Plug-In Specimens for Measurement of the Corrosion Rateof Mg Alloys

ZHIMING SHI,1 ARVIND PRASAD,1 and ANDREJ ATRENS1,2

1.—Division of Materials, The University of Queensland, Brisbane, QLD 4072, Australia.2.—e-mail: Andrejs.atrens@uq.edu.au

Magnesium alloy corrosion is often nonlinear. The corrosion rate acceleratesto steady state after an initial period of low corrosion. Plug-in specimenspermit simultaneous measurement of the corrosion rate using hydrogen evo-lution, PH, and Tafel extrapolation of cathodic polarization curves, Pi. More-over, weight loss allows independent verification. PH is consistently greaterthan Pi. The data, for short exposure periods up to 10 days, are consistent withthe unipositive Mg+ ion being a short-lived intermediate. Tafel extrapolationneeds to be used with caution for estimation of Mg corrosion, as the measuredcorrosion rate can have a significant contribution from crevice corrosion.Furthermore, measurements made at short immersion times may not reflectthe steady-state corrosion rate, and the corrosion reaction at the Mg surfacemay be decoupled from the electrochemical measurement.

INTRODUCTION

Corrosion limits more widespread use of Mgalloys in aircraft and automobiles. In contrast,complete corrosion within a finite period allows Mgto be considered for use in biodegradable medicalimplants. Our research on Mg corrosion,1–8 Mgflammability,9 and stress corrosion cracking10,11 hasprovided many key insights. This paper summarizesrecent advances at the University of Queens-land12–15 regarding measurement of the Mg corro-sion rate and mechanism.

The weight loss rate, designated PW (mm year�1),provides a benchmark. This is a simple method, al-though experimental skill is required in the case ofMg, particularly to ensure removal of all corrosionproducts. The evaluated corrosion rate is less thanthe actual corrosion rate if some corrosion productsremain on the surface. Some literature resultsappear to be erroneous due to this effect.12 Our workhas found13 that chromic acid solution removes noMg metal, and removes only the corrosion products.One drawback of this approach is that the weightloss rate is an average over the experiment durationand does not easily allow for measurement of thetime variation of the corrosion rate.

The Mg corrosion mechanism1–8 is summarized inFig. 1. Corrosion mainly occurs at breaks in a partlyprotective surface film. The anodic partial reactionoccurs in two steps, (1) and (2). In the second step,

some fraction, k, of unipositive Mg+ ions undergoeselectrochemical anodic oxidation to the equilibriumMg++ ion. The complement fraction, 1 � k, reactschemically by reaction (3) to produce the stableMg++ ion and H2. The two anodic partial reactions(1) and (2) are balanced by the cathodic partialreaction (4).

Mg!Mgþ þ e anodic partial reactionð Þ (1)

k Mgþ ! k Mgþþ þ ke anodic partial reactionð Þ (2)

1� kð Þ Mgþ þ 1� kð Þ Hþ ! 1� kð ÞMgþþ

þ 1� kð Þ 1=2H2 chemical reactionð Þ ð3Þ

1þ kð Þ Hþ þ 1þ kð Þ e! 1þ kð Þ 1=2H2

cathodic partial reactionð Þ ð4Þ

Equations 1–4 are summed to give the followingoverall reaction:

Mgþ 2Hþ !Mgþþ þH2 (5)

The overall corrosion reaction indicates thatmeasurement of the evolved hydrogen1–8 can be

JOM, Vol. 64, No. 6, 2012

DOI: 10.1007/s11837-012-0335-z� 2012 TMS

(Published online June 3, 2012) 657

used to characterize Mg corrosion (Fig. 2).14 Thismethod provides an instantaneous measurement ofthe corrosion rate, and allows the study of how thecorrosion rate varies with time. The instantaneouscorrosion rate, designated PH (mm year�1), is givenby the slope of a curve such as those of Fig. 2.Figure 2 presents the corrosion behavior (opensymbols) of AZ91 immersed at 37�C in Hank’ssolution, a synthetic body fluid with chloride con-centration similar to that of blood. The pH (fullsymbols) was maintained nearly constant by bub-bling CO2 of 0.9 atm pressure through the solution.This made use of the buffer system between CO2

and the bicarbonate ions in the Hank’s solution.This is the same buffer that maintains blood pH at aconstant value. For AZ91 immersed in Hank’ssolution at 37�C, there was an incubation period oflow corrosion rate followed by an acceleration of thecorrosion rate to a steady-state corrosion rate.

Prior research1–8 has indicated that the hydrogenproduced in the overall corrosion reaction (5) has all(or nearly all) been evolved as hydrogen gas. How-ever, some of the evolved hydrogen may be dissolvedin the Mg metal; for example, Zainal Abidin et al.14

found, for Mg alloys (Mg-2Zn-0.2Mn, ZE41, andAZ91) immersed in Hank’s solution at 37�C, thatthe average corrosion rate evaluated from thehydrogen evolution rate was linearly related to, butsomewhat less than, the corrosion rate measuredfrom weight loss (Fig. 3). This indicated that somehydrogen was dissolved in the Mg specimen ratherthan being evolved as hydrogen gas. The hydrogenproduced by Mg corrosion is produced at the cor-roding (oxide-free) Mg surface, most probably asatomic hydrogen. Two atomic hydrogen atoms cancombine to form a hydrogen molecule, and a numberof hydrogen molecules can combine and be evolvedas a hydrogen gas bubble from the Mg surface.Alternatively, atomic hydrogen can enter the metaland be dissolved in the metal lattice. This is possiblebecause Mg can store significant amounts ofhydrogen. Nogita et al.16 describe a particular alloythat has a reversible storage capacity of 6 wt.%hydrogen and is currently being commercialized forhydrogen storage. The concentration of hydrogendissolved in the Mg metal was estimated14 andfound to be small relative to the solubility of Hin Mg.

The corrosion product, Mg(OH)2, has low solubil-ity. Its precipitation during immersion tests, usingunbuffered solutions, causes the pH to increase to10.5. Similarly, the pH also tends to increase duringthe measurement of a cathodic polarization curve.Mg alloys usually contain second phases. Thesesecond phases cause microgalvanic acceleration ofthe alpha Mg matrix,1–8 so that Mg alloys typicallycorrode faster than high-purity (HP) Mg. High

Fig. 2. Corrosion behavior14 as characterized by hydrogen evolution(open symbols) of AZ91 immersed in Hank’s solution at 37�C. ThepH of the solution is shown by the full symbols. The pH was main-tained nearly constant by bubbling CO2 of 0.9 atm pressure throughthe solution.14

Fig. 3. In Hank’s solution at 37�C, the average corrosion rate eval-uated from the hydrogen evolution rate was linearly related to butsomewhat less than the corrosion rate measured from weight loss.14

This indicated that some hydrogen was dissolved in the Mgspecimen.

Some H dissolves in Mg

H2O

Mg2+ + H2

Mg+

H2

Mg

e

Fig. 1. Schematic of the Mg corrosion mechanism. Adapted fromRef. 2.

Shi, Prasad, and Atrens658

purity for Mg alloys in the corrosion sense means aMg alloy with impurity elements below their toler-ance limits.7 For Mg alloys that are not high purity,the corrosion behavior is dominated by the impuritycontent, regardless of the content of other alloyingelements in the Mg alloy.7

Tafel extrapolation from cathodic polarizationcurves is a standard technique in electrochemistry.However, it was recognized2 in 2003 that the Tafelextrapolation method was problematical when usedto measure the corrosion rate for Mg. Furthermore,the corrosion rate for Mg evaluated by Tafelextrapolation of polarization curves has notagreed12 with the corrosion rate evaluated fromweight loss and hydrogen evolution.

MOUNTED SPECIMENS

Figure 4 illustrates a typical mounted speci-men.13 A piece of the Mg alloy is mounted in met-allurgical resin and polished so that one surface ofabout 10 mm 9 10 mm is exposed to the solution.An electrical connection is made to the specimen viaan insulated copper wire within the metallurgicalmount. The mounted specimen is easy to make. It iseasy to prepare the exposed surface to any desiredspecification. Mounted specimens or equivalent arewidely used in electrochemical testing of Mg.17–34

They can also be used to characterize corrosion byhydrogen evolution and by weight loss.

Shi and Atrens13 measured numerous cathodicpolarization curves for mounted specimens of HPMg immersed in 3.5% NaCl saturated withMg(OH)2. For each cathodic curve, Tafel extrapola-tion was used to estimate the corrosion current

density, icorr, which was converted to a corrosionrate, Pi. The average value was

Pi ¼ 1:38� 0:63 mm year�1: (6)

During the measurement of these cathodicpolarization curves, it was noticed that corrosionoften started at the edge of the specimen (Fig. 4).This indicated that the measurement of the corro-sion behavior was influenced in some way by theexperimental arrangement of the mounted speci-men. To eliminate the influence of the specimenencapsulation, the fishing line specimen wasdeveloped.

FISHING LINE SPECIMENS

Figure 5 illustrates a fishing line specimen.13 Thespecimen is supported within a cradle made fromfishing line. The specimen has little physical contactwith the fishing line, thereby minimizing anyinfluence of the specimen support. The specimen issupported within the corrosion medium, and thearrangement allows characterization of the corro-sion rate of the specimen by hydrogen evolution, PH,and by weight loss, PW. Shi and Atrens13 carried outnumerous measurements with HP Mg immersed in3.5% NaCl saturated with Mg(OH)2. The averagevalue of the corrosion rate evaluated by hydrogenevolution was

PAH ¼ 0:35� 0:20 mm year�1: (7)

Fig. 4. A typical mounted specimen.13 HP Mg after immersion in3.5% NaCl saturated with Mg(OH)2.

Fig. 5. A fishing line specimen13 of HP Mg immersed in 3.5% NaClsaturated with Mg(OH)2.

Plug-In Specimens for Measurement of the Corrosion Rate of Mg Alloys 659

The average value of the corrosion rate evaluatedby hydrogen evolution was in good agreement withthe averagecorrosion rate measured from weight loss:

PW ¼ 0:38� 0:24 mm year�1: (8)

Moreover, each individual measurement of thecorrosion rate evaluated by hydrogen evolution wasin good agreement with the corresponding corrosionrate measured from weight loss. This gave comfortin the validity of both the corrosion rate measuredby hydrogen evolution, PH, and that measured byweight loss, PW.

There was a significant difference between thecorrosion rate measured with the mounted speci-mens (6) and that measured with the fishing linespecimens (7, 8) for HP Mg immersed in 3.5% NaClsaturated with Mg(OH)2. The higher corrosion rateof the mounted specimens indicated that there wasadditional corrosion in the mounted specimens.Examination of the mounted specimen sides wasmade possible by the use of a transparent metal-lurgical mounting. Examination of the mountedspecimen sides indicated that there was crevicecorrosion on the specimen sides that were notintentionally exposed to the solution. Such amounted specimen was broken open, and the sidesurfaces were examined using visual examination,optical microscopy, and scanning electron micros-copy (SEM). Figure 6 shows a typical SEM view ofthe crevice-corroded surface. Of significant interestis the appearance of acicular features that areidentified as MgH2 because their appearance wassimilar to hydrides in zirconium.35 However, themechanism of crevice corrosion in Mg alloys wasbeyond the scope of the investigation.13

PLUG-IN SPECIMENS

Figure 7 shows the preparation of a plug-in speci-men for Mg.13 These specimens were designed tohave no crevice and to allow simultaneous measure-ment of polarization curves and the evolved hydro-

gen. They were also designed so that the averagecorrosion rate could be measured by weight loss. Thebared end of a polyvinylchloride (PVC)-insulatedelectrical copper wire was pressed into an undersizedhole drilled in the edge of the specimen (20 mm 915 mm 9 6 mm) until the PVC insulation was pres-sed firmly against the Mg specimen, with the poly-tetrafluoroethylene (PTFE) plumber’s tape ensuringthat there was no crevice associated with the Cu wire.The copper wire outer diameter (OD) at 1.78 mm wasslightly larger than the hole in the Mg specimen withinner diameter (ID) of 1.70 mm. This ensured thatsome force was needed to push the copper wire intothe hole in the Mg specimen, that the copper wire washeld firmly in place, and that there was significantpressure between the Mg specimen and the PVCinsulation at the end of the wire. The OD of the PVCwas 3.9 mm so there was an annulus of PVC of1.06 mm firmly pressed onto the Mg specimen, sep-arating the Cu conductor from the solution. The endof the PVC insulation was cut square to ensure thatthere was a good seal between the PVC and the Mgspecimen. This seal did not allow electrolyte ingressand ensured there was no crevice. PTFE plumber’stape was wrapped several times around the end of thePVC insulation before the Cu wire was pressed intothe Mg. The PTFE tape helped ensure that there wasno crevice, and that no solution seeped under thePVC and reached the Cu conductor.

CATHODIC POLARIZATION CURVES

Cathodic polarization curves were measured forHP Mg in 3.5% NaCl saturated with Mg(OH)2 using

Fig. 6. Crevice corrosion in a mounted specimen.13

Fig. 7. Plug-in Specimen.13

Shi, Prasad, and Atrens660

plug-in specimens.13 The measured cathodic polar-ization curves were extrapolated10 by Tafel extrap-olation to yield the corrosion current density icorr

and the corresponding corrosion rate Pi. Figure 8shows data for Pi and the corrosion rate evaluatedfrom hydrogen evolution, PH. PH was consistentlygreater than Pi. The data averaged over the first10 days indicated an apparent valence for Mg of1.4513 consistent with the unipositive Mg+ ion1–8

being a short-lived intermediate, see Eqs. 1–4.Figure 8 shows that, after about 14 days, there wasa significant increase in PH, without any corre-sponding change in Pi. This is attributed to the cor-rosion products and evolved hydrogen insulating thecorrosion processes from the measured electro-chemical behavior. Under these conditions, there isno relationship between the measured electrochem-istry and the corrosion behavior. Figure 9 shows atypical surface view of the plug-in specimens after31 days of immersion. This surface appearance isconsistent with linked-up corrosion cavities, at thebottom of which there was typically a dense coverageof crack-like features. There were remains of theoriginal surface with little corrosion attack.

ANODIC POLARIZATION CURVES

Shi et al.15 used plug-in specimens to measuregalvanostatic anodic polarization curves for high-purity (HP) Mg in 3.5% NaCl saturated withMg(OH)2, whilst also measuring the evolvedhydrogen and weight loss. Thus, at each appliedcurrent density, the following were measured: (i)the evolved hydrogen, and (ii) the total weight loss.

Figure 10 presents two times the average Mgweight loss rate, 2NW, plotted against the flux ofelectrons, Ne, corresponding to the applied galva-nostatic current density, iapplied, during the mea-surement of the galvanostatic anodic polarizationcurves.15 The weight loss initially increased slowly

with increasing Ne. Thereafter, there was a linearrelationship on the log–log axes. The weight lossrate was always significantly greater than the linedrawn on Fig. 10 for

2NW ¼ Ne: (9)

This indicates that the Mg weight loss was signif-icantly larger than expected if the applied currentdensity, on the basis of Faraday’s law, was directlyoxidizing metallic Mg to the equilibrium ion, Mg++.This is strong support for the unipositive Mg+ corro-sion mechanism as summarized by Eqs. 1–4. Thegreater than expected production rate of Mg++ iscaused by the fact that part of the corrosion of Mgis chemical, so that the rate of Mg++ production isgreater than expected from the flux of electrons, Ne.

Figure 11 indicates that the rate of evolvedhydrogen increased with increasing weight loss

Fig. 8. Comparison of Pi and PH for HP Mg plug-in specimensimmersed for 7 days, 14 days, and 31 days in 3.5% NaCl saturatedwith Mg(OH)2.13

Fig. 9. SEM images of the surface after removal of corrosion prod-ucts for a HP Mg plug-in specimen immersed in 3.5% NaCl saturatedwith Mg(OH)2 for 31 days.13

Fig. 10. Two times the average Mg weight loss rate, 2NW, plottedagainst the flux of electrons, Ne, corresponding to the applied gal-vanostatic current density, iapplied, during galvanostatic testing for HPMg in 3.5% NaCl saturated with Mg(OH)2.15

Plug-In Specimens for Measurement of the Corrosion Rate of Mg Alloys 661

rate, caused by the increasing applied current den-sity.15 This is consistent with the chemical reactionof the unipositive Mg+ ion to produce H2 via Eq. 3.

This research also found that some hydrogendissolved in the Mg just as in the research of ZainalAbiden et al.11 summarized above.

Furthermore, it was possible to estimate the self-corrosion rate, and to compare it with the corrosionrate associated directly with the applied currentdensity.15 Figure 12 presents the results plottedagainst the flux of electrons, Ne, corresponding tothe applied galvanostatic current density, iapplied,during the measurement of the galvanostatic anodicpolarization curves. Figure 12 shows that self-cor-rosion was more important than the applied currentdensity in causing weight loss throughout the ano-dic polarization curve.

ACKNOWLEDGEMENTS

The research on Mg flammability, corrosion, andstress corrosion cracking is supported by BoeingResearch and Technology, CAST CRC, the Aus-tralian Research Council Centre of ExcellenceDesign of Light Alloys, and GM Global Researchand Development. CAST CRC was establishedunder, and is funded in part by, the AustralianFederal Government’s Cooperative Research Cen-tre scheme.

REFERENCES

1. G.L. Song and A. Atrens, Adv. Eng. Mater. 1, 11 (1999).2. G.L. Song and A. Atrens, Adv. Eng. Mater. 5, 837 (2003).3. A. Atrens, M. Liu, and N.I. Zainal Abidin, Mater. Sci. Eng.,

B 176, 1609 (2011).4. A. Atrens and W. Dietzel, Adv. Eng. Mater. 9, 292 (2007).5. A. Atrens, M. Liu, N.I. Zainal Abidin, and G. Song, Corro-

sion of Magnesium Alloys, ed. G.L. Song (Cambridge:Woodhead, 2011), p. 117.

6. M. Liu, P. Schmutz, P.J. Uggowitzer, G. Song, and A. Atrens,Corros. Sci. 52, 3687 (2010).

7. M. Liu, P.J. Uggowitzer, A.V. Nagasekhar, P. Schmutz, M.Easton, G.L. Song, and A. Atrens, Corros. Sci. 51, 602(2009).

8. M.C. Zhao, M. Liu, and G. Song, Corros. Sci. 50, 1939 (2008).9. A. Prasad, Z. Shi, and A. Atrens, Corros. Sci. 54, 153 (2012).

10. N. Winzer, A. Atrens, G. Song, E. Ghali, W. Dietzel, K.U.Kainer, N. Hort, and C. Blawert, Adv. Eng. Mater. 7, 659(2005).

11. A. Atrens, N. Winzer, W. Dietzel, P.B. Srinivasan, andG. Song, Corrosion of Magnesium Alloys, ed. G.L. Song(Cambridge: Woodhead, 2011), p. 299.

12. Z. Shi, M. Liu, and A. Atrens, Corros. Sci. 52, p 579 (2010).13. Z. Shi and A. Atrens, Corros. Sci. 53, 226 (2011).14. N.I. Zainal Abidin, A.D. Atrens, D. Martin, and A. Atrens,

Corros. Sci. 53, 3542 (2011).15. Z. Shi, J.X. Jia, and A. Atrens, Corrosion Science.

10.1016/j.corsci.2011.12.002.16. K. Nogita, S. Ockert, J. Pierce, M.C. Greaves, C.M. Gourlay,

and A.K. Dahle, Int. J. Hydrogen Energy 34, 7686 (2009).17. Y. Song, D. Shan, R. Chen, and E.-H. Han, Corros. Sci. 51,

1087 (2009).18. Y. Song, D. Shan, R. Chen, and E.-H. Han, Corros. Sci. 52,

1830 (2010).19. S. Izumi, M. Yamasaki, and Y. Kawamura, Corros. Sci. 51,

395 (2009).20. N. Liu, J. Wang, Y. Wu, and L. Wang, J. Mater. Sci. 43, 2550

(2008).21. D.A. Jones, Principles and Prevention of Corrosion (Upper

Saddle River, NJ: Prentice Hall, 1992).22. E. Zhang, D. Yin, L. Xu, L. Yang, and K. Yang, Micro-

structure. Mater. Sci. Eng., C 29, 987 (2009).23. W. He, E. Zhang, and K. Yang, Mater. Sci. Eng., C 30, 167

(2010).24. L. Yang, Y. Wei, L. Hou, and D. Zhang, Corros. Sci. 52, 345

(2010).25. D. Song, A. Ma, J. Jiang, P. Lin, D. Yang, and J. Fan,

Corros. Sci. 53, 481 (2010).26. W. Zhou, T. Shen, and N.N. Aung, Corros. Sci. 52, 1035

(2010).27. G. Song, Corros. Sci. 51, 2063 (2009).28. W.C. Neil, M. Forsyth, P.C. Howlett, C.R. Hutchinson, and

B.R.W. Hinton, Corros. Sci. 51, 387 (2009).29. V. Lisitsyn, G. Ben-Hamu, D. Eliezer, and K.S. Shin, Corros.

Sci. 51, 776 (2009).30. N. Birbilis, M.A. Easton, A.D. Sudholtz, S.M. Zhu, and

M.A. Gibson, Corros. Sci. 51, 683 (2009).31. C. Op’tHoog, N. Birbilis, and Y. Estrin, Adv. Eng. Mater. 6,

579 (2008).

Fig. 11. The average hydrogen evolution rate, NH, plotted againstthe average Mg weight loss rate, NW, during galvanostatic testing forHP Mg in 3.5% NaCl saturated with Mg(OH)2.15

Fig. 12. The ratio of self-corrosion rate to the total corrosion rateplotted against the flux of electrons, Ne, corresponding to the appliedgalvanostatic current density, iapplied, during galvanostatic testing forHP Mg in 3.5% NaCl saturated with Mg(OH)2.15

Shi, Prasad, and Atrens662

32. H.B. Yao, Y. Li, and A.T.S. Wee, Electrochim. Acta 58, 4197(2003).

33. A.D. Sudholz, K. Gusieva, X.B. Chen, B.C. Muddle,M.A. Gibson, and N. Birbilis, Corros. Sci. 53, 2277 (2011).

34. D. Orlov, K.D. Ralston, N. Birbilis, and Y. Estrin, ActaMater. 59, 6176 (2011).

35. C.D. Cann and A. Atrens, J. Nuclear Mater. 88, 42(1980).

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