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Cu-coated Mg alloy (AZ91D) in 0.1 M H2SO4 with different concentrations of NaCl
Corrosion Science 52 (2010) 1326–1332
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Corrosion behavior of Cr/Cu-coated Mg alloy (AZ91D) in 0.1 M H2SO4 with different concentrations of NaCl Ching An Huang *, Che Kuan Lin, Yu Hu Yeh Department of Mechanical Engineering, Chang Gung University, Taoyuan 333, Taiwan
a r t i c l e
i n f o
Article history: Received 31 August 2009 Accepted 14 September 2009 Available online 11 December 2009 Keywords: A. Mg alloy A. Cr deposit C. Corrosion resistance C. Blister
a b s t r a c t An electroplating process was proposed for obtaining a protective Cr/Cu deposit on the two-phase Mg alloy AZ91D. The corrosion behavior of Cu-covered and Cr/Cu-covered AZ91D specimens was studied electrochemically in 0.1 M H2SO4 with different NaCl concentrations. Experimental results showed that the corrosion resistance of an AZ91D specimen improved significantly after Cr/Cu electrodeposition. The corrosion resistance of Cr/Cu-covered AZ91D decreased with increasing NaCl concentration in 0.1 M H2SO4 solution. After immersion in a 0.1 M H2SO4 with a NaCl-content above 3.5 wt.%, the surface of Cr/Cu-covered AZ91D suffered a few blisters. Cracks through the Cr deposit provided active pathways for corrosion of the Cu and the AZ91D substrate. Formation of blisters on the Cr/Cu-covered AZ91D surface was confirmed based on the results of an open-circuit potential test, which detected an obvious potential drop from noble to active potentials. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium alloys are widely used as components in the electronic, automobile and aerospace engineering industries, due to their low density and high specific strength, as well as their excellent heat dissipation and electromagnetic interference (EMI) shielding properties. However, many potential applications of Mg alloys are prohibited or severely limited, due to their high chemical activity and corrosion susceptibility. In wet environments or even in air, magnesium oxide or hydroxide films develop easily on Mg alloys [1]. These Mg oxide or hydroxide films are loose and porous, and therefore do not provide corrosion protection [1]. Thus, formation of a protective coating or film on a component made of Mg alloy has been an important point of study in recent years. Several surface treatments have been examined for the protection of Mg alloys, including chemical conversion [2], anodization [3], micro-arc oxidation [4], electroless plating [5], and electroplating [6]. Among these, the electroplating process has some distinct advantages, such as coating reliability, convenience, and relatively low cost. Therefore, development of a protective coating process for Mg alloys using electroplating would be both useful and practical. Due to its relatively high strength, amenability to casting, and corrosion resistance, AZ91D is commonly die-cast to form components for automobiles, computers, communication hardware, and consumer electronics (3C). Generally, two phases, eutectic a and * Corresponding author. Tel.: +886 3 2118800x5655. E-mail address: [email protected] (C.A. Huang). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.12.010
intermetallic b (Mg17Al12), are identifiable in AZ91D. Together, these two phases act as a galvanic couple and they have different discharging capacities in aqueous solution. As a result, a non-uniform surface morphology is typically obtained when AZ91D is immersed in an aqueous solution, or in an electroplating bath [6]. Therefore, it is relatively difficult to use electroplating to deposit a protective coating for AZ91D. Many researchers [7,8] have proposed that a Zn or Ni could be electroplated directly onto a Mg or Mg alloy and then used as an undercoat for subsequent protective coating or surface treatment. However, in this electroplating method, some complicated pretreatments are required; moreover, these pretreatments are normally carried out in baths with highly toxic chromate or hydrofluoric acid [9,10]. Recently, we developed an electroplating process to obtain a protective Ni/Cu coating for pure Mg and Mg alloys using more environmentally benign baths [11,12]. In our electroplating procedure, an alkaline Cu-plating bath was used not only as an activation agent, but also as an electroplating bath. A Mg alloy specimen was activated by galvanostatic etching, and then Cu was electroplated directly on the specimen in the same bath. The Cu-coated Mg alloy could then be further electroplated in acidic Cu and Ni baths to achieve a Ni/Cu protective coating. Building on our previous results, in this work, we electroplated Cu onto AZ91D using an environmentally benign electroplating process. We then electrodeposited Cr using a trivalent Cr bath onto Cu-coated AZ91D, to improve its corrosion resistance. The corrosion behavior of the Cu-coated and Cr/Cu-coated AZ91D specimens was evaluated in 0.1 M H2SO4 containing different concentrations of NaCl. The morphologies of Cr/Cu-coated AZ91D specimens cor-
C.A. Huang et al. / Corrosion Science 52 (2010) 1326–1332
roded in 0.1 M H2SO4 containing 3.5 wt.% NaCl were investigated particularly.
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men were calculated using ten measurements taken on cross section of the specimen mounted in epoxy.
2. Experimental procedure 3. Results and discussion Commercial Mg–Al alloy (AZ91D) in bar form was shaped into a disc with a diameter of 12 mm and a thickness of 2.5 mm. The AZ91D disc specimen was used as the substrate for Cu and Cr electroplating. The chemical composition of the AZ91D specimen is given in Table 1. All electroplating processes and corrosion tests were conducted in an electrochemical three-electrode cell. The AZ91D disc specimen was used as the working electrode, while plantized Ti-mesh and Ag/AgCl in saturated KCl solution were used as the counter and the reference electrodes, respectively. Before Cu electroplating, the surface of the AZ91D substrate was mechanically ground with 600-grit emery paper, cleaned with de-ionized water, and dried with an air blaster. To obtain a uniform and dense Cu film on the AZ91D disc substrate, the latter was etched galvanostatically with a current density of 2 A dm 2 for 120 s and then electroplated in an alkaline Cu-sulfate solution bath [11]. The Cucoated AZ91D specimen was then further electroplated in an acid Cu bath with a current density of 4 A dm 2, to increase the thickness of the Cu layer. Finally, an anti-corrosion Cr coating was carried out by electroplating onto the Cu-coated AZ91D using an environmentally friendly trivalent Cr bath. Analytical grade H2SO4, NaCl, and de-ionized water were used to make 0.1 M H2SO4 solutions containing different NaCl concentrations for the corrosion tests. The responses of AZ91D, Cu-coated AZ91D and Cr/Cu-coated AZ91D to these solutions were evaluated using anodic polarization and open-circuit potential (OCP) tests at 25 °C. The anodic polarization behavior of a specimen was potentiodynamically evaluated at a scan rate of 5 mV/s from 0.25 V (vs. Ecorr) to 1.6 V (vs. Ag/AgClsat.), using a potentiostat/galvanostat (EG&G Model 263A). Variations in the OCP of Cu-coated, as-plated Cr/Cu-coated, and flame-heated Cr/Cu-coated AZ91D specimens were tested in 0.1 M H2SO4 with different NaCl concentrations for 2 h. The surface morphologies of Cr/Cu-coated specimens were examined with a scanning electron microscope (SEM, HITACH S3000 N). The hardnesses of AZ91D substrates and of as-plated and flame-heated Cr-coated specimens were measured via a micro-hardness tester (Matsuzawa Digital, Model MXT-a7e) with a 50 g load. Mean hardness and standard deviation for each speci-
Table 1 Chemical composition of AZ91D used in this study. Element
Al
Zn
Mn
Mg
Weight percent
8.88
0.66
0.13
Balance
3.1. Surface morphologies and interfacial structure Figs. 1(a) and (b) show the surface morphology and cross section of Cr/Cu-coated AZ91D that was electroplated in alkaline followed by acidic Cu-plating baths and then in a trivalent Cr plating bath. The Cr coating typically had a nodular surface morphology, which is shown in Fig. 1(a). It is visible in the cross-sectional SEM micrograph shown in Fig. 1(b), the AZ91D substrate was uniformly covered by a dense 4 lm-thick Cu layer and then a 20 lm-thick Cr layer. No interfacial defects were found, either at the boundary between the Cu deposit and AZ91D, or between the Cr and Cu. This finding demonstrates that Cu can be successfully electroplated onto an AZ91D specimen using our proposed electroplating process, in spite of the known difficulties in achieving uniform metal deposits on AZ91D by electroplating. As mentioned, the Cr electroplating process was preceded by a treatment of galvanostatic etching using a constant anodic current density of 2 A cm 2, followed by Cu electroplating in the same alkaline Cu-plating bath. We found the oxides that had formed on the Mg alloy surfaces were nearly removed using the galvanostatic etching, leading to an active surface for Cu electroplating [11,12]. As is clear from the SEM imaging, relatively few cracks are found in the 20 lm thick Cr deposit. Many researchers have demonstrated [13] that the Cr film obtained from electroplating in a trivalent bath has an amorphous structure with a few cracks that run completely through the deposit. These cracks cause Cr-coated specimens to have comparatively bad corrosion resistance. In this study, cracks in the Cr deposit were found to be relatively few and narrow when Cr deposited on a Cu-deposited AZ91D substrate. The pattern of crack information could be attributed to the soft Cu undercoat between the Cr deposit and the AZ91D substrate. Ni had the same effect on the morphology of the cracks in Cr deposits, as evidenced by our previous work [14]. Therefore, it may be expected that the Cr/Cu deposit should have relatively high corrosion resistance, due to minimization of the density and width of cracks completely spanning the electroplated layer. Figs. 2(a) and (b) show the surface morphology and cross section, respectively, of a Cr/Cu deposit on AZ91D after flame heating for 0.5 s. Flame heating was found to widen both superficial and penetrating cracks in the Cr/Cu coating. Therefore, corrosion resistance of as-plated Cr/Cu-coated AZ91D specimens may be expected to worsen after flame heating, due to the broadening of these cracks.
Fig. 1. The (a) surface morphology and (b) cross section of as-plated Cr/Cu-coated AZ91D specimen.
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Fig. 2. The (a) surface morphology and (b) cross section of flame-heated Cr/Cu-coated AZ91D specimen.
3.2. Hardness test The hardness values of AZ91D and of as-plated and flameheated Cr/Cu-coated AZ91D, are presented in Fig. 3. The hardness of AZ91D was only 90 Hv. A considerable increase in hardness, up to 740 Hv, was observed in the as-plated Cr deposit; moreover, the hardness increased, up to 1050 Hv, after flame heating for 0.5 s. The experimental results show that the surface hardness of AZ91D greatly increased after Cr electrodeposition and flame heating. In our previous study [15], we found that the noticeable increase in the hardness of flame-heated Cr deposits could be attributed to precipitation of diamond membranes in amorphous Cr deposits. This may also be the case here. The diamond membranes are transformed from amorphous carbon during crystallization of amorphous Cr deposits using flame heating. Based on the results of the hardness test, we might expect that the flame-hardened Cr/ Cu-coated AZ91D specimen should have high wear resistance. Although the wear resistance of the Cr/Cu coating on AZ91D is an important issue, it is not the focus of this work. 3.3. Electrochemical corrosion test Fig. 4 shows anodic polarization curves for AZ91D, as-plated Cr/ Cu-coated AZ91D, and flame-heated Cr/Cu-coated AZ91D specimens in 0.1 M H2SO4 solution. A low corrosion potential of
1.8 V (vs. Ag/AgClsat.) and a high corrosion current density of 2.3 10 3 A cm 2 were detected for the AZ91D substrate. The corrosion behavior indicates that the AZ91D substrate had a high corrosion rate and high chemical reactivity. After Cu electroplating in an acidic Cu-plating bath, the corrosion potential of AZ91D increased to 0.15 V. The Cu-coated AZ91D had a low corrosion current density of 4.2 10 6 A cm 2. However, Cu coatings are easily oxidized in atmosphere [16]. In this work, Cr electrodeposition was used as a protective coating for AZ91D because of its high oxidation resistance. As-expected, the corrosion current density of Cr/ Cu-coated AZ91D was only a few 10 6 A cm 2. Similar results for different protective coatings on Mg alloy specimens have been reported by several researchers, who also used an anti-corrosion coating as the final electroplating process to increase the corrosion resistance of a metal substrate [17,18]. That is, the Cr layer could act as an anti-corrosion coating for AZ91D. The corrosion potential of as-plated Cr/Cu-coated AZ91D was 0.27 V and shifted to 0.15 V after flame heating for 0.5 s. The corrosion current densities of as-plated and flame-heated Cr/Cu-coated AZ91D were almost equal. Fig. 5 illustrates open-circuit potentials (OCP) observed in 0.1 M H2SO4 for the AZ91D substrate, the Cu-coated AZ91D, and both asplated and flame-hardened Cr/Cu-coated AZ91D specimens. All specimens exhibited stable OCPs during 2-h immersion. As expected, the AZ91D substrate has the lowest OCP of ca. 1.8 V. This
1250
Hardness (Hv)
1000
750
500
250
0 AZ 91D
As-plated Cr/Cu-coated Specimen
Flame-heated Cr/Cu-coated
Fig. 3. Hardness values of AZ91D substrate, as-plated and flame-heated Cr coatings.
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1.6 1.2
AZ91D substrate As-plated Cu coated As-plated Cr/Cu coated Flame-hardened Cr/Cu coated
Potential (V) v.s. Ag/AgCl (sat.)
0.8 0.4 0.0 -0.4 -0.8 -1.2 -1.6 -2.0 1E-6
1E-5
1E-4
1E-3
0.01
0.1
-2
Current density (A cm ) Fig. 4. Anodic polarization behavior of AZ91D substrate, Cu-coated, as-plated and flame-hardened Cr/Cu-coated AZ91D specimens in 0.1 M H2SO4 solution.
AZ91D substrate As-plated Cu-coated
1.5
As-plated Cr/Cu-coated Potential (V) vs. Ag/AgCl(sat.)
1.0
Flame-hardened Cr/Cu-coated
0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 0
1200
2400
3600 4800 Immersion time (s)
6000
7200
Fig. 5. Variation of OCP of AZ91D substrate, Cu-coated, as-plated and flame-hardened Cr/Cu-coated AZ91D specimens in 0.1 M H2SO4 solution over 7200 s.
indicates that the AZ91D substrate was very active in the solution. As shown in Fig. 5, OCP values rose noticeably after Cu or Cr electroplating. The OCPs of as-plated and flame-heated Cr/Cu-coated AZ91D specimens were nearly identical and ca. 0.2 V. A slightly high OCP of 0.1 V was observed for Cu-deposited AZ91D. For applications, an understanding of the corrosion behavior of as-plated Cr/Cu-coated AZ91D is necessary, because such materials may be used directly following Cr electroplating. We studied mainly the corrosion behavior of as-plated Cr/Cu-coated AZ91D in 0.1 M H2SO4 solution, varying the NaCl content up to 7 wt.%. Variations of OCP values of as-plated Cr/Cu-coated AZ91D specimens under these conditions are shown in Fig. 6. Evidently, the OCP of as-plated Cr/Cu-coated AZ91D decreased with increasing NaCl content, and a marked potential drop was observed when the NaCl content was raised above 3.5 wt.%. The potential dropped from approximately 0.2 V to 0.6 V, and this drop occurred more
early with increasing NaCl content. The potential drop observed in the solution with 7 wt.% NaCl took place near the beginning of immersion, indicating that the as-plated Cr/Cu-coated AZ91D was corroded immediately in this solution. 3.4. Corrosive morphology study As illustrated in Fig. 7, a few blisters were observed on the surface of as-plated Cr/Cu-coated AZ91D, after a potential drop was induced by immersion in 0.1 M H2SO4 with 3.5 wt.% NaCl. Since the AZ91D substrate has a low corrosion potential, it may be expected that the AZ91D substrate was exposed directly to the solution after the potential drop. However, the SEM data showed only a surface with a few blisters. This implies that the corrosion of asplated Cr/Cu-coated AZ91D occurred primarily within blisters, leading to a relatively low corrosion potential.
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0.1 M H2SO4 solution + Without NaCl 1 wt% NaCl 3.5 wt% NaCl 5 wt% NaCl 7 wt% NaCl
Potential (V) vs. Ag/AgCl(sat.)
0.2
0.0
-0.2
-0.4
-0.6
-0.8 0
1200
2400 3600 4800 Immersion time (s)
6000
7200
Fig. 6. Variation of OCP of as-plated Cr/Cu-coated AZ91D specimens in 0.1 M H2SO4 with different NaCl concentrations.
Fig. 7. Surface morphology of an as-plated Cr/Cu-coated AZ91D specimen after observation of a potential drop in 0.1 M H2SO4 with 3.5 wt.% NaCl.
To study the corrosion behavior of as-plated Cr/Cu-coated AZ91D immersed in 0.1 M H2SO4 with 3.5 wt.% NaCl, surface morphologies and cross sections were examined after immersion for periods I, II and III, corresponding to those before, during and after the potential drop as indicated in Fig. 8. Figs. 9(a) and (b) show the surface morphology and cross-sectional view of as-plated Cr/Cu-coated AZ91D immersed in 0.1 M H2SO4 with 3.5 wt.% NaCl during period I. A slightly etched surface was observed, as in Fig. 9(a), but Fig. 9(b) shows that the Cu layer adjacent to the Cr layer was preferentially etched, particularly at sites directly under cracks penetrating through the Cr layer. These etching results indicate that such penetrating cracks provided corrosive pathways for the solution to attack the Cu deposit. Interestingly, the corrosion potential of Cu-coated AZ91D was a little higher than that of Cr/Cu-coated AZ91D, as illustrated in Fig. 5. Nevertheless, the Cu layer was etched preferentially when galvanically coupled to the Cr layer. This implies the Cr deposit became more passive than the Cu deposit when immersed in 0.1 M H2SO4 with 3.5 wt.% NaCl.
0.0
Potential (V) vs. Ag/AgCl(sat.)
I -0.2
II
III
-0.4
-0.6
-0.8 0
1200
2400
3600
4800
6000
7200
Immersion time (s) Fig. 8. Immersion periods I, II and III corresponding to those before, during, and after a potential drop was observed in 0.1 M H2SO4 with 3.5 wt.% NaCl.
C.A. Huang et al. / Corrosion Science 52 (2010) 1326–1332
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Fig. 9. The (a) surface morphology and (b) cross section of as-plated Cr/Cu-coated AZ91D specimen after immersion for period I (before potential drop) in 0.1 M H2SO4 with 3.5 wt.% NaCl.
Fig. 10. The (a) surface morphology and (b) cross section of as-plated Cr/Cu-coated AZ91D specimen after immersing for period II (within potential drop) in 0.1 M H2SO4 with 3.5 wt.% NaCl.
When immersed for period II, many blisters could be seen on the surface of as-plated Cr/Cu-coated AZ91D. As shown in Fig. 10(a), in views of the surface morphology, the blisters seem unbroken. From the sample cross section, as shown in Fig. 10(b), the AZ91D substrate alongside the Cu layer was severely etched. The severely etched AZ91D can be found directly beneath the engraved Cu layer, which itself was preferentially corroded near penetrating cracks in the Cr layer. As the AZ91D was etched, the nearby Cr/Cu bilayer swelled and lifted off of the AZ91D surface, resulting in formation of a blister on the Cr/Cu surface. When the AZ91D was exposed in 0.1 M H2SO4 with 3.5 wt.% NaCl, the speci-
men was dissolved substantially, and the corrosion potential of asplated Cr/Cu-coated AZ91D decreased rapidly at the period II. As shown in Fig. 11, a broken Cr/Cu layer was seen in the surface morphology of as-plated Cr/Cu-coated AZ91D after immersion for period III. Broken sites were above blisters. Due to the relatively large exposed area of AZ91D under the broken sites, low corrosion potentials were detected after immersion for longer than period III (as defined in Fig. 8). The results of the OCP tests in 0.1 M H2SO4 with 3.5 wt.% NaCl for as-plated and flame-hardened Cr/Cu-coated AZ91D specimens are shown in Fig. 12. A potential drop is clearly observable. The blistered morphology of corroded, flame-hardened Cr/Cu-coated AZ91D was similar to that of as-plated samples after observation of a potential drop. However, the immersion period required for observation of the potential drop was relatively short for the flame-hardened Cr/Cu-coated AZ91D specimen, as shown in Fig. 8. The potential data indicate that the corrosion resistance of as-plated Cr/Cu-coated AZ91D decreased after flame heating for 0.5 s. irrespective of its high hardness. This can be explained by the increased presence and width of penetrating cracks in the flame-hardened samples (see Fig. 2), since corrosion occurred mainly through these cracks. 4. Conclusions
Fig. 11. The surface morphology of as-plated Cr/Cu-coated AZ91D specimen after immersion for period III (after potential drop) in 0.1 M H2SO4 with 3.5 wt.% NaCl.
An environmentally benign electroplating process was proposed to obtain a protective Cr/Cu deposit on an AZ91D substrate. The corrosion behavior of Cu-coated, as-plated Cr/Cu-coated, and flame-heated Cr/Cu-coated AZ91D specimens were electrochemically evaluated in 0.1 M H2SO4 solution with different NaCl concentrations. The corrosion resistance of an AZ91D specimen could be significantly improved after electrodeposition of Cr layer. Cracks
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0.0
As-plated Cr/Cu-coated
Potential (V) vs. Ag/AgCl(sat.)
Flame-hardened Cr/Cu-coated -0.2
-0.4
-0.6
-0.8 0
1000
2000
3000
4000
5000
6000
7000
Immersion time (s) Fig. 12. Variation of OCP of as-plated and flame-hardened Cr/Cu-coated AZ91D specimens in 0.1 M H2SO4 with 3.5 wt.% NaCl.
penetrating the Cr layer provided active pathways for corrosion of the Cu deposit, and subsequently the AZ91D substrate, in 0.1 M H2SO4 solutions with NaCl content higher than 3.5 wt.%. A few blisters were visible on the surface of Cr/Cu-coated AZ91D after immersion in 0.1 M H2SO4 solution with a NaCl-content above 3.5 wt.%. Formation of blisters on the surfaces was confirmed by the results of the open-circuit potential test, in which an obvious drop of open-circuit potential from noble to active potentials was detected. Acknowledgements The author, Ching An Huang, would like to thank the National Science Council ROC, for financial support for this research under contract number: NSC 97-2221-E-182-007. References [1] P. Schmutz, V. Guillaumin, R.S. Lillard, J.A. Lillard, G.S. Frankel, J. Electrochem. Soc. 150 (4) (2003) B99.
[2] K.Z. Chong, T.S. Shih, Mater. Chem. Phys. 80 (2003) 191. [3] Y. Mizutani, S.J. Kim, R. Ichino, M. Okido, Surf. Coat. Technol. 169–170 (2003) 143. [4] Y.Q. Wang, M.Y. Zheng, K. Wu, Mater. Lett. 59 (2005) 1727. [5] Y.W. Song, D.Y. Shan, E.H. Han, Electrochim. Acta (2008) 2135. [6] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88. [7] A.K. Sharma, H. Narayanamurthy, H. Bhojarej, J.M. Mohideem, Met. Finish. 91 (1993) 34. [8] J.L. Luo, N. Cui, J. Alloys Compd. 265 (1998) 305. [9] R. Ambat, W. Zhou, Surf. Coat. Technol. 179 (2004) 124. [10] Z. Liu, W. Gao, Surf. Coat. Technol. 200 (2006) 3553. [11] C.A. Huang, T.H. Wang, T. Weirich, V. Neubert, Electrochim. Acta 53 (2008) 7235. [12] C.A. Huang, T.H. Wang, T. Weirich, V. Neubert, Corros. Sci. 50 (2008) 1385. [13] M.E. Bahrololoom, A. Hoveidaei, Surf. Eng. 15 (6) (1999) 502. [14] C.A. Huang, Ui Wei Lieu, Chin Huo Chuang, Surf. Coat.Technol. 203 (2009) 2921. [15] C.A. Huang, Yu Wei Liu, Ching Hao Chuang, Thin Solid Films 517 (2009) 4902. [16] O. Kubaschewski, B.E. Hopkins, in: G.T. Burstein, L.L. Shreir, R.A. Jarman (Eds.), Corrosion, vol. 1, Elsevier, Butterworth-Heinemann, 1994 (chapter 4). [17] C. Gu, J. Lian, J. He, Z. Jiang, Q. Jiang, Surf. Coat. Technol. 200 (2006) 5413. [18] Z.A. Hamid, I.M. Ghayad, Mater. Lett. 53 (2002) 238.
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Corrosion behavior of Cr/Cu-coated Mg alloy (AZ91D) in 0.1 M H2SO4 with different concentrations of NaCl Ching An Huang *, Che Kuan Lin, Yu Hu Yeh Department of Mechanical Engineering, Chang Gung University, Taoyuan 333, Taiwan
a r t i c l e
i n f o
Article history: Received 31 August 2009 Accepted 14 September 2009 Available online 11 December 2009 Keywords: A. Mg alloy A. Cr deposit C. Corrosion resistance C. Blister
a b s t r a c t An electroplating process was proposed for obtaining a protective Cr/Cu deposit on the two-phase Mg alloy AZ91D. The corrosion behavior of Cu-covered and Cr/Cu-covered AZ91D specimens was studied electrochemically in 0.1 M H2SO4 with different NaCl concentrations. Experimental results showed that the corrosion resistance of an AZ91D specimen improved significantly after Cr/Cu electrodeposition. The corrosion resistance of Cr/Cu-covered AZ91D decreased with increasing NaCl concentration in 0.1 M H2SO4 solution. After immersion in a 0.1 M H2SO4 with a NaCl-content above 3.5 wt.%, the surface of Cr/Cu-covered AZ91D suffered a few blisters. Cracks through the Cr deposit provided active pathways for corrosion of the Cu and the AZ91D substrate. Formation of blisters on the Cr/Cu-covered AZ91D surface was confirmed based on the results of an open-circuit potential test, which detected an obvious potential drop from noble to active potentials. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium alloys are widely used as components in the electronic, automobile and aerospace engineering industries, due to their low density and high specific strength, as well as their excellent heat dissipation and electromagnetic interference (EMI) shielding properties. However, many potential applications of Mg alloys are prohibited or severely limited, due to their high chemical activity and corrosion susceptibility. In wet environments or even in air, magnesium oxide or hydroxide films develop easily on Mg alloys [1]. These Mg oxide or hydroxide films are loose and porous, and therefore do not provide corrosion protection [1]. Thus, formation of a protective coating or film on a component made of Mg alloy has been an important point of study in recent years. Several surface treatments have been examined for the protection of Mg alloys, including chemical conversion [2], anodization [3], micro-arc oxidation [4], electroless plating [5], and electroplating [6]. Among these, the electroplating process has some distinct advantages, such as coating reliability, convenience, and relatively low cost. Therefore, development of a protective coating process for Mg alloys using electroplating would be both useful and practical. Due to its relatively high strength, amenability to casting, and corrosion resistance, AZ91D is commonly die-cast to form components for automobiles, computers, communication hardware, and consumer electronics (3C). Generally, two phases, eutectic a and * Corresponding author. Tel.: +886 3 2118800x5655. E-mail address: [email protected] (C.A. Huang). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.12.010
intermetallic b (Mg17Al12), are identifiable in AZ91D. Together, these two phases act as a galvanic couple and they have different discharging capacities in aqueous solution. As a result, a non-uniform surface morphology is typically obtained when AZ91D is immersed in an aqueous solution, or in an electroplating bath [6]. Therefore, it is relatively difficult to use electroplating to deposit a protective coating for AZ91D. Many researchers [7,8] have proposed that a Zn or Ni could be electroplated directly onto a Mg or Mg alloy and then used as an undercoat for subsequent protective coating or surface treatment. However, in this electroplating method, some complicated pretreatments are required; moreover, these pretreatments are normally carried out in baths with highly toxic chromate or hydrofluoric acid [9,10]. Recently, we developed an electroplating process to obtain a protective Ni/Cu coating for pure Mg and Mg alloys using more environmentally benign baths [11,12]. In our electroplating procedure, an alkaline Cu-plating bath was used not only as an activation agent, but also as an electroplating bath. A Mg alloy specimen was activated by galvanostatic etching, and then Cu was electroplated directly on the specimen in the same bath. The Cu-coated Mg alloy could then be further electroplated in acidic Cu and Ni baths to achieve a Ni/Cu protective coating. Building on our previous results, in this work, we electroplated Cu onto AZ91D using an environmentally benign electroplating process. We then electrodeposited Cr using a trivalent Cr bath onto Cu-coated AZ91D, to improve its corrosion resistance. The corrosion behavior of the Cu-coated and Cr/Cu-coated AZ91D specimens was evaluated in 0.1 M H2SO4 containing different concentrations of NaCl. The morphologies of Cr/Cu-coated AZ91D specimens cor-
C.A. Huang et al. / Corrosion Science 52 (2010) 1326–1332
roded in 0.1 M H2SO4 containing 3.5 wt.% NaCl were investigated particularly.
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men were calculated using ten measurements taken on cross section of the specimen mounted in epoxy.
2. Experimental procedure 3. Results and discussion Commercial Mg–Al alloy (AZ91D) in bar form was shaped into a disc with a diameter of 12 mm and a thickness of 2.5 mm. The AZ91D disc specimen was used as the substrate for Cu and Cr electroplating. The chemical composition of the AZ91D specimen is given in Table 1. All electroplating processes and corrosion tests were conducted in an electrochemical three-electrode cell. The AZ91D disc specimen was used as the working electrode, while plantized Ti-mesh and Ag/AgCl in saturated KCl solution were used as the counter and the reference electrodes, respectively. Before Cu electroplating, the surface of the AZ91D substrate was mechanically ground with 600-grit emery paper, cleaned with de-ionized water, and dried with an air blaster. To obtain a uniform and dense Cu film on the AZ91D disc substrate, the latter was etched galvanostatically with a current density of 2 A dm 2 for 120 s and then electroplated in an alkaline Cu-sulfate solution bath [11]. The Cucoated AZ91D specimen was then further electroplated in an acid Cu bath with a current density of 4 A dm 2, to increase the thickness of the Cu layer. Finally, an anti-corrosion Cr coating was carried out by electroplating onto the Cu-coated AZ91D using an environmentally friendly trivalent Cr bath. Analytical grade H2SO4, NaCl, and de-ionized water were used to make 0.1 M H2SO4 solutions containing different NaCl concentrations for the corrosion tests. The responses of AZ91D, Cu-coated AZ91D and Cr/Cu-coated AZ91D to these solutions were evaluated using anodic polarization and open-circuit potential (OCP) tests at 25 °C. The anodic polarization behavior of a specimen was potentiodynamically evaluated at a scan rate of 5 mV/s from 0.25 V (vs. Ecorr) to 1.6 V (vs. Ag/AgClsat.), using a potentiostat/galvanostat (EG&G Model 263A). Variations in the OCP of Cu-coated, as-plated Cr/Cu-coated, and flame-heated Cr/Cu-coated AZ91D specimens were tested in 0.1 M H2SO4 with different NaCl concentrations for 2 h. The surface morphologies of Cr/Cu-coated specimens were examined with a scanning electron microscope (SEM, HITACH S3000 N). The hardnesses of AZ91D substrates and of as-plated and flame-heated Cr-coated specimens were measured via a micro-hardness tester (Matsuzawa Digital, Model MXT-a7e) with a 50 g load. Mean hardness and standard deviation for each speci-
Table 1 Chemical composition of AZ91D used in this study. Element
Al
Zn
Mn
Mg
Weight percent
8.88
0.66
0.13
Balance
3.1. Surface morphologies and interfacial structure Figs. 1(a) and (b) show the surface morphology and cross section of Cr/Cu-coated AZ91D that was electroplated in alkaline followed by acidic Cu-plating baths and then in a trivalent Cr plating bath. The Cr coating typically had a nodular surface morphology, which is shown in Fig. 1(a). It is visible in the cross-sectional SEM micrograph shown in Fig. 1(b), the AZ91D substrate was uniformly covered by a dense 4 lm-thick Cu layer and then a 20 lm-thick Cr layer. No interfacial defects were found, either at the boundary between the Cu deposit and AZ91D, or between the Cr and Cu. This finding demonstrates that Cu can be successfully electroplated onto an AZ91D specimen using our proposed electroplating process, in spite of the known difficulties in achieving uniform metal deposits on AZ91D by electroplating. As mentioned, the Cr electroplating process was preceded by a treatment of galvanostatic etching using a constant anodic current density of 2 A cm 2, followed by Cu electroplating in the same alkaline Cu-plating bath. We found the oxides that had formed on the Mg alloy surfaces were nearly removed using the galvanostatic etching, leading to an active surface for Cu electroplating [11,12]. As is clear from the SEM imaging, relatively few cracks are found in the 20 lm thick Cr deposit. Many researchers have demonstrated [13] that the Cr film obtained from electroplating in a trivalent bath has an amorphous structure with a few cracks that run completely through the deposit. These cracks cause Cr-coated specimens to have comparatively bad corrosion resistance. In this study, cracks in the Cr deposit were found to be relatively few and narrow when Cr deposited on a Cu-deposited AZ91D substrate. The pattern of crack information could be attributed to the soft Cu undercoat between the Cr deposit and the AZ91D substrate. Ni had the same effect on the morphology of the cracks in Cr deposits, as evidenced by our previous work [14]. Therefore, it may be expected that the Cr/Cu deposit should have relatively high corrosion resistance, due to minimization of the density and width of cracks completely spanning the electroplated layer. Figs. 2(a) and (b) show the surface morphology and cross section, respectively, of a Cr/Cu deposit on AZ91D after flame heating for 0.5 s. Flame heating was found to widen both superficial and penetrating cracks in the Cr/Cu coating. Therefore, corrosion resistance of as-plated Cr/Cu-coated AZ91D specimens may be expected to worsen after flame heating, due to the broadening of these cracks.
Fig. 1. The (a) surface morphology and (b) cross section of as-plated Cr/Cu-coated AZ91D specimen.
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Fig. 2. The (a) surface morphology and (b) cross section of flame-heated Cr/Cu-coated AZ91D specimen.
3.2. Hardness test The hardness values of AZ91D and of as-plated and flameheated Cr/Cu-coated AZ91D, are presented in Fig. 3. The hardness of AZ91D was only 90 Hv. A considerable increase in hardness, up to 740 Hv, was observed in the as-plated Cr deposit; moreover, the hardness increased, up to 1050 Hv, after flame heating for 0.5 s. The experimental results show that the surface hardness of AZ91D greatly increased after Cr electrodeposition and flame heating. In our previous study [15], we found that the noticeable increase in the hardness of flame-heated Cr deposits could be attributed to precipitation of diamond membranes in amorphous Cr deposits. This may also be the case here. The diamond membranes are transformed from amorphous carbon during crystallization of amorphous Cr deposits using flame heating. Based on the results of the hardness test, we might expect that the flame-hardened Cr/ Cu-coated AZ91D specimen should have high wear resistance. Although the wear resistance of the Cr/Cu coating on AZ91D is an important issue, it is not the focus of this work. 3.3. Electrochemical corrosion test Fig. 4 shows anodic polarization curves for AZ91D, as-plated Cr/ Cu-coated AZ91D, and flame-heated Cr/Cu-coated AZ91D specimens in 0.1 M H2SO4 solution. A low corrosion potential of
1.8 V (vs. Ag/AgClsat.) and a high corrosion current density of 2.3 10 3 A cm 2 were detected for the AZ91D substrate. The corrosion behavior indicates that the AZ91D substrate had a high corrosion rate and high chemical reactivity. After Cu electroplating in an acidic Cu-plating bath, the corrosion potential of AZ91D increased to 0.15 V. The Cu-coated AZ91D had a low corrosion current density of 4.2 10 6 A cm 2. However, Cu coatings are easily oxidized in atmosphere [16]. In this work, Cr electrodeposition was used as a protective coating for AZ91D because of its high oxidation resistance. As-expected, the corrosion current density of Cr/ Cu-coated AZ91D was only a few 10 6 A cm 2. Similar results for different protective coatings on Mg alloy specimens have been reported by several researchers, who also used an anti-corrosion coating as the final electroplating process to increase the corrosion resistance of a metal substrate [17,18]. That is, the Cr layer could act as an anti-corrosion coating for AZ91D. The corrosion potential of as-plated Cr/Cu-coated AZ91D was 0.27 V and shifted to 0.15 V after flame heating for 0.5 s. The corrosion current densities of as-plated and flame-heated Cr/Cu-coated AZ91D were almost equal. Fig. 5 illustrates open-circuit potentials (OCP) observed in 0.1 M H2SO4 for the AZ91D substrate, the Cu-coated AZ91D, and both asplated and flame-hardened Cr/Cu-coated AZ91D specimens. All specimens exhibited stable OCPs during 2-h immersion. As expected, the AZ91D substrate has the lowest OCP of ca. 1.8 V. This
1250
Hardness (Hv)
1000
750
500
250
0 AZ 91D
As-plated Cr/Cu-coated Specimen
Flame-heated Cr/Cu-coated
Fig. 3. Hardness values of AZ91D substrate, as-plated and flame-heated Cr coatings.
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1.6 1.2
AZ91D substrate As-plated Cu coated As-plated Cr/Cu coated Flame-hardened Cr/Cu coated
Potential (V) v.s. Ag/AgCl (sat.)
0.8 0.4 0.0 -0.4 -0.8 -1.2 -1.6 -2.0 1E-6
1E-5
1E-4
1E-3
0.01
0.1
-2
Current density (A cm ) Fig. 4. Anodic polarization behavior of AZ91D substrate, Cu-coated, as-plated and flame-hardened Cr/Cu-coated AZ91D specimens in 0.1 M H2SO4 solution.
AZ91D substrate As-plated Cu-coated
1.5
As-plated Cr/Cu-coated Potential (V) vs. Ag/AgCl(sat.)
1.0
Flame-hardened Cr/Cu-coated
0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 0
1200
2400
3600 4800 Immersion time (s)
6000
7200
Fig. 5. Variation of OCP of AZ91D substrate, Cu-coated, as-plated and flame-hardened Cr/Cu-coated AZ91D specimens in 0.1 M H2SO4 solution over 7200 s.
indicates that the AZ91D substrate was very active in the solution. As shown in Fig. 5, OCP values rose noticeably after Cu or Cr electroplating. The OCPs of as-plated and flame-heated Cr/Cu-coated AZ91D specimens were nearly identical and ca. 0.2 V. A slightly high OCP of 0.1 V was observed for Cu-deposited AZ91D. For applications, an understanding of the corrosion behavior of as-plated Cr/Cu-coated AZ91D is necessary, because such materials may be used directly following Cr electroplating. We studied mainly the corrosion behavior of as-plated Cr/Cu-coated AZ91D in 0.1 M H2SO4 solution, varying the NaCl content up to 7 wt.%. Variations of OCP values of as-plated Cr/Cu-coated AZ91D specimens under these conditions are shown in Fig. 6. Evidently, the OCP of as-plated Cr/Cu-coated AZ91D decreased with increasing NaCl content, and a marked potential drop was observed when the NaCl content was raised above 3.5 wt.%. The potential dropped from approximately 0.2 V to 0.6 V, and this drop occurred more
early with increasing NaCl content. The potential drop observed in the solution with 7 wt.% NaCl took place near the beginning of immersion, indicating that the as-plated Cr/Cu-coated AZ91D was corroded immediately in this solution. 3.4. Corrosive morphology study As illustrated in Fig. 7, a few blisters were observed on the surface of as-plated Cr/Cu-coated AZ91D, after a potential drop was induced by immersion in 0.1 M H2SO4 with 3.5 wt.% NaCl. Since the AZ91D substrate has a low corrosion potential, it may be expected that the AZ91D substrate was exposed directly to the solution after the potential drop. However, the SEM data showed only a surface with a few blisters. This implies that the corrosion of asplated Cr/Cu-coated AZ91D occurred primarily within blisters, leading to a relatively low corrosion potential.
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0.1 M H2SO4 solution + Without NaCl 1 wt% NaCl 3.5 wt% NaCl 5 wt% NaCl 7 wt% NaCl
Potential (V) vs. Ag/AgCl(sat.)
0.2
0.0
-0.2
-0.4
-0.6
-0.8 0
1200
2400 3600 4800 Immersion time (s)
6000
7200
Fig. 6. Variation of OCP of as-plated Cr/Cu-coated AZ91D specimens in 0.1 M H2SO4 with different NaCl concentrations.
Fig. 7. Surface morphology of an as-plated Cr/Cu-coated AZ91D specimen after observation of a potential drop in 0.1 M H2SO4 with 3.5 wt.% NaCl.
To study the corrosion behavior of as-plated Cr/Cu-coated AZ91D immersed in 0.1 M H2SO4 with 3.5 wt.% NaCl, surface morphologies and cross sections were examined after immersion for periods I, II and III, corresponding to those before, during and after the potential drop as indicated in Fig. 8. Figs. 9(a) and (b) show the surface morphology and cross-sectional view of as-plated Cr/Cu-coated AZ91D immersed in 0.1 M H2SO4 with 3.5 wt.% NaCl during period I. A slightly etched surface was observed, as in Fig. 9(a), but Fig. 9(b) shows that the Cu layer adjacent to the Cr layer was preferentially etched, particularly at sites directly under cracks penetrating through the Cr layer. These etching results indicate that such penetrating cracks provided corrosive pathways for the solution to attack the Cu deposit. Interestingly, the corrosion potential of Cu-coated AZ91D was a little higher than that of Cr/Cu-coated AZ91D, as illustrated in Fig. 5. Nevertheless, the Cu layer was etched preferentially when galvanically coupled to the Cr layer. This implies the Cr deposit became more passive than the Cu deposit when immersed in 0.1 M H2SO4 with 3.5 wt.% NaCl.
0.0
Potential (V) vs. Ag/AgCl(sat.)
I -0.2
II
III
-0.4
-0.6
-0.8 0
1200
2400
3600
4800
6000
7200
Immersion time (s) Fig. 8. Immersion periods I, II and III corresponding to those before, during, and after a potential drop was observed in 0.1 M H2SO4 with 3.5 wt.% NaCl.
C.A. Huang et al. / Corrosion Science 52 (2010) 1326–1332
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Fig. 9. The (a) surface morphology and (b) cross section of as-plated Cr/Cu-coated AZ91D specimen after immersion for period I (before potential drop) in 0.1 M H2SO4 with 3.5 wt.% NaCl.
Fig. 10. The (a) surface morphology and (b) cross section of as-plated Cr/Cu-coated AZ91D specimen after immersing for period II (within potential drop) in 0.1 M H2SO4 with 3.5 wt.% NaCl.
When immersed for period II, many blisters could be seen on the surface of as-plated Cr/Cu-coated AZ91D. As shown in Fig. 10(a), in views of the surface morphology, the blisters seem unbroken. From the sample cross section, as shown in Fig. 10(b), the AZ91D substrate alongside the Cu layer was severely etched. The severely etched AZ91D can be found directly beneath the engraved Cu layer, which itself was preferentially corroded near penetrating cracks in the Cr layer. As the AZ91D was etched, the nearby Cr/Cu bilayer swelled and lifted off of the AZ91D surface, resulting in formation of a blister on the Cr/Cu surface. When the AZ91D was exposed in 0.1 M H2SO4 with 3.5 wt.% NaCl, the speci-
men was dissolved substantially, and the corrosion potential of asplated Cr/Cu-coated AZ91D decreased rapidly at the period II. As shown in Fig. 11, a broken Cr/Cu layer was seen in the surface morphology of as-plated Cr/Cu-coated AZ91D after immersion for period III. Broken sites were above blisters. Due to the relatively large exposed area of AZ91D under the broken sites, low corrosion potentials were detected after immersion for longer than period III (as defined in Fig. 8). The results of the OCP tests in 0.1 M H2SO4 with 3.5 wt.% NaCl for as-plated and flame-hardened Cr/Cu-coated AZ91D specimens are shown in Fig. 12. A potential drop is clearly observable. The blistered morphology of corroded, flame-hardened Cr/Cu-coated AZ91D was similar to that of as-plated samples after observation of a potential drop. However, the immersion period required for observation of the potential drop was relatively short for the flame-hardened Cr/Cu-coated AZ91D specimen, as shown in Fig. 8. The potential data indicate that the corrosion resistance of as-plated Cr/Cu-coated AZ91D decreased after flame heating for 0.5 s. irrespective of its high hardness. This can be explained by the increased presence and width of penetrating cracks in the flame-hardened samples (see Fig. 2), since corrosion occurred mainly through these cracks. 4. Conclusions
Fig. 11. The surface morphology of as-plated Cr/Cu-coated AZ91D specimen after immersion for period III (after potential drop) in 0.1 M H2SO4 with 3.5 wt.% NaCl.
An environmentally benign electroplating process was proposed to obtain a protective Cr/Cu deposit on an AZ91D substrate. The corrosion behavior of Cu-coated, as-plated Cr/Cu-coated, and flame-heated Cr/Cu-coated AZ91D specimens were electrochemically evaluated in 0.1 M H2SO4 solution with different NaCl concentrations. The corrosion resistance of an AZ91D specimen could be significantly improved after electrodeposition of Cr layer. Cracks
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0.0
As-plated Cr/Cu-coated
Potential (V) vs. Ag/AgCl(sat.)
Flame-hardened Cr/Cu-coated -0.2
-0.4
-0.6
-0.8 0
1000
2000
3000
4000
5000
6000
7000
Immersion time (s) Fig. 12. Variation of OCP of as-plated and flame-hardened Cr/Cu-coated AZ91D specimens in 0.1 M H2SO4 with 3.5 wt.% NaCl.
penetrating the Cr layer provided active pathways for corrosion of the Cu deposit, and subsequently the AZ91D substrate, in 0.1 M H2SO4 solutions with NaCl content higher than 3.5 wt.%. A few blisters were visible on the surface of Cr/Cu-coated AZ91D after immersion in 0.1 M H2SO4 solution with a NaCl-content above 3.5 wt.%. Formation of blisters on the surfaces was confirmed by the results of the open-circuit potential test, in which an obvious drop of open-circuit potential from noble to active potentials was detected. Acknowledgements The author, Ching An Huang, would like to thank the National Science Council ROC, for financial support for this research under contract number: NSC 97-2221-E-182-007. References [1] P. Schmutz, V. Guillaumin, R.S. Lillard, J.A. Lillard, G.S. Frankel, J. Electrochem. Soc. 150 (4) (2003) B99.
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