particular outer fiber stress level, give a failure/no-failure result. When testing multiple C-ring test specimens at varying stress levels, an apparent threshold stress for EC can be obtained.

10.1.1 This section sets forth the procedure for C-ring testing at room temperature and atmospheric pressure. Special considerations for testing at elevated temperature and pressure are set forth in Section 7.

10.2 Test Specimen 10.2.1 An unnotched C-ring test specimen in accordance with ASTM G3818 shall be used.

Sizes for C-rings may vary over a wide range, but C-rings with an outside diameter (OD) of less than about 15.9 mm (0.625 in) should not be used because of increased difficulties in machining and decreased precision in stressing. A typical C-ring test specimen is shown in Figure 9.

10.2.2 The circumferential stress may vary across the width of the C-ring; the variation extent depends on the width-to-thickness (w/t) and diameter-to-thickness (d/t) ratios of the C-ring. The w/t ratio shall be between 2 and 10, and the d/t ratio shall be between 10 and 100.

10.2.3 The material used in the bolting fixtures shall be of the same material as that of the Cring test specimen or be electrically isolated from the C-ring test specimen to minimize any galvanic effects, unless specific galvanic effects are desired.

10.2.4 Machining should be done in stages: the final two passes should remove a total of no more than 0.05 mm (0.002 in) of metal, and the final cut should leave the principal surface with a finish of 0.81 Âľm (32 Âľin) or finer. After machining, test specimens shall be stored in a lowhumidity area, in a desiccator, or in uninhibited oil until ready for testing.

10.2.4.1 A high-quality machined surface is normally used for corrosion test purposes. However, the as-fabricated surface of a tube or bar also may be evaluated by Cring test specimens. Using any finishing process other than machining shall be reported with the test data.

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ANSI/NACE TM0177-2016

Figure 9: Dimensional Drawing of the C-Ring Test Specimen 10.2.5 Test Specimen Identification The C-ring test specimen end segments may be stamped or vibratory stenciled.

10.2.6 Test Specimen Cleaning 10.2.6.1 Before testing, C-ring test specimens shall be degreased with solvent and rinsed with acetone.

10.2.6.2 The C-ring test specimen shall not be contaminated after cleaning. Test specimens should be handled with new disposable gloves. Powdered gloves shall be cleaned of powder prior to handling test specimens.

10.3 Test Solutions—see Section 6.

10.4 Test Equipment 10.4.1 The test equipment necessary for stressing C-ring test specimens shall include calipers or equivalent equipment capable of measuring to the nearest 0.025 mm (0.0010 in), wrenches sized to the bolting fixtures used, and a clamping device. C-ring test specimens shall be clamped during stressing by the bolting fixtures or the tips of the C-ring. No clamping shall take place in the central test section of the C-ring.

10.4.2 The C-ring test specimen shall be so supported that nothing except the test solution contacts the stressed area.

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ANSI/NACE TM0177-2016 10.4.2.1 The supporting fixture shall be constructed of material compatible with the test solution.

10.4.2.2 Galvanic effects between the C-ring test specimens, supporting fixtures, and test vessel shall be avoided. For example, an isolating bushing or washer may be used to isolate the C-ring electrically from the supporting fixtures.

10.4.3 Test Vessel 10.4.3.1 The test vessel should be sized to maintain a test solution volume of 30 ± 10 mL/cm2 of test specimen surface area.

10.4.3.2 A fritted glass bubbler should be used to introduce the inert gas and H2S below the array of C-ring test specimens. The bubbles should not impinge on the C-ring test specimens.

10.5 Deflection Calculations 10.5.1 The deflection necessary to obtain the desired stress on the C-ring test specimen shall be calculated using Equation (6): D=

πd ( d-t) S

(6)

4tE

where: D = deflection of C-ring test specimen across bolt holes; d = C-ring test specimen outer diameter; t = C-ring test specimen thickness; S = desired outer fiber stress; and E = elastic modulus.

10.5.1.1 Deflections calculated by Equation (6) should be limited to stresses below the material elastic limit. For many CRAs, the elastic limit is well below the 0.2% offset proof (yield) stress. Deflection values beyond the elastic limit can be calculated from information obtained from the stress-strain curve of the material and the strain-deflection characteristics of the specific C-ring geometry being used.

10.5.1.2 Equation (6) may be used for carbon and low-alloy steels to calculate the deflection necessary to stress the test specimen to 100% of the 0.2% offset yield strength (SY) by substituting SY + E (0.002) for S in the original equation. This relationship is not valid for all alloy systems and should be checked before use.

10.5.1.3 No equation exists to calculate the deflection needed to stress C-ring test specimens to values between the material’s elastic limit and its 0.2% offset proof (yield) stress.

10.5.2 The deflection can be determined directly by using electrical resistance strain gauges applied to the C-ring test specimen.

10.5.2.1 Each C-ring shall be strain-gauged on the outside diameter at a point 90° opposite the axis of the C-ring bolt. The bolt shall be tightened to the appropriate strain by monitoring the strain gauge output, then the strain gauge and glue residue shall be NACE International

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ANSI/NACE TM0177-2016 removed. The C-ring shall then be recleaned using the same procedures given in Paragraph 10.2.6.

10.6 Testing Sequence 10.6.1 The C-ring test specimen dimensions shall be measured, and the corresponding C-ring deflections shall be calculated.

10.6.2 C-ring test specimens shall be stressed by tightening bolting fixtures to calculated deflections measured to the nearest 0.025 mm (0.0010 in).

Deflections shall be measured at the center line of the bolting fixture. These measurements may be taken at the outer diameter, inner diameter, or midwall with care to maintain consistency in the points of measurement. If the desired deflection is exceeded, the test shall be run at the higher deflection or discarded.

10.6.3 The C-ring test specimens shall be cleaned and placed into the test vessel.

10.6.4 Deaeration requirements are given in Paragraph 6.7.

10.6.5 The test solution shall then be saturated with H2S at a minimum flow rate per a documented procedure that has been validated to obtain 2,300 mg/L minimum H2S concentration or other minimum value H2S concentration that is proportional to less than 1 bar H2S partial pressures (at the extent of the test specimen locations). When Solution D is used, the minimum H2S concentration shall be 160 mg/L. Saturation shall occur within an hour of contact with specimens. For larger test vessels, saturation can require more than two hours.

Analysis shall be done using iodometric titration (See Appendix C or other suitable methods).

The validation of the documented procedure shall be performed after saturation at the start of test, after 24 hours, weekly, and at the end of the test. A continuous flow of H2S through the test vessel and outlet trap shall be maintained with a positive pressure of H2S throughout the test that prevents air from entering the test vessel.

NOTE: Laboratories at high elevations may find it necessary to compensate for lower atmospheric pressure in order to achieve the required saturation levels.

10.6.6 The termination of the test shall be at all C-ring test specimens’ failure or after 720 hours, whichever occurs first.

10.7 Failure Detection 10.7.1 Highly stressed C-rings of alloys that are appreciably susceptible to EC tend to fracture through the entire thickness or to crack in a way that is conspicuous. However, with more-ECresistant alloys, cracking frequently begins slowly and is difficult to detect. Small cracks may initiate at multiple sites and be obscured by corrosion products. It is preferable to report the first crack, if detected at 10X magnification, as the criterion of failure. An alternative method of exposing cracking in C-rings after exposure is to stress the C-ring beyond the tested stress level. Cracks resulting from EC can be differentiated from mechanically induced cracks by the corroded nature of the crack surface.

10.8 Reporting of Results 10.8.1 Failure/no-failure data shall be reported from each stress level for each specimen. If time-to-failure data are recorded, they shall be reported.

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ANSI/NACE TM0177-2016 10.8.2 If known, the chemical composition, heat treatment, mechanical properties, and other data taken shall be reported.

10.8.3 Table 3 shows the recommended format for reporting the data.

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ANSI/NACE TM0177-2016 _____________________________________________________________________________ Table 3 NACE Uniform Material Testing Report Form (Part 1): Testing in Accordance with NACE Standard TM0177(A) Method C—NACE Standard C-Ring Test Submitting Company Submitted by Alloy Designation Type

Submittal Date Testing Lab General Material

Telephone No.

He at Nu mb er /I den tif i cat ion Che mi st r y C Mn Si P S Ni Cr Mo V Al Ti Nb N Cu Other Material Processing History Melt Practice (e.g., OH, BOF, EF, AOD)(B) Product Form Heat Treatment (Specify time, temperature, and cooling mode for each cycle in process.)

Other Mechanical, Thermal, Chemical, or Coating Treatment(C) (A)

Test method must be fully described if not in accordance with TM0177.

Melt practice: open-hearth (OH), basic oxygen furnace (BOF), electric furnace (EF), argon-oxygen decarburization (AOD).

(C) E.g., cold work, plating, nitriding, prestrain.

(B)

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ANSI/NACE TM0177-2016

Table 3 (continued) NACE Uniform Material Testing Report Form (Part 2): Testing in Accordance with NACE Standard TM0177 Method C—NACE Standard C-Ring Test Tested per NACE Standard TM0177(A)

Lab Data for Material: Test Specimen Geometry: Outside Diameter Test Equipment: Bolting Material Same as Specimen Correction for Yield Applied Chemistry: Solution

Test Solution A

Outlet Trap to Exclude Oxygen

Test Solution B

Test Solution C (define)

Temperature Maintained 24 °C ± 3 °C (75 °F ± 5 °F)

Test Specimen Properties

Width

Test Solution D

Temperature Maintained

Applied Stress (% of Yield Strength)

Test Solution pH(E)

Applied Heat Treatmen t

Other Test

±3 °C (±5 °F)

Remarks (Including Surface Condition and H2S Level)

Finish

Start

Hardness ( )

Reduction In Area (%)

Elongation (%)

Ultimate Tensile Strength ( ) Yield Strength(D) ( )

Orientation(C

)

Location(B)

Material Identification

Wall/Thickness

Time-to-Failure (Hours) NF = No Failure at 720 hours

(A)Test

method must be fully described if not in accordance with NACE Standard TM0177. of test specimen may be: tubulars—outside diameter (OD), midwall (MW), or inside diameter (ID); solids—surface (S), quarter-thickness (QT), midradius (MR), center (C), or edge (E).

(C)Orientation may be longitudinal (L) or transverse (T).

(D) Open parentheses must be filled with metric or English units, as appropriate to the data. Yield strength is assumed to be 0.2% offset unless otherwise noted.

(E) Enter pH for test conducted on nonfailed C-ring test specimen at highest stress if summarizing data.

(B)Location

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ANSI/NACE TM0177-2016 ____________________________________________________________________________ Section 11: Method D—NACE Standard Double Cantilever Beam Test 11.1 Method D, the NACE Standard DCB Test, provides for measuring the resistance of metallic materials to propagation of EC, expressed in terms of a critical stress intensity factor, KISSC for SSC and KIEC for the more general case of EC, using a crack-arrest type of fracture mechanics test.

Method D does not depend on the uncertainty of pitting and/or crack initiation, because a crack is always initiated in a valid test. For SSC testing of carbon and low-alloy steels, this method requires little time. Method D gives a direct numerical rating of crack propagation resistance and does not depend on evaluation of failure/no-failure results.19 The subject of fracture mechanics testing for evaluation of EC resistance is currently under consideration by NACE TG 085 and Work Group (WG) 085c, and ASTM Committees E8.06.02 and G1.06.04. The user of this standard should maintain contact with these groups and their technical activities for knowledge of current state-ofthe-art testing techniques.

11.1.1 KISSC is not an intrinsic material property, but depends on the environmental exposure conditions and method of testing. Nevertheless, the values obtained by carefully adopting this standard can be used for comparative purposes.

11.1.2 This section sets forth the procedure for DCB testing at room temperature and atmospheric pressure and enables computation of KISSC. When the special considerations set forth in Section 7 for testing at elevated temperature and pressure are observed, the computed stress intensity factor should be written as KIEC. The equations needed to compute KIEC are the same as those set forth in Paragraph 11.6 for KISSC. However, the following descriptions of material behavior under SSC conditions may not be accurate for the more general conditions of EC. 11.2 Test Specimen 11.2.1 The standard DCB test specimen design shall be in accordance with Figure 11(a). A double-tapered wedge shall be used to load the DCB test specimen (see Figure 11 [b]). The double-tapered wedge shall be made of the same material as the DCB test specimen or of the same class of material as the DCB test specimen. The wedge material may be heat treated or cold worked to increase its hardness and thereby help to prevent galling during wedge insertion. Wedges may be shielded with polytetrafluoroethylene (PTFE) tape to reduce corrosion in the wedge section. After machining, test specimens and wedges shall be stored in a low-humidity area, in a desiccator, or in uninhibited oil until ready for testing.

11.2.2 The standard DCB test specimen thickness shall be nominally 9.53 mm (0.375 in); complete dimensions are shown in Figure 11(a). The surface roughness of the test specimen shall not exceed 0.81 µm (32 µin) finish. When the material being tested is too thin to meet this requirement, optional thicknesses as noted in Figure 11(a) may be considered. Subsize DCB test specimens of some carbon and low-alloy steels may give lower KISSC values than the standard DCB test specimens.

11.2.3 Full-thickness DCB test specimens may be prepared from tubular products if the ratio of the tubular outside diameter to the wall thickness is greater than 10. The side grooves should be 20% of the wall thickness, thus maintaining a web thickness (Bn) equal to 60% of the wall thickness.

11.2.4 The side grooves must be machined carefully to avoid overheating and cold working.

The final two machining passes should remove a total of 0.05 mm (0.002 in) of metal. Grinding is also acceptable if the process does not harden or temper the material.

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ANSI/NACE TM0177-2016 11.2.5 If testing materials of low KISSC (below 22 to 27 MPa√m [20 to 25 ksi√in]) or materials in which crack initiation is difficult, e.g., lower-yield-strength materials, introducing the electrodischarge-machined (EDM) slot noted in Figure 11(a) or a fatigue precrack is very helpful in avoiding sidearm cracking and in initiating SSC, respectively. Fatigue precracking of the DCB specimen should be conducted under load control at a convenient frequency. The acceptable range for a fatigue precrack is 1 to 3 mm (0.04 to 0.12 in) past the base of the chevron or end of the EDM slot. To avoid erroneous high results, the maximum precracking load shall be the lesser of 70% of the expected initial KI imparted by the wedge or 30 MPa√m (27 ksi√in). The ratio of minimum to maximum load shall be in the range of 0.1 to 0.2. The precrack should be sharpened at two-thirds of the maximum precracking load for approximately 20,000 cycles to extend the crack through any zone of residual compressive stresses that might have developed.

11.2.6 Test Specimen Identification Each sidearm of the DCB test specimen should be identified by stamping or vibratory stenciling, either near the two holes or on the end that is not wedge loaded.

NOTE: To maintain identification during machining, the specimen should initially be marked on the end that is not wedge loaded.

11.2.7 Hardness Measurement When specified, three or more hardness readings shall be taken on each test specimen. The average value and face tested shall be reported with the test data. The hardness should be measured within the final 50 mm (2 in) of length on the long narrow faces of standard size and 6.35 mm (0.250 in) thick specimens, (see Figure 10) or on the broad faces near the side groove of thinner subsized specimens.

Figure 10: Location of Hardness Impressions on DCB Specimen 11.2.8 Dimensional Check Dimensions B, Bn, 2h, and the distance of the hole centers from the near end of the DCB test specimen shall be measured. (A blade micrometer should be used for measuring Bn). Any values that lie outside the limits shown in Figure 11 (a) shall be recorded for later use in computing KISSC (see Paragraph 11.6.3).

11.2.9 Test Specimen Cleaning Test specimens shall be degreased with solvent and rinsed in acetone. Test specimen degreasing should take place before fatigue precracking and wedge insertion. All degreased test specimens shall be handled in such a manner to avoid contamination. All degreased test specimens should be handled with new powder-free disposable gloves. If used, powdered gloves shall be cleaned of powder prior to handling DCB test specimens.

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ANSI/NACE TM0177-2016

Dimension B Bn D E F G h J K L M N R S U W Z

Size (mm) 9.53 ± 0.05 5.72 ± 0.05 4.85 6.35 + 0.25/–0.0 6.35 ± 0.10 1.91 ± 0.05 12.70 ± 0.05 38.10 ± 0.76 3.18 ± 0.25 101.60 ± 1.60 51 ± 13 6.35 ± 0.10 0.25 ± 0.05 2.39 ± 0.05 127 25.40 ± 0.05 ± 0.05

(in) 0.375 ± 0.002 0.225 ± 0.002 0.191 (No. 11 Drill) 0.25 + 0.01/–0.00 0.250 ± 0.004 0.075 ± 0.002 0.500 ± 0.002 1.500 ± 0.030 0.125 ± 0.010 4.000 ± 0.0625 2.0 ± 0.5 0.250 ± 0.004 0.010 ± 0.002 0.09375 ± 0.002 5 1.000 ± 0.002 ± 0.002

Optional Thicknesses B Bn 4.76 mm (0.188 in) 2.86 mm (0.113 in) 6.35 mm (0.250 in) 3.81 mm (0.150 in) 12.7 mm (0.500 in) 7.62 mm (0.300 in) Full Wall 0.6*B Figure 11(a): Dimensional Drawing of the DCB Specimen 39

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Figure 11(b): DCB Specimen—Double-Tapered Wedge

11.3 Test Solutions—see Section 6.

11.4 Test Equipment 11.4.1 The test vessel shall be sized to maintain a test solution volume of 10-12 mL/cm2 of test specimen area (generally, 1 L of test solution per DCB specimen is used). The maximum volume of the test vessel should be 10 L for ease of test solution saturation.

11.4.2 A slotted base plate or other test specimen holder (electrically isolating) should be used to ensure uniform spacing and orientation of the DCB test specimens.

11.4.3 A small wedge-loading fixture may be attached to the jaws of a bench vise to facilitate full wedge insertion, flush with the end of the DCB test specimen. Alternatively, the arms may be spread by a tensile testing machine. Care shall be taken to avoid over spreading.

11.4.4 A fritted glass bubbler should be used to introduce the inert gas and H2S below the array of DCB test specimens. The bubbles should not impinge on the DCB test specimens.

11.5 Testing Sequence 11.5.1 Clean the DCB test specimens as indicated in Paragraph 11.2.9.

11.5.2 Select wedges to achieve specified arm displacement.

11.5.2.1 Measure the slot thickness.

11.5.2.2 The arm displacement shall be carefully controlled because initial stress intensity affects the final KISSC.20 The arm displacement shall not exceed the limits listed in Tables 4 and 5. The acceptable arm displacements given in Table 4 may provide guidance in the selection of arm displacements for testing other grades of carbon and low-alloy steels. In a similar way, Table 6 provides guidance in choosing arm displacements for low-alloy steel products not listed in Table 4 and for other alloys.

NOTE: For nonstandard products and fit-for-purpose testing, some experimentation may be necessary to determine the required arm displacement range, i.e., one causing crack growth of about 12.7 to 25.4 mm (0.5 to 1.0 in).

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ANSI/NACE TM0177-2016 11.5.2.2.1 When testing steels with yield strengths below 550 MPa (80 ksi), the arm displacement (ď ¤) may be computed using Equation (7a) or (7b), in which YS is yield strength:

(7a)

(7b) 11.5.2.2.2 For tests in Solution A and 100% H2S, the arm displacement range is listed in Tables 4 and 6.

11.5.2.2.3 For tests in Solution D and 7% H2S, the arm displacement range for API 5CT Grade C110 is listed in Table 5.

Table 4 Arm Displacements for API and Other Grade Oilfield Tubular Steels in Solution A and 100%H2S Acceptable Grade(A) Arm Displacement (ď ¤) Yield Strength Range MPa

(ksi)

L80 552-655 (80-95) C90 621-724 (90-105) T95 655-758 (95-110) R95 655-758 (95-110) Grade 100(B) 689-793 (100-115) Grade 105(B) 724-827 (105-120) C-110 758-827 (110-120) P-110 758-965 (110-140) Q-125 862-1,030 (125-150) (A) API grades unless noted otherwise.

(B) Non-API grades.

41

mm

(0.001 in)

0.71-0.97 0.71-0.79 0.66-0.74 0.66-0.74 0.51-0.76 0.46-0.71 0.46-0.54 0.25-0.64 0.25-0.51

(28-38) (28-31) (26-29) (26-29) (20-30) (18-28) (18-21) (10-25) (10-20)

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ANSI/NACE TM0177-2016 Table 5 Arm Displacement for API 5CT Grade C110 in Solution D and 7% H2S Grade(A) Yield Strength Range Acceptable Arm Displacement () MPa

(A)

(ksi)

C110 758-827 (110-120) API grades unless noted otherwise.

mm

(0.001 in)

0.84-0.92

(33-36)

Table 6 Suggested Arm Displacements for Selected Alloys and Strength Levels in Solution A and 100% H2S Material Arm Displacement (), mm (0.001 in.)(A) Yield Strength (B) (ksi) Low-Alloy Steels

Duplex Stainless UNS N10276 Ti-3-8-6-4-4 Steels 552 (80) 0.71-1.07 (28-42) 0.64-0.89 (25-35) — — — 621 (90) 0.58-0.89 (23-35) 0.46-0.71 (18-28) — — — 689 (100) 0.48-0.79 (19-31) 0.20-0.46 (8-18) — — — 827 (120) 0.33-0.58 (13-23) — 0.89-1.02 (35-40) — — 965 (140) 0.20-0.46 (8-18) — 0.64-0.89 (25-35) — 1.02-1.52 (40-60) 1,100 (160) 0.18-0.38 (7-15) — — 1.27-1.78 (50-70) — 1,240 (180) 0.15-0.30 (6-12) — — 1.02-1.27 (40-50) — (A) These values apply at the indicated yield strength, not over a range of yield strengths. Therefore, the user should interpolate or extrapolate to the actual yield strength of the alloy being used.

(B) For oilfield tubular steels, use Table 4.

MPa

UNS J91540

11.5.2.2.4 On both sides of the test specimen, measure the initial spacing of the loading holes in the DCB test specimen for use in determining actual arm displacement at the load line. Initial spacing may be measured using lines inscribed vertically along the load line of each side of the test specimen (see Figure 11[a]). Other methods of determining the actual arm displacement may be used if they have been demonstrated to achieve the required accuracy. The average of the dimensional measurements taken from each side shall be used as the initial spacing value.

11.5.2.2.5 Select a wedge thickness that would produce an expected arm displacement in the desired range.

NOTE: For materials with high cracking resistance or testing in a higher-pH environment, fatigue precracking and/or higher initial arm displacements (δ) should be used.

11.5.2.2.6 Press the wedge into the slot and flush with the end of the DCB test specimen.

11.5.2.2.7 On both sides of the test specimen, re-measure the spacing of the holes at the load line, use the average of the dimensional measurements taken from each side as the post-wedge insertion spacing, and then compute the actual arm displacement (δ). Remeasurement may be performed using the same inscribed line described in Paragraph 11.5.2.2.4 above. For the SSC-resistant carbon and low-alloy steels in Table 6, if the actual arm displacement lies below the intended range, a new wedge may be inserted to achieve an acceptable displacement. If the actual arm displacement lies above the intended range, the wedge shall be removed, and the EDM slot or fatigue precrack shall be extended through the zone of plastically deformed material before reloading the DCB test specimen.

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ANSI/NACE TM0177-2016

11.5.3 Degrease the DCB test specimens with solvent and rinse in acetone. All degreased test specimens shall be handled in such a manner as to avoid contamination. All degreased test specimens should be handled with new powder-free disposable gloves. If used, powdered gloves shall be cleaned of powder prior to handling DCB test specimens.

NOTE: Fatigue precracked DCB test specimens shall be cleaned by wiping, not by using immersion.

11.5.4 Place the DCB test specimens in the test vessel. The test specimen shall be exposed to H2S within 24 hours of wedge insertion.

11.5.5 Deaeration requirements are given in Paragraph 6.7.

11.5.6 The test solution shall then be saturated with H2S at a minimum flow rate per a documented procedure that has been validated to obtain 2,300 mg/L minimum H2S concentration or other minimum value H2S concentration that is proportional to less than 1 bar H2S partial pressures (at the extent of the test specimen locations). When Solution D is used, the minimum H2S concentration shall be 160 mg/L. Saturation shall occur within two hours of contact with specimens. Analysis shall be done using iodometric titration (See Appendix C or other suitable methods). The validation of the documented procedure shall be performed after saturation at the start of test, after 24 hours, weekly, and at the end of the test. A continuous flow of H2S through the test vessel and outlet trap shall be maintained with a positive pressure of H2S throughout for the duration of the test that prevents air from entering the test vessel.

NOTE: Laboratories at high elevations may find it necessary to compensate for lower atmospheric pressure in order to achieve the required saturation levels.

11.5.7 The temperature of the test solution shall be maintained within the range of 24 ± 1.7 °C (75 ± 3.0 °F) throughout the test because KISSC values for low-alloy steels have been found to vary significantly with temperature in the vicinity of room temperature. Temperature of the test solution shall be monitored, and the average reported.

11.5.8 The test duration for carbon and low-alloy steels shall be at least 14 days. The test duration for carbon and low-alloy steels in Solution D shall be at least 17 days. For stainless steels, Ni-, Ni/Co-, Ti-, or Zr-based alloys, a longer test may be required to ensure that the crack has stopped growing. For these materials, the test duration should be at least 720 hours.

For testing steels at very high strength levels, at low temperatures or mild environments, it may be necessary to test for at least 21 days or longer. The duration of the test shall be mutually agreed upon among all interested parties and shall be reported with the results.

11.5.9 After the exposure, remove corrosion products from the DCB test specimen by vapor honing, by bead- or sand-blasting lightly, or by any other means that do not remove a significant amount of metal.

11.5.10 Obtain the load-vs.-displacement curve and remove the wedge. The equilibrium wedge-load (P) is located at the abrupt change in slope of this “lift-off” curve. The lift-off load should be taken directly from the curve instead of attempting to correct for small (<5%) compliance error that is introduced during the measuring process.21 Lift-off load shall be determined within three days after exposure.

11.5.10.1 The test machine load shall be calibrated per ASTM E4 (Standard Practices for Force Verification of Testing Machines).22 Prior to applying a load, the testing machine and graph shall be set up in such a manner that zero force indication signifies a state of zero force on the specimen.

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11.5.10.2 The displacement rate shall not exceed 0.5 mm/min (0.02 in/min).

11.5.10.3 When determining the equilibrium wedge-load (P), the resolution of the load on the graph or digital display shall allow load determination within ±1.0 % of the final wedgeload (P).

11.5.11 After wedge removal, open the DCB test specimen mechanically to expose the crack surfaces. This procedure may require the use of a tensile machine. For ferritic steels, this process can be made easier by chilling the DCB test specimen in liquid nitrogen and splitting the arms apart with a hammer and chisel.

11.6 Determination of KISSC 11.6.1 The fracture surface shall be examined for the following characteristics: (a) The fracture surface shall show at least 2.5 mm (0.10 in) crack growth beyond the base of the chevron, EDM notch, or fatigue precrack; (b) The crack front should be at least 25 mm (1.0 in) from the unslotted end; (c) If the fracture is nonplanar at any location along its length, the raised surface shall not project beyond the “V” portion of the side groove; however, a deviation at a small point shall not invalidate the test. (d) The crack front shall not be pinned; causes of pinning include internal cracks and fissures; (e) The crack front should be free of significant disturbance created when the main crack dips below, or rises above, a previously formed (darker) triangular edge crack (“shark’s tooth”).23 The combined width of any edge crack(s) at the crack front on the fracture surface shall not exceed 25% of the web thickness Bn for a valid test. Edge cracks lying away from the crack front (that formed earlier in the test) shall be given one-fourth the weight of those at the crack front. Thus, a DCB specimen whose earlier-formed edge cracks have a combined width exceeding 100% of the web thickness shall be invalid. For DCB test specimens with both earlier-formed edge cracks and edge cracks at the crack front, the sum of the combined widths of the earlier-formed edge cracks divided by four, plus the widths of those at the crack front, shall not exceed 25% of the web thickness, Bn, for the test to be considered valid. See Figure 12 for an example of an invalid test caused by excessive edge cracking.

NOTE: Edge cracks may not affect results in the mechanical assurance curve (see Appendix D [Nonmandatory]). (f) The crack should not branch into one or both of the sidearms (side crack); (g) Dry cracks are lighter colored SSC cracks that are characterized by the same topology, and extend beyond, the black SSC crack (see Figure 13). The crack length used to calculate the KISSC shall include any dry cracking. (h) The crack front of full-thickness (curved) DCB test specimens must not lead on the edge of the fracture surface closer to the tubular internal surface.

11.6.2 If the fracture surface is satisfactory with respect to all of the characteristics in Paragraph 11.6.1, the test shall be considered valid. The distance from the slotted end of the NACE International

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ANSI/NACE TM0177-2016 DCB test specimen to the mean position of the crack front shall be measured under magnification using a dial caliper, digital caliper or comparable instrument. It is recommended, along the crack front, to use five equally spaced points across the width starting and ending at each edge and then use the average of the three middle points with the average of the two edges as the mean position of the crack front. From that distance, 6.35 mm (0.250 in) shall be subtracted to obtain the crack length (a). The location of the SSC/brittle (overload) fracture boundary may be checked, if in doubt, by a staining technique before sidearm separation or scanning electron microscopy after separation.

11.6.3 The stress intensity factor for SSC of flat DCB test specimens shall be calculated using Equation (8): 1

KISSC (flat DCB test specimen) =

Pa(2 3 + 2.38h/a)(B/Bn )

3

(8)

3/2

Bh

where: KISSC = threshold stress intensity factor for SSC; P = measured lift-off load; a = crack length, as described in Paragraph 11.6.2; h = height of each arm; B = DCB test specimen thickness; and Bn = web thickness.

Calculation example: Data:

B = 9.53 mm Bn = 5.72 mm h = 12.7 mm P = 1,870 N a = 46.48 mm

(0.375 in); (0.225 in); (0.500 in); (421 lbf); (1.830 in); and

Result: KISSC = 35.3 MPa√m (32.1 ksi√in).

11.6.4 The stress intensity factor for SSC of full-wall thickness (curved) DCB test specimens shall be computed using Equation (9):24 12

 3I  KISSC (curved DCB test specimen) =  3  KISSC (flat DCB test specimen)  Bh 

(9)

where: KISSC (flat DCB test specimen) is calculated using Equation (8); and Quantity I is computed from Equation (10) as follows:

D I=   4 4

2

h



 h   2

D 4 32 



  

2

2

D h

h

2

1/2



D

4

128

1

sin

 2h  h   D 4

      

h D 8 2

45

3/2

2

B

D 2

1/2

2

B

2

h



1 D

8

2

4

B

1

sin

   D 2

3/2

2

B

2

h

(10)

 2h     D  2B  NACE International

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ANSI/NACE TM0177-2016 where: B and h have values in accordance with Paragraph 11.6.3; and D = tubular outside diameter.

Mathematical analysis and experimental results have shown that the correction factor for curved DCB test specimens results in only a few percentage increase, at most, in KISSC value.

Calculation example: Data:

D = 193.7 mm (7.625 in); and Other data as in Paragraph 11.6.3 calculation example.

Result: KISSC = 35.4 MPa√m (32.2 ksi√in).

11.7 Reporting of Test Results: 11.7.1 For each set of DCB test specimens, all individual values of KISSC for valid tests shall be reported. The arm displacement for each DCB test specimen shall be reported.

11.7.2 If known, the chemical composition, heat treatment, mechanical properties, and other data taken shall be reported.

11.7.3 Table 6 shows the recommended format for reporting the data. In addition to items in Table 6 (Part 2), report the following for each specimen: 11.7.3.1 Actual Dimensions: height (h), specimen thickness (B), web thickness (B n), crack length (a) and actual arm displacement (δ); 11.7.3.2 Lift-off load (P); 11.7.3.3 Start and end of test H2S concentrations (when specified); 11.7.3.4 Three HRC hardness indentations results (when specified); 11.7.3.5 Actual test duration.

11.8 The recommended quality assurance method of evaluating the final mechanical state and validity within the test parameters of each DCB test specimen is in Appendix D. 11.9 A recommended method for determining KLIMIT and KIapplied is provided in Appendix E (Nonmandatory). KLIMIT represents the crack initiation toughness for a specific material tested in a specific environment and estimates the minimum KISSC. KIapplied is the applied stress intensity factor obtained after wedge insertion and before the environmental exposure.

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ANSI/NACE TM0177-2016

Figure 12: Example of Invalid Test due to Edge Cracks Using Low-Angle Fiber Optics Illumination (See Paragraph 11.6.1[e])

Figure 13: Example of Dry Cracking (See Paragraph 11.6.1[g])

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ANSI/NACE TM0177-2016 Table 7 NACE Uniform Material Testing Report Form (Part 1): Testing in Accordance with NACE Standard TM0177(A) Method Dâ&#x20AC;&#x201D;NACE Standard DCB Test Submitting Company Submitted by Alloy Designation

Telephone No.

Submittal Date Testing Lab General Material Type

He at Nu mb er /I den tif i cat ion

Che mi st r y C Mn Si P S Ni Cr Mo V Al Ti Nb N Cu Other Material Processing History Melt Practice (e.g., OH, BOF, EF, AOD)(B) Product Form Heat Treatment (Specify time, temperature, and cooling mode for each cycle in process.)

Other Mechanical, Thermal, Chemical, or Coating Treatment(C) (A)

Test method must be fully described if not in accordance with TM0177.

Melt practice: open-hearth (OH), basic oxygen furnace (BOF), electric furnace (EF), argonoxygen decarburization (AOD).

(C) E.g., cold work, plating, nitriding, prestrain.

(B)

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Table 7 (continued) NACE Uniform Material Testing Report Form (Part 2): Testing in Accordance with NACE Standard TM0177 Method D—NACE Standard DCB Test Lab Data for Material: Tested in accordance with NACE Standard TM0177(A) DCB Specimen Geometry:

Standard Nonstandard: Thickness Galvanic Couple

Starter Elastic Modulus Chemistry: Test Solution A Solution Outlet Trap to Exclude Oxygen Maintained ±1.7 °C (±3.0 °F)

Test Solution B

Height (2h) Fatigue Precrack

Test Solution C (define)

EDM Crack

Test Solution D

Other Test

Temperature Maintained 24° ± 1.7 °C (75° ± 3.0 °F)

DCB Test Specimen Properties(D)

Temperature

Data for Valid Tests

KISSC or KIEC ( )

Arm Displacement () ( )

Finish

Test Remarks Solution (Including pH Surface Condition and H2S Level)

Start

Hardness(

Reduction in Area (%) Elongation (%)

)

Ultimate Tensile Strength ( ) Yield Strength(D) ( ) Orientation(C)

Location(B)

Material Identification

Length

1

2

3

4

1

2

3

4

Mean Std. Dev

(A)

Test method must be fully described if not in accordance with NACE Standard TM0177.

Location of test specimen may be: tubulars—outside diameter (OD), midwall (MW), or inside diameter (ID); solids—surface (S), quarter-thickness (QT), midradius (MR), center (C), or edge (E).

(C) Orientation may be longitudinal (L) or transverse (T).

(D) Open parentheses must be filled with metric or English units, as appropriate to the data. Yield strength is assumed to be 0.2% offset unless otherwise noted.(E) Enter pH for test conducted on nonfailed DCB test specimen at highest stress if summarizing data.

(B)

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ANSI/NACE TM0177-2016 _____________________________________________________________________________ References 1. ANSI/NACE MR0175/ISO 15156 (latest revision), “Petroleum, Petrochemical, and Natural Gas Industries—Materials for Use in H2S-Containing Environments in Oil and Gas Production” (Houston, TX: NACE).

2. European Federation of Corrosion (EFC)(5) Publication #16, (latest revision), “Guidelines on Materials Requirements for Carbon and Low Alloy Steel for H2S-Containing Environments in Oil and Gas Production” (London, UK: EFC).

3. API Specification 5CT/ISO 11960 (latest revision), “Specification for Casing and Tubing” (Washington, DC: API).

4. ISO 11960 (latest revision), “Petroleum and natural gas industries—Steel pipes for use as casing or tubing for wells” (Geneva, Switzerland: ISO).

5. NACE Standard TM0284 (latest revision), “Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking” (Houston, TX: NACE).

6. ASTM D1193 (latest revision), “Standard Specification for Reagent Water” (West Conshohocken, PA: ASTM).

7. ASTM A370 (latest revision), “Standard Test Methods and Definitions for Mechanical Testing of Steel Products” (West Conshohocken, PA: ASTM).

8. ASTM E18 (latest revision), “Standard Test Methods for Rockwell Hardness of Metallic Materials” (West Conshohocken, PA: ASTM).

9. ASTM E384, (latest revision), “Standard Test Method for Knoop and Vickers Hardness of Materials” (West Conshohocken, PA: ASTM).

10. G. Steinbeck, W. Bruckoff, M. Koehler, H. Schlerkmann, G.A. Schmitt, “Test Methodology for Elemental Sulfur Resistant Advanced Materials for Oil and Gas Field Equipment,” CORROSION/95, paper no. 47 (Houston, TX: NACE, 1995).

11. Steel-Iron Test Specification of the Institute for Iron and Steel (VDEh/Germany) SEP 1865: “Test Methods for the Evaluation of the Corrosion Performance of Steels and Non-Ferrous Alloys in the System: Water—Hydrogen Sulfide—Elemental Sulfur” (Düsseldorf, GE: Steel Institute VDEh).(6) 12. EFC Publication #17, (latest revision), “Corrosion Resistant Alloys for Oil and Gas Production: Guidance on General Requirements and Test Methods for H2S Service” (London, UK: EFC).

13. J.L. Crolet, M.R. Bonis, “How to Pressurize Autoclaves for Corrosion Testing under Carbon Dioxide and Hydrogen Sulfide Pressure,” Corrosion 56, 2 (2000): p. 167.

14. J.B. Greer, “Results of Interlaboratory Sulfide Stress Cracking Using the NACE T-1F-9 Proposed Test Method,” MP 16, 9 (1977): p. 9.

15. ISO 4287 (latest revision), “Geometrical Product Specifications (GPS) - Surface texture: Profile method - Terms, definitions and surface texture parameters” (Geneva, Switzerland: ISO).

(5) European

Federation of Corrosion (EFC), The Institute of Metals, 1 Carlton House Terrace, London, SW1Y 5DB, UK. (6) Steel Institute VDEh, Sohnstraße 65, 40237 Düsseldorf, Germany.

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ANSI/NACE TM0177-2016 16. ASTM G49 (latest revision), “Standard Practice for Preparation and Use of Direct Tension Stress-Corrosion Test Specimens” (West Conshohocken, PA: ASTM).

17. J.P. Fraser, G.G. Eldredge, R.S. Treseder, “Laboratory and Field Methods for Quantitative Study of Sulfide Corrosion Cracking,” H2S Corrosion in Oil and Gas Production—A Compilation of Classic Papers, R.N. Tuttle, R.D. Kane, eds. (Houston, TX: NACE, 1981), p. 283. Original Publication: Corrosion 14, 11 (1958): p. 37.

18. ASTM G38 (latest revision), “Standard Practice for Making and Using C-Ring Stress-Corrosion Test Specimens” (West Conshohocken, PA: ASTM).

19. R.B. Heady, “Evaluation of Sulfide Corrosion Cracking Resistance in Low Alloy Steels,” Corrosion 33, 3 (1977): p. 98.

20. D.L. Sponseller, “Interlaboratory Testing of Seven Alloys for SSC Resistance by the Double Cantilever Beam Method,” Corrosion 48, 2 (1992): p. 159.

21. K. Szklarz, T. Perez, “Observations on the Use of the Double Cantilever Beam Specimen for Sulfide Stress Corrosion Tests,” CORROSION/95, paper no. 48 (Houston, TX: NACE, 1995).

22. ASTM E4 (latest revision), “Standard Practices for Force Verification of Testing Machines” (West Conshohocken, PA: ASTM).

23. D.L. Sponseller, C.J. Padfield, B.E. Urband, “Factors Affecting Crack Path, Edge Cracking, and KISSC Rating during Testing of Low-Alloy Steels by the NACE Double-Cantilever-Beam Method of TM0177-96(D),” CORROSION/2003, paper no. 103 (Houston, TX: NACE, 2003).

24. S.W. Ciaraldi, “Application of a Double Cantilever Beam Specimen to Laboratory Evaluation of Sour Service High-Alloy Tubulars,” CORROSION/83, paper no. 162 (Houston, TX: NACE, 1983).

25. OSHA(7) Rules and Regulations (Federal Register, Vol. 37, No. 202, Part II, October 18, 1972).

26. Chemical Safety Data Sheet SD-36: Hydrogen Sulfide (Washington, DC: MCA,(8) 1950).

27. N. Irving Sax, Dangerous Properties of Industrial Materials (New York, NY: Reinhold Book Corp., 1984).

28. K. Szklarz, “Interpreting and Using the NACE Double Cantilever Beam (DCB) Test,” CORROSION/2008, paper no. 108 (Houston, TX: NACE, 2008).

29. H.A. Ernst, R. Bravo, T. Perez, C. Morales, K. Szklarz, G. Lopez Turconi, “Assessment of the Effect of Different Test Variables on the Measured KISSC Value,” CORROSION/2005, paper no. 87 (Houston, TX: NACE, 2005).

(7) Occupational

Safety and Health Administration (OSHA), U.S. Department of Labor, 200 Constitution Ave. NW, Washington, DC 20210.

(8) American Chemistry Council (ACC) (formerly known as the Manufacturing Chemists Association [MCA] and then as the Chemical Manufacturers Association), 700 Second St. NE, Washington, DC 20002.

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ANSI/NACE TM0177-2016 _____________________________________________________________________________ Appendix A Safety Considerations in Handling H2S Toxicity (Nonmandatory) This appendix is considered nonmandatory, although it may contain mandatory language. It is intended only to provide supplementary information or guidance. The user of this standard is not required to follow, but may choose to follow, any or all of the provisions herein.

H2S is perhaps responsible for more industrial poisoning accidents than is any other single chemical. A number of these accidents have been fatal. H2S must be handled with caution and any experiments using it must be planned carefully. The OSHA maximum allowable concentration of H2S in the air for an eight-hour work day is 20 parts per million (ppm), well above the level detectable by smell.25 However, the olfactory nerves can become deadened to the odor after exposure of 2 to 15 min, depending on concentration, so that odor is not a completely reliable alarm system.

Briefly, the following are some of the human physiological reactions to various concentrations of H2S. Exposure to concentrations in the range of 150 to 200 ppm for prolonged periods may cause edema of the lungs. Nausea, stomach distress, belching, coughing, headache, dizziness, and blistering are symptoms of poisoning in this range of concentration. Pulmonary complications, such as pneumonia, are strong possibilities from such subacute exposure.

At 500 ppm, unconsciousness may occur in less than 15 min, and death within 30 min. At concentrations above 1,000 ppm, a single inhalation may result in instantaneous unconsciousness, complete respiratory failure, cardiac arrest, and death.

Additional information on the toxicity of H2S can be obtained from the Chemical Safety Data Sheet SD-3626 and from Dangerous Properties of Industrial Materials.27 Fire and Explosion Hazards H2S is a flammable gas and yields poisonous sulfur dioxide (SO2) as a combustion product. In addition, its explosive limits range from 4 to 46% in air. Appropriate precautions shall be taken to prevent these hazards from developing.

Safety Procedures During Test All tests shall be performed in a hood with adequate ventilation to exhaust all of the H2S. The H2S flow rates during the test should be kept low to minimize the quantity exhausted. A 10% caustic absorbent solution for effluent gas can be used to further minimize the quantity of H2S gas exhausted. This caustic solution needs periodic replenishing. Provision shall be made to prevent backflow of the caustic solution into the test vessel if the H2S flow is interrupted. Suitable safety equipment shall be used when working with H2S. Because the downstream working pressure frequently rises as corrosion products, debris, etc., accumulate and interfere with regulation at low flow rates, particular attention should be given to the output pressure on the pressure regulators. Gas cylinders shall be securely fastened to prevent tipping and breaking of the cylinder head. Because H2S is in liquid form in the cylinders, the highpressure gauge must be checked frequently, because relatively little time elapses after the last liquid evaporates and the pressure drops from 1.7 MPa (250 psig) to atmospheric pressure. The cylinder shall be replaced by the time it reaches 0.5 to 0.7 MPa (75 to 100 psig) because the regulator control may become erratic. Flow shall not be allowed to stop without closing a valve or disconnecting the tubing at the test vessel because the test solution continues to absorb H2S and NACE International

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ANSI/NACE TM0177-2016 move upstream into the flowline, regulator, and even the cylinder. A check valve in the line should prevent the problem if the valve works properly. However, if such an accident occurs, the remaining H2S should be vented as rapidly and safely as possible and the manufacturer notified so that the cylinder can be given special attention. _____________________________________________________________________________ Appendix B Explanatory Notes on Environmental Cracking Test Method (Nonmandatory) This appendix is considered nonmandatory, although it may contain mandatory language. It is intended only to provide supplementary information or guidance. The user of this standard is not required to follow, but may choose to follow, any or all of the provisions herein.

Reasons for Reagent Purity Water impurities of major concern are alkaline- or acid-buffering constituents that may alter the pH of the test solution and organic and inorganic compounds that could change the nature of the corrosion reaction. Oxidizing agents could also convert part of the H2S to soluble products, such as polysulfides and polythionic acids, which may also affect the corrosion process.

Alkaline materials (such as magnesium carbonate and sodium silica aluminate) are often added to (or not removed from) commercial grades of sodium chloride to ensure free-flowing characteristics and can greatly affect the pH. Trace oxygen impurities in the purge gas are much more critical than water impurities if nitrogen (or other inert gas) is continuously mixed with H2S to obtain a lower partial pressure of H2S in the gas and hence a lower H2S concentration in the test solution. Oxidation products could accumulate, resulting in changes in corrosion rate and/or hydrogen entry rate (see the paragraph below on Reasons for Exclusion of Oxygen).

Preparation of Test Specimen All machining operations shall be performed carefully and slowly so that overheating, excessive gouging, and cold work do not alter critical physical properties of the material. Uniform surface condition is critical to consistent SSC test results.

Reasons for Exclusion of Oxygen Obtaining and maintaining an environment with minimum dissolved oxygen contamination is considered very important because of significant effects noted in field and laboratory studies: (1) Oxygen contamination in brines containing H2S can result in drastic increases in corrosion rates by as much as two orders of magnitude. Generally, the oxygen can also reduce hydrogen evolution and entry into the metal. Systematic studies of the parameters affecting these phenomena (as they apply to EC) have not been reported in the literature.

(2) Small amounts of oxygen or ammonium polysulfide are sometimes added to aqueous refinery streams in conjunction with careful pH control near 8 to minimize both corrosion and hydrogen blistering. The effectiveness is attributed to an alteration of the corrosion product.

In the absence of sufficient data to define and clarify the effects of these phenomena on EC, all reasonable precautions to exclude oxygen shall be taken. The precautions cited in this standard minimize the effects of oxygen with little increase in cost, difficulty, or complexity.

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ANSI/NACE TM0177-2016

Cautionary Notes Cleaning solvents such as acetone and other hydrocarbon liquids can be hazardous if the vapors are inhaled or absorbed through the skin. Many chlorinated hydrocarbon compounds are suspected of being carcinogenic and should be used only with the proper safeguards. _____________________________________________________________________________ Appendix C Determination of H2S Concentration in Test Solution by Iodometric Titration (Nonmandatory) This appendix is considered nonmandatory, although it may contain mandatory language. It is intended only to provide supplementary information or guidance. The user of this standard is not required to follow, but may choose to follow, any or all of the provisions herein.

This procedure details an acceptable method that may be used for the determination of the H2S concentration in the test solution by iodometric titration.

Test Equipment 10 mL and 25mL volumetric pipettes 50 mL and 100 mL volumetric flasks 250 mL conical (Erlenmeyer) flask 100 mL beaker 25 mL burette 50 mL syringe (graduated to 60 mL) Test Reagents Standard 0.1 N iodine solution (0.0995 - 0.1005 N, Certified) Standard 0.1 N sodium thiosulfate solution (0.0995 - 0.1005 N, Certified) Standard 0.01 N iodine solution (0.0095 - 0.0105 N, Certified) Standard 0.01 N sodium thiosulfate solution (0.0095 - 0.0105 N, Certified) Concentrated HCl (approx. 37 wt% HCl) Starch solution (approx. 1 wt%) Analytical grade reagents shall be used.

Titration Procedure C1 Table C1 gives the titration parameters required for determination of the test solution H 2S concentration, for a series of H2S partial pressure ranges from 1.00 bar to less than 0.01 bar.

The limits for each range in the series have been chosen to ensure that analytical accuracy is maintained. The range selected shall be appropriate to the test H2S partial pressure.

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ANSI/NACE TM0177-2016 Table C1 Titration Parameters for Determination of the Test Solution H2S Concentration.

920 – 2300 370 – 920 185 – 460 92 – 230

Test Solution Sample Volume mL 10 25 50 100

Iodine / Na2S203 Concentration N 0.1 0.1 0.1 0.1

37 – 92 18 – 46 < 23

25 50 100

0.01 0.01 0.01

pH2S bar

xH2S mole-%

cH2S mg/L

0.40 – 1.00 0.16 – 0.40 0.08 – 0.20 0.04 – 0.10

40 – 100 16 – 40 8.0 – 20 4.0 – 10

0.016 – 0.04 0.008 – 0.02 < 0.01

1.6 – 4.0 0.8 – 2.0 < 1.0

C2 Pipette 25 mL of the selected (0.1 N or 0.01 N) standard iodine solution into a conical flask.

C3 Acidify with a few drops of concentrated HCl. If sampling using a syringe (see Paragraph C4.3), weigh the conical flask containing the acidified iodine solution and record the result.

C4.1 When sampling using a volumetric pipette, transfer an initial 25-50 mL of test solution from the test vessel into a clean beaker. Rinse the beaker with the test solution and discard. Transfer a further 50-100 mL of test solution to the beaker. Fill the pipette from the beaker, rinse with the test solution and discard. Extract the required volume of test solution from the beaker via the pipette, and immediately transfer to the conical flask containing the acidified iodine solution.

Record the volume of test solution transferred.

C4.2 When sampling using a volumetric flask, transfer an initial 25-50 mL of test solution from the test vessel into the flask. Rinse the flask with the test solution and discard. Transfer sufficient test solution to fill the flask to the required volume, and immediately transfer to the conical flask containing the acidified iodine solution. Record the volume of test solution transferred.

C4.3 When sampling using a syringe, extract an initial 25-50 mL of test solution from the test vessel. Rinse the syringe with the test solution and discard. Refer to the graduations on the syringe to extract the approximate volume of test solution required, and transfer directly to the pre-weighed conical flask containing the acidified iodine solution. Re-weigh the conical flask and record the result. Determine the exact volume of test solution transferred.

NOTE: Care should be taken to ensure that dissolved gases do not break out of the test solution when extracting samples via syringe.

C5 Titrate with the selected (0.1 N or 0.01 N) standard sodium thiosulfate solution until the solution changes color from dark yellow/tan to pale yellow.

C6 Add a few drops of starch solution.

NOTE: The starch addition shall be made near the end point of the titration when most of the iodine has been removed, and the color of the solution starts to fade. This is necessary due to the insolubility of the blue starch-iodine complex which may otherwise prevent some of the iodine from reacting.

C7 Continue to titrate slowly with the selected (0.1 N or 0.01 N) standard sodium thiosulfate solution until the dark blue color disappears, and the end point is reached. The end point is a 55

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ANSI/NACE TM0177-2016 milky yellow suspension of colloidal sulfur. thiosulfate solution used.

Record the total volume of standard sodium

C8 Calculate the H2S concentration (mg/L) using Equation (C1): (C1) where: A = Normality of standard iodine solution (equivalents/L) times the volume used (L) B = Normality of standard sodium thiosulfate solution (equivalents/L) times the volume used (L) C = Volume of test solution sample (L)

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ANSI/NACE TM0177-2016 _____________________________________________________________________________ Appendix D Recommendations for Determining Mechanical Quality Assurance of Test Results for Method D (DCB Test) 28 (Nonmandatory) This appendix is considered nonmandatory, although it may contain mandatory language. It is intended only to provide supplementary information or guidance. The user of this standard is not required to follow, but may choose to follow, any or all of the provisions herein.

The purpose of this informative appendix is to provide a recommended quality assurance method of evaluating the final mechanical state of the NACE TM0177 Method D (DCB) test specimen.

This allows the interpreter of the DCB test results to evaluate whether the mechanical state (specimen dimensions, loading, etc.) of the DCB specimen were within the parameters of the test method after environmental exposure.

Method Procedure The following information is required for each DCB test specimen: Per material/grade of material (for suite of tests): δmin = minimum allowable arm displacement minus 0.0254 mm (0.001 in); δmax = maximum allowable arm displacement plus 0.0254 mm (0.001 in); and E = measured elastic modulus or default of 2.07 x 105 MPa (30,000 ksi) for carbon and low alloy steels.

NOTE: Minimum and maximum allowable arm displacement shall be applied for casing and tubing material. For heavy wall coupling stock, such limits may not be applicable because of the scatter in the DCB test results. Therefore, more work is required to validate the arm displacement limits, and it should be agreed between manufacturer and customer how to proceed with the limits for mechanical compliance analysis.

Per each DCB test specimen: P = measured lift-off load; af = measured final crack length; B = specimen thickness; and h = arm height (½ specimen height).

Graphing On a graph of af/h versus P/B, plot curves of constant δmax and constant δmin using Equation (D1).

Pi/B = δmE/ (– 26.232 + 51.866 ai/h + 8.523 (ai/h)2 + 8.5178 (ai/h)3 ) (D1) where: δm is δmax or δmin and (Pi/B, ai/h) are points on the curve of δmax or δmin

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ANSI/NACE TM0177-2016 Each DCB test result should be recorded as a point of (P/B, af/h) on the graph. Equation (D1) is valid for both Chevron and EDM notch.

Evaluation If the DCB test specimen crack surface is valid, then test results plotted between the δ max and δmin curves are considered mechanically valid results. This indicates the physical dimensions of the DCB specimen are correct and the required arm displacement was correctly applied by wedge insertion.

Figure D1 gives a generic example of a mechanical quality assurance graph. If cracking is valid, test results between the curves indicate the DCB test was conducted within the test parameters chosen. Test results outside the curves may have numerous causes (see Paragraph 11.6.1), including:       

DCB dimensions Machining Improper wedge placement Wrong arm displacement Invalid crack plane Lift-off load or final crack length improperly measured Wrong modulus used

Plotting a Minimum KISSC Criterion If a minimum KISSC criterion is being applied to the test data, then that criterion can be plotted on the quality assurance graph using Equation (D2): P/B = KISSC h1/2/([2√3 a/h + 2.38] [B/Bn]1/√3)

6

(D2)

KISSC minimum criterion

Crack Length / Half Height Crack Length/Half Height

5.5 5 δ min 4.5

Reject

4 Reject

3.5

δ max 3 2.5 2 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Lift-off Load / Thickness

NACE International

Figure D1: Example of Quality Assurance Plot

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ANSI/NACE TM0177-2016 _____________________________________________________________________________ Appendix E Recommended Method for Determining KIapplied and KLIMIT for the Method D (DCB) Test29 (Nonmandatory) This appendix is considered nonmandatory, although it may contain mandatory language. It is intended only to provide supplementary information or guidance. The user of this standard is not required to follow, but may choose to follow, any or all of the provisions herein.

The purpose of this appendix is to provide a recommended method for determining KIapplied and KLIMIT for the NACE Method D (DCB) test.

Definition KLIMIT is an estimate of the minimum KISSC. Therefore, KLIMIT represents the crack initiation toughness for a specific material tested in a specific environment. The KLIMIT is observed with decreasing arm displacement when KISSC equals KIapplied. The use of KLIMIT as a parameter for design is under study.

KIapplied is the applied stress intensity factor obtained after wedge insertion and before the environmental exposure.

Determination of KLIMIT: Optional Selection of Arm Displacement: For the specific material and environment of exposure, KISSC results from 3 different arm displacements are required. It is recommended that valid tests at each arm displacement be conducted in triplicate. The middle arm displacement should be that specified in Section 11. The minimum difference between two arm displacements shall be at least 0.15 mm (0.006 in). Note that too small arm displacement may give no result.

Required Information: For each DCB specimen, the following is necessary: KISSC as obtained in accordance with Section 11.6; and KIapplied as defined in the following paragraph.

Determination of KIapplied: If the fracture surface is satisfactory with respect to all the characteristics in Paragraph 11.6.1, the test shall be considered valid. The distance from the slotted end of the DCB test specimen to the mean position of the crack starter or fatigue precrack front should be measured using a dial caliper or other suitable measuring device; this measurement minus 6.35 mm (0.25 in) is the initial crack length (ai). Use the Chevron effective starting crack of 30.02 mm (1.182 in) for any KIapplied or KLIMIT calculations for the Chevron DCB test specimen.

The following procedure shall be used to determine KIapplied based on final valid KISSC results for greater consistency.

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ANSI/NACE TM0177-2016 The final arm displacement, δf, shall be calculated from the lift-off load and final crack length by following Equation (E1) for flat DCB test specimens: δf = P (– 26.232 + 51.866 af/h + 8.523 (af/h)2 + 8.5178 (af/h)3)/EB

(E1)

The calculated initial load, Pi, shall be calculated by the using Equation (E2) for flat DCB test specimens: Pi = δf EB/ (– 26.232 + 51.866 ai/h + 8.523 (ai/h)2 + 8.5178 (ai/h)3)

(E2)

The KIapplied of flat DCB test specimens shall be calculated using Equation (E3): KIapplied (flat DCB test specimen) = Pi ai (2 √3 + 2.38 h/ai)(B/Bn)1/√3 Bh 3/2

(E3)

where: B, Bn and h have values in accordance with Paragraph 11.6.3; P = measured lift-off load; af = measured final crack length; δf = calculated final arm displacement; Pi = calculated initial load; E = measured elastic modulus or default of 2.07 x 10 5 MPa (30,000 ksi) for carbon and low alloy steels; and ai = starting (initial) crack length.

On a graph of KISSC versus KIapplied, plot the line of KISSC = KIapplied. Then plot the KISSC versus KIapplied result for each individual DCB test. Perform a linear least squares analysis on the DCB test data and determine the best fit linear equation (line). Extrapolate the best fit line to intersect with the KISSC = KIapplied line. The intersection is defined as KLIMIT. See Figure E1 for an example graph. The statistics of KLIMIT are the same as the statistics for the best fit line.

The best fit line is given by Equation (E4): KISSC = m KIapplied + c

(E4)

where: m = slope of line; and c = intercept of the line.

The KLIMIT is calculated by using Equation (E5): KLIMIT = c / (1 – m)

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(E5)

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ANSI/NACE TM0177-2016

Example of KLIMIT 50

48

46 y = 0.3231x + 21.922

KISSC, MPa m1/2

44

42

40

38

36

34

32

KLIMIT

30 30

35

40

45

50

55

60

65

70

75

80

KIapplied. MPa m1/2

Figure E1: Example of KLIMIT Determination

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ANSI/NACE TM0177-2016

62

ISBN 1-57590-036-X NACE International NACE International

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Paper No.

11115

2011

NACE CORROSION 2011 STG 32 – Advances in Materials for Oil & Gas Production Mitigation of Sulfide Stress Cracking in Down Hole P110 Components via Low Plasticity Burnishing

Jeremy Scheel, Doug Hornbach, Paul Prevéy Lambda Technologies 5521 Fair Lane Cincinnati, OH, 45227-3401 USA Darrel Chelette, Peter Moore U.S. Steel Tubular Products, Inc. 10343 North Sam Houston Park Drive #120 Houston, TX 77064 USA

ABSTRACT Sulfide stress cracking (SSC) along with hydrogen embrittlement (HE) prevents the use of less expensive high strength carbon steel alloys in the recovery of fossil fuels in H2S containing ‘sour’ service environments that are commonly seen in deep well fossil fuel recovery efforts. High magnitude tensile stresses are generated by connection interferences created during power make-up of down hole tubular components. When subject to service loads the stresses are increased further providing the high tensile stresses necessary for SSC initiation. Because these alloys processed into high strength grades are not suited for fully saturated sour service environments, the current solution is to use or develop much more expensive alloys with increased corrosion-cracking resistance or limit their use to significantly weaker sour environments or higher operating temperatures.

©2011 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

1

Introduction of stable, high magnitude compressive residual stresses into less expensive carbon steel alloys alleviates the tensile stresses and mitigates SSC while also improving fatigue strength. This could allow the potential of using less expensive alloys in sour environments. Low plasticity burnishing (LPB) is highly effective when applied to metallic components using a proven reproducible process of producing deep, high magnitude compressive residual stresses in complex geometric components without altering the geometry, design or chemistry.

The LPB process, applied with advanced control systems, is presently being employed to treat components resulting in a substantial increase in service life through SSC mitigation and improved fatigue life. The benefits of LPB have been evaluated on full size specimens of uni-axial hoop stress loaded coupling blanks and C-ring specimens manufactured from quench and tempered API P110 grade steel with a yield strength of 132 ksi (910 MPa). Specimens were exposed to 100% NACE TM0177-Solution A at 1 bara H2S in both the LPB treated and untreated condition. The time to failure was documented along with the increase in life resulting from LPB treatment. LPB was successful in completely mitigating SSC in each test specimen up to 85% SMYS hoop tension; and in each case met or exceeded the 720-hour exposure time defined in NACE TM0177. At an applied fiber stress of 90% SMYS, the C-ring samples have exceeded exposures of 840 hours without failure. The initial results indicate that LPB processing of down hole tubular components may provide an alternative economical means of SSC mitigation and greatly reducing risk of component failure in sour environments.

Key Words: low plasticity burnishing, sulfide stress cracking, fatigue, residual stress, sour service

INTRODUCTION Surface enhancement of metals, inducing a layer of surface compressive residual stresses in metallic components, has long been recognized1-4 to enhance fatigue strength and mitigate stress cracking. The fatigue strength of many engineering components is often improved by methods including rolling or shot peening. Modern surface enhancement treatments such as low plasticity burnishing (LPB),5 laser shock peening (LSP),6 and ultrasonic peening,7 have emerged that in varying degrees benefit fatigue and stress corrosion prone components.

Maximum benefits are obtained when deep compression is achieved with minimal cold working of the surface.

Environmentally assisted cracking (EAC) in the form of Sulfide Stress Cracking (SSC), Stress Corrosion Cracking (SCC) and Hydrogen Embrittlement (HE) prevent the use of less expensive high strength carbon steel alloys in the recovery of fossil fuels in corrosive-cracking environments commonly seen in offshore and deep well recovery efforts. Tensile residual stresses generated from straightening, machining and connection make-up when added to applied stresses during down hole operations in high-pressure environments are significant contributors to EAC and fatigue failure. Because these alloys at high strength levels are not suited for sulfide or chloride environments, the current solution is to use or create much more expensive alloys with increased corrosion-cracking resistance to mitigate the problems or limit their use to significantly weaker sour environments.

Introducing compressive residual stresses into less expensive carbon steel alloys can dramatically reduce the risk of failure, mitigate SSC, HE and SCC, and improve fatigue

2

strength.8 This could allow the potential of using less expensive alloys in harsh environments where they currently are unable to be used. Low plasticity burnishing (LPB) is highly effective, reliable and reproducible method of producing deep compressive residual stresses in complex geometric components. With advanced control systems LPB can be applied using a closed loop feedback surface enhancement method capable of introducing a customized compressive residual stress field specifically tailored for each application. LPB so applied produces a very smooth surface finish, which aids in nondestructive inspection and examination. LPB tooling can be integrated with existing equipment used for manufacture and repair of down hole tubular products.

SSC susceptibility in high strength API P110 grade tubular products prevents their use in 100% H2S sour environments at temperatures less than 79°C (175 °F).9,10 As more deep wells and offshore resources are probed and recovered it is imperative to mitigate the problem of EAC in a cost effective manner. Laboratory testing in standard 100% NACE TM0177-Solution A 11 at 1 bara H2S liquid environment has shown that the LPB process can be employed to treat components with the effect of providing a substantial increase in service life, and SSC mitigation. The LPB technology is now being evaluated to determine if it can play a pivotal role in creating more reliable and efficient fossil fuel recovery systems that are capable of safely and reliably operating in aggressive environments. LPB processing has successfully been used to mitigate EAC in high strength steel, stainless steels and aluminums used in both the aerospace and nuclear industries. LPB technology was developed in conjunction with NASA’s SBIR program and is currently used in production of parts used in the aerospace, medical, and nuclear industries and on many different metal 12-16 alloys.

EXPERIMENTAL PROCEDURE Material C- ring specimens and full size coupling blank specimens were sectioned from a length of API P110 grade quench and tempered coupling stock. Figure 1 shows the coupling stock used to manufacture the specimens. Figure 2 shows an example of each of the 2 geometries tested in this investigation.

Figure 1: API P110 Quench + Temper coupling stock.

3

A

B

Figure 2: Examples of tested geometries: (A) C-ring specimen, and (B) full sized coupling blank in test fixture.

Specimen Processing LPB process parameters were developed to achieve nominally 0.040 in. (1 mm) depth of compression. Samples were processed on a CNC mill or lathe to allow positioning of the LPB tool in a series of passes along the region to be processed while controlling the burnishing pressure to develop the pre-determined magnitude of compressive stress with controlled low cold working. The full lengths of the outer diameter of the full sized coupling blanks were LPB processed. The C-ring specimens were processed on the exposed section of the outside diameter. The LPB process has been previously documented in detail.17 X-ray Diffraction Residual Stress Analysis X-ray diffraction residual stress measurements were made at the surface and at several depths below the surface on the outside diameter of both LPB and untreated specimens to characterize the residual stress distributions. Measurements were made in the axial direction employing a sin2Ď&#x2C6; technique and the diffraction of chromium KÎą1 radiation from the (211) planes of steel.

Material was removed electrolytically for subsurface measurement in order to minimize possible alteration of the subsurface residual stress distribution. The measurements were corrected for both the penetration of the radiation into the subsurface stress gradient and for stress relaxation caused by layer removal. The value of the x-ray elastic constants required to calculate the macroscopic residual stress from the strain normal to the (211) planes of steel were determined in accordance with ASTM E1426-9.18,19 Systematic errors were monitored per ASTM specification E915.20 Surface Roughness The improvement in surface roughness was documented for LPB vs. un-treated coupling material. Surface roughness measurements were performed on both the untreated and LPB treated coupling blanks using a standard surface roughness tester. The Ra surface roughness was calculated over a 0.50 in. (12.7 mm) evaluation length in the axial direction.

4

SSC Testing SSC Testing was conducted on 4 ½ in. (114.3 mm) API P110 quench and temper coupling stock (5 in. (127 mm) outside diameter). The coupling stock was sectioned into C-Ring specimens per NACE TM0177-Method C for testing and the full sized coupling stock blanks were machined to create an inside diameter of 4.375 in (111.1 mm) and provide a sealing surface for the internal pressure seals. Specimens were tested in both the un-treated condition as well as after LPB processing to determine the differential effects resulting from LPB treatment.

C-ring Testing: Testing was performed on LPB treated and un-treated C-ring specimens per NACE TM0177Method C. All testing was performed in 100% NACE TM0177-Solution A at 1 bara H2S at 25° C, the pH was monitored continuously throughout testing to ensure conformance to NACE TM0177. Specimens were sectioned from a length of API P110 coupling stock. The specimens were loaded initially to nominally 45% of SMYS. After exposure to at least 720 hours the specimens were tested at 80%, 85% and 90% of SMYS. Stress on the specimens was monitored continuously using strain gage rosettes placed on the inner diameter opposite the exposed location of maximum applied tension. The entire specimen except the outer gage region was coated in a polymer based stop off coating after loading and prior to immersion in solution. Figure 3 shows a C-ring specimen ready for testing.

Figure 3: C-ring specimen prior to testing.

Full Sized Pressurized Coupling blank Testing: Testing of full size coupling blanks was performed using a custom made holding fixture connected to a pressurizing test station. Specimens were tested in both the LPB treated and un-treated conditions. All tests were performed in 100% NACE TM0177-Solution A at 1 bara H2S at room temperature with the pH continuously monitored. The full sized coupling blanks were internally pressurized hydraulically to impart the desired amount of applied hoop stress.

Test solution was monitored for pH and refreshed as needed to conform to the NACE TM0177 standard. Testing was conducted until specimen failure or a run out life of 720 hours (30 days) or more was achieved per NACE TM0177 standard. Specimens were tested at 45%, 80%, and

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85% of SMYS. Pressure was monitored continuously throughout the test and a timer was placed in the circuit to trip upon sample failure.

Figure 4: Full size coupling blank pressurized test apparatus and setup.

RESULTS AND DISCUSSION

X-ray Diffraction Residual Stress Analysis X-ray diffraction residual stress vs. depth results for untreated and LPB processed API P110 coupling blanks are presented graphically in Figure 5. Compressive stresses are shown as negative values, and tensile stresses as positive, in units of ksi (103 psi) and MPa (106 N/m2).

Compared to the untreated condition, LPB produced a compressive residual stress field with a much greater magnitude of compression (>10X) and over 2X the depth of compression. The magnitude of compression is near the SMYS of 110 ksi (759 MPa) at the surface. LPB produces much less cold working than conventional processes and ensures the deep fiber layers remain in stable compression, even at high temperature or in the case of mechanical overload as has been demonstrated in prior work.21

6

-3

20

0

200

Depth (x 10 mm) 400 600 800

1000

1200 100 0

0

-100

Residual Stress (ksi)

Quench + Temper Untreated

-40

LPB

-300 -400

-60

-500

-80

-600

-100 -120

-200

Residual Stress (MPa)

-20

-700 -800 0

10

20

30

40

50

-3

Depth (x 10 in.)

Untreated (Quench + Temper)

LPB

Figure 5: Residual stress comparison for LPB processed and un-treated material.

Surface Roughness The improvement in surface roughness after LPB processing was quantified using the Ra surface roughness. LPB improved the surface finish by a factor of 2.6X. This can aid in NDI examination as well as reduce friction in service. Figure 6 displays the results graphically.

Each value is an average of three repeat measurements.

7

300

AVERAGED SURFACE ROUGHNESS API P110 Steel Coupling Average of 3 Measurements

7

Surface Roughness - Ra (Îź in)

241.7 200

6 5 4

150

3 100 2

92.0 50

Surface Roughness - Ra (Îźm)

250

1

0

0

UNTREATED

LPB PROCESSED

Figure 6: Surface roughness for LPB processed and un-treated API P110 coupling blank.

SSC Testing The SSC testing data is presented graphically below in Figures 7 & 8. The un-treated C-ring specimen with an OD exposed surface failed in 10 hours at a stress of 45% SMYS, The LPB processed specimens exceeded the run-out life of 720 hours at 45%, 80%, 85% and 90% of SMYS exceeding typical hold-time requirements for testing in a sour service environment. The full sized coupling blank test results are very similar with the un-treated coupling blank failing after the entire OD surface was exposed for 37.5 hours. The LPB processed specimens exceeded 720 hours at 45%, 80% and 85% SMYS stress levels while surpassing typical holdtime requirements for sour service testing. The second full sized LPB coupling blank ran for a total of 1454.75 hours in solution before testing was terminated and the specimen was removed from solution for dye inspection, which revealed no cracking. These test results demonstrate the dramatic improvement achieved by the LPB treatment compared to the untreated P110 material.

A macro photo comparison of a failed untreated C-ring specimen and a run out LPB C-ring specimen is shown in Figure 9. Dye penetrent was used to reveal the axial SSC failure in the un-treated coupling blank shown in Figure 10. Figure 11 shows the LPB coupling blank after timed run out at 85% SMYS, with and without FDI developer, documenting that there are no cracks of any size beginning to initiate on the specimen. The SSC testing results show definitively that LPB is able to mitigate SSC cracking in common API P110 steel and dramatically increase the life. The 85% SMYS stress level is regarded as an aggressive performance test for metal that is in direct contact with a 100% H2S saturated environment.

API Specification 5CT uses 80% SMYS stress levels for C90 and T95 tensile specimens and the next edition will likely add 85% SMYS stress level for a new C110 grade.

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API P110 STEEL C-RING TESTING NACE A Solution, 25° C LPB RUN-OUT PROCESSED EXCEEDED NACE TM0177 841 Hours 90% SMYS LPB RUN-OUT PROCESSED EXCEEDED NACE TM0177 820 Hours 85% SMYS LPB RUN-OUT PROCESSED EXCEEDED NACE TM0177 822 Hours 80% SMYS LPB PROCESSED 45% SMYS

RUN-OUT (EXCEEDED NACE TM0177 by > 2X) 1,719 Hours Test Stopped

UNTREATED FAILED (10 Hrs) (Quench +Temper) 45% SMYS 0

200

400

NACE TM0177 RUN-OUT 720 Hrs

600

800

1000

1200

1400

1600

1800

TIME (Hours)

Figure 7: C-ring testing results.

API P110 STEEL COUPLING PRESSURE TEST NACE A Solution, 25° C LPB PROCESSED 85% SMYS

RUN - OUT (734.5 hrs) 1454.75 hrs = Total Time Exposed

LPB PROCESSED 80% SMYS

RUN - OUT (720.25 hrs)

UNTREATED (Quench+Temper) 45% SMYS

FAILED (37.5 Hrs) NACE TM0177 RUN-OUT 720 Hrs 0

100

200

300

400

500

600

700

TIME (Hours)

Figure 8: Full Sized Coupling blank Test Results.

9

800

UNTREATED (FAILED)

LPB

Figure 9: Comparison of LPB treated and un-treated C-ring specimens after testing. The untreated specimen failed in 10 hours at 45% of SMYS. The LPB specimens ran-out at 45%, 80%, 85%, and 90% SMYS with no cracking.

AXIAL FAILURE

AXIAL FAILURE

Figure 10: FDI inspection of failed un-treated coupling blank revealing thru wall axial SSC.

Figure 11: LPB processed coupling blank and test fixture after run-out at 85% SMYS. FDI developer showing no signs of crack initiation.

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CONCLUSIONS • • • • •

LPB imparted a deep compressive layer of stable residual compression over 2X deeper and 10X greater in magnitude than the untreated coupling blanks.

LPB was able to completely mitigate SSC failure in all tested specimens. The full sized coupling blank test exceeded the NACE TM0177 720 hour NACE A exposure time requirement at 45%, 80%, and 85% of the SMYS of 110 ksi (759 MPa).

The LPB processed C-ring tests exceeded NACE TM0177 time requirements at stresses levels equal to 45%, 80%, 85%, and 90% of SMYS. The untreated coupling blanks and c-ring specimens failed in 33 hrs and 10 hrs respectively at a stress load of 45% SMYS. Use of an engineered deep compressive stress field using LPB to mitigate SSC was successful on API P110 quench and temper coupling blank specimens.

REFERENCES 1. Frost, N.E. Marsh, K.J. Pook, L.P., (1974), Metal Fatigue, Oxford University Press.

2. Fuchs, H.O. and Stephens, R.I., (1980), Metal Fatigue In Engineering, John Wiley & Sons.

3. Berns, H. and Weber, L., (1984), "Influence of Residual Stresses on Crack Growth," Impact Surface Treatment, edited by S.A. Meguid, Elsevier, 33-44.

4.

Ferreira, J.A.M., Boorrego, L.F.P., and Costa, J.D.M., (1996), "Effects of Surface Treatments on the Fatigue of Notched Bend Specimens," Fatigue, Fract. Engng. Mater., Struct., Vol. 19 No.1, pp 111-117.

5. Prevéy, P.S. Telesman, J. Gabb, T. and Kantzos, P., (2000), “FOD Resistance and Fatigue Crack Arrest in Low Plasticity Burnished IN718,” Proc of the 5th National High Cycle Fatigue Conference, Chandler, AZ. March 7-9.

6. Clauer, A.H., (1996), "Laser Shock Peening for Fatigue Resistance," Surface Performance of Titanium, J.K. Gregory, et al, Editors, TMS Warrendale, PA, pp 217-230.

7. T. Watanabe, K. Hattori, et al,, (2002), “Effect of Ultrasonic Shot Peening on Fatigue Strength of High Strength Steel,” Proc. ICSP8, Garmisch-Partenkirchen, Germany, Ed. L. Wagner, pg 305310.

8. Paul S. Prevéy, N Jayaraman "Overview of Low Plasticity Burnishing for Mitigation of Fatigue Damage Mechanisms," Proceedings of ICSP 9, Paris, Marne la Vallee, France, Sept. 6-9,2005.

9. Snape, E.: “Sulfide Stress Corrosion of Some Medium and Low Alloy Steels,” Corrosion (June 1967) 23, 326-332.

10. Carter, C.S. and Hyatt, M.V.: “Review of Stress Corrosion Cracking in Low Alloy Steels With Yield Strength Below 150 ksi,” SCC and Hydrogen Embrittlement of Iron Base Alloy, NACE Reference Book No. 5 (1977) 524-600.

11. NACE Standard TM0177-2005: Laboratory Testing of Metals to Specific Forms of Environmental Cracking, NACE International.

12. J. Scheel, D. Hornbach, P. Prevey, “Mitigation of Stress Corrosion Cracking in Nuclear Weldments Using Low Plasticity Burnishing,” Proceedings of the 16th International Conference on Nuclear Engineering (ICONE16), May 11-15, 2008, Orlando, FL. 13. N. Jayaraman, P. Prevéy, “An Overview of the use of Engineered Compressive Residual Stresses to Mitigate SCC and Corrosion Fatigue,” Proceedings of 2005 Tri-Service Corrosion Conference, Orlando, FL, Nov. 14-18, 2005.

14. D.H. Hornbach and P.S. Prevéy, “Tensile Residual Stress Fields Produced in Austenitic Alloy Weldments,” Proceedings: Energy Week Conference Book IV, Jan. 28-30, Houston, TX, ASME International, 1997.

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15. P.S. Prevey, et al. “Effect of Prior Machining Deformation on the Development of Tensile residual Stresses in Weld Fabricated Nuclear Components” Journal of Materials Engineering and Performance, vol. 5(1), Materials Park, OH; ASM International, 1996 pp. 51-56.

16. D. Hornbach, P. Prevéy, “Reducing Corrosion Fatigue and SCC Failures in 300M Steel Landing Gear Using Low Plasticity Burnishing,” Proceedings of 2007 SAE AeroTech Congress & Exhibition, Los Angeles, CA, September 17-20, 2007.

17. P. Prevey., “Burnishing Method and Apparatus for Providing a Layer of Compressive Residual Stress in the Surface of a Workpiece.” US Patent # 5,826,453, Oct. 27, 1998.

18. Cullity, B.D., (1978) Elements of X-ray Diffraction, 2nd ed., (Reading, MA: Addison-Wesley), pp.

447-476.

19. Prevéy, P.S., (1986), “X-Ray Diffraction Residual Stress Techniques,” Metals Handbook, 10, (Metals Park, OH: ASM), pp 380-392.

20. ASTM Standard E915, 2010, "Standard Test Method for Verifying the Alignment of X-Ray Diffraction Instrumentation for Residual Stress Measurement," ASTM International, West Conshohocken, PA, 2003, DOI: 10.1520/E0915-10, www.astm.org.

21. Paul S. Prevéy, “The Effect of Cold Work on the Thermal Stability of Residual Compression in Surface Enhanced IN718”, Proceedings of the 20th ASM Materials Solutions Conference and Exposition, St. Louis, MO, Oct. 10-12, 2000.

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Designation: G 38 – 01

Standard Practice for

Making and Using C-Ring Stress-Corrosion Test Specimens1 This standard is issued under the fixed designation G 38; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript epsilon (e) indicates an editorial change since the last revision or reapproval.

This standard has been approved for use by agencies of the Department of Defense.

1. Scope 1.1 This practice describes the essential features of the design and machining, and procedures for stressing, exposing, and inspecting C-ring type of stress-corrosion test specimens.

An analysis is given of the state and distribution of stress in the C-ring.

1.2 Specific considerations relating to the sampling process and to the selection of appropriate test environments are outside the scope of this practice.

1.3 The values stated in SI units are to be regarded as standard. The inch-pound units are provided for information.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

2. Referenced Documents 2.1 NACE Document: NACE TM0177–96 Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments2 3. Summary of Practice 3.1 This practice involves the preparation of and the quantitative stressing of a C-ring stress-corrosion test specimen by application of a bending load. Characteristics of the stress system and the distribution of stresses are discussed. Guidance is given for methods of exposure and inspection.

FIG. 1 Sampling Procedure for Testing Various Products

4. Significance and Use 4.1 The C-ring is a versatile, economical specimen for quantitatively determining the susceptibility to stress-corrosion cracking of all types of alloys in a wide variety of product forms. It is particularly suitable for making transverse tests of tubing and rod and for making short-transverse tests of various products as illustrated for plate in Fig. 1.

5. Sampling 5.1 Test specimens shall be taken from a location and with an orientation so that they adequately represent the material to be tested.

5.2 In testing thick sections that have a directional grain structure, it is essential that the C-ring be oriented in the section so that the direction of principal stress (parallel to the stressing bolt) is in the direction of minimum resistance to stress-corrosion cracking. For example, in the case of aluminum alloys (1),3 this is the short-transverse direction relative to

1 This practice is under the jurisdiction of ASTM Committee G01 on Corrosion of Metals and is the direct responsibility of Subcommittee G01.06 on StressCorrosion Cracking and Corrosion Fatigue.

Current edition approved May 10, 2001. Published May 2001. Originally published as G 38 - 73. Last previous edition G 38 - 73 (1995)e1.

2 Available from National Association of Corrosion Engineers (NACE), P.O. Box 218340, Houston, TX 77218–8340.

3 The boldface numbers in parentheses refer to the list of references at the end of this practice.

Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.

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HYDROGEN EMBRITTLEMENT OF CARBON STEEL AND STAINLESS STEEL Presented by Professor Roy Johnsen Qatar Petroleum, Research and Technology Department

Norwegian University of Science and Technology

Materials and Science Engineering Symposium 2012

roy.johnsen@ntnu.no

Read More

Fion Zhang/ Charlie Chong

TO BE PRESENTED  What is Hydrogen Embrittlement (HE)?  Examples of HE failures from the oil and gas

industry

 How to test the resistance against HE?  Example of test results  Effect of temperature on HE susceptibility for

stainless steel under cathodic protection.

 What is needed to get HE and how to avoid HE?

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ISO 21457 Material selection for oil and gas

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HYDROGEN EMBRITTLEMENT According to ISO 21457

SULFIDE STRESS CRACKING (SSC) Cracking of metal involving corrosion and tensile stress (residual and/or applied) in the presence of water and H2S. Hydrogen from acid corrosion on the metal surface diffusing into the metal.

HYDROGEN STRESS CRACKING (HSC = HISC1)) Cracking that occurs in a sensitive metal due to a combination of hydrogen and stress. Hydrogen normally generated by a cathodic reaction as a result of cathodic protection or galvanic coupling.

1)

Hydrogen Induced Stress Cracking

HYDROGEN INDUCED CRACKING (HIC) Cracking that occurs in carbon and low alloy steels when atomic hydrogen diffuses into the steel and then combines to form molecular hydrogen at trap sites (no stress/strain needed).

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HYDROGEN SOURCES  H2S in wellfluid  Cathodic Protection (CP)  Galvanic corrosion  Welding  Pre-treatment/cleaning with acids  Electrolytic coating (with current)  Sulfate Reducing Bacteria (SRB)

Hydrogen embrittlement cracks in carbon steel Materials and Science Engineering Symposium 2012

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2.5” WELLHEAD CONNECTOR BOLT MADE FROM CARBON STEEL  Bolt exposed subsea connected

to cathodic protection

 High strength steel bolts (12.9

quality)

 Bolt hardness: 384HV

 Yield strength: 1148 MPa

 The fracture surface has a

classic HE apperance with secondary cracks, intergranular

fracture morphology and several locations were cracking have initiated.

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SUBSEA MANIFOLD MADE FROM 22% Cr DUPLEX STAINLESS STEEL

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SUBSEA MANIFOLD MADE FROM 22% Cr DUPLEX STAINLESS STEEL, cont.

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SUBSEA PIPELINE MADE FROM S13% Cr

12 km long subsea pipeline Materials and Science Engineering Symposium 2012

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SUBSEA HUB MADE FROM 25% Cr SUPER DUPLEX STAINLESS STEEL

Hydrogen Embritlement cracks

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W.H.Johnson 1875 ..some remarkable changes produced in iron by the action of hydrogen and acids â&#x20AC;Ś â&#x20AC;&#x153;The change is at once made evident to any one by the extraordinary decrease in toughness and breaking strain of the iron so treated, and is all the more remarkable as it is not permanent, but only temporary in character, for with lapse of time the metal slowly regains its original toughness and strengthâ&#x20AC;?.

Proceedings of the Royal Society of London 1875

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HE CRACKING MECHANISMS Hydrogen Enhanced Local Plasticity - HELP Interstitial hydrogen enhances dislocation mobility at the crack tip â&#x20AC;&#x201C; causes local softening (micro void cracking) or localized slip that appears macroscopically brittle.

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HE CRACKING MECHANISMS, cont.

Hydrogen Enhanced De-cohesion - HEDE Interstitial hydrogen lowers the cohesive strength by dilatation of the atomic lattice â&#x20AC;&#x201C; causes decrease in energy barrier for cleavage or grain boundary brittle fracture.

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SUFIDE STRESS CRACKING (SSC) TESTING  NACE TM0177-2005 “Laboratory testing of

metals for resistance to Sulfide Stress Corrosion Cracking and Stress Corrosion Cracking in H2S environment”

 NACE MR0175/ISO 15156 “Petroleum and

natural gas industries – Materials for use in H2S containing environments in oil and gas production” Part 1, Part 2 and Part 3

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TESTING OF HYDROGEN EMBRITTLEMENT 4-point bending Test specimen under load

Autoclaves for exposure under actual conditions Materials and Science Engineering Symposium 2012

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TESTING OF HYDROGEN EMBRITTLEMENT Constant Load (CL) Mills Measure

Timer Proof ring Exposure chamber

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TESTING OF HTDROGEN EMBRITTLEMENT Slow Strain Rate testing

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WHAT HAPPEN DURING CATHODIC PROTECTION? Hydrogen induced stress cracking in steel structures due to absorbed hydrogen from cathodic protection  During CP at low potentials, hydrogen atoms H+ are formed

on the steel surface

Hurray

 Some recombines to H2(gas) and escape  Others absorb into the steel lattice

Sorry

 Hydrogen stays diffusible in the lattice

or get trapped at reversible or irreversible traps  Reversible traps are typically, dislocations, vacations and grain

boundaries

 Typical irreversible traps are the interface between steel and

precipitates like Al2O3 and TiC

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SSRT OF STAINLESS STEELS UNDER CP Is there a temperature level where HE is prevented?

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TEST PROGRAM MATERIALS  SMSS (12Cr, 6Ni, 2Mo)  SDSS (25%Cr, 4%Mo, 6%Ni,

0.3%N, 1%W)

TEST CONDITIONS  SSRT (10-6 s-1  2.7 m/min)  Exposure in air and cathodic

polarized to -1050 mV vs. SCE in 3.5% NaCl solution

 Temperature 40C  1500C

SDSS MICROSTRUCTURE

 Two parallel specimens

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SSRT

Stress â&#x20AC;&#x201C; Elongation curves

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AFTER SSRT

Test temperature 40C

Air

2 mm

CP

2 mm

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AFTER SSRT Test temperature 1500C

Air

CP

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SUMMARY SEM EVALUATION AIR-EXPOSURE

The specimens show high degree of ductility â&#x20AC;&#x201C; ductile fracture (dimples combined with tearing).

CATHODIC POLARISATION -1050 mV vs. SCE

40C: Little degree of ductility in initial fracture region

1000C: Still brittle fracture, but some regions (10-15%) with ductile appearance in the initial fracture region 1500C: As for 1000C, but higher ratio of ductile appearance in the initial fracture region (ď ž30%) For all temperatures: trans crystalline brittle fracture Materials and Science Engineering Symposium 2012

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SSRT

Summary of results SMSS - Air

SMSS - CP

SDSS - Air

SDSS - CP

70 60

RA (%)

50 40 30 20 10 0 1

2

4 Deg. C

80 Deg. C

3

4

100 Deg. C

150 deg. C

Reduction Area (RA) = (Astart – Afinal)/Astart*100 % Materials and Science Engineering Symposium 2012

roy.johnsen@ntnu.no

CONCLUSIONS FROM THE TEST PROGRAM 1.Is SMSS/SDSS more susceptible to HE at low

temperature than at high temperature?

Yes â&#x20AC;&#x201C; higher degree of brittle fracture

2.Is there a max. temperature where HE is

eliminated?

Test results indicate that HE still occurs at 1500C

SMSS: S13% Cr martensittic stainless steel

SDSS: 25% Cr super duplex stainless steel

Materials and Science Engineering Symposium 2012

roy.johnsen@ntnu.no

NEW CONSTANT LOAD TEST METHOD DEVELOPED

Reference paper: Roy Johnsen, Bård Nyhus, Stig Wästberg: Hydrogen Indused Stress Cracking of Stainless Steels under Cathodic Protection in Sea Water – Presentation of a new test method. OMAE 2009-79325.

Materials and Science Engineering Symposium 2012

roy.johnsen@ntnu.no

CRACK IN 25% Cr SDSS

Under cathodic polarisation at 40C, 90% of ď łAYS Ferrite

Crack

Materials and Science Engineering Symposium 2012

Austenite

roy.johnsen@ntnu.no

HYDROGEN DIFFUSION MEASUREMENTS  Electrochemical method of hydrogen permeation was first

proposed in 1962 and applied to Pd membranes.

 Now it is applied to different metals and alloys

(ASTM G148-97(2003); ISO 17081:2004) Counter electrode

Ref. electrode

-1050 mVAg/AgCl

Ref. electrode

Counter electrode

+300 mVAg/AgCl

Specimen Materials and Science Engineering Symposium 2012

roy.johnsen@ntnu.no

PhD CANDIDATE IN FRONT OF HER EQUIPMENT

Materials and Science Engineering Symposium 2012

roy.johnsen@ntnu.no

HYDROGEN DIFFUSION CELL AT NTNU Electrochemical method of hydrogen permeation has been adopted in the new equipment

 Loading unit: 30 kN  Constant load, fatigue load, SSRT  Temperature: 4-80 ºC  Pressure: max. 100 bar  Ti autoclave – H2S, CO2

Materials and Science Engineering Symposium 2012

roy.johnsen@ntnu.no

HYDROGEN EMBRITTLEMENT What is needed?

 Access to hydrogen  Cathodic protection or

corrosion in H2S environment

 Certain strain/stress level

HYDROGEN

STRESS/STRAIN

MATERIAL

(global and local)

 Load and geometry  Susceptible microstructure  Selected material Materials and Science Engineering Symposium 2012

All these are needed at the same time!

TO AVOID HE Remove one of the elements! roy.johnsen@ntnu.no

STRESS/STRAIN

An Industry Design Guideline that defines the best practice for design and fabrication of duplex stainless steels for subsea equipment exposed to Cathodic Protection.

Materials and Science Engineering Symposium 2012

roy.johnsen@ntnu.no

ANY QUESTIONS?

Materials and Science Engineering Symposium 2012

roy.johnsen@ntnu.no

G 38 the grain structure. If the ring is not so oriented it will tend to crack off-center at a location where the stress is unknown.

Fig. 4 where the transverse tensile stress at the mid-width of a 19.00 mm (0.748 in.) outside diameter by 1.537 mm (0.0605 in.) thick by 19.0 mm (0.75 in.) wide C-ring of aluminum alloy 7075-T6 was equal to about one third of the circumferential tensile stress. In this example the circumferential stress was uniform over most of the width of the C-ring; measurements were not made at the extreme edge.

7.3 In the case of the notched C-ring (Fig. 3(d)) a triaxial stress state is present adjacent to the root of the notch (5). In addition, the circumferential stress at the root of the notch will be greater than the nominal stress and generally may be expected to be in the plastic range.

7.4 The possibility of residual stress should always be considered, especially when C-rings are machined from products that contain appreciable residual stress or when C-rings over about 6.35 mm (1⁄4 in.) thick are heat treated after being machined. It is generally not advisable to heat treat finishmachined C-rings because of the likelihood of developing residual stresses in the ring.

6. Specimen Design 6.1 Sizes for C-rings may be varied over a wide range, but C-rings with an outside diameter less than about 16 mm (5⁄8 in.) are not recommended because of increased difficulties in machining and decreased precision in stressing. The dimensions of the ring can affect the stress state, and these considerations are discussed in Section 7. A typical shop drawing for the manufacture of a C-ring is shown in Fig. 2.

7. Stress Considerations 7.1 The stress of principal interest in the C-ring specimen is the circumferential stress. It should be recognized that this stress is not uniform (2, 3). First, there is a gradient through the thickness, varying from a maximum tension on one surface to a maximum compression on the opposite surface. Secondly, the stress varies around the circumference of the C-ring from zero at each bolt hole to a maximum at the middle of the arc opposite the stressing bolt; the nominal stress is present only along a line across the ring at the middle of the arc. Thus, when the specimen is stressed by measuring the strain on the tension surface of the C-ring, the strain gage should be positioned at the middle of the arc in order to indicate the maximum strain.

Thirdly, the circumferential stress may vary across the width of the ring, the extent of the variation depending on the widthto-thickness and diameter-to-thickness ratios of the C-ring. In general, when loaded as shown in Fig. 3 (a, b), the tensile stress on the outer surface will be greater at the extreme edge than at the center, while when loaded as shown in Fig. 3 (c), the tensile stress on the inner surface will be less at the edge than at the center (4).

7.2 Another characteristic of the stress system in the C-ring is the presence of biaxial stresses; that is, transverse as well as circumferential stresses are developed on the critical test section. The transverse stress will vary from a maximum at the mid-width to zero at the edges, and will be the same sign as the circumferential stress. In general, the transverse stress may be expected to decrease with decreasing width to thickness and increasing diameter to thickness ratios. An example is shown in

NOTE 1—When specimens are exposed to corrosive media at elevated temperatures, the possibility of relaxation of stress during the exposure period should be investigated. Relaxation can be estimated from known creep data for both the ring and the stressing bolt.

7.5 An advantage of the C-ring is that it can be stressed with high precision and bias by application of a measured deflection. The sources of error in stressing are those that are inherent with the use of measuring instruments (micrometers, strain gages, etc.) as discussed in 7.2-7.4 and Annex A1.

7.6 The calculated stress applies only to the state of stress before initiation of cracks. Once cracking has initiated the stress at the tip of the crack, as well as in uncracked areas, has changed.

8. Stressing Methods 8.1 The C-ring, as generally used, is a constant-strain specimen with tensile stress produced on the exterior of the ring by tightening a bolt centered on the diameter of the ring.

However, a nearly constant load can be developed by the use of a calibrated spring placed on the loading bolt. C-rings also can be stressed in the reverse direction by spreading the ring and creating a tensile stress on the inside surface. These methods of

NOTE 1—If stock is undersize or tube stock is used dimensions can be varied to suit size of section from which the specimen must be cut.

FIG. 2 C-Ring Type of Stress-Corrosion Specimen

2

G 38

NOTE 1—For Fig 3 (d) a similar notch could be used on the tension side of (b) or (c).

FIG. 3 Methods of Stressing C-Rings

eC eT

= circumferential strain, and = transverse strain.

NOTE 2—When using electrical strain gages with thin-walled C-rings, a correction should be allowed for the displacement of the gage from the surface of the ring. All traces of the gage and the adhesive must be removed from the C-ring before it is exposed.

NOTE 3—Stresses may be calculated from measured strains using the modulus of elasticity, provided the stresses and strains do not exceed the proportional limit.

8.3 When several rings of the same alloy and dimensions are to be loaded, it is convenient to determine a calibration curve of circumferential stress versus ring deflection as in Fig. 4 to avoid the inconvenience of strain gaging each ring.

8.4 The amount of compression required on the C-ring to produce elastic straining only, and the degree of elastic strains can be predicted theoretically (2, 3). Therefore, C-rings may be stressed by calculating the deflection required to develop a desired elastic stress by using the individual ring dimensions in a modified curved beam equation as shown in Table A1.1. The accuracy of calculated stresses is shown in Fig. 4 by the agreement of the calculated curve and the actual data points.

See Annex A1 for the equation for stressing C-ring specimens.

8.5 In the case of notched specimens a nominal stress is assumed using the ring outside diameter measured at the root of the notch. Consideration then should be given to the stress concentration factor (KT) for the specific notch when calculating the D required to develop the intended stress.

FIG. 4 Stresses in 7075-T6 Aluminum Alloy C-Ring StressCorrosion Specimen (4)

stressing are illustrated in Fig. 3. Proper choice of a minimum bolt diameter or a spring constant is, of course, required to assure achieving true constant strain or constant load stressing.

8.2 The most accurate stressing procedure is to attach circumferential and transverse electrical strain gages to the surface stressed in tension and to tighten the bolt until the strain measurements indicate the desired circumferential stress. The circumferential (sC) and transverse (sT), stresses are calculated as follows:

NOTE 4—The National Association of Corrosion Engineers (NACE) Standard TM0177–96 provides procedures for stressing C-Rings to the 0.2% offset yield strength of the material to be tested. Experimentation under the review and scrutiny of the ASTM subcommittee holding jurisdiction of this standard was conducted to assess the accuracy and validity of such procedures. It was found that for a wide range of alloy systems, heat treatments, and test specimen dimensions, errors in the target strain associated with the 0.2% offset yield strength occurred which would be of significance. However, it was also determined that in all cases the actual strain realized following the procedures exceeded that associated with the 0.2% offset yield stress, rendering results following such procedures conservative from an engineering analysis standpoint.

sC 5 E/~1 2 µ2!·~eC 1 µeT!, and sT 5 E/~1 2 µ2!·~eT 1 µeC!

where: E = Young’s modulus of elasticity, µ = Poisson’s ratio, 3

G 38 12.2 Care must be exercised to avoid galvanic effects between the C-ring, the stressing bolt, and exposure racks. It is essential also to prevent crevice corrosion that could develop corrosion products between ring and bolt and alter the stress in the C-ring. Protection can readily be applied by means of suitable coatings or by insulating bushing as shown in Fig. 5.

Consideration must be given to the selection of coatings or insulators that will neither contaminate the corroding medium nor be deteriorated by it. An insulating bushing, for example, that would deteriorate or creep, and thus allow the stress in the specimen to decrease, would be unsatisfactory.

9. Machining 9.1 When rings are machined from solid stock, precautions should be taken to avoid practices that overheat, plastically deform, or develop residual stress in the metal surface.

Machining should be done in stages so that the final cut leaves the principal surface with a clean finish of 0.7 µm (30 µin.) rms or better. Necessary machining sequences, type of tool, feed rate, etc., depend upon the alloy and temper of the test piece.

Lapping, mechanical polishing, and similar operations that produce flow of the metal should be avoided.

10. Surface Preparation 10.1 A high-quality machined surface is the most desirable for corrosion test purposes unless one wants to test the as-fabricated surface of a tube or bar; it should, of course, be degreased before exposing the specimen. In order to remove heat treat films or thin layers of surface metal that may have become distorted during machining, chemical or electrochemical etches may be used. The choice of such a treatment will depend upon the alloy of the test piece. Care should be exercised to choose an etchant that will not selectively attack constituents in the metal or will not deposit undesirable residues on the surface. Etching or pickling should not be used for alloys that may undergo hydrogen embrittlement.

10.2 It is generally the best procedure to complete the surface preparation before the C-ring is stressed except for a possible final degreasing of the critically stressed area.

10.3 Every precaution should be taken to maintain the integrity of the surface after the final preparation; that is, avoid finger printing and any rough handling that could mar the finish.

NOTE 5—Specimens should be placed in the intended corrosive environment as soon as possible after being stressed, as some alloys may crack in moderately humid air.

NOTE 6—Hemispheric glazed ceramic insulators (S-151 Steatite) that are excellent for use outdoors and in neutral aqueous solutions can be obtained from Saxonburg Ceramics, Inc., P. O. Box 157, Saxonburg, PA 16056. Beeswax, and other adherent wax-type coatings, are suitable for room temperature tests in aqueous solutions. For tests in acidic or alkaline solutions, fast drying vinyl-type lacquers have been used successfully; an example is an electroplaters stop-off, “Micro Shield”, available from the Michigan Chrome and Chemical Co., Dept. T-R, 8615 Grinnell Ave., Detroit, MI 48213.

12.3 Determination of cracking time is a subjective procedure involving visual examination that under some conditions can be very difficult, as noted in Section 13, and depends on the skill and experience of the inspector.

13. Inspection 13.1 Highly stressed C-rings of alloys that are appreciably susceptible to stress-corrosion cracking tend to fracture through the entire thickness or to crack in a way that is conspicuous. Frequently, however, with lower applied stresses, or with more stress-corrosion-resistant alloys, cracking begins slowly and is difficult to detect. Small cracks may initiate at multiple sites and be obscured by corrosion products, and an arbitrary decision must be made to declare a specimen “failed.”

Inasmuch as C-rings do not always fracture, it is preferable to report the first crack as the criterion of failure. It is common practice to make this inspection with the naked eye or at a low magnification. If there are indications noted that cannot be established definitely as a crack by this type of examination, the investigator should either (a) note the date of this first suspicion of cracking and continue the exposure of the specimen, watching for further growth that will confirm the first indication as the failure date, or (b) discontinue exposure of the specimen and perform a metallographic examination of a cross

11. Specimen Identification 11.1 Specimen numbers may be scribed on one of the tips adjacent to the cut-away segment of the C-ring. No markings of any kind should be made on the critically stressed arc between the bolt holes. Nonmetallic tags may be attached to the stressing bolt by means of a second nut.

12. Exposure Methods 12.1 The C-ring, because of its small size and the simple methods of stressing, can be exposed to almost any kind of corrosive environment (6). The specimens should be supported in such a way that nothing except the corrosive medium comes in contact with the critically stressed area. No part of an exposure rack should be allowed to touch the surface or the edges of the critically stressed region.

FIG. 5 Protection Against Galvanic Effects

4

G 38 section taken through the suspected crack to establish whether there is cracking. Metallographic examination of fractured or cracked C-rings can also be helpful in determining whether the failure was caused by stress-corrosion cracking or by some other form of localized corrosion.

14.1.6 Test duration, and 14.1.7 Criterion of failure.

14.2 Full information should also be reported about the alloy(s) being tested, including the following: 14.2.1 Alloy designation or specification number, 14.2.2 Composition of the test lot, 14.2.3 Fabrication history, 14.2.4 Heat treatment, and 14.2.5 Mechanical properties.

14. Report 14.1 In addition to reporting the number of specimens failed and the time to “failure” of each specimen, particulars should be reported concerning the following: 14.1.1 Stressing methods, 14.1.2 Magnitude of applied stress, 14.1.3 Specimen orientation, 14.1.4 Dimensions and surface preparation, 14.1.5 Test medium,

15. Keywords 15.1 constant load; constant strain; C-rings; notches; quantitative stress; stress-corrosion cracking; stress-corrosion test specimen

ANNEX (Mandatory Information) A1. EQUATION FOR STRESSING C-RING SPECIMENS

A1.1 Calculate the final diameter (ODf) required to give the desired stress using the following equations:

NOTE A1.1—Tables such as Table A1.1 can be developed to avoid repetitive calculations for investigations involving many tests of a given nominal size C-ring.

NOTE A1.2—The main source of error in this procedure lies in the measurements of the C-ring dimensions. If in a typical example of a 19.05 mm (0.750 in.) OD by 1.52 mm (0.060 in.) wall thickness C-ring the measurements are made to the nearest 0.03 mm (0.001 in.), the random error in the calculated value of D should not exceed about 3 %; and the error would be less for larger and thicker rings. An error of 0.001 in. in measuring OD and ODf, however, will have a variable effect upon the stress actually developed, depending upon the magnitudes of the desired stress and the OD of the ring. For the size of ring mentioned the percent error in applying D would be 63 % for f = 345 MPa (50 ksi) ranging to 630 % for f = 34 MPa (5 ksi).

ODf 5 OD 2 D, and D 5 fpD2/4EtZ

where: OD = outside diameter of C-ring before stressing, in. (or mm), ODf = outside diameter of stressed C-ring, in. (or mm), f = desired stress, MPa (or psi) (within the proportional limit), D = change of OD giving desired stress, mm (or in.), D = mean diameter (OD − t), mm (or in.), t = wall thickness, mm (or in.), E = modulus of elasticity, MPa (or psi), and Z = a correction factor for curved beams (see Fig. A1.1).

5

G 38

FIG. A1.1 Correction Factor for Curved Beams

6

G 38 TABLE A1.1 Deflections for a C-Ring of Nominal 0.750 in. OD by 0.060 in. Wall Thickness and Alloy with a Modulus of Elasticity of 68 900 MPa (10 000 ksi) for Stressing to 689 MPa (100 ksi)

NOTE 1—To obtain the deflection required to develop the intended stress, f, in a particular C-ring, locate the number corresponding to the actual OD and t for that particular C-ring and multiply it by f 3 10−5; for example, for a C-ring with an OD of 0.7520 in. and a t of 0.0620 in., multiply 0.0642 by f/1000 3 1/100.

NOTE 2—For alloys with a different modulus of elasticity another table could be calculated, or divide the calculated value of D by E 3 10−7.

Actual OD, mm

Actual t, mm 1.422 1.435 1.448 1.460 1.473 1.486 1.499 1.511 1.524 1.537 1.549 1.562 1.575 1.587 1.600 1.613 1.626

18.974

18.999

19.025

19.050

19.075

19.101

19.126

19.152

19.177

19.202

1.808 1.791 1.773 1.755 1.737 1.720 1.702 1.687 1.669 1.653 1.638 1.620 1.605 1.590 1.577 1.562 1.547

1.816 1.796 1.778 1.760 1.742 1.725 1.707 1.692 1.674 1.659 1.643 1.626 1.610 1.595 1.580 1.565 1.552

1.819 1.801 1.783 1.765 1.748 1.730 1.712 1.697 1.679 1.664 1.646 1.631 1.615 1.600 1.585 1.570 1.557

1.826 1.806 1.788 1.770 1.753 1.735 1.717 1.702 1.684 1.669 1.651 1.636 1.620 1.605 1.590 1.575 1.560

1.831 1.811 1.793 1.775 1.758 1.740 1.722 1.707 1.689 1.671 1.656 1.641 1.626 1.610 1.595 1.580 1.565

1.836 1.816 1.798 1.780 1.763 1.745 1.727 1.709 1.694 1.676 1.661 1.646 1.631 1.615 1.600 1.585 1.570

1.841 1.821 1.803 1.786 1.768 1.750 1.732 1.714 1.699 1.681 1.666 1.650 1.633 1.618 1.603 1.590 1.575

1.846 1.826 1.808 1.791 1.773 1.755 1.737 1.720 1.704 1.687 1.671 1.656 1.638 1.623 1.608 1.593 1.580

1.852 1.834 1.814 1.796 1.778 1.760 1.742 1.725 1.709 1.692 1.676 1.659 1.643 1.628 1.613 1.598 1.582

1.857 1.839 1.819 1.801 1.783 1.765 1.747 1.730 1.714 1.697 1.681 1.664 1.648 1.633 1.618 1.603 1.588

0.7470

0.7480

0.7490

0.7500

0.7510

0.7520

0.7530

0.7540

0.7550

0.7560

0.0712 0.0705 0.0698 0.0691 0.0684 0.0677 0.0670 0.0664 0.0657 0.0651 0.0645 0.0638 0.0632 0.0626 0.0621 0.0615 0.0609

0.0715 0.0707 0.0700 0.0693 0.0686 0.0679 0.0672 0.0666 0.0659 0.0653 0.0647 0.0640 0.0634 0.0628 0.0622 0.0616 0.0611

0.0716 0.0709 0.0702 0.0695 0.0688 0.0681 0.0674 0.0668 0.0661 0.0655 0.0648 0.0642 0.0636 0.0630 0.0624 0.0618 0.0613

0.0719 0.0711 0.0704 0.0697 0.0690 0.0683 0.0676 0.0670 0.0663 0.0657 0.0650 0.0644 0.0638 0.0632 0.0626 0.0620 0.0614

0.0721 0.0713 0.0706 0.0699 0.0692 0.0685 0.0678 0.0672 0.0665 0.0658 0.0652 0.0646 0.0640 0.0634 0.0628 0.0622 0.0616

0.0723 0.0715 0.0708 0.0701 0.0694 0.0687 0.0680 0.0673 0.0667 0.0660 0.0654 0.0648 0.0642 0.0636 0.0630 0.0624 0.0618

0.0725 0.0717 0.0710 0.0703 0.0696 0.0689 0.0682 0.0675 0.0669 0.0662 0.0656 0.0650 0.0643 0.0637 0.0631 0.0626 0.0620

0.0727 0.0719 0.0712 0.0705 0.0698 0.0691 0.0684 0.0677 0.0671 0.0664 0.0658 0.0652 0.0645 0.0639 0.0633 0.0627 0.0622

0.0729 0.0722 0.0714 0.0707 0.0700 0.0693 0.0686 0.0679 0.0673 0.0666 0.0660 0.0653 0.0647 0.0641 0.0635 0.0629 0.0623

0.0731 0.0724 0.0716 0.0709 0.0702 0.0695 0.0688 0.0681 0.0675 0.0668 0.0662 0.0655 0.0649 0.0643 0.0637 0.0631 0.0625

Actual OD, in.

Actual t, in.

0.0560 0.0565 0.0570 0.0575 0.0580 0.0585 0.0590 0.0595 0.0600 0.0605 0.0610 0.0615 0.0620 0.0625 0.0630 0.0635 0.0640

REFERENCES (1) Sprowls, D. O., and Brown, R. H., “What Every Engineer Should Know About Stress Corrosion of Aluminum,” Metal Progress, Vol 81, No. 4, April 1962, pp. 79–85, and Vol 81, No. 5, May 1962, pp. 77–83.

(2) Timoshenko, S., Strength of Materials, Part II, 2nd ed., D. Van Nostrand, New York, NY, 1952, Chapter 2.

(3) Fernandex, S. O., and Tisinai, G. F., “Stress Analysis of Unnotched C-Rings Used for Stress Cracking Studies,” Journal of Engineering for Industry, Vol 90, 1968, pp. 147–152.

(4) Kelsey, R. A., “Unpublished Work,” Alcoa Research Laboratories, Aluminum Company of America, New Kensington, PA, 1969.

(5) Williams, F. S., Beck, W., and Jankowsky, E. J., “A Notched Ring Specimen for Hydrogen Embrittlement Studies,” Proceedings, ASTM, Vol 60, 1960, p. 1192.

(6) Romans, H. B., “Stress Corrosion Test Environments and Test Duration,” Symposium on Stress Corrosion Testing, ASTM STP 425, ASTM, 1967, pp. 182–208.

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G 38 The American Society for Testing and Materials takes no position respecting the validity of any patent rights asserted in connection with any item mentioned in this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the risk of infringement of such rights, are entirely their own responsibility.

This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years and if not revised, either reapproved or withdrawn. Your comments are invited either for revision of this standard or for additional standards and should be addressed to ASTM Headquarters. Your comments will receive careful consideration at a meeting of the responsible technical committee, which you may attend. If you feel that your comments have not received a fair hearing you should make your views known to the ASTM Committee on Standards, at the address shown below.

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8