corrosion resistance of aged die cast magnesium alloy...

Materials Science and Engineering A366 (2004) 74–86

Corrosion resistance of aged die cast magnesium alloy AZ91D

Guangling Song∗, Amanda L. Bowles, David H. StJohn

CRC for Cast Metals Manufacturing (CAST), Division of Materials Engineering, School of Engineering,The University of Queensland, Brisbane, Qld 4072, Australia

Received 14 April 2003; received in revised form 25 August 2003

Abstract

The corrosion behaviour of die cast magnesium alloy AZ91D aged at 160◦C was investigated. The corrosion rate of the alloy decreaseswith ageing time in the initial stages and then increases again at ageing times greater than 45 h. The dependence of the corrosion rate onageing time can be related to the changes in microstructure and local composition during ageing. Precipitation of the� phase (Mg17Al12)occurs exclusively along the grain boundaries during ageing. The� phase acts as a barrier, resulting in a decreasing corrosion rate in the initialstages of ageing. In the later stages, the decreasing aluminium content of� grains makes the� matrix more active, causing an increase in thecorrosion rate. Electrochemical testing results also confirm the combined effects of the changes in� and� phases on the corrosion resistanceof the aged die cast AZ91D alloy.© 2003 Elsevier B.V. All rights reserved.

Keywords:Magnesium; Corrosion; Microstructure; Heat treatment

1. Introduction

The use of magnesium alloys in the automotive industryis primarily driven by energy and environmental concerns.Magnesium alloys have a high strength-to-weight ratio, andthus appear to be promising alternatives to aluminium andsteel alloys used in the automotive industry. As automotivecomponents, magnesium alloys are likely to be exposed tomoderate temperatures in the range of 60–200◦C [1]. Ex-posure to temperatures in this range results in precipitationof the� phase exclusively around the grain boundary areas[2,3]. These changes in the microstructure occur concomi-tantly with changes in the mechanical properties. However,the magnitude of the changes in the mechanical propertiesis dependent on the original, as-cast microstructure[2,3].

The effect of microstructure on the corrosion resistanceof magnesium alloys has been widely reported[4–20]. Par-ticularly, the role of the� phase in corrosion is extensivelyaddressed for AZ91, and it is generally accepted that the� phase is a corrosion barrier and its presence in an AZalloy is beneficial to the corrosion resistance of the alloy[4–6,8,11,13]. Hence, solution-heat-treated and aged AZ91,

∗ Corresponding author. Tel.:+61-7-33-65-41-97;fax: +61-7-33-65-38-88.

E-mail address:[email protected] (G. Song).

which has∼18 wt.% � phase (equilibrium), is more cor-rosion resistant than as-cast or solution heat-treated alloys.Song and Atrens[16], and Song et al.[18] proposed thatthe� phase in an AZ alloy can act as either a galvanic cath-ode or a barrier to corrosion, depending on the amount anddistribution of the� phase. In die cast AZ91, the� precipi-tates are finely and continuously distributed along the grainboundaries in the surface region of the casting, so the barriereffect of the� phase is more effective in impeding corro-sion. This explains why the surface of a die cast AZ91 ismore corrosion resistant than its interior.

The above findings suggest that the� phase is criticalto corrosion resistance and any changes in the amount anddistribution of the� phase in an AZ alloy can result in adifferent corrosion resistance. Since long-term exposure tomoderate temperatures results in a change in microstructureof magnesium alloys[2,3,7,21–24], it is of interest then toknow whether the corrosion resistance significantly changesduring exposure to moderate temperatures.

There is very limited work on the effect of ageing oncorrosion resistance of die cast AZ91. The only publishedresults are by Suman[7], who showed that ageing at tem-peratures up to 230◦C for 36 h had little effect on corrosionresistance. However, ageing at a temperature above 230◦Csignificantly reduced the corrosion resistance of die castAZ91 under a salt spraying condition. However, the change

0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2003.08.060

G. Song et al. / Materials Science and Engineering A366 (2004) 74–86 75

in corrosion resistance with ageing was not explained in thatstudy; the correlation between the change in microstructureof die cast AZ91 and the increase in corrosion rate has notbeen illustrated. So far, there is no relevant published dataon the effect of the long-term moderate temperature ageingon corrosion resistance of die cast AZ91 for times correlat-ing to actual service conditions. From a practical view point,it is of significance to know the effect of long-term moder-ate temperature ageing on the corrosion resistance. Further-more, it is hoped that an investigation of the mechanismsrelating to the change in corrosion performance of die castAZ91 during ageing will lead to a better understanding ofthe correlation between microstructure and corrosion resis-tance in die cast AZ alloys.

2. Experimental

2.1. Specimens

Die cast AZ91D and solution-heat-treated permanentmould cast binary Mg–Al alloys were used in this study.

Magnesium alloy AZ91D was cold chamber high pres-sure die cast on a 250 tonne Toshiba machine into tensilespecimens with a gauge length of 50 mm and a rectangularcross-section, 10 mm in width and 5 mm in thickness. Spec-imens were aged in air from the as-cast condition at 160◦Cfor periods up to 585 h. The oxidation of the specimen sur-face was not significant. The surfaces became only slightlydarker after the ageing.

Four Mg–Al alloys with aluminium contents: 2.00, 3.89,5.78 and 8.95 wt.% Al were permanent mould cast. Thespecimens were solution heat treated in an argon envi-ronment at 413◦C for 24 h and quenched in water. Theas-quenched microstructures of these alloys are single�phase.

The chemical compositions of the die cast and permanentmould cast alloys were determined by ICP–AES analysis andare given inTable 1. The castings have similar iron impuritylevels, and the copper and nickel impurities in all the alloysare below the instrument detectable levels. Except for thealuminium content, the differences in composition betweenthe die cast AZ91D and permanent mould cast Mg–Al alloysare zinc and manganese.

The aged die cast tensile specimens and the solutiontreated Mg–Al alloys were cut into small coupons. The high

Table 1Chemical compositions of alloys

Al(wt.%)

Zn(wt.%)

Mn(wt.%)

Fe(wt.%)

Cu(wt.%)

Ni(wt.%)

Die cast AZ91D 9.05 0.797 0.19 0.01 <0.002 <0.0022.00 wt.% Al 2.00 <0.005 0.01 0.01 <0.005 <0.0023.89 wt.% Al 3.89 <0.005 0.01 0.01 <0.002 <0.0015.78 wt.% Al 5.78 <0.005 0.01 0.01 <0.002 <0.0018.95 wt.% Al 8.95 <0.005 0.01 0.01 <0.002 <0.001

pressure die casting specimens were approximately 1 cm×1 cm× 0.5 cm and the binary Mg–Al alloys were approxi-mately 2 cm× 1 cm× 0.5 cm, respectively, in size. Some ofthe coupons were welded with electrical wire and embeddedin epoxy resin as electrodes for electrochemical testing, theothers were used in immersion and salt spraying tests.

2.2. Solutions

Five weight percent NaCl salt solution was used in saltspray and immersion tests. The solution was prepared withAR grade NaCl and demineralised water. For electrochem-ical measurements, the salt solution was saturated withMg(OH)2 which had a stable pH value of around 11 toassure reproducible electrochemical results. The Mg(OH)2saturated solution also to some extent represents the thinliquid layer adjacent to the AZ91D surface in NaCl solu-tion. As magnesium dissolves quickly, so it is quite easy tobuild up a local high pH value at its surface.

2.3. Weight loss measurement

The specimens were initially cleaned with acetone anddemineralised water. After air drying, the specimens wereweighed (original weight,w0) before corrosion testing. Af-ter salt spraying or immersion tests, the corroded specimenswere cleaned with distilled water and dried. They were thenimmersed in a chromate acid (200 g/l CrO3+10 g/l AgNO3)at ambient temperature for 5–10 min to remove the corrosionproducts. It has been repeatedly demonstrated in the labora-tory by using not corroded specimens in the acid under thesame conditions that the chromate acid can cause almost noweight loss to a not corroded AZ alloy specimen, i.e., thechromate acid can remove the corrosion products on AZ al-loy without etching its metallic substrate or non-corrosion ar-eas. The specimens were then quickly washed with distilledwater and dried again. The specimens were then weighedfor the final weight (w1). The difference betweenw0 andw1 is the corrosion weight loss (�w).

2.4. Immersion test

Several coupon specimens were polished with emery pa-per and cleaned with demineralised water. The coupons werethen submerged in 450 ml of the salt solution by hangingthe coupons in 500 ml beakers. The tests were conducted at25◦C.

2.5. Salt spraying

Several polished and cleaned specimens were placed in asalt spray chamber (Vostsch VSC450) where salt sprayingwas conducted according to ASTM B117. A special PVCholder was made to support the specimens in the chamber,to ensure the position of the specimens was as per ASTMB117.

76 G. Song et al. / Materials Science and Engineering A366 (2004) 74–86

2.6. Polarisation and electrochemical impedance spectrum

Polarisation curves and electrochemical impedance spec-tra of the specimens were measured in an electrolytic cellcontaining about 500 ml of Mg(OH)2 saturated 5 wt.% NaClsolution using a Solatron 1287+ 1255B electrochemicalmeasurement system. The polarisation started from a ca-thodic potential of about−200 mV relative to the corrosionpotential and stopped at an anodic potential 50 mV positiveto the corrosion potential. The scanning rate was 10 mV/min.Normally, for a metallic electrode, most electrochemical in-formation on its active corrosion processes can be obtainedin a potential range from−150 to+120 mV relative to thecorrosion potential. However, the magnesium electrode wastoo active in the solution, so cathodic polarisation had to becarried out immediately after the electrode was immersed into the solution. Considering some transient processes mayoccur after a polarisation potential was applied to the elec-trode, which could affect the readings of steady polarisationcurrents, the polarisation curve measured in the first few min-utes should better be discarded. Hence, in the experiments,the cathodic polarisation actually started from−200 mV rel-ative to the corrosion potential. The polarisation curves inthe range from−200 to −150 mV were discarded. In theanodic region, the polarisation current always increased dra-matically due to the “pitting” corrosion. At+50 mV relativeto the corrosion potential, severe localised corrosion damagealready became very evident. There was no need for furtheranodically polarising the electrode.

AC impedance measurements were conducted at the cor-rosion potentials of the electrodes. The amplitude of appliedAC signal was 5 mV, and the measured frequency range wasfrom 1 mHz to 1 kHz.

During the polarisation and AC impedance measurements,stationary electrodes were used, because oxygen diffusionin solution has no significant influence on the corrosion pro-cess[8]. The experimental temperature was 25◦C. All thepotentials referred to in this paper are relative to the sil-ver/saturated silver chloride electrode.

2.7. SEM

Carbon coated specimens were examined in a PhilipsXL30 SEM.

2.8. Quantification of theβ phase fraction and aluminiumcontent in theα matrix

Nuclear magnetic resonance (NMR) was used to deter-mine the fraction of� phase and to measure the amount ofaluminium in solid solution during ageing. NMR analysisproduces a spectrum for a specific atomic species, wherethe particular atomic environment of the species governs thepositions of the peaks. NMR can be used to identify thephases which contain a particular atomic species. The inten-sity (area) of an NMR peak is also proportional to the num-

ber of the type of nuclei that is being detected in the sam-ple. It is possible then to determine the relative proportionof different phases in a specimen. NMR was used to mea-sure the ratio of aluminium in solid solution to that in the�phase, allowing calculation of the mass fraction of precipi-tate [25,26] and the average amount of aluminium in solidsolution. The sample preparation for NMR involved filingapproximately 300 mg from the specimens under alcohol (toprevent heating). NMR spectroscopy was carried out on aBruker MSL 400, with a magnetic field of 9.4 T, operatingat a frequency of 104.32 MHz to excite27Al. A comprehen-sive review of NMR spectroscopy and in particular the useof NMR to study the ageing characteristics of Mg–Al alloysis given by Bastow and co-worker[27] Bastow and Smith[28].

3. Results and discussion

3.1. Corrosion of aged die cast AZ91D

Figs. 1 and 2show the effect of ageing on the corro-sion resistance of die cast AZ91D under immersion and saltspraying conditions. The dependence of the corrosion rateon ageing time under these two corrosion conditions is sim-ilar. In both cases, the corrosion rate of die cast AZ91D de-creases with ageing time up to approximately 45 h, then be-gins to increase with ageing time. The minimum corrosionrate occurred between 15 and 45 h.

A minimum in the corrosion rate during ageing appearsto be contradictory to Suman’s[7] conclusion that exposureof die cast AZ91 to temperatures up to 230◦C for severalhours could be tolerated without a significant effect on thecorrosion resistance. In fact, Suman might have misinter-preted his results. When plotting the corrosion rate of die castAZ91 aged at 177◦C versus ageing time, Suman assumed alinear relationship. The distribution of the data points givenby Suman is more likely to be a curve with a minimum be-tween 10 and 30 h. The minimum corrosion rate observedin this work between 14 and 45 h is in effect consistent withthe reported results[7].

The change in corrosion resistance with ageing time cor-responds to the change in yield stress for die cast AZ91Dunder the same ageing conditions[2]. The most interestingfinding is that the minimum corrosion rate appears at al-most the same ageing time as the maximum yield stress[2].Bowles [2] and Bowles et al.[3] have investigated the ef-fect of ageing on the mechanical properties of the die castAZ91D and found that, in general the yield stress increasesand the ductility decreases with ageing time. These changesin the mechanical properties can be related to the precipi-tation of the� phase in three morphologies (eutectic, dis-continuous, and rod shaped� phase). The striking similaritybetween the curves of yield stress versus ageing time andthe corrosion rate versus ageing time strongly suggests thatthe change in corrosion resistance is closely associated with


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Fig. 1. Average weight loss rate of die cast AZ91D aged at 160◦C after immersion in 5 wt.% NaCl solution for 7 days.

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Fig. 2. Average weight loss rate of die cast AZ91D aged at 160◦C after exposure to salt spraying for 8 days.


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Fig. 3. Relationship between corrosion rate and yield stress of AZ91Daged at 160◦C (the ageing hours are specified in the figure).

the change in microstructure that also determines the yieldstress. To illustrate the relationship between corrosion rateand yield strength, the corrosion rates of the die cast AZ91Daged at 160◦C measured in this paper (Figs. 1 and 2) areplotted against the yield strengths measured by Bowles[2]and shown inFig. 3. It is clearly shown that there is a rela-tionship between corrosion rate and yield strength. This re-lationship is such that, for the casting and ageing conditionsused in this study, yield strength could be a prediction ofrelative corrosion resistance. It would be of interest to test abroader range of casting and ageing conditions to determinewhether this general relationship holds in all cases. The im-plication is that both yield strength and corrosion resistanceare controlled by the same microstructural factors.

3.2. Effect of microstructure

The changes in microstructure of die cast AZ91D by age-ing treatment are displayed inFig. 4. In the as-cast condi-tion the microstructure consists of primary� grains with thegrain boundaries decorated by large� phase particles. Thereis significant solute segregation in the� grains. The alu-minium content increases towards the grain boundaries asa result of coring during solidification. The increasing alu-minium content towards the edges of the� grains is seen asan increasing brightness in the SEM images. During ageing,precipitation of the� phase occurs. In the initial stages ofageing the volume fraction of� phase increases rapidly (see15 h,Fig. 5). The� precipitates appear in two morphologies(see the enlarged typical microstructure inFig. 4), lamellarplates (as discontinuous precipitation) and small rod shapedprecipitates (likely to be continuous precipitation of the�phase[29,30]). The precipitation reactions occur exclusivelyin the grain boundary areas, so that there is an almost con-tinuous network of� phase along the grain boundaries. Withextended ageing, the precipitation reactions slow down asthe matrix becomes depleted of aluminium, and the reaction(rod shaped precipitates) moves towards the grain interiors[2]. As the continuous precipitation reaction moves furtherfrom the grain boundaries the continuity of� phase networkalong the grain boundaries does not increase.

From examination of the microstructure, the decrease inthe corrosion rate in the first 45 h of exposure can be ex-plained. It has been demonstrated[4–6,11,13]that the cor-rosion resistance of AZ alloys increases as the amount of�phase increases by solution heat treatment then ageing treat-ment (T6), because the� phase acts as a barrier. The mostconvincing experimental evidence for the barrier effect of�phase was first given by Lunder et al.[4]. They comparedthe corrosion rates of as-cast, solution heat treated (T4), andsolution-heat-treated then aged (T6) AZ91 specimens, andrelated the corrosion resistance to the amount of� phase.Song et al.’s dual role mechanism[16,18] further specifiedthe importance of the continuity of the� phase, i.e. the bar-rier effect only dominates the corrosion process when the�precipitates are finely distributed, building up a certain de-gree of continuity in the barrier. FromFigs. 4 and 5both theamount and the continuity of the� phase along the grainboundaries increases with ageing time; the corrosion rateshould therefore decrease with ageing time according to ei-ther the barrier effect or dual role mechanisms.

The micro-morphologies of corroded specimens areshown inFig. 6, directly verifying the barrier effect of the� phase. It can be seen that corrosion mainly occurs in the� matrix. The� precipitates, including eutectic and discon-tinuous, are stable in corrosion, and corrosion is confinedby the� precipitates.

However, the barrier effect of the� phase can only ex-plain the decrease in corrosion rate with ageing time. Theincreasing corrosion rate with ageing times longer than 45 hcan not be ascribed to the� phase alone. According to thebarrier effect or the dual mechanism model, at long ageingtimes a larger amount of� phase will be present in the mi-crostructure and the continuity of the� barrier does not de-crease along the grain boundaries. As a result, the corrosionrate should not increase with ageing time.

3.3. Influence of theα matrix

The chemical composition of the matrix is as critical as thephase constituents to the corrosion performance of an alloy.For an AZ alloy, the chemical composition of the� phase isalmost constant,∼44 wt.% aluminium below 120◦C [31].However, the composition of the� matrix changes widelywith temperature. It has been reported[9] that the aluminiumcontent of� phase can vary from 1.5 wt.% aluminium in thegrain centre to about 12 wt.% aluminium in the vicinity ofthe� phase[9,32]. In this study, it was found that the averagealuminium content of the� matrix dramatically decreasedfrom about 6.5 to 2 wt.% aluminium in the first 45 h of age-ing, and then slowly decreased with extended ageing downto about 1.5 wt.% aluminium after 585 h (seeFig. 5). It canbe expected that in the grain interior the aluminium contentcould be as low as 1.5 wt.% and some regions along the grainboundaries may retain a relatively high super-saturation ofaluminium even after long term ageing. The decrease in theaverage aluminium concentration in the� matrix does not


Fig. 4. Influence of the ageing at 160◦C on the microstructure of die cast AZ91D. A high magnification image is shown in the bottom right corner. Thewhite arrow indicates a region of discontinuous precipitation and the area marked “A” shows rod shaped precipitates.

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Fig. 5. Volume fraction of� phase in aged die cast AZ91D and the average aluminium concentration in� matrix.


Fig. 6. Micro-morphologies of die cast AZ91D specimens after immersion in 5 wt.% NaCl solution for 4 h. In photo (c), the white tiny dots in thecorroded areas are the rod shaped continuous� precipitates.

necessarily mean that the aluminium content decreases allover the grain. It could indeed indicate a reduction of alu-minium content in the areas rich in aluminium along thegrain boundaries.

The beneficial effect of aluminium in the� phase hasbeen postulated in several studies, and has been success-fully employed in interpreting the phenomenon that corro-sion stops at the grain boundary before reaching the� phasein some cases[4,10,20]. However, so far corrosion testingresults that directly support this postulation are not suffi-cient. In this study, corrosion rates of several single� phasealloys with various aluminium contents were measured af-

ter salt immersion testing to verify this postulation. The re-sults are shown inFig. 7. It can be seen that the corrosionresistance of� phase decreases as the aluminium level inthe� phase decreases, experimentally confirming the abovepostulation.

Since a decrease in aluminium content can lead to anincreased corrosion rate in the� matrix, it is expected that thedecreasing average aluminium concentration in the� matrixduring ageing would result in a decrease in the corrosionresistance of die cast AZ91. This then may be responsiblefor the increasing corrosion rate of the alloy after 45 h ofageing.


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Fig. 7. Average weight loss rates of Mg–Al single phase alloys with various aluminium contents after immersion in 5 wt.% NaCl solution for 3 h.

3.4. Combined influence of the changes inα andβ phaseson the corrosion rate

There are two contradictory tendencies in the corrosionresistance caused by ageing. First, the corrosion rate de-creases due to the precipitation of the� phase along thegrain boundaries. The second tendency is the increasing cor-rosion rate resulting from the decreased aluminium contentin the� matrix. The overall change in the corrosion rate ofthe alloy depends on the combined influences of the changesin the aluminium content in the� phase and the proportionof grain boundary� phase. The microstructure of the as-diecast AZ91 consists predominantly of eutectic intergranular� phase, and a small amount of discontinuous� along thegrain boundaries (seeFig. 5(as-cast)). The grain boundarieshave a high level of aluminium in solid solution (12 wt.%)[9], and in general have a higher corrosion resistance thanthe lower aluminium grain interiors[4,10,20]. In as-die castAZ91, the� phase mainly acts as a barrier to corrosion[18].Therefore, the corrosion micro-morphology of as-die castAZ91 in the corroded areas is confined by the eutectic�phase and high aluminium regions of the� matrix. Corro-sion stops in the vicinity of the� phase, a few micrometersbefore the intergranular� phase.Fig. 6(a)shows a typicalmorphology.

In the early stage of ageing, precipitation of discontinu-ous� phase occurs in the high aluminium regions of the�matrix—along the grain boundaries. The original high alu-

minium grain boundaries become covered by� phase pre-cipitates and the amount of aluminium in solid solution inthese areas decreases dramatically (as a result of the pre-cipitation reactions). More importantly, the� precipitatesform an almost continuous� phase barrier along the grainboundaries, which can, to some extent, stop the develop-ment of corrosion[17]. Hence, the replacement of the highaluminium areas in the� phase by� precipitates will leadto a decrease in corrodible surface and an increase in thecontinuity (effective length) of the corrosion barrier.

The discontinuous (lamellar)� phase (separated by thediscontinuous lamellar� precipitates) may have a lower alu-minium content and could be less corrosion resistant thanthe original high aluminium� phase. Yet, the discontinuous� phase is so close to the discontinuous� precipitates thatcorrosion could be quickly stopped by the adjacent discon-tinuous� precipitates.

Therefore, in the early stages of ageing, the beneficial ef-fect of the increased fraction of� precipitates suppressesthe detrimental effect of the decreased aluminium content inthe � matrix. The overall effect of turning high aluminium� phase into discontinuous� precipitates is beneficial tothe overall corrosion resistance of the alloy. The corrosionmorphology shown inFig. 6(b)is evidence of the combinedeffects of the changes in� and� phases. The corroded re-gions are confined by the� phase (including the discon-tinuous precipitates). In the corroded regions, there is littlediscontinuous� phase remaining. This is slightly different


from the morphology for the as-die cast alloy as shown inFig. 6(a), in which corrosion stops at the high aluminiumregions before reaching the� phase.

As ageing progresses, a significant fraction of rod shaped� phase precipitates appear. The rod shaped precipitates oc-cur adjacent to the grain boundary discontinuous precipi-tates (seeFig. 4); previous work has shown that the rodshaped precipitates occur after the initiation of discontinu-ous precipitation at the grain boundaries a lower tempera-tures (100–140◦C) [2]. The grain boundary area has an ini-tially high aluminium content. This and the presence of thegrain boundary favours the formation of discontinuous pre-cipitates. The rod shaped� occurs further out in the grain(although this reaction is still confined to the high aluminiumat the exterior of the grains)[2,3]. The precipitation of therod shaped� phase away from the grain boundaries doesnot build up an effective barrier to corrosion. Therefore, ex-tended ageing does not reinforce the beneficial barrier effectof the grain boundary� precipitation. In contrast, the pre-cipitation of the rod shaped� phase further reduces the alu-minium content of the� matrix, which leads to an increasedcorrosion rate of the� matrix. Fig. 6(c) shows a corrodedarea of a specimen aged for 585 h. In this area, all the�phase was corroded and some rod shaped� precipitates areleft. This suggests that in this area the rod shaped� precipi-tates cannot stop corrosion, and the surrounding� matrix iseasily corroded. The detrimental influence of the reductionof aluminium content in the� matrix on corrosion counter-acts the beneficial influence of the increase in� precipitates.The aluminium content in the� matrix mainly governs theoverall change in corrosion rate at this stage of ageing.

3.5. Electrochemical investigations

Corrosion damage results from electrochemical reactions,so electrochemical measurements can often reveal the cor-rosion mechanism. The electrochemical impedance spec-troscopy (EIS) is a useful technique in the study of corrosion.The corrosion mechanism of magnesium sometimes can beestimated through analysing the measured electrochemicalimpedance spectrum[17,33]. The diameter of the capacitivesemicircle of a measured Nyquist spectrum is closely relatedto the corrosion rate. Makar et al.[34] compared corrosionrates calculated from the first capacitive semicircle in thehigh frequency region with weight loss results for variousmagnesium alloys, and found that most of the EIS resultsmatched very well with the weight loss data.

In this study, the EIS results of Mg–Al single,� phasealloys were measured and are presented inFig. 8. All the al-loys have inductive loops in the low frequency region. TheirEIS spectra are similar except for the difference in the diam-eters of the loops. This means that the corrosion mechanismsof these single-phase alloys are the same, but their corro-sion rates could be different. If the first capacitive semicir-cles are taken as the impedance representing the corrosionresistance of the single� phase alloys, then the dependence

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Fig. 8. Electrochemical impedance spectra of Mg–Al single� phase alloysin 5 wt.% NaCl saturated with Mg(OH)2.

of the corrosion resistance on the aluminium content in the� phase is consistent with the weight loss results (Fig. 7),i.e. corrosion resistance increases as the aluminium contentin � matrix increases.

Fig. 9 displays the EIS spectra of die cast AZ91D. Theas-die cast AZ91D has a capacitive semicircle in the highfrequency region and some inductive data points in the lowfrequency region. The inductive points could be associatedwith the corrosion of the� matrix as shown inFig. 8. Thechanges in the spectrum caused by ageing can be sum-marised by the following two aspects. First, the inductivepoints change into capacitive points as the ageing time in-creases. Second, the diameter of the capacitive semicircle inthe high frequency region increases first, then decreases withageing time (the spectrum for 150 h slightly deviates fromthe trend). The maximum appears at an ageing time of 15 h.

The change from inductive into capacitive points indicatesthat the dominating process or step involved in corrosionmight have changed during ageing. A possible explanationfor this change is that ageing has led to more� precipitates,so the electrochemical signals from� phase overwhelms the� phase information. Inert� phase normally shows a capac-itive spectrum[17]. The change in the EIS characteristicsfurther verifies the influence of the precipitation of� phaseduring ageing on corrosion.

The minimum corrosion rate appearing at a certain ageingtime is further verified by the fact that the changing tendencyof corrosion resistance as represented by the diameters ofthe capacitive loops (seeFig. 9) is almost the same as thatof the weight loss rate (seeFigs. 1 and 2).

The polarisation behaviours of the Mg–Al single� phasealloys and aged die cast AZ91D are shown inFigs. 10 and11, respectively. The apparent changes in the curves causedby decreasing the aluminium content in the single� phasealloys and by ageing AZ91D are to shift the polarisationcurves to more negative potentials. In fact, the shifts resultfrom the different “pitting” potentials (Ept) of these alloys.It has been demonstrated that[33] AZ alloys normally startlocalised corrosion (pitting) at a potential slightly more


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Fig. 9. Electrochemical impedance spectra of die cast AZ91D aged at 160◦C for various periods of time in 5 wt.% NaCl saturated with Mg(OH)2.

negative than their corrosion potentials in a salt solution.On a polarisation curve obtained by the potentiodynamicmethod, and starting potential scanning from a cathodicpotential, the “pitting” potential Ept is usually indicatedby a sudden drop in cathodic current which is immediately

Fig. 10. Polarisation curves of Mg–Al single� phase alloys in 5 wt.% NaCl saturated with Mg(OH)2.

followed by anodic current around the corrosion potential.At the “pitting” potential, the initiation of localised corro-sion is accompanied by dramatic visible hydrogen evolutionfrom the corroding sites on the electrode surface during po-larisation curve measurement. InFig. 10, the Ept is marked


Fig. 11. Polarisation curves of die cast AZ91D aged at 160◦C for various periods of time in 5 wt.% NaCl saturated with Mg(OH)2.

on each polarisation curve. The Ept is an important electro-chemical parameter. It indicates the tendency to localisedcorrosion. A more positive Ept means less likely localisedcorrosion.

Figs. 12 and 13show the dependence of Ept on alu-minium content in the� phase and on ageing time for diecast AZ91D. The� phase, with a higher aluminium contenthas a more positive Ept (Fig. 12). This means that the�phase with a high aluminium content is more stable than

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-1.51

-1.5

0 2 4 6 8 10Aluminium concentration of alloy (wt.%)

Ep

t (V

)

Fig. 12. Dependence of “pitting” potential (Ept) on aluminium content inMg–Al single � phase alloys.

� with a low aluminium content, which is consistent withthe weight loss results (Fig. 7). The decrease in corrosionrate could be due to the strong passivating capacity of alu-minium [6]. For die cast AZ91D, the shift of Ept to a morenegative potential by ageing (Fig. 13) could be attributedto the decrease in aluminium content in� matrix (Fig. 5).The corrosion of AZ91 is normally initiated from a lowaluminium region in the� matrix. Therefore, the initiationof corrosion in die cast AZ91 is actually determined by the

-1.55

-1.54

-1.53

-1.52

-1.51

0 200 400 600 800

Ageing time (h)

Ep

t (V

)

Fig. 13. Dependence of “pitting” potential (Ept) on ageing time for diecast AZ91D aged at 160◦C.


corrosion resistance of the� phase, particularly the lowaluminium areas. Ageing results in a decrease in the aver-age aluminium concentration in the� matrix (Fig. 5), andit will certainly lead to a decrease in the Ept (Fig. 12).

In as-die cast AZ91D, the original distribution of alu-minium in the� matrix is non-uniform[2,9], and in the den-drite cores, the aluminium level could be as low as 1.5 wt.%,even lower than that in the 2 wt.% aluminium alloy. The pre-cipitation of � phase reduces the average aluminium con-centration in the� matrix, and hence reduces the areas richin aluminium and further increases the low aluminium ar-eas where corrosion is likely to initiate. Since the Ept is de-termined by the stability of these areas and the aluminiumcontent in these areas could be even lower than that in the2 wt.% aluminium alloy, the Ept for aged die cast AZ91D ingeneral should be more negative than Mg–Al single phasealloy (seeFigs. 12 and 13).

4. Summary

Moderate temperature ageing of die cast AZ91D leads toprecipitation of� phase in the aluminium rich areas of the� matrix. The volume fraction of� phase increases and theconcentration of aluminium in the� matrix decreases. The� phase precipitates along the grain boundaries and acts asa barrier retarding the development of corrosion in the�matrix. As a result of the precipitation reactions the corro-sion rate of the aged die cast AZ91 decreases with ageingtime in the early stages. During extended ageing, precipi-tation of rod shaped� phase further away from the grainboundaries does not effectively increase the barrier effect. Incontrast, the� matrix is activated because of the decrease inaluminium content resulting from� precipitation. This wasverified by measuring the corrosion rate of single� phase al-loys where it was found that the corrosion rate increased sig-nificantly as the aluminium content decreased. Hence, in thelow aluminium regions of the� matrix of die cast AZ91D,initiation of corrosion becomes easier and corrosion devel-ops more quickly. Therefore, the corrosion rate of the ageddie cast AZ91 increases again with extended ageing.

An interesting observation was made in this study whereit was found that there is a direct relationship between yieldstrength and corrosion rate for the die cast AZ91D. The yieldstress first increases and then decreases with ageing. Thisrelationship implies that the yield strength and the corrosionrate of AZ91D die cast specimens are both controlled by thesame microstructural factors.

Acknowledgements

The study was supported by the CRC for Cast MetalsManufacturing (CAST). CAST was established under andis supported by the Australian Government’s CooperativeResearch Centres Program (CRC). It is also acknowledgedthat Mr. David Forrestal and Dr. Sarath Hapugoda partici-

pated in the performance of salt spraying, immersion andelectrochemical tests.

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