corrosion 2010 manuscript caseres et al comparative study for corrosion inhibitor evaluation in co2

CORROSION

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A COMPARATIVE STUDY OF GRAVIMETRIC AND ELECTROCHEMICAL TECHNIQUES FOR THE EVALUATION OF CORROSION INHIBITOR ACTIVIY ONSET AND EFFICENCY IN PIPELINE CO2 ENVIRONMENTS

Journal:

Manuscript Type:

Complete List of Authors:

03-Nov-2010

Cáceres, Andrea; UNAM, Facultad de Química; ICF-UNAM, Materiales

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Key Words:

Discussion

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Date Submitted by the Author:

CJ-1011-DS-0322

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Manuscript ID:

Corrosion

carbon steel, coupons, inhibitors, instrumentation, testing, weight loss

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A COMPARATIVE STUDY OF GRAVIMETRIC AND ELECTROCHEMICAL TECHNIQUES FOR THE EVALUATION OF CORROSION INHIBITOR ACTIVIY ONSET AND EFFICENCY IN PIPELINE CO2 ENVIRONMENTS

A. CACERES1, M. CASALES2, J. G. GONZALEZ-RODRIGUEZ3 L. MARTINEZ2 1 Departamento de Ingeniería Química Metalúrgica. Facultad de Química. Universidad Nacional Autónoma de México, 04510, México, D.F., México 2 Universidad Nacional Autónoma de México, Instituto de Ciencias Físicas, Av. Universidad S/N, Cuernavaca, Mor., México 3 Universidad Autonoma del Estado de Morelos, C.I.I.C.A.P., Av. Universidad 1001, Cuernavaca, Mor., México

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ABSTRACT

Monitoring of internal corrosion of pipelines is a critical element of the pipeline maintenance programs.

We have performed a benchmarking study of enhanced techniques for real time corrosion monitoring

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such as; a combination of, Linear Polarization Resistance and Harmonic Distortion Analysis, and enhanced Electrical Resistance in conjunction with standard metal coupon. The transition of the corrosion

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processes occurring in metals immersed in corrosive electrolytes at the onset of inhibitor action after its injection was found a revealing item for study. This study considered electrolytes of 3 % NaCl solution

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with and without 10% diesel, saturated with CO2 at 50°C and standard AISI 1018 coupons and electrodes. A set of generic corrosion imidazoline based inhibitors. The benefits of employing real time corrosion monitoring devices include shortened test times, increased number of inhibitors evaluated, and an increase in data quality and quantity. The test show overestimate of the corrosion rates resulting

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from linear polarization resistance in the solution without inhibitor, while the Harmonic Distortion Analysis and enhanced Electrical Resistance present similar result with Standard Coupon. The enhanced electrical resistance monitoring proved also to be a good device to reveal the onset of corrosion protection after the inhibitor is applied.

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Keywords: Internal corrosion, real time corrosion monitoring, corrosion inhibitors, linear polarization resistance, harmonic analysis, electrical resistance and coupons.

1. INTRODUCTION In the oil and gas industry, the monitoring of internal corrosion of pipelines must be a top priority. This requirement has led to the evolution of corrosion monitoring tools toward real-time data, process control tools, knowledge-based systems, and smart structures. Corrosion monitoring is more complex than the

CORROSION

monitoring of most other process parameters because the different corrosion types existents, there is no single measurement technique that will detect all of these various conditions. An extensive range of corrosion monitoring techniques and systems has evolved, particularly in the last two decades, for detecting, measuring, and predicting corrosion damage. The developments of efficient corrosion monitoring techniques and user-friendly software have permitted us to field new techniques enables a near instantaneous appraisal of the corrosivity of produced and transported fluids [1, 2].

The techniques for real time corrosion monitoring usually employment are associated with electrochemical technical such as linear polarization resistance, and weight loss employing coupons weight and electrical resistance. The coupons of corrosion are only capable of providing information about long term [3]. The object of this study was to evaluate comparatively the following techniques:

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linear polarization resistance (LPR), Harmonic Distortion Analysis (HDA), electrical resistance (ER) and coupons of weight loss in corrosion system with CO2.

It is well known that weight loss coupons are used to evaluate the corrosion rates over long periods of

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time, this technique represents global rates corrosion in the system unlike electrochemical techniques trough which one can follow changes in kinetic of the process, however, the weight loss coupons is of the oldest techniques and widely used in industry. The monitoring techniques such as electrical resistance

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(ER) and Linear Polarization Resistance (LPR) can be used to provide further quick information about instantaneous corrosion in the system. The principle of the widely used ER technique is quite simple, that

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is, the electrical resistance of a sensing element increases as its cross sectional area is reduced by corrosion damage, the electrical resistance technique (ER) by corrosion monitoring method grew quickly after the correlation with corrosion rates was recognized in the 1950s [4, 5]. In the linear polarization

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resistance a small potential perturbation (typically 10–20 or even 30 mV) is applied to the working electrode and the resulting current is measured. The ratio of the potential to current perturbations is known as the polarization resistance that can be itself related to the corrosion current with the Stern–

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Geary relationship, where Taffel slopes are used anodic and cathodic pre-established, this is 120 mV/dec. respectively. Harmonic distortion is a measure of the non-linear current distortion arising during the LPR measurements. The data is analyzed with the use of Fast Fourier Transform analysis, and so obtained a measure of the corrosion current and provide an on-line estimate of the corrosion rate calculation of Stern-Geary constant. The Stern and Geary constant is calculated front the Harmonic Analysis data, with first, second and third harmonics in the sinewave signal of frequency W [6].

2. EXPERIMENTAL PROCEDURES

2.1 Test material and solution

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AISI 1018 carbon steel samples were employed in this study. Standard three identical electrodes with a triangular configuration were used in the Linear Polarization Resistance and Harmonic Distortion analysis techniques. The electrical resistance was used a Microcor probes in the cylindrical element form.

Gravimetric coupons by metal samples® were used for the weight loss test. The surface samples were polished with 600 grit SiC paper and finally rinsed with distilled water, acetone and ethanol.

Experiments were realized in a basic solution of 3 wt % NaCl, in tri-distilled water and a mixed which consisted of 90 vol. % of this solution and 10 vol. % diesel were used as testing solutions. The solutions were saturated with CO2 for 2 hours before testing and to obtain a CO2 atmosphere during testing. The CO2 gas was bubbled for 2 hours prior to testing in order to remove the dissolved oxygen from the test solution. During the experiment, the CO2 flow was kept constant and in this way the deareation of the

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solution, the electrolyte was saturated with CO2 by continuous sparging with CO2, the saturation with CO2 and constant positive pressure of CO2 were always ensured. For all experiments the temperature was constant at 50 ± 2°C.

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2.2 Weight loss and Electrochemical measurements

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All test were obtained in a glass cell, in weight loss test were obtained in a glass cell with coupons of

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weight loss immersed, the coupons were removed for each one of the test time, the time total to exposition was 336 hours. Electrical Resistance probes were obtained by CORRDATA® Corrosion Monitoring System. Electrochemical techniques employed included, Linear Polarization Resistance, LPR, and Harmonic Distortion Analysis (HDA), these probes were obtained by multiple technique the SmartCET

TM

System.

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Linear Polarization Resistance and Harmonic Analysis were conducted with three electrodes mode.

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Linear Polarization Resistance technique involves measurements of the polarization resistance of a corroding electrode using a small amplitude polarization of the electrodes. The slope of the response at the corrosion potential, the polarization response, is then inversely proportional to the corrosion current.

Harmonic Analysis technique is a measurement of the non-linear current distortion arising during the LPR measurement. The data is analyzed with the using Fast Fourier Transform analysis to provide a measure of the corrosion current, and to provide an on-line estimate of the corrosion rate calculation about the Stern-Geary constant.

The inhibitors used were a commercial inhibitors type hidroxietil-imidazoline, in concentration of 25 ppm.

The working electrode was allowed to pre-corrode in the corrosive electrolyte for 2 hours before the corrosion inhibitor was added.

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3. RESULTS AND DISCUSSION 3.1 Coupons Weight loss The corrosion rate by coupons weight loss can be calculated through the equation (1):

(1) 6

Where CR is the corrosion rate (mpy), K is a constant (3.45*10 ), W is weight loss during exposition time 2 3 (g), A is exposure area (cm ), T is the exposition time (hours) and D is the density (g/cm )

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Table 1 summarizes the weight loss results after exposing 1018 carbon steel during 336 hours to a CO2saturated 3% NaCl solution and a mixed which of 90 vol. % of this solution and 10 vol. % diesel at 50°C with and without 25 ppm. of inhibitors commercial inhibitors type hidroxyethyl-imidazoline, in

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concentration of 25 ppm. The high efficiency of these inhibitors is evident, since the corrosion rate is decreased from 57 mpy for the uninhibited 3% NaCl solution to 1.44 mpy, 0.41 mpy and 1.43 mpy in presence of the inhibitor type hidroxyehyil imidazoline, obtaining an efficiency value of 97.47 %, 99.28 %

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and 97.49% respectively. The corrosion products morphology of specimen exposed to the uninhibited solution after 192 and 336 hours of exposure, Fig.1, consists of a porous layer of FeCO3, Fig. 2, mainly

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siderite with equiaxial grains between 10-15 µm long. On the other hand, specimens exposed to the inhibited solution.

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a.

b.

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Fig.1.-SEM micrograph of steel covered by FeCO after a)192 h and b) 336 h of exposure to uninhibited CO2-saturated 3% NaCl 3

solution at 50 °C.

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3% NaCl 10 vol.% solution with

3% NaCl solution*

diesel 90 vol. % *

Corrosion Rate mpy

Efficiency Inhibitor (%)

Corrosion Rate mpy

Without

57.0

------

54.9

------

HEIO

1.44

97.47

0.951

98.27

HEIE

0.41

99.28

0.399

99.27

97.49

0.978

98.22

INHIBIDOR

HEIC

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Efficiency Inhibitor (%)

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300

FeCO3 192 h

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(104)

Intensity (a.u.)

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200

(012)

100 (018) (110)

(113) (202) (024)

(122)(214)

(300)

0 10

20

30

40

50

2Θ (degrees)

60

70

Tabl e 1.W eight loss tests resul ts

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Fig.2.-XRD patterns of the carbon steel coupons covered by FeCO3 in the uninhibited CO2-saturated 3% NaCl solution at 50°.

All peaks can be attributed to siderite)

3.2 Electrical resistance (ER) Corrosion rate was calculated by electrical resistance, with this technique, the electrical resistance of a metal specimen and its cross sectional area decreases, the corresponding increase in electrical

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resistance is related to the corrosion which has occurred. The electrical resistance of a metal or alloy element is given through the equation (2):

(2)

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2

Where L is the probe element length (cm); A is the cross-sectional area (cm ); and r is the specific

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resistance of the probe metal (Ω*cm). Reduction or metal loss in the element cross section, A due to corrosion will be accompanied by a proportionate increase in the element electrical resistance (R). To obtain the corrosion rate, a series of measurements are made over a period of time, and the results are

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plotted as a function of exposure time. The corrosion rate can be determined from the slope of the resulting plot [5,6].

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The results from the Electrical Resistance curves for 1018 carbon steel exposed to the CO2-saturated 3% NaCl solution at 50°C with and without 25 ppm of inhibitors type hydroxyethyl imidazoline are shown on Fig. 3. In this system the average corrosion rate value for the uninhibited solution was 72.6 mpy, with inhibitor addition the average corrosion rate were 0.65 mpy with HEIO inhibitor, 0.3 mpy with HEIE inhibitor and 1.24 mpy with HEIC inhibitor, with this technique, the higher efficiency of this corrosion inhibitors was clear, obtaining an efficiencies values of 98.95 % for HEIO inhibitor, 99.61% by HEIE inhibitor and 98.39 % by HEIC inhibitor

The electrical resistance curves for 1018 carbon steel exposed to the CO2-saturated in mixture of 90 vol.% of 3%NaCl and 10 vol. % diesel at 50°C with and without inhibitors commercial inhibitors type hidroxyethyl imidazoline in concentration of 25 ppm are shown on fig. 4. The average corrosion rate value in the environment brine-oil satured with CO2, was 77.3 mpy, once the corrosion inhibitor was added the

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average corrosion rate were 0.7 mpy with HEIO inhibitor, 0.3 mpy with HEIE inhibitor and 1.5 mpy with HEIC inhibitor, as in the system 3% NaCl solution, the higher efficiency of this corrosion inhibitors was clear obtaining an efficiencies values of 99.07 % for HEIO inhibitor, 99.62% by HEIE inhibitor and 98.02 % by HEIC inhibitor. This higher efficiency can be seen once the corrosion inhibitors were added. The industrial use of ER monitoring has been documented for various authors, G.R. Cameron and L.G. Coker [7] showed that sulfide environments present difficulties for the ER technique; electrically conducting iron sulfide corrosion products cause underestimates of metal loss. Gareau [8] illustrated the use of a computerized system for data transmission from several remote sites to a central control facility. At this central facility, the ER data were monitored in real time, manipulated, and analyzed to obtain information on corrosion at the remote sites Thierry, et al.[9] compared the results of weight-loss-coupon exposures, ER measurements, and linear polarization resistance (LPR) measurements in cooling water systems.

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They concluded that the ER measurements indicated corrosion rates greater than those observed on the coupons when there was significant localized corrosion of the coupons, concluding that ER technique is sensitive to pitting attack and provides a rate that is some intermediate value between the general corrosion rate and the penetration rate.

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HEIO

HEIC

HEIE

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Metal loss (mils)

0.16

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0.00 0

300

600

900

1200

Time (min)

Figure 3. Metal loss data for Electrical Resistance probes with and without inhibitors type hydroxy ethyl imidazolinas in 3% NaCl solution

CORROSION

Without inhibitor

HEIO

HEIC

HEIE

Metal loss (mils)

0.3

0.2

0.1

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0

300

600

900

1200

Time (min)

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Figure 4. Metal loss data for Electrical Resistance probes with and without inhibitors type hydroxyl ethyl imidazolinas in 3% NaCl 10 vol. % solution with diesel 90 vol. %

3.3 Electrochemical techniques

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The electrochemical nature of corrosion, allows that measurements of the electrical properties of the metal solution interface, electrochemical monitoring methods involve the determination of specific interface properties. In linear polarization resistance technique a small potential perturbation is applied to

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the working electrode and the resulting current is measured. The ratio between the potential and current perturbations is known as the polarization resistance (Rp). That can be it related to the corrosion current through following Stern–Geary equation (3):

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(3)

Where Rp is the polarization resistance; icorr is the uniform corrosion current; and B is an empirical polarization resistance constant that is related to the anodic (ba) and cathodic (bc) Taffel slopes through the equation (4);

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(4)

The corrosion rate obtained by linear polarization resistance are for 1018 carbon steel exposed to the CO2-saturated 3% NaCl solution at 50째C with and without 25 ppm is showed in figures 5, the average corrosion rate without inhibitors was 150.87 mpy, once the corrosion inhibitor was added the corrosion rate obtained were 0.81 mpy with HEIO inhibitor, 0.14 mpy with HEIE inhibitor and 1.89 mpy with HEIC inhibitor. The figure 6 showed the corrosion rate in the system with CO2-saturated in mixture of 90 vol.%

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of 3%NaCl and 10 vol. % diesel at 50째C, the average corrosion rate without inhibitor was 144.47 mpy, with inhibitor the average corrosion rates were 0.04 mpy with HEIO inhibitor, 0.062 mpy with HEIE inhibitor and 0.16 mpy with HEIC inhibitor. For all systems the efficiency of inhibitor corrosion was evident, this was more than 99%.

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160

HEIC

HEIE

120

On

80

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Corrosion Rate (mpy)

HEIO

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40

0 0

300

600

900

1200

1500

Time (min)

Figure 5. Corrosion rate by LPR probes with and without inhibitors type hydroxyl ethyl imidazolinas in 3% NaCl solution

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without inhibitor

180

HEIO

HEIC

HEIE

Corrosion Rate (mpy)

150

120

90

60

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0

0

300

600

900

1200

1500

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Time (min)

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Figure 6. Corrosion Rate by LPR probes with and without inhibitors type hydroxyl ethyl imidazolinas in 3% NaCl 10 vol. % solution with diesel 90 vol. %

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Harmonic Distortion Analysis technique, a low frequency sinusoidal potential is applied to a three electrode measurement system, and the resulting current is measured three different frequencies is used

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to verify the repeatability of the technique. The amplitude is in the range of 10–30-mV peak to peak. The frequency used is typically 0.1–10 Hz. Analyzing the primary frequency and the harmonics allow extraction of the required kinetic parameters of the corrosion process. Based on Stern-Geary model of the

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electrical double layer, a polarized electrode near the corrosion potential by a sinusoidal voltage of

angular frequency ω and voltage amplitude Uo, over a period of time; t, gives a faradaic current density i,

given through the equation (5);

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i = icor (exp(2.3Uo sinωt/ba) – exp(2.3Uosinωt/bc)

(5)

The current density of the Faradaic process will have a distorted sinusoinal form due to the non linear nature of the anodic and cathodic partial processes and will include higher harmonics having kω angular frequency (k=2,3,….). This phenomenon is termed Faradaic distortion [10,11,12,13,14].

The amplitudes of the harmonic components are obtained by Fourier’s series expansion of the

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exponential terms, equation (6):

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If=io + i1sinωt – i2cos2ωt – i3sin3ωt + …..

(6)

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The first term of the series is the io D.C component (Faradaic rectification), the coefficients of the other terms are the amplitudes of the harmonic components. For the first three harmonics components are given by equations (7,8,9):

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(7)

(8)

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(9)

Where βa=ba/2.303, βc=bc/2.303, the first order modified Bessel functions are noted by In (n=1,2,3….).

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This formulae can be simplified if Uo amplitude is limited to the extent (Uo < βa and Uo < βc) that the Besel

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functions could be approximated with the small arguments. Thus, the equations derivate of above formula where simplified by Devay and Mészáros as follows by equations (13,14,15):

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On (11)

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(12)

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Solving the equations system, Knowing amplitudes i1, i2, and i3, Devay and Mészáros proposed the

following equations for the determination of corrosion current for the determination of icorr , βa and βc.

quations (13,14)

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(13)

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The figures 7 and 8 showed the corrosion rate obtained for harmonic distortion analysis. In figure 7 for 1018 carbon steel exposed to the CO2-saturated 3% NaCl solution at 50°C with and without 25 ppm, the

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CORROSION

average corrosion rate without inhibitor was 53.21 mpy, once the corrosion inhibitor was added the average corrosion rate obtained were 0.78 mpy with HEIO inhibitor, 0.29 mpy with HEIE inhibitor and 0.86 mpy with HEIC inhibitor. The figure 8 showed the corrosion rate in the system with CO2-saturated in mixture of 90 vol.% of 3%NaCl and 10 vol. % diesel at 50°C, the average corrosion rate without inhibitor was 54 mpy, with inhibitor the average corrosion rates were 0.42 mpy with HEIO inhibitor, 0.088 mpy with HEIE inhibitor and 0.46 mpy with HEIC inhibitor. For all systems the efficiency of inhibitor corrosion was evident, this was more than 99%.

CORROSION

without inhibitor

HEIO

HEIC

HEIE

Corrosion Rate (mpy)

80

40

r Fo 0

0

200

400

600

800

1000

1200

1400

Time (minutes)

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Figure 7. Corrosion rate measured by Harmonic Distortion Analysis with and without inhibitors type hydroxyl ethyl imidazolinas in 3% NaCl solution

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Without inhibitor

HEIO

HEIC

HEIE

Corrosion Rate (mpy)

90

60

30

r Fo 0

0

Figure 8.

300

600

900

1200

1500

Time (min)

Corrosion Rate measured by Harmonic Distortion Analysis with and without inhibitors type hydroxyl ethyl

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imidazolinas in 3% NaCl 10 vol.% solution with diesel 90 vol. %

The corrosion rate obtained by linear polarization resistance in system without inhibitor presented

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overestimate of corrosion rate respect to harmonic distortion analysis, similar results were obtained by Durnie et al. they demonstrated that Harmonic Distortion Analysis yields comparables corrosion rate data

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to LPR under a variety of carbon dioxide corrosion conditions, but a regular error of approximately 100% was evident in comparison of HAD corrosion rates against comparable data obtained by using Linear Polarization Resistance, this was attributed to the assumed Taffel slopes in the LPR, while HAD is capable of determined Tafel Slopes in each measurement cycle.

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4. Conclusions

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CORROSION

The results showed good correlation between Electrical Resistance, Harmonic Distortion Analysis and weight loss coupons in corrosion system without corrosion inhibitors.

The Harmonic Distortion Analysis technique present a advantage respect to Linear Polarization Resistance due the simultaneous production of Taffel slopes, eliminating the approximation imposed by using assumed Tafel slopes in Linear Polarization Resistance.

The techniques of monitoring online of Electrical resistance, Linear Polarization Resistance and Harmonic Distortion Analysis represents a fundamental breakthrough, these techniques offers the possibility of measuring changes in corrosion rate rapidly and accurately in a corrosive system with CO2-saturated in

CORROSION

3% NaCl solution and mixture of 90 vol. % of 3%NaCl and 10 vol. % diesel at 50°C, with and without used to inhibitors corrosion.

Acknowledgments: Andrea Caceres would like to acknowledge to Consejo Nacional de Ciencia y Tecnología (CONACYT, México) by Ph. D. graduate by scholarship.

The authors would like to acknowledge to Q. Ivan Puente for their technical support in scanning electron microscopy and would like acknowledge to M. L. Ramon Garcia for their technical support in Diffraction ray-x.

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REFERENCES

[1] Corrosion inspection and monitoring. Pierre R. Roberge. A John Wiley & Sons, Inc., Publication.

Ontario-Canada 2007.

[2] Abdullatif Abdulhadi, Mohammed F. Al-Subaie, Ahmed M. Al-Zahrani, Majed M Al-Qarni. NACE corrosion 2007.Paper No. 07265.Nace International Houston-Texas.

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[3] Dawn C. Eden and David A. Eden, CORROSION NACExpo2006. Paper no. 06321. ©2006 NACE International. Houston, Texas.

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[4] Freedman aj, Troscinski es, and Dravnieks A. An Electrical Resistance Method of Corrosion Monitoring in Refinery Equipment. Corrosion 1958.

[5] Standard Guide for On-Line Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods). ASTM Standards. G 96-90. 2001. Philadelphia, PA, American Society for Testing of Materials.

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[6] J. Dévay and L. Mészáros. Acta Chim. Acad. Sci. Hung. Tomus 100 (1-4), pp 183-202. 1979.

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[7] G.R. Cameron and L.G. Coker, ASTM STP908, Corrosion Monitoring in Industrial Plant, 1986, pp. 251-265.

[8] Gareau, F. S., “Remote Corrosion Monitoring Using an Interactive Data Acquisition and Transfer System,” Paper 107, Corrosion 88, NACE, Houston, 1988.

[9] Thierry, D., Thoren, A., and Leygraf, C., “Corrosion Monitoring Techniques Applied to Cooling Water and District Heating Systems,” Paper 463, Corrosion 87, NACE, Houston, 1987

[10] G.L. Cooper, ASTM STP908, Corrosion Monitoring in Industrial Plants, 1986, pp. 237-250.

[11] A. Pirnát, L. Mészáros and B. Lengyel. Corrosion Science, Vol. 37, No. 6, pp 963-973, 1995 [12] William Durnie, Toland De Marco, Alan Jefferson, Brian Kinsella. Corrosion Science 44 (2002) 12131221.

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[13] S. Sathiyanarayanan, K. Balakrishanan, British Corros. J. 29 (2) (1994), 152.

[14] J. P. Diard, G. Le Gorrec, C. Montella. J. Electrochem. Soc. 142 (10) (1995), 3612 [15] S. K. Rangajaran, J. Electroanal. Chem. 62 (1) (1975) 31.

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