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International Journal of Metallurgical & Materials Science and Engineering (IJMMSE) ISSN 2278-2516 Vol. 3, Issue 2, Jun 2013, 21-32 Š TJPRC Pvt. Ltd.

EFFECT OF SOME ALLOYING ELEMENTS AND HEAT TREATMENT ON THE CORROSION BEHAVIOR OF AZ91 AND ZM60 MAGNESIUM ALLOYS IBRAHIM M. GHAYAD, NABIL N. GIRGIS & AHMED N. ABDUL-AZIM Central Metallurgical Research & Development Institute (CMRDI), Helwan, Cairo, Egypt

ABSTRACT In the present work the corrosion behavior of Mg-Al alloy (AZ 91), with additions of Ca, Sr and rare earth elements (mesh metal, MM), as well as Mg-Zn alloy (ZM 60), with addition of Cu aand MM, were investigated. Since the anodic polarization behavior of Mg alloys was complicated, the anodic Tafel constants were difficult to be accurately estimated, so polarization resistance (Rp, ohm cm2) was taken as a corrosion parameter instead of calculating corrosion current (Icorr). The addition of Ca, Sr and MM in different amounts singly or in combinations improved corrosion resistance of AZ91 alloy due to significant grain refinement which means smaller grains and higher grain boundary areas. The highest resistance was obtained for the alloy AZ91-0.4 Ca-0.14 Sr-1.2 MM. Cu addition enhances corrosion resistance of ZM60 alloy till 2 wt% but further increase of copper had a determintal effect on ressitance due to galvanic action. Rare earths imroved corrosion resitance of ZM60 alloys containing Cu. These alloys are more corrosion resistant than AZ91 alloys in 3.5% NaCl. Heat treatment, T6, imroved corrosion resistance of both magnesium alloy groups due to altering the size, amount and distribution of the precipitated phases.

KEYWORDS: Magnesium Alloys, Corrosion, Polarization Resistance, Heat Treatment, Microstructure INTRODUCTION The need for weight reduction, particularly in aerospace, automobile, telecommunication and portable microelectronics industries is important for materials selection. Magnesium alloys, with one quarter of the density of steel and only two-thirds that of aluminium, and a strength to weight ratio that far exceeds either are a promising alternatives. Unfortunately, the use of these alloys is limited due to their poor corrosion resistance. Corrosion resistance is especially poor when a magnesium alloy contains specific metallic impurities or when magnesium alloy is exposed to aggressive electrolyte species such as Cl- ions [1-3]. There are two main reasons for the poor corrosion resistance of magnesium alloys. Firstly there is internal galvanic corrosion caused by second phase or impurities. Secondly, the quasi-passive hydroxide film on magnesium is much less stable than passive films which form on metals such as aluminum and stainless steel. This quassi-passive provides only poor pitting resistance for magnesium and magnesium alloys. In addition, the corrosion behavior of produced magnesium alloys is influenced by phase distribution, grain size, solidification rate, heat treatment, microporosity and casting methods influence [4-7]. Even though corrosion of magnesium alloys is currently a serious problem. The prospect of magnesium alloys is still promising because of their advantages and potential applications. Magnesium alloys with aluminium and zinc have found wide spread application in automobile sector. Corrosion performance is therefore quite important as it concerns the


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Ibrahim M. Ghayad, Nabil N. Girgis & Ahmed N. Abdul-Azim

operating life of magnesium alloy components in a vehicle. The addition of rare earth elements, calcium, strontium and copper is an effective way to improve the mechanical and corrosion performance of magnesium alloys [8-17]. The present paper addresses the effect of alloying elements Ca, Sr, and MM on the corrosion behavior of Mg–Al alloy (ZA91) as well as the effect of additions of alloying elements Cu and MM on the corrosion behavior of Mg–Zn alloy (ZM60).

EXPERIMENTAL Material Two groups of magnesium-base alloys namely: AZ91 alloy with different minor additions of calcium (Ca), strontium (Sr) and rare earth elements (MM), and ZM60 alloy with different minor additions of copper (Cu) and rare earth elements (MM), were prepared and tested for microstructure and corrosion behavior. The chemical composition of these magnesium alloys are shown in Tables 1 and 2. Rare earths (MM) are typically added to the Mg–alloys as Cerium–based mesh metal, MM, containing lanthanum, neodymium, and praseodymium. A typical composition of MM is 50% Ce, 25 % La, 20 % Nd, and 3 % Pr. These elements have very low solubilities in Mg (Ce, 0.09; La, 0.14; Nd, 0.10 ; and Pr, 0.09 at.%) [18]. Table 1: Chemical Composition of AZ91 Magnesium Alloys

AZ91 AZ91 AZ91 AZ91 AZ91 AZ91 AZ91

Alloy Composition ( wt. % ) ( 9 Al + 0.7 Zn + 0.3 Mn + bal. Mg ) + 0.2 Ca + 0.4 Ca + 0.6 Ca + 0.4 Ca + 0.4 Sr + 0.4 Ca + 0.14 Sr + 1.2 MM + 1.2 MM

Table 2: Chemical Composition of ZM60 Magnesium Alloys Alloy Composition ( wt. % ) ZM60 ( 6Zn + 0.5 Mn + bal. Mg ) ZM60 + 2.5 MM ZM60 + 3.5 MM ZM60 + 1 Cu ZM60 + 2 Cu ZM60 + 3 Cu ZM60 + 4 Cu ZM60 + 3 Cu + 2.5 MM ZM60 + 3 Cu + 3.5 MM Corrosion Measurements According to Stern – Geary relationship [19]: β a βc = ----------------------------2.3(βa + βc) Rp

I corr

Where : I corr

=

B ---------Rp

is the corrosion current

Rp

is the polarization resistance

βa , βc

are the anodic and cathodic Tafel constants


Effect of Some Alloying Elements and Heat Treatment on the Corrosion Behavior of AZ91 and ZM60 Magnesium Alloys

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Since the anodic polarization behavior of magnesium alloys is complicated, βa and consequently Icorr are difficult to be accurately estimated. Fortunately, βa, βc and hence B may be assumed to be constants for a particular metal in a given environment. So, Rp is inversely proportional to Icorr and can be taken as an electrochemical parameter for the Mg– alloy system under investigation [20-21]. For measuring the polarization resistance (Rp, Ω cm2), specimens of 4.0 cm diameter and 3.0 mm thickness were cut from different magnesium alloys prepared in the form of rods of the same diameter. They were polished using 220,400, and 800 grade emery paper, degreased, washed with distilled water and immersed in 3.5% NaCl testing solution in the corrosion cell. A conventional three–electrode corrosion cell was used with a saturated calomel electrode (SCE), platinum electrode and magnesium alloy specimen electrode as reference, counter and working electrodes; respectively (Figure 1). The specimen area exposed to the 3.5% NaCl solution was 2.54 cm2. Leakage of electrolyte was prevented by a viton o-ring. An Autolab potentiostat / Galvanostat (PGSTAT30) corrosion measurement system was used to scan the potential at a rate of 0.5 mV/s over the range of (Ecorr–20) to (Ecorr+ 20). The current within this range varied linearly with applied potential. The polarization resistance was determined from the slope of this plot.

Figure 1: Schematic Representation of the Electrochemical Cell Used in the Corrosion Measurements Heat Treatment It was carried out by heating specimens to 410°C for 20 h followed by water quenching (T4 treatment), then aged at 200°C for 17 h (T6 Treatment). Metallography Specimens from the different Mg alloys were chosen for metallographic examination. Wet grinding performed with different grades of silicon carbide abrasive (120-1000) papers. After grinding, mechanical polishing was performed in two stages: rough and finish by using standard methods. Light scratches and cold worked surface metal on the polished specimen can be removed by alternate light etching and light repolishing. The specimens were etched with 3% HNO3, and 95% ethyl alcohol (natal). Microscopic observation of the cast specimens were observed by an optical microscope (OM) and scanning electron microscope (SEM). All specimens for microstructural characterization were cut from the same positions in the ingot. Surface morphology of corroded samples was investigated under the SEM after seven days immersion in 3.5% NaCl.

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Ibrahim M. Ghayad, Nabil N. Girgis & Ahmed N. Abdul-Azim

RESULTS AND DISCUSSIONS Effect of Ca, Sr, and MM on the Corrosion Resistance of AZ91–Base Alloys Table 3 shows the polarization resistance (Rp, Ωcm2) of alloy AZ91 with different alloying elements; calcium (Ca), strontium (Sr),as well as rare earth metals (MM) in 3.5% NaCl solution. The results show that a slight increase of polarization resistance, i.e. corrosion resistance, of AZ91–0.20 Ca (18.9 Ωcm2) compared with the polarization resistance of AZ91 magnesium alloy (17.8 Ωcm2). On increasing calcium from 0.2 to 0.6 the polarization resistance was increased in the following order: AZ91-0.2 Ca

<

AZ91-0.4 Ca

<

AZ91-0.6 Ca

Addition of 0.4 Sr to AZ91-0.4 Ca alloy increases polarization resistance from 22.7 Ωcm2 to 45.2 Ωcm2). However, the highest resistance (68.2 Ωcm2) was obtained for the alloy AZ91-0.4 Ca–0.14 Sr-1.2 MM. Another increase in the polarization resistance was observed with the addition of 1.2 MM to AZ91 magnesium alloy. The main form of MM in the as cast structure is Al4MM intermetallics [22]. Because of high structural stability of MM, they were combined with Al to form Al4MM until all the available MM is used. Compared to the AZ91 alloy the microstructure of AZ91-MM alloy shows considerable grain refinement, Figure 2-3. It can be concluded from the above results that additions of Ca, Sr, and MM in different amounts singly or in combinations resulted in a significant grain refinement, Figure 2-6. Moreover, addition of Ca or MM in the range studied resulted in a stabilization of the grain size of AZ91 magnesium alloy. It is evident that these additions refined also the dendrite cell size and size of the Mg17 Al12 phase. The AZ91–X alloys with fine grains would be more likely to have a continuous ß phase, Mg17 Al12, along the α boundaries, Figure 8. Therefore the much better corrosion performance of AZ91–base alloys can be ascribed to the finer α grain size and the more continuous ß-phase caused by some additions such as Ca, Sr, and MM in small amounts. Figure 7 [4] schematically presents the possible corrosion process on a AZ91 magnesium alloy and the effect of continuous ß–phase. It is observed that α–grains are fine, the gaps between ß–precipitates are narrow and the ß–phase is nearly continuous. The corrosion of α–phase is then quite much reduced by the barrier offered by the continuous ß–phase at the grain boundaries. Again, Ca, Sr, or MM cause significant grain refinement which means smaller grains and higher grain boundary areas i.e. decreases the anode to cathode area ratio; the condition for reduced corrosion current. It is well established that the film formed on the top of a finer grains of the matrix underneath is more stable, adherent and protective to the surface. The corrosion resistance of Mg mainly depends on the surface oxide film ,i.e., the corrosion property of Mg should be affected by MM, Ca & Sr addition since the surface oxide film will be changed. One of the important effects of MM on the corrosion resistance of Mg is the so-called scavenger effect, i.e., some impurity elements in Mg, such as Fe, severely deteriorates its corrosion resistance, and MM is said to cancel their influence by the formation of intermetallic compounds with the impurities. Table 3: Polarization Resistance ( Rp, Ωcm2) of AZ91- X Alloys in 3.5 % NaCl Solution Alloy AZ91 AZ91 - 0.2 Ca AZ91 - 0.4 Ca AZ91 - 0.6 Ca AZ91 – 0.4 Ca – 0.4 Sr AZ91 – 0.4 Ca – 0.14 Sr – 1.2 MM AZ91 – 1.2 MM

Polarization Resistance ( Rp,Ωcm2) 17.8 18.9 22.7 27.3 45.2 68.2 28.2


Effect of Some Alloying Elements and Heat Treatment on the Corrosion Behavior of AZ91 and ZM60 Magnesium Alloys

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Ibrahim M. Ghayad, Nabil N. Girgis & Ahmed N. Abdul-Azim


Effect of Some Alloying Elements and Heat Treatment on the Corrosion Behavior of AZ91 and ZM60 Magnesium Alloys

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Effect of Cu and/or MM on the Corrosion Resistance of ZM60–Base Alloys Table 4 shows the polarization resistance of ZM60 alloys with different amount of alloying elements; copper (Cu) and / or rare earths

(MM) in 3.5 % NaCl solution. Polarization resistance of the alloy Mg–6Zn–0.5 Mn–2.5MM

2

increased to 150.2 Ωcm compared to 140.6 Ωcm2 obtained for Mg–6Zn–0.5Mn. Generally, binary Mg–6Zn–0.5Mn is relatively has a coarse grained in the cast condition, Figure 8, while the Mg–Zn–MM has a fine grained, contains a large volume fraction of quite finely divided α+( Mg Zn MM) eutectic, Figure 9.The improved corrosion behavior of these alloys is attributed to the refined microstructure and formation of a protective film on the surface of magnesium alloy as a result of reaction of saline solution with MM, and the inertness of the second phase particles, Mg Mn MM [23]. With increasing amount of MM from 2.5 to 3.5 wt.%, a slight increase in the corrosion rate was observed and the polarization resistance of this alloy decreased from 150.2 Ωcm2 to 145.3 Ωcm2. This decrease in corrosion resistance may be attributed to the formation of massive grain boundary phase containing zinc and rare earth element, Figure 8 , which cause embrittlement of the Mg–6 Zn–3.5 MM alloy. On the other hand, the addition of 1-2 wt. % copper increases the polarization resistance of the magnesium alloy, Table 4. The highest increase of corrosion resistance obtained with the addition of 2 wt. % copper. However, further additions of copper, 3- 4 wt.%, has a detrimental effect on corrosion resistance, this is presumably because most of the copper with addition from 1- 2 wt. % is incorporated in the eutectic phase Mg (Cu, Zn)2 , Figure 10. With more copper additions, 3–4 wt. %, besides the presence of eutectic phase some elemental copper was found which forms a galvanic action and accordingly increased corrosion rate [24]. Moreover, addition of MM to Mg–Zn–Cu alloy had a detrimental effect on corrosion resistance. The polarization resistance was decreased from 120.3 Ωcm2 for Mg–Zn–Cu alloy to 112.4 & 92.3 Ωcm2 for 2.5 & 3.5 MM containing alloys; respectively. It is worthy to note that Mg–Zn alloys are more corrosion resistant than Mg–Al alloys, Tables 3 & 4. Table 4: Polarization Resistance ( Rp, Ωcm2) of Mg – Zn - X Alloys in 3.5 % NaCl Solution Alloy Mg – 6Zn – 0.5 Mn Mg – 6Zn – 0.5 Mn – 2.5 MM Mg – 6Zn – 0.5 Mn – 3.5 MM Mg – 6Zn – 0.5 Mn – 1 Cu Mg – 6Zn – 0.5 Mn – 2 Cu Mg – 6Zn – 0.5 Mn – 3 Cu Mg – 6Zn – 0.5 Mn – 4 Cu Mg – 6Zn – 0.5 Mn – 3 Cu – 2.5 MM Mg – 6Zn – 0.5 Mn – 3 Cu – 3.5 MM

Polarization Resistance (Rp, Ωcm2) 140.6 150.2 145.3 155.7 160.2 120.3 98.5 112.4 92.3

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Ibrahim M. Ghayad, Nabil N. Girgis & Ahmed N. Abdul-Azim

Effect of Heat Treatment on the Corrosion Resistance of AZ91-Base and ZM60â&#x20AC;&#x201C;Base Alloys The results of polarization resistance for AZ91-base and ZM60-base alloys in the cast and T6 condition in 3.5% NaCl solution are shown in Table 5. It can be seen that the polarization resistance increases with heat treatment, T6 condition, for all the specimens of the two groups of alloys. Heat treatment can dramatically alter the size, amount and distribution of the precipitated phases which inturn alters the corrosion behavior of magnesiumâ&#x20AC;&#x201C;based alloys. It was also found that a solution and aging treatment, T6 condition, for these two groups of magnesium alloys have a great impact on corrosion behavior due to the modification of the microstructure in the presence of different elements. Artificial aging at 200oC causes precipitation of either Mg17Al12 or MgZn2 phases along the grain boundaries of either AZ91-base or ZM60base alloys. When the aging time increased, 17 hrs, these precipitates were found to increase in amounts and grow through the grains discontinuously. The longer the aging time the longer the amount of the precipitates formed in the structure of


Effect of Some Alloying Elements and Heat Treatment on the Corrosion Behavior of AZ91 and ZM60 Magnesium Alloys

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these alloys. The presence of different alloying additions had a dramatic influence on the nucleation of the precipitating phases resulting in a much refined and more homogeneous distribution of the precipitates in the microstructure when compared with AZ91 or ZM60 alone. These precipitates have a high resistance to corrosion, so it may play the role of reducing the corrosion rate by forming a barrier of the precipitated phases once the less noble phase is dissolved [25]. Table 5: Polarization Resistances (Rp, Ωcm2) of Mg – Al – X and Mg – Zn - X Alloys at Different Conditions in 3.5 % NaCl Solution Alloy AZ91 AZ91 - 0.2 Ca AZ91 - 0.4 Ca AZ91 - 0.6 Ca AZ91 – 0.4 Ca – 0.4 Sr AZ91 – 0.4 Ca – 0.14 Sr – 1.2 MM AZ91 – 1.2 MM Mg – 6Zn – 0.5 Mn Mg – 6Zn – 0.5 Mn – 2.5 MM Mg – 6Zn – 0.5 Mn – 3.5 MM Mg – 6Zn – 0.5 Mn – 1 Cu Mg – 6Zn – 0.5 Mn –2 Cu Mg – 6Zn – 0.5 Mn – 3 Cu Mg – 6Zn – 0.5 Mn – 4 Cu Mg – 6Zn – 0.5 Mn – 3 Cu – 2.5 MM Mg – 6Zn – 0.5 Mn – 3 Cu – 3.5 MM

Rp, Ωcm2 (Cast) 17.8 18.9 22.7 27.3 45.2 68.2 28.2 140.6 150.2 145.3 155.7 160.2 120.3 98.5 112.4 92.3

Rp, Ωcm2 (After T6) 58.92 71.88 67.33 72.18 97.32 110.4 63.65 220.2 280.6 230.2 291.4 305.2 198.6 145.4 175.3 161.6

CONCLUSIONS •

The addition of calcium, strontium and rare earth elements in different amounts singly or in combinations improved corrosion resistance of AZ91 alloy due to significant grain refinement. The highest resistance was obtained for the alloy AZ91–0.4 Ca–0.14 Sr–1.2 MM.

Rare earth elements additions improved the corrosion resistance of ZM60 alloys due to grain refinement. But increasing MM from 2.5 to 3.5 wt. % decreased the corrosion resistance due to the formation of massive grain boundary phase containing zinc and rare earth element.

Copper enhances corrosion resistance of ZM60 alloys till 2 wt% but further increase of copper had a detrimental effect on resistance due to galvanic action. Rare earths impaired corrosion resistance of ZM60 alloys containing copper.

ZM60 alloys are more corrosion resistant than AZ91 alloys in 3.5 % NaCl solution.

Heat treatment, T6 , improved corrosion resistance of both magnesium alloy groups due to altering the size, amount and distribution of the precipitated phases.

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