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Influence of surface roughness on the corrosion behaviour of magnesium alloy
Materials and Design 32 (2011) 2350–2354
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Influence of surface roughness on the corrosion behaviour of magnesium alloy R. Walter, M. Bobby Kannan ⇑ Discipline of Chemical Engineering, School of Engineering and Physical Sciences, James Cook University, Townsville, Queensland 4811, Australia
a r t i c l e
i n f o
Article history: Received 3 September 2010 Accepted 6 December 2010 Available online 13 December 2010
a b s t r a c t In this study, the influence of surface roughness on the passivation and pitting corrosion behaviour of AZ91 magnesium alloy in chloride-containing environment was examined using electrochemical techniques. Potentiodynamic polarisation and electrochemical impedance spectroscopy tests suggested that the passivation behaviour of the alloy was affected by increasing the surface roughness. Consequently, the corrosion current and the pitting tendency of the alloy also increased with increase in the surface roughness. Scanning electron micrographs of 24 h immersion test samples clearly revealed pitting corrosion in the highest surface roughness (Sa 430) alloy, whereas in the lowest surface roughness (Sa 80) alloy no evidence of pitting corrosion was observed. Interestingly, when the passivity of the alloy was disturbed by galvanostatically holding the sample at anodic current for 1 h, the alloy underwent high pitting corrosion irrespective of their surface roughness. Thus the study suggests that the surface roughness plays a critical role in the passivation behaviour of the alloy and hence the pitting tendency. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium alloys have gained significant interest for automotive and aerospace applications for their high-strength-to-weight ratio. However, their wide-spread use is limited by their high corrosion susceptibility, particularly in chloride-containing environment. In recent years, a substantial amount of work has been carried out to improve the corrosion resistance of magnesium and its alloys through alloying and surface coatings for various applications [1–5]. A few researchers have also studied the influence of microstructure on the corrosion behaviour of magnesium alloys [6,7]. Generally, the surface roughness plays a role on the corrosion behaviour of metallic materials. It has been reported that an increase in the surface roughness of stainless steels increases the pitting susceptibility [8–10] and general corrosion rate [11]. A similar trend has been reported for other metals, such as copper [12], and titanium-based alloys [13]. However, the literature on the effect of surface roughness on the corrosion behaviour of magnesium and its alloys is limited. Interestingly, the only work by Alvarez et al. [14] on AE44 magnesium alloy is in contrast to the trend reported for other metallic materials. Based on immersion test results, the authors [14] reported that the general corrosion decreased as the alloy’s surface roughness increased. Further they reported that the polished alloy allowed greater initial pitting and higher pitting volume than the semi-polished alloy. Typically, the general and localized corrosion behaviour of alloys would depend on their passivation behaviour. Hence, it is important to know ⇑ Corresponding author. Tel.: +61 7 4781 5080; fax: +61 7 4781 6788. E-mail address: [email protected] (M.B. Kannan). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.12.016
the passivation behaviour of magnesium alloys with different surface finish to correlate the surface roughness to their general corrosion and pitting tendency. In this study the corrosion behaviour of AZ91 magnesium alloy was studied using electrochemical techniques, such as potentiodynamic polarisation and electrochemical impedance spectroscopy, especially to understand the passivation behaviour of the alloy with different surface roughness. 2. Experimental procedure Sand-cast AZ91 magnesium alloy was used as the test material in this study. The chemical composition of the alloy is given in Table 1. For obtaining different surface roughness, the samples were grinded/polished with different grits of silicon carbide (SiC) (i.e., 320, 600 and 1200) and 3 lm diamond-paste. The surface roughness was analysed using atomic force microscopy (AFM). The corrosion behaviour of the alloy was studied using electrochemical techniques, such as potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS). A potentiostat and frequency response analyser (Model VersaSTAT 3) driven by VersaStudio software were used for potentiodynamic polarisation and EIS experiments, respectively. A typical three electrode system consisting of graphite as a counter electrode, Ag/AgCl electrode as a reference electrode and sample (0.75 cm2 exposed area) as a working electrode were used. The test electrolyte was 0.5 wt.% NaCl solution. Prior to testing, the samples were allowed for 2 h to reach a relatively stable open circuit potential. EIS experiments were performed over the frequency range of 105–102 Hz at AC amplitude of 5 mV. Potentiodynamic polarisation experiments were done at a scan rate of 0.5 mV/s. In addition, immersion tests
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R. Walter, M.B. Kannan / Materials and Design 32 (2011) 2350–2354 Table 2 Surface roughness of AZ91 magnesium alloy with different surface finish.
Table 1 Chemical composition of the AZ91 alloy.
AZ91 sand-cast
Al
Zn
Mn
Si
Fe
Mg
Sample finish
Sa (nm)
Standard deviation (nm)
9.18
0.78
0.20
0.01
0.002
Bal.
320 SiC 600 SiC 1200 SiC 3 lm diamond-paste
430 248 145 80
183 86 42 21
were carried out for 24 h, and galvanostatic tests were carried out at 1.25 mA/cm2 anodic current for 1 h, and the post-corrosion samples were examined using scanning electron microscope (SEM) to identify the mode of corrosion attack. 3. Results and discussion The AFM images for the different surface finishes are shown in Fig. 1. It was clearly evident that the surface roughness of the alloy decreased as the grit size of the grinding paper used was increased. Further, the diamond-paste polished alloy showed a relatively smooth surface as compared to the SiC grinded alloy. The measured surface roughness and standard deviation are listed in Table 2. As expected the standard deviation in the surface roughness values decreased as the surface of the alloy became smoother. Fig. 2 shows the Nyquist plots of the alloy with different surface roughness tested in chloride-containing environment. In the SiC grinded samples, a similar behaviour was observed i.e., a high frequency capacitive loop and low frequency inductive loop. It is reported in literature that the high frequency capacitive loop corresponds to the charge transfer and film effect [15,16] and the low frequency inductive loop indicates pitting of the alloy [17]. Interestingly, the diamond-paste polished alloy showed one capacitive loop at high frequency and another depressed capacitive loop at medium frequency. Moreover, the alloy showed no evidence of an inductive loop, which suggested that the sample with smooth surface has not undergone pitting. It is reported in the literature that for rare-earth containing magnesium alloys, an observation of a
Fig. 2. Nyquist plots of AZ91 magnesium alloy, with different surface roughness, tested in 0.5 wt.% NaCl.
second capacitive loop in the mid frequency is related to the relaxation of mass transport through the corrosion product layer, or in other words, suggests the presence of a protective film [15]. It also means that the absence or lack of evidence of the second capacitive loop in the mid frequency is an indication of scarcely/ no protective film. Although the formation of magnesium hydroxide via the reaction: 2Mg þ 2H2 O ! MgðOHÞ2 þ H2 , is plausible under different surface roughness conditions, the formation of a continu-
Fig. 1. Surface topography of AZ91 magnesium alloy grinded/polished up to (a) 320 grit SiC; (b) 600 grit SiC; (c) 1200 grit SiC; and (d) 3 lm diamond-paste.
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Fig. 3. Potentiodynamic polarisation curves of AZ91 alloy, with different surface roughness, tested in 0.5 wt.% NaCl.
ous protective film would be higher on a smooth surface alloy than on an irregular surface. Hence, the alloy with relatively high surface roughness showed no evidence of the second capacitive loop, but instead showed an inductive loop in the low frequency indicative of pitting. In the case of the low surface roughness alloy, the evidence of depressed second capacitive loop and the non-existence of low frequency inductive loop suggested that smooth surface alloy
Table 3 Electrochemical corrosion parameters of AZ91 alloy (with different surface roughness) obtained from potentiodynamic polarisation curves. Surface finish
icorr (lA/cm2)
Ecorr (V)
Ebd (V)
Epass (mV)
320 grit SiC 600 grit SiC 1200 grit SiC 3 lm diamond-paste
6.92 4.79 3.73 2.19
1.447 1.427 1.418 1.378
1.447 1.392 1.362 1.299
0 35 56 79
exhibits higher passivation tendency and pitting resistance than the high surface roughness alloy. The polarisation curves of AZ91 alloy, with different surface roughness, tested in chloride-containing solution are shown in Fig. 3. The electrochemical corrosion parameters from the polarisation curves are listed in Table 3. The corrosion potential (Ecorr) of the alloy shifted towards the noble direction as the surface roughness decreased. Notably, the corrosion current (icorr) decreased as the surface roughness decreased. However, there was no significant difference in the cathodic current within the samples having different surface roughness, which suggested that the shift in the Ecorr and the difference in icorr were solely due to the anodic behaviour of the alloy. The alloy with the highest surface roughness showed a sharp increase in the anodic current just above the Ecorr. This phenomenon (i.e., sharp increase in the anodic current) was an indication of pitting, and in this case the pitting potential was pinned with the Ecorr. Interestingly, as the surface roughness decreased the alloy revealed a passive-like behaviour before a sharp break-down. It was noticed that the passive-potential region (Epass, the difference between the corrosion potential and the break-down potential, Ebd) increased when the surface roughness of the alloy decreased. Hence, the alloy with the lowest surface roughness showed a passive-potential region of about 79 mV, whereas the highest surface roughness alloy showed no evidence of passive region. Although the passivity could be mainly due to the formation of magnesium hydroxide, the presence of aluminium (forms aluminium oxide in aqueous environment) in the alloy may also have some influence on the passivation behaviour. The SEM micrographs of the samples immersed in chloridecontaining solution at the open circuit potential for 24 h (Fig. 4) clearly revealed that the alloy with the highest surface roughness underwent high pitting corrosion, whereas the alloy with the lowest surface roughness showed no evidence of localized attack. The alloy having mid-range surface roughness did show some evidence of pitting corrosion, however substantially lower than in the alloy with highest surface roughness.
Fig. 4. SEM micrographs of AZ91 magnesium alloy grinded/polished (for different surface roughness) up to (a) 320 grit SiC; (b) 600 grit SiC; (c) 1200 grit SiC; and (d) 3 lm diamond-paste, and immersed in 0.5 wt.% NaCl for 24 h.
R. Walter, M.B. Kannan / Materials and Design 32 (2011) 2350–2354
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Fig. 5. SEM micrographs of AZ91 magnesium alloy grinded/polished up to (a) 320 grit SiC; (b) 600 grit SiC; (c) 1200 grit SiC; and (d) 3 lm diamond-paste, after galvanostatic testing.
Interestingly, the SEM micrographs of galvanostatically-held alloy revealed a large number of pits, irrespective of their surface roughness (Fig. 5). Alvarez et al. [14] also found pitting in both polished and semi-polished AE44 magnesium alloy. Interestingly, they reported that the density of pitting was relatively higher in polished alloy as compared to semi-polished alloy. In fact, a closer look at the SEM micrographs of galvanostatically-held alloy, suggested that the alloy with lowest surface roughness exhibits a slightly higher number of pits as compared to the alloy with highest surface roughness. However, Alvarez et al. [14] observed larger pits in semi-polished alloy as compared to polished alloy. In order to understand the differences in the pitting behaviour of magnesium alloy with different testing methods, the fundamental corrosion mechanism of magnesium has been reviewed. It is well documented in the literature that magnesium dissolution increases the local pH at cathodic sites of the sample, which tends to facilitate corrosion-product film, or in other words passivates the alloy [1]. However, in the presence of chloride ions the passive film on magnesium breaks down, causing pitting corrosion. Alvarez et al. [14] conducted the testing in 3.5% NaCl solution, which is not different to the test solution in this study; however it should be noted that they have aerated the solution throughout the experiment. Although oxygen (in air) has no significant influence on the corrosion behaviour of the magnesium [18], the stirring effect caused by aeration could reduce the local pH change and consequently affect the passivation tendency of the alloy. Hence, they observed pitting corrosion even in the polished alloy under immersion testing. However, in the case of the galvanostatically-held alloy, the anodic current was high enough to break the passive film of the alloy under all surface roughness (refer Fig. 3), and hence pitting corrosion was observed in all the samples irrespective of their surface roughness. 4. Conclusions The study clearly suggests that the surface roughness plays a critical role in the corrosion behaviour of AZ91 magnesium alloy
in chloride-containing environment. The electrochemical experiments showed that an increase in the surface roughness of the alloy affects the passivation tendency and consequently increases the pitting susceptibility of the alloy. However, when the passivity of the alloy is disturbed then the influence of surface roughness on the pitting corrosion susceptibly becomes less significant. References [1] Song G, Atrens A. Understanding magnesium corrosion – a framework for improved alloy performance. Adv Eng Mater 2003;5:837–58. [2] Bobby Kannan M, Dietzel W, Blawert C, Atrens A, Lyon P. Stress corrosion cracking of rare-earth containing magnesium alloys ZE41, QE22 and electron 21 (EV31A) compared with AZ80. Mater Sci Eng A 2007;480:529–39. [3] Bobby Kannan M, Raman RK. In vitro degradation and mechanical integrity of calcium-containing magnesium alloys in modified-simulated body fluid. Biomaterials 2008;29:2306–14. [4] Gray JE, Luan B. Protective coatings on magnesium and its alloys – a critical review. J Alloys Compd 2002;336:88–113. [5] Bobby Kannan M, Gomes D, Dietzel W, Abetz V. Polyoxadiazole-based coating for corrosion protection of magnesium alloy. Surf Coat Technol 2008;202: 4598–601. [6] Song G, Atrens A, Dargusch M. Influence of microstructure on the corrosion of diecast AZ91D. Corros Sci 1998;41:249–73. [7] Bobby Kannan M. Influence of microstructure on the in-vitro degradation behaviour of magnesium alloys. Mater Lett 2010;62:739–42. [8] Hong T, Nagumo M. Effect of surface roughness on early stages of pitting corrosion of type 301 stainless steel. Corros Sci 1997;39:1665–72. [9] Zuo Y, Wang H, Xiong J. The aspect ratio of surface grooves and metastable pitting of stainless steel. Corros Sci 2002;44:25–35. [10] Sasaki K, Burstein GT. The generation of surface roughness during slurry erosion–corrosion and its effect on the pitting potential. Corros Sci 1996;38: 2111–20. [11] Shahryari A, Kamal W, Omanovic S. The effect of surface roughness on the efficiency of the cyclic potentiodynamic passivation (CPP) method in the improvement of general and pitting corrosion resistance of 316LVM stainless steel. Mater Lett 2008;62:3906–9. [12] Li W, Li DY. Influence of surface morphology on corrosion and electronic behaviour. Acta Mater 2006;54:445–52. [13] Cabrini M, Cigada A, Rondelli G, Vicentini B. Effect of different surface finishing and of hydroxyapatite coatings on passive and corrosion current of Ti6Al4V alloy in simulated physiological solution. Biomaterials 1997;18: 783–7. [14] Alvarez RB, Martin HJ, Horstemeyer MF, Chandler MQ, Williams N, Wang PT, et al. Corrosion relationships as a function of time and surface roughness on a structural AE44 magnesium alloy. Corros Sci 2010;52:1635–48.
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[15] Zucchi F, Grassi V, Frignani A, Monticelli C, Trabanelli G. Electrochemical behaviour of a magnesium alloy containing rare earth elements. J Appl Electrochem 2006;36:195–204. [16] Guo LF, Yue TM, Man HC. Excimer laser surface treatment of magnesium alloy WE43 for corrosion resistance improvement. J Mater Sci Lett 2005;40:3531–3.
[17] Jin S, Amira S, Ghali E. Electrochemical impedance spectroscopy evaluation of the corrosion behavior of diecast and thixocast AXJ530 magnesium alloy in chloride solution. Adv Eng Mater 2007;9:75–83. [18] Makar GL, Kruger J. Corrosion of magnesium. Int Mater Rev 1993;38: 138–53.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Influence of surface roughness on the corrosion behaviour of magnesium alloy R. Walter, M. Bobby Kannan ⇑ Discipline of Chemical Engineering, School of Engineering and Physical Sciences, James Cook University, Townsville, Queensland 4811, Australia
a r t i c l e
i n f o
Article history: Received 3 September 2010 Accepted 6 December 2010 Available online 13 December 2010
a b s t r a c t In this study, the influence of surface roughness on the passivation and pitting corrosion behaviour of AZ91 magnesium alloy in chloride-containing environment was examined using electrochemical techniques. Potentiodynamic polarisation and electrochemical impedance spectroscopy tests suggested that the passivation behaviour of the alloy was affected by increasing the surface roughness. Consequently, the corrosion current and the pitting tendency of the alloy also increased with increase in the surface roughness. Scanning electron micrographs of 24 h immersion test samples clearly revealed pitting corrosion in the highest surface roughness (Sa 430) alloy, whereas in the lowest surface roughness (Sa 80) alloy no evidence of pitting corrosion was observed. Interestingly, when the passivity of the alloy was disturbed by galvanostatically holding the sample at anodic current for 1 h, the alloy underwent high pitting corrosion irrespective of their surface roughness. Thus the study suggests that the surface roughness plays a critical role in the passivation behaviour of the alloy and hence the pitting tendency. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium alloys have gained significant interest for automotive and aerospace applications for their high-strength-to-weight ratio. However, their wide-spread use is limited by their high corrosion susceptibility, particularly in chloride-containing environment. In recent years, a substantial amount of work has been carried out to improve the corrosion resistance of magnesium and its alloys through alloying and surface coatings for various applications [1–5]. A few researchers have also studied the influence of microstructure on the corrosion behaviour of magnesium alloys [6,7]. Generally, the surface roughness plays a role on the corrosion behaviour of metallic materials. It has been reported that an increase in the surface roughness of stainless steels increases the pitting susceptibility [8–10] and general corrosion rate [11]. A similar trend has been reported for other metals, such as copper [12], and titanium-based alloys [13]. However, the literature on the effect of surface roughness on the corrosion behaviour of magnesium and its alloys is limited. Interestingly, the only work by Alvarez et al. [14] on AE44 magnesium alloy is in contrast to the trend reported for other metallic materials. Based on immersion test results, the authors [14] reported that the general corrosion decreased as the alloy’s surface roughness increased. Further they reported that the polished alloy allowed greater initial pitting and higher pitting volume than the semi-polished alloy. Typically, the general and localized corrosion behaviour of alloys would depend on their passivation behaviour. Hence, it is important to know ⇑ Corresponding author. Tel.: +61 7 4781 5080; fax: +61 7 4781 6788. E-mail address: [email protected] (M.B. Kannan). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.12.016
the passivation behaviour of magnesium alloys with different surface finish to correlate the surface roughness to their general corrosion and pitting tendency. In this study the corrosion behaviour of AZ91 magnesium alloy was studied using electrochemical techniques, such as potentiodynamic polarisation and electrochemical impedance spectroscopy, especially to understand the passivation behaviour of the alloy with different surface roughness. 2. Experimental procedure Sand-cast AZ91 magnesium alloy was used as the test material in this study. The chemical composition of the alloy is given in Table 1. For obtaining different surface roughness, the samples were grinded/polished with different grits of silicon carbide (SiC) (i.e., 320, 600 and 1200) and 3 lm diamond-paste. The surface roughness was analysed using atomic force microscopy (AFM). The corrosion behaviour of the alloy was studied using electrochemical techniques, such as potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS). A potentiostat and frequency response analyser (Model VersaSTAT 3) driven by VersaStudio software were used for potentiodynamic polarisation and EIS experiments, respectively. A typical three electrode system consisting of graphite as a counter electrode, Ag/AgCl electrode as a reference electrode and sample (0.75 cm2 exposed area) as a working electrode were used. The test electrolyte was 0.5 wt.% NaCl solution. Prior to testing, the samples were allowed for 2 h to reach a relatively stable open circuit potential. EIS experiments were performed over the frequency range of 105–102 Hz at AC amplitude of 5 mV. Potentiodynamic polarisation experiments were done at a scan rate of 0.5 mV/s. In addition, immersion tests
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R. Walter, M.B. Kannan / Materials and Design 32 (2011) 2350–2354 Table 2 Surface roughness of AZ91 magnesium alloy with different surface finish.
Table 1 Chemical composition of the AZ91 alloy.
AZ91 sand-cast
Al
Zn
Mn
Si
Fe
Mg
Sample finish
Sa (nm)
Standard deviation (nm)
9.18
0.78
0.20
0.01
0.002
Bal.
320 SiC 600 SiC 1200 SiC 3 lm diamond-paste
430 248 145 80
183 86 42 21
were carried out for 24 h, and galvanostatic tests were carried out at 1.25 mA/cm2 anodic current for 1 h, and the post-corrosion samples were examined using scanning electron microscope (SEM) to identify the mode of corrosion attack. 3. Results and discussion The AFM images for the different surface finishes are shown in Fig. 1. It was clearly evident that the surface roughness of the alloy decreased as the grit size of the grinding paper used was increased. Further, the diamond-paste polished alloy showed a relatively smooth surface as compared to the SiC grinded alloy. The measured surface roughness and standard deviation are listed in Table 2. As expected the standard deviation in the surface roughness values decreased as the surface of the alloy became smoother. Fig. 2 shows the Nyquist plots of the alloy with different surface roughness tested in chloride-containing environment. In the SiC grinded samples, a similar behaviour was observed i.e., a high frequency capacitive loop and low frequency inductive loop. It is reported in literature that the high frequency capacitive loop corresponds to the charge transfer and film effect [15,16] and the low frequency inductive loop indicates pitting of the alloy [17]. Interestingly, the diamond-paste polished alloy showed one capacitive loop at high frequency and another depressed capacitive loop at medium frequency. Moreover, the alloy showed no evidence of an inductive loop, which suggested that the sample with smooth surface has not undergone pitting. It is reported in the literature that for rare-earth containing magnesium alloys, an observation of a
Fig. 2. Nyquist plots of AZ91 magnesium alloy, with different surface roughness, tested in 0.5 wt.% NaCl.
second capacitive loop in the mid frequency is related to the relaxation of mass transport through the corrosion product layer, or in other words, suggests the presence of a protective film [15]. It also means that the absence or lack of evidence of the second capacitive loop in the mid frequency is an indication of scarcely/ no protective film. Although the formation of magnesium hydroxide via the reaction: 2Mg þ 2H2 O ! MgðOHÞ2 þ H2 , is plausible under different surface roughness conditions, the formation of a continu-
Fig. 1. Surface topography of AZ91 magnesium alloy grinded/polished up to (a) 320 grit SiC; (b) 600 grit SiC; (c) 1200 grit SiC; and (d) 3 lm diamond-paste.
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Fig. 3. Potentiodynamic polarisation curves of AZ91 alloy, with different surface roughness, tested in 0.5 wt.% NaCl.
ous protective film would be higher on a smooth surface alloy than on an irregular surface. Hence, the alloy with relatively high surface roughness showed no evidence of the second capacitive loop, but instead showed an inductive loop in the low frequency indicative of pitting. In the case of the low surface roughness alloy, the evidence of depressed second capacitive loop and the non-existence of low frequency inductive loop suggested that smooth surface alloy
Table 3 Electrochemical corrosion parameters of AZ91 alloy (with different surface roughness) obtained from potentiodynamic polarisation curves. Surface finish
icorr (lA/cm2)
Ecorr (V)
Ebd (V)
Epass (mV)
320 grit SiC 600 grit SiC 1200 grit SiC 3 lm diamond-paste
6.92 4.79 3.73 2.19
1.447 1.427 1.418 1.378
1.447 1.392 1.362 1.299
0 35 56 79
exhibits higher passivation tendency and pitting resistance than the high surface roughness alloy. The polarisation curves of AZ91 alloy, with different surface roughness, tested in chloride-containing solution are shown in Fig. 3. The electrochemical corrosion parameters from the polarisation curves are listed in Table 3. The corrosion potential (Ecorr) of the alloy shifted towards the noble direction as the surface roughness decreased. Notably, the corrosion current (icorr) decreased as the surface roughness decreased. However, there was no significant difference in the cathodic current within the samples having different surface roughness, which suggested that the shift in the Ecorr and the difference in icorr were solely due to the anodic behaviour of the alloy. The alloy with the highest surface roughness showed a sharp increase in the anodic current just above the Ecorr. This phenomenon (i.e., sharp increase in the anodic current) was an indication of pitting, and in this case the pitting potential was pinned with the Ecorr. Interestingly, as the surface roughness decreased the alloy revealed a passive-like behaviour before a sharp break-down. It was noticed that the passive-potential region (Epass, the difference between the corrosion potential and the break-down potential, Ebd) increased when the surface roughness of the alloy decreased. Hence, the alloy with the lowest surface roughness showed a passive-potential region of about 79 mV, whereas the highest surface roughness alloy showed no evidence of passive region. Although the passivity could be mainly due to the formation of magnesium hydroxide, the presence of aluminium (forms aluminium oxide in aqueous environment) in the alloy may also have some influence on the passivation behaviour. The SEM micrographs of the samples immersed in chloridecontaining solution at the open circuit potential for 24 h (Fig. 4) clearly revealed that the alloy with the highest surface roughness underwent high pitting corrosion, whereas the alloy with the lowest surface roughness showed no evidence of localized attack. The alloy having mid-range surface roughness did show some evidence of pitting corrosion, however substantially lower than in the alloy with highest surface roughness.
Fig. 4. SEM micrographs of AZ91 magnesium alloy grinded/polished (for different surface roughness) up to (a) 320 grit SiC; (b) 600 grit SiC; (c) 1200 grit SiC; and (d) 3 lm diamond-paste, and immersed in 0.5 wt.% NaCl for 24 h.
R. Walter, M.B. Kannan / Materials and Design 32 (2011) 2350–2354
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Fig. 5. SEM micrographs of AZ91 magnesium alloy grinded/polished up to (a) 320 grit SiC; (b) 600 grit SiC; (c) 1200 grit SiC; and (d) 3 lm diamond-paste, after galvanostatic testing.
Interestingly, the SEM micrographs of galvanostatically-held alloy revealed a large number of pits, irrespective of their surface roughness (Fig. 5). Alvarez et al. [14] also found pitting in both polished and semi-polished AE44 magnesium alloy. Interestingly, they reported that the density of pitting was relatively higher in polished alloy as compared to semi-polished alloy. In fact, a closer look at the SEM micrographs of galvanostatically-held alloy, suggested that the alloy with lowest surface roughness exhibits a slightly higher number of pits as compared to the alloy with highest surface roughness. However, Alvarez et al. [14] observed larger pits in semi-polished alloy as compared to polished alloy. In order to understand the differences in the pitting behaviour of magnesium alloy with different testing methods, the fundamental corrosion mechanism of magnesium has been reviewed. It is well documented in the literature that magnesium dissolution increases the local pH at cathodic sites of the sample, which tends to facilitate corrosion-product film, or in other words passivates the alloy [1]. However, in the presence of chloride ions the passive film on magnesium breaks down, causing pitting corrosion. Alvarez et al. [14] conducted the testing in 3.5% NaCl solution, which is not different to the test solution in this study; however it should be noted that they have aerated the solution throughout the experiment. Although oxygen (in air) has no significant influence on the corrosion behaviour of the magnesium [18], the stirring effect caused by aeration could reduce the local pH change and consequently affect the passivation tendency of the alloy. Hence, they observed pitting corrosion even in the polished alloy under immersion testing. However, in the case of the galvanostatically-held alloy, the anodic current was high enough to break the passive film of the alloy under all surface roughness (refer Fig. 3), and hence pitting corrosion was observed in all the samples irrespective of their surface roughness. 4. Conclusions The study clearly suggests that the surface roughness plays a critical role in the corrosion behaviour of AZ91 magnesium alloy
in chloride-containing environment. The electrochemical experiments showed that an increase in the surface roughness of the alloy affects the passivation tendency and consequently increases the pitting susceptibility of the alloy. However, when the passivity of the alloy is disturbed then the influence of surface roughness on the pitting corrosion susceptibly becomes less significant. References [1] Song G, Atrens A. Understanding magnesium corrosion – a framework for improved alloy performance. Adv Eng Mater 2003;5:837–58. [2] Bobby Kannan M, Dietzel W, Blawert C, Atrens A, Lyon P. Stress corrosion cracking of rare-earth containing magnesium alloys ZE41, QE22 and electron 21 (EV31A) compared with AZ80. Mater Sci Eng A 2007;480:529–39. [3] Bobby Kannan M, Raman RK. In vitro degradation and mechanical integrity of calcium-containing magnesium alloys in modified-simulated body fluid. Biomaterials 2008;29:2306–14. [4] Gray JE, Luan B. Protective coatings on magnesium and its alloys – a critical review. J Alloys Compd 2002;336:88–113. [5] Bobby Kannan M, Gomes D, Dietzel W, Abetz V. Polyoxadiazole-based coating for corrosion protection of magnesium alloy. Surf Coat Technol 2008;202: 4598–601. [6] Song G, Atrens A, Dargusch M. Influence of microstructure on the corrosion of diecast AZ91D. Corros Sci 1998;41:249–73. [7] Bobby Kannan M. Influence of microstructure on the in-vitro degradation behaviour of magnesium alloys. Mater Lett 2010;62:739–42. [8] Hong T, Nagumo M. Effect of surface roughness on early stages of pitting corrosion of type 301 stainless steel. Corros Sci 1997;39:1665–72. [9] Zuo Y, Wang H, Xiong J. The aspect ratio of surface grooves and metastable pitting of stainless steel. Corros Sci 2002;44:25–35. [10] Sasaki K, Burstein GT. The generation of surface roughness during slurry erosion–corrosion and its effect on the pitting potential. Corros Sci 1996;38: 2111–20. [11] Shahryari A, Kamal W, Omanovic S. The effect of surface roughness on the efficiency of the cyclic potentiodynamic passivation (CPP) method in the improvement of general and pitting corrosion resistance of 316LVM stainless steel. Mater Lett 2008;62:3906–9. [12] Li W, Li DY. Influence of surface morphology on corrosion and electronic behaviour. Acta Mater 2006;54:445–52. [13] Cabrini M, Cigada A, Rondelli G, Vicentini B. Effect of different surface finishing and of hydroxyapatite coatings on passive and corrosion current of Ti6Al4V alloy in simulated physiological solution. Biomaterials 1997;18: 783–7. [14] Alvarez RB, Martin HJ, Horstemeyer MF, Chandler MQ, Williams N, Wang PT, et al. Corrosion relationships as a function of time and surface roughness on a structural AE44 magnesium alloy. Corros Sci 2010;52:1635–48.
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[15] Zucchi F, Grassi V, Frignani A, Monticelli C, Trabanelli G. Electrochemical behaviour of a magnesium alloy containing rare earth elements. J Appl Electrochem 2006;36:195–204. [16] Guo LF, Yue TM, Man HC. Excimer laser surface treatment of magnesium alloy WE43 for corrosion resistance improvement. J Mater Sci Lett 2005;40:3531–3.
[17] Jin S, Amira S, Ghali E. Electrochemical impedance spectroscopy evaluation of the corrosion behavior of diecast and thixocast AXJ530 magnesium alloy in chloride solution. Adv Eng Mater 2007;9:75–83. [18] Makar GL, Kruger J. Corrosion of magnesium. Int Mater Rev 1993;38: 138–53.