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Increasing the wear and corrosion resistance of magnesium alloy (AZ91D) with electrodeposition from eco-friendly copper- and trivalent chromium-plating baths
Surface & Coatings Technology 205 (2010) 139–145
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Increasing the wear and corrosion resistance of magnesium alloy (AZ91D) with electrodeposition from eco-friendly copper- and trivalent chromium-plating baths 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 22 December 2009 Accepted in revised form 5 June 2010 Available online 15 June 2010 Keywords: Mg alloy Trivalent Cr electrodeposition Wear resistance Corrosion resistance
a b s t r a c t Mg alloy, AZ91D, which has a two-phase structure, was successfully electroplated in an alkaline Cu-plating bath. The Cu-coated Mg alloy specimen was further electroplated in eco-friendly acidic Cu and then trivalent Cr baths to obtain an anti-wear and anti-corrosion Cr/Cu coating. Experimental results show that the wear and corrosion resistance of the Mg alloy specimen was considerably improved by trivalent Cr electrodeposition. The hardness of the as-plated Cr deposit was drastically increased by using reductionflame heating for 0.5 s. The above-mentioned results were measured via bonding strength, hardness, wear and corrosion tests. A superior wear and corrosion resistance was obtained when a Cu-coated Mg alloy specimen was electroplated with a trivalent Cr deposit, followed by heating with reduction-flame heating for 0.5 s. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Mg alloys have been used as components in the electronic, automobile, and aerospace engineering industries because they have low density, a high specific strength ratio, good heat dissipation, and electromagnetic interference (EMI) shielding. Applications of Mg and Mg alloys are severely limited due to their high chemical activity. Schmutz et al. [1] pointed out that magnesium oxides or hydroxide layers can easily develop on the surfaces of Mg alloys exposed to wet environments, or even in air. Therefore, it is necessary to apply a protective coating or film on structural components made of Mg or an Mg alloy. Several surface treatments have been adopted to generate protective films or surface coatings for Mg and Mg alloys [2–4]. These surface films not only improve the corrosion resistance of Mg alloys but also provide an undercoat for further surface treatments [5–7]. However, most of the treatments have been carried out with a pretreatment involving soaking the Mg alloy in chromate or hydrofluoric acid for surface activation [8,9]. Typical surface treatments, including chemical conversion coating [10], anodizing [11], micro-arc oxidation treatment [12], and electroplating [13], have been proposed for Mg alloys. Among them, electroplating has some distinct advantages, such as reliable coating quality, convenient operation and a relatively low cost. Therefore, developing a protective coating for Mg alloys using electroplating would be a practical and useful accomplishment.
⁎ Corresponding author. Tel.: + 886 3 2118800x5655. E-mail address: [email protected] (C.A. Huang). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.06.012
Due to its relatively high strength, good casting quality and corrosion resistance, AZ91D is an appropriate Mg alloy for die casting to fabricate automobile parts and construction materials for computers, communications, and consumer-electronic (3C) products. Nevertheless, the eutectic α and intermetallic β (Mg17Al12) phases in the AZ91D structure could act as a galvanic couple leading to a non-uniform surface during surface treatment in an aqueous solution. Gray et al. [13] demonstrated that the non-uniform surface morphology results from different discharging capacities between α and β phases. Therefore, it is difficult to develop a suitable pretreatment to obtain an undercoat on which a protective metal deposit can be electroplated. Many researchers [14,15] have proposed that zinc or nickel could be directly plated onto magnesium and used as an undercoat for a subsequent protective coating. Most pretreatment processes proposed for Mg alloys are relatively complicated [16]; furthermore, pretreatment is performed in a bath with highly toxic chromate or hydrofluoric acid [8,9]. We have proposed an eco-friendly electroplating process to achieve a protective Ni/Cu deposit on pure Mg, AZ31, and AZ61 Mg alloys [17,18]. In our proposed electroplating process, an activated surface is generated on a Mg alloy via galvanostatic etching in an alkaline Cu-plating bath. Cu electroplating is performed immediately after galvanostatic etching in the same alkaline Cu-plating bath. Then the Cu-coated Mg alloy can be further electroplated in acidic Cu and Ni baths to achieve a Ni/Cu protective coating. Based on our previous experimental results, in this work we attempted to electroplate a Cu deposit on AZ91D with an eco-friendly electroplating process. Furthermore, trivalent Cr electrodeposition was conducted on Cu-coated AZ91D to improve its wear and corrosion resistance. The wear resistance, corrosion resistance, hardness and bonding strength were evaluated for Cu-coated and Cr/Cu-coated AZ91D specimens.
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Table 1 Chemical composition of AZ 91D used in this study. Element
Al
Zn
Mn
Mg
Weight percent
8.88
0.66
0.13
Balance
2. Experimental procedure Commercial Mg–Al alloy (AZ91D) in bar form was shaped into a disc form 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 were conducted in a twoelectrode cell. The AZ91D disc specimen was used as the working electrode and a plantized Ti mesh as the counter electrode. 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 remove the oxides and obtain a uniform Cu deposit on the AZ91D disc substrate, the substrate was galvanostatically etched with a current density of 2 A dm− 2 for 120 s and then electroplated with a current density of 4 A dm− 2 for 300 s to obtain a ca. 3-μm-thick Cu deposit in an alkaline Cu-sulfate bath [17]. To obtain a Cr/Cu-coated AZ91D specimen, the Cu-coated specimen was further electroplated in an acidic Cu-sulfate bath with a current density of 4 A dm− 2 for 300 s followed by trivalent Cr electroplating in a bath [19,20] with a current density of 30 A dm− 2 for 420 s. All the Cu and trivalent Cr electroplating baths are eco-friendly without any highly toxic chemical components. The alkaline Cu-sulfate bath contained 40 g L− 1 CuSO4, 150 g L− 1 KNaC4H4O6, 20 g L− 1 H3BO3 and a small amount of phosphate to maintain its pH value at 9.8. The acidic Cu-plating bath was composed of 80 g L− 1 CuSO4, and 0.4 vol.% H2SO4. The Cr-plating bath consisted of 0.8 M CrCl3 6H2O with urea as a complexing agent, and a small amount of buffer salts to maintain the pH at 1.1 [17]. Some of the Cr/Cu-coated AZ91D specimens were heated with a reduction-flame for about 0.5 s. The torch length of the reduction-flame was approximately 20 cm long. The Cr/Cu-coated specimen was heated approximately with the center at the torch length so that the whole deposited surface could be heated at the same time. To evaluate the bonding strength of Cu- and Cr/Cu-coated AZ91D specimens, the ASTM D3359-02 Standard Test Method for Measuring Adhesion by Tape Test was carried out. The test was accomplished by scratching six 1-mm-wide parallel lines with a diamond knife in both longitudinal and latitudinal directions on the surfaces of Cu- and Cr/
Cu-coated AZ91D specimens. Subsequently, 3 M tape (3 M Core Series 4-1000) was adhered to the scratched specimen for ca. 60 s, and then the tape was perpendicularly and rapidly peeled off from the deposited surface. According to the ASTM D3359-02 standard, bonding strength can be divided into 6 classes, namely from class 0 to class 5. Specimens in the higher classes have higher bonding strength. The corrosion behavior of the deposited AZ91D specimen was evaluated in an electrochemical three-electrode cell by means of the anodic polarization test in 0.1 M H2SO4 solution at 27 °C. The deposited specimen was used as a working electrode. A plantized Ti mesh and an Ag/AgCl electrode in saturated KCl solution were used as counter and reference electrodes, respectively. The anodic polarization behavior of the Cu- and Cr/Cu-coated AZ91D specimens was evaluated by potentiodynamic scanning with a scan rate of 5 mV s− 1 from − 0.25 V (vs. open circuit potential) in the noble potential direction until breakdown of the deposit. The hardness values of AZ91D substrate, Cu and as-plated and flame-heated Cr deposits were measured with a micro-hardness tester (Matsuzawa Digital, Model MXT-α7e) using a 50 g load. Mean hardness and its standard deviation for a specimen were calculated according to ten measurements. The wear resistance of Cu- and Cr/Cucoated AZ91D specimens was evaluated with a ball-on-plate wear tester in which a 5 mm steel ball counterpart with a hardness value of ca. 450 Hv was utilized. A constant load of 10 N was applied normally to the Cu- or Cr/Cu-coated AZ91D specimen under unlubricated condition at 25 °C. Each wear resistance testing was conducted with a circular track of 3 mm in diameter, a frequency of 10 Hz and a total sliding distance of 50 m. The wear resistance of a specimen was evaluated from its weight-loss value after wear test. 3. Results and discussion 3.1. Surface morphologies and cross sections Fig. 1(a) and (b) show the surface morphology and cross section of Cu-coated AZ91D, which was electroplated in alkaline then followed by acidic Cu-plating baths. As shown in Fig. 1(a), a visually bright surface with a typical nodular surface morphology was found on the Cu-coated AZ91D. From the cross-sectional micrograph, the AZ91D substrate was uniformly covered by a dense Cu deposit with a thickness of 4 μm. No interfacial defects were seen along the interface between the Cu deposit and AZ91D. That is, a Cu deposit was successfully electroplated on the AZ91D specimen with our proposed
Fig. 1. (a) Surface morphology and (b) cross-sectional view of the Cu-coated AZ91D specimen.
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Fig. 2. Potential response of galvanostatic etching followed by Cu electroplating in an alkaline Cu-plating bath for preparation of the Cu-coated AZ 91D specimen.
electroplating process, even though it is accepted by many researchers that it is difficult to obtain a uniform metal deposit on AZ91D using electroplating. We have proposed an electroplating process [17] in
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Fig. 5. Hardness values of AZ 91D, as-plated Cu-coated, as-plated Cr/Cu-coated and flame-heated Cr/Cu-coated AZ91D specimens.
which galvanostatic etching with a current density of 2 A dm− 2 was applied to pure Mg, AZ31 and AZ61 specimens in an alkaline Cuplating bath. We found that the oxides that formed on Mg and Mg
Fig. 3. SEM micrographs of (a) surface morphology and (b) cross section of a Cr/Cu-coated AZ91D specimen.
Fig. 4. Surface morphologies of (a) Cu- and (b) Cr/Cu-coated AZ91D specimens after the ASTM D3359-02 Standard Test Method for Measuring Adhesion by Tape Test.
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Fig. 6. Weight-loss values of as-plated Cu-, as-plated Cr/Cu- and flame-heated Cr/Cu-coated AZ91D specimens as well as AZ91D after the wear test.
alloy surfaces could be removed using galvanostatic etching, leading to an active surface for Cu electroplating. The effect of galvanic etching on the surfaces of Mg and Mg alloys can be recognized by their potential variation during etching. The potential variation of AZ91D during galvanostatic etching and Cu electroplating in the alkaline Cuplating bath is presented in Fig. 2. The potential variation from period I to III has the same trend as those of Mg, AZ31 and AZ61 [14,15], which implies that an activated surface of AZ91D could be achieved by galvanostatic etching. However, the bonding strength, wear and
corrosion resistance of Cu-coated AZ91D specimen still needed to be tested. To obtain an anti-wear Cr deposit, the Cu-coated AZ91D was electroplated with a current density of 30 A dm− 2 in a plating bath with trivalent chromium ions. Fig. 3(a) and (b) show the surface morphology and cross section of the Cr/Cu-coated AZ91D. It can be clearly seen that a 20-μm-thick Cr deposit with relatively few cracks and a typical nodular surface morphology was distributed uniformly on the Cu deposit. Many researchers [21] have shown that the Cr deposit obtained from a trivalent bath has an amorphous structure with some through-deposit cracks. Because of the through-deposit cracks, trivalent Cr-coated specimens have comparatively bad corrosion resistance. However, we have demonstrated that a Ni undercoat between the Cr deposit and the steel substrate significantly reduces the crack density and even narrows the cracks developed in the Cr deposit [22]. This could be probably attributed to removal of internal stress in the Cr deposit to a degree. In this study, we found the same effect for Cu as that of a Ni undercoat in reducing the crack density in Cr deposit. That is, relatively few and narrow cracks were found in the Cr deposit when it was electroplated on a Cu-coated AZ91D substrate. Therefore, it can be expected that the Cr/Cu deposit has relatively high corrosion resistance because no through-deposit cracks were observed in the Cr deposit. 3.2. Bonding strength and hardness tests As expected, a very good bonding strength was detected between the Cu deposit and the AZ91D substrate. As can be seen in Fig. 4(a) in which the Cu-coated AZ91D is estimated to be of class 5 according to the ASTM D3359-02 standard. However, a slight decrease in bonding strength for the Cr/Cu-coated AZ91D specimen was observed, as presented in Fig. 4(b). There were a few positions with detached
Fig. 7. Surface morphology of Cu-coated AZ91D specimen observed with (a) OM and (b) SEM; (c) and (d) are EDS-spectra of the as-plated area A and ground area B, respectively, on the SEM micrograph shown in (b).
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deposits along cut marks after peeling the adhesive tape from the specimen. According to the ASTM D3359-02 standard [23], the bonding strength of Cr/Cu-coated AZ91D lay between class 3 and class 4. The bonding strength between the Cr/Cu deposit and the steel substrate will be further evaluated with ball-on-disc wear tester in following section. Fig. 5 shows the hardness values of AZ91D, the Cu deposit, and the as-plated and flame-heated Cr deposits. The hardness of AZ91D was 90 Hv and that of the Cu deposit was 180 Hv. A hardness of 740 Hv was detected with the as-plated Cr deposit; furthermore, the Cr deposit obtained a very high hardness of 1000 Hv after reduction-flame heating for 0.5 s. The experimental results show that the surface hardness of Cu-coated AZ91D was greatly increased after trivalent Cr electrodeposition. Moreover, a further increase in hardness of asplated Cr/Cu deposits could be achieved by using reduction-flame heating for 0.5 s. In our previous study [24], we found that the obvious increase in hardness of flame-heated Cr deposits could be attributed to precipitation of diamond membranes in amorphous Cr deposits. That is, the diamond membranes are transformed from amorphous carbon membranes during crystallization of the amorphous Cr deposits using flame heating. Thus, we could expect high wear resistance of the Cr/Cucoated AZ91D specimen and, moreover, superior wear resistance of the flame-heated Cr/Cu-coated specimen. 3.3. Wear and corrosion tests The weight-loss values of AZ91D, Cu-coated AZ91D, and as-plated and flame-hardened Cr/Cu-coated AZ91D specimens are given in Fig. 6. As expected, low weight-loss values for the Cr/Cu-coated AZ91D and high weight-loss values for AZ91D and Cu-coated AZ91D were detected, which implies that the Cr/Cu-coated AZ91D specimen has
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high wear resistance. These results are in full agreement with those from the hardness test. As the specimens with higher the hardness values had higher wear resistance. The weight-loss values of AZ91D and Cu-coated AZ91D were roughly evaluated to be 3 mg. An order reduction in the weight-loss value to 0.37 mg was detected when the Cu-coated AZ91D specimen was electroplated with a Cr deposit. Furthermore, an additional order reduction of the weight-loss value to 0.05 mg resulted when the Cr/Cu-coated AZ91D specimen was flameheated for 0.5 s. That is, Cr/Cu-electrodeposition is helpful for improving the wear resistance of AZ91D. Superior wear resistance could be obtained when an as-plated Cr/Cu-coated AZ91D specimen was flame-heated for 0.5 s. The surface morphologies of Cu-, as-plated Cr/Cu-, and flamehardened Cr/Cu-coated AZ91D specimens after the wear test are shown in Figs. 7–9. Fig. 7(a) and (b) show the unground and ground surfaces of Cu-coated AZ91D in optical and SEM images. Unsurprisingly, deeply ground marks in a circular track can be clearly seen on the surface of the Cu-coated AZ91D specimen after the wear test. Fig. 7 (c) and (d) show the results of EDS analyses on areas A and B, which correspond to the unground and ground portions of Cu-coated AZ91D, respectively. In the analysis of the unground area A, the only element detected was Cu; in contrast, in the analysis of ground area B only Mg was observed, which means that the Cu deposit was fully worn away from the AZ91D surface after the wear test. That is, Cu-coated AZ91D has low wear resistance. Fig. 8(a) and (b) show the ground surfaces of the as-plated Cr/Cucoated AZ91D specimen after the wear test. As presented in the optical image in Fig. 8(a), more than half of the Cr-coated surface had a nodular morphology, similar to the as-plated Cr deposit. There were no observable deeply ground surfaces and the ground tracks did not be continuous. An EDS analysis at position A in Fig. 8(b) on the nodular
Fig. 8. Surface morphology of Cr/Cu-coated AZ91D specimen observed with OM (a) and SEM (b). (c) and (d) are EDS-spectra of the positions A and B, respectively, on the SEM micrograph shown in (b).
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Fig. 9. Surface morphologies observed with (a) OM and (b) SEM of flame-heated Cr/Cu-coated AZ91D specimen after the wear test.
surface showed a chemical composition consisting mainly of Cr with a minor amount of Fe (see Fig. 8(c)). In contrast, the result of EDS analysis on the ground track at position B in Fig. 8(b) show that more abundant Fe was detected, as shown in Fig. 8(d). Because the hardness of the asplated Cr deposit was higher than that of steel, the Fe detected on the surface of Cr/Cu-coated AZ91D must be from the steel counterpart. That is, the ground track in the Cr/Cu-coated AZ91D is the cold-welded steel ball. Consequently, the as-plated Cr/Cu-coated AZ91D specimen had a high wear resistance. As presented in the optical image in Fig. 8(a), more than half of the surface of the flame-hardened Cr after the wear test was ground tracks, which means much more cold-welded or worn steel ball was observed. The cold-welded areas on the flame-heated Cr/ Cu-coated surface were larger than those found on the as-plated one, as shown in Fig. 9(a) and (b), which implies that the flame-heated Cr/Cucoated AZ91D has high hardness and superior wear resistance. As shown in Figs. 8 and 9, the cold-welded surfaces of as-plated and flameheated Cr/Cu-coated AZ91D could possibly pick up a few weight gains from the steel counterpart. However, the wear resistance of all tested specimens can be evaluated from their weight-loss values after the wear test (see Fig. 6). Zeng et al. [25] have demonstrated that better wear resistance of a trivalent Cr-coated specimen can be achieved by annealing at 200 °C for 1 h. However, the wear resistance of the trivalent Cr-coated specimen was worse than that of a hexavalent Cr-coated specimen, owing to relatively low hardness after 200 °C annealing. In this study, a very high hardness of ca. 1000 Hv, which is much higher than that of a hexavalent Cr deposit, could be obtained when the as-plated trivalent Cr deposit was flame-heated for 0.5 s. Moreover, the flameheated Cr/Cu-coated AZ91D had a superior wear resistance compared to a steel counterpart. That is, the flame-heated Cr/Cu-coated AZ91D had a suitable bonding strength, high hardness and superior wear resistance.
Fig. 10 shows the anodic polarization behavior of the AZ91D substrate, Cu-, as-plated Cr/Cu-, 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 high corrosion current density of 2.3×10− 3 A cm− 2 were detected for the AZ91D substrate. Its logarithmic anodic current density increased with increasing anodic overpotential, which indicates that the AZ91D substrate had a high corrosion rate and chemical reactivity. The corrosion potential of AZ91D increased to −1.12 V after Cu electroplating in an alkaline Cu-plating bath. However, from anodic polarization behavior, we can clearly see that the Cu-coated AZ91D obtained from the alkaline Cu-plating bath had poor corrosion resistance, which could be possibly attributed to a too-thin Cu deposit on AZ91D. The Cu-coated AZ91D had a high corrosion potential of −0.15 V and a corrosion current density of 4.2×10− 6 A cm− 2 when it was further electroplated in an acidic Cu-plating bath. Although the Cu-coated AZ91D specimen had a low corrosion current density, it could be easily oxidized in the atmosphere [26]. Therefore, trivalent Cr electrodeposition was used as a protective coating for AZ91D owing to its high oxidation resistance. The corrosion current density of Cr/Cu-coated AZ91D was also in the range of a few 10− 6 A cm− 2. Similar results for an anti-corrosion coating for Mg alloy specimens have been reported by several researchers [27,28], who also used an anti-corrosion coating as the final electroplating process to increase the corrosion resistance of the Mg alloy. The corrosion potential of as-plated Cr/Cu-coated AZ91D was −0.27 V and shifted to −0.15 V after flame heating. That is, trivalent Cr electrodeposition could act as an anti-corrosion coating for AZ91D. 4. Conclusions An eco-friendly electroplating process to obtain an anti-wear and anti-corrosion Cr/Cu coating on Mg alloy AZ91D was proposed in this work. Specifically, an AZ91D specimen was successfully electroplated
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Acknowledgement The authors would like to thank the National Science Council of Republic of China (Taiwan) for the financial support for this research under contract number NSC 97-2221-E-182-007.
References
Fig. 10. Anodic polarization behavior of Cu-, as-plated Cr/Cu-, and flame-heated Cr/Cudeposited AZ91D specimens as well as an AZ91D substrate in 0.1 M H2SO4 solution.
in an alkaline Cu-plating bath by using a pretreatment of galvanostatic etching. Then, the Cu-coated AZ91D specimen was further electroplated in an acidic Cu bath followed by a trivalent Cr bath. Based on the results of the wear test, the wear resistance of the AZ91D specimen was significantly improved after trivalent Cr electrodeposition. Moreover, the hardness of the Cr deposit was markedly increased from 760 Hv to 1000 Hv using reduction-flame heating for 0.5 s. A superior wear resistance was also achieved when the Cu-coated AZ 91D specimen was electroplated with a trivalent Cr deposit and then heated with reduction-flame for 0.5 s. Furthermore, an anodic polarization test showed that the AZ91D specimen had high corrosion resistance when electroplated with a protective Cr/Cu coating.
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Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Increasing the wear and corrosion resistance of magnesium alloy (AZ91D) with electrodeposition from eco-friendly copper- and trivalent chromium-plating baths 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 22 December 2009 Accepted in revised form 5 June 2010 Available online 15 June 2010 Keywords: Mg alloy Trivalent Cr electrodeposition Wear resistance Corrosion resistance
a b s t r a c t Mg alloy, AZ91D, which has a two-phase structure, was successfully electroplated in an alkaline Cu-plating bath. The Cu-coated Mg alloy specimen was further electroplated in eco-friendly acidic Cu and then trivalent Cr baths to obtain an anti-wear and anti-corrosion Cr/Cu coating. Experimental results show that the wear and corrosion resistance of the Mg alloy specimen was considerably improved by trivalent Cr electrodeposition. The hardness of the as-plated Cr deposit was drastically increased by using reductionflame heating for 0.5 s. The above-mentioned results were measured via bonding strength, hardness, wear and corrosion tests. A superior wear and corrosion resistance was obtained when a Cu-coated Mg alloy specimen was electroplated with a trivalent Cr deposit, followed by heating with reduction-flame heating for 0.5 s. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Mg alloys have been used as components in the electronic, automobile, and aerospace engineering industries because they have low density, a high specific strength ratio, good heat dissipation, and electromagnetic interference (EMI) shielding. Applications of Mg and Mg alloys are severely limited due to their high chemical activity. Schmutz et al. [1] pointed out that magnesium oxides or hydroxide layers can easily develop on the surfaces of Mg alloys exposed to wet environments, or even in air. Therefore, it is necessary to apply a protective coating or film on structural components made of Mg or an Mg alloy. Several surface treatments have been adopted to generate protective films or surface coatings for Mg and Mg alloys [2–4]. These surface films not only improve the corrosion resistance of Mg alloys but also provide an undercoat for further surface treatments [5–7]. However, most of the treatments have been carried out with a pretreatment involving soaking the Mg alloy in chromate or hydrofluoric acid for surface activation [8,9]. Typical surface treatments, including chemical conversion coating [10], anodizing [11], micro-arc oxidation treatment [12], and electroplating [13], have been proposed for Mg alloys. Among them, electroplating has some distinct advantages, such as reliable coating quality, convenient operation and a relatively low cost. Therefore, developing a protective coating for Mg alloys using electroplating would be a practical and useful accomplishment.
⁎ Corresponding author. Tel.: + 886 3 2118800x5655. E-mail address: [email protected] (C.A. Huang). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.06.012
Due to its relatively high strength, good casting quality and corrosion resistance, AZ91D is an appropriate Mg alloy for die casting to fabricate automobile parts and construction materials for computers, communications, and consumer-electronic (3C) products. Nevertheless, the eutectic α and intermetallic β (Mg17Al12) phases in the AZ91D structure could act as a galvanic couple leading to a non-uniform surface during surface treatment in an aqueous solution. Gray et al. [13] demonstrated that the non-uniform surface morphology results from different discharging capacities between α and β phases. Therefore, it is difficult to develop a suitable pretreatment to obtain an undercoat on which a protective metal deposit can be electroplated. Many researchers [14,15] have proposed that zinc or nickel could be directly plated onto magnesium and used as an undercoat for a subsequent protective coating. Most pretreatment processes proposed for Mg alloys are relatively complicated [16]; furthermore, pretreatment is performed in a bath with highly toxic chromate or hydrofluoric acid [8,9]. We have proposed an eco-friendly electroplating process to achieve a protective Ni/Cu deposit on pure Mg, AZ31, and AZ61 Mg alloys [17,18]. In our proposed electroplating process, an activated surface is generated on a Mg alloy via galvanostatic etching in an alkaline Cu-plating bath. Cu electroplating is performed immediately after galvanostatic etching in the same alkaline Cu-plating bath. Then the Cu-coated Mg alloy can be further electroplated in acidic Cu and Ni baths to achieve a Ni/Cu protective coating. Based on our previous experimental results, in this work we attempted to electroplate a Cu deposit on AZ91D with an eco-friendly electroplating process. Furthermore, trivalent Cr electrodeposition was conducted on Cu-coated AZ91D to improve its wear and corrosion resistance. The wear resistance, corrosion resistance, hardness and bonding strength were evaluated for Cu-coated and Cr/Cu-coated AZ91D specimens.
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Table 1 Chemical composition of AZ 91D used in this study. Element
Al
Zn
Mn
Mg
Weight percent
8.88
0.66
0.13
Balance
2. Experimental procedure Commercial Mg–Al alloy (AZ91D) in bar form was shaped into a disc form 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 were conducted in a twoelectrode cell. The AZ91D disc specimen was used as the working electrode and a plantized Ti mesh as the counter electrode. 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 remove the oxides and obtain a uniform Cu deposit on the AZ91D disc substrate, the substrate was galvanostatically etched with a current density of 2 A dm− 2 for 120 s and then electroplated with a current density of 4 A dm− 2 for 300 s to obtain a ca. 3-μm-thick Cu deposit in an alkaline Cu-sulfate bath [17]. To obtain a Cr/Cu-coated AZ91D specimen, the Cu-coated specimen was further electroplated in an acidic Cu-sulfate bath with a current density of 4 A dm− 2 for 300 s followed by trivalent Cr electroplating in a bath [19,20] with a current density of 30 A dm− 2 for 420 s. All the Cu and trivalent Cr electroplating baths are eco-friendly without any highly toxic chemical components. The alkaline Cu-sulfate bath contained 40 g L− 1 CuSO4, 150 g L− 1 KNaC4H4O6, 20 g L− 1 H3BO3 and a small amount of phosphate to maintain its pH value at 9.8. The acidic Cu-plating bath was composed of 80 g L− 1 CuSO4, and 0.4 vol.% H2SO4. The Cr-plating bath consisted of 0.8 M CrCl3 6H2O with urea as a complexing agent, and a small amount of buffer salts to maintain the pH at 1.1 [17]. Some of the Cr/Cu-coated AZ91D specimens were heated with a reduction-flame for about 0.5 s. The torch length of the reduction-flame was approximately 20 cm long. The Cr/Cu-coated specimen was heated approximately with the center at the torch length so that the whole deposited surface could be heated at the same time. To evaluate the bonding strength of Cu- and Cr/Cu-coated AZ91D specimens, the ASTM D3359-02 Standard Test Method for Measuring Adhesion by Tape Test was carried out. The test was accomplished by scratching six 1-mm-wide parallel lines with a diamond knife in both longitudinal and latitudinal directions on the surfaces of Cu- and Cr/
Cu-coated AZ91D specimens. Subsequently, 3 M tape (3 M Core Series 4-1000) was adhered to the scratched specimen for ca. 60 s, and then the tape was perpendicularly and rapidly peeled off from the deposited surface. According to the ASTM D3359-02 standard, bonding strength can be divided into 6 classes, namely from class 0 to class 5. Specimens in the higher classes have higher bonding strength. The corrosion behavior of the deposited AZ91D specimen was evaluated in an electrochemical three-electrode cell by means of the anodic polarization test in 0.1 M H2SO4 solution at 27 °C. The deposited specimen was used as a working electrode. A plantized Ti mesh and an Ag/AgCl electrode in saturated KCl solution were used as counter and reference electrodes, respectively. The anodic polarization behavior of the Cu- and Cr/Cu-coated AZ91D specimens was evaluated by potentiodynamic scanning with a scan rate of 5 mV s− 1 from − 0.25 V (vs. open circuit potential) in the noble potential direction until breakdown of the deposit. The hardness values of AZ91D substrate, Cu and as-plated and flame-heated Cr deposits were measured with a micro-hardness tester (Matsuzawa Digital, Model MXT-α7e) using a 50 g load. Mean hardness and its standard deviation for a specimen were calculated according to ten measurements. The wear resistance of Cu- and Cr/Cucoated AZ91D specimens was evaluated with a ball-on-plate wear tester in which a 5 mm steel ball counterpart with a hardness value of ca. 450 Hv was utilized. A constant load of 10 N was applied normally to the Cu- or Cr/Cu-coated AZ91D specimen under unlubricated condition at 25 °C. Each wear resistance testing was conducted with a circular track of 3 mm in diameter, a frequency of 10 Hz and a total sliding distance of 50 m. The wear resistance of a specimen was evaluated from its weight-loss value after wear test. 3. Results and discussion 3.1. Surface morphologies and cross sections Fig. 1(a) and (b) show the surface morphology and cross section of Cu-coated AZ91D, which was electroplated in alkaline then followed by acidic Cu-plating baths. As shown in Fig. 1(a), a visually bright surface with a typical nodular surface morphology was found on the Cu-coated AZ91D. From the cross-sectional micrograph, the AZ91D substrate was uniformly covered by a dense Cu deposit with a thickness of 4 μm. No interfacial defects were seen along the interface between the Cu deposit and AZ91D. That is, a Cu deposit was successfully electroplated on the AZ91D specimen with our proposed
Fig. 1. (a) Surface morphology and (b) cross-sectional view of the Cu-coated AZ91D specimen.
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Fig. 2. Potential response of galvanostatic etching followed by Cu electroplating in an alkaline Cu-plating bath for preparation of the Cu-coated AZ 91D specimen.
electroplating process, even though it is accepted by many researchers that it is difficult to obtain a uniform metal deposit on AZ91D using electroplating. We have proposed an electroplating process [17] in
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Fig. 5. Hardness values of AZ 91D, as-plated Cu-coated, as-plated Cr/Cu-coated and flame-heated Cr/Cu-coated AZ91D specimens.
which galvanostatic etching with a current density of 2 A dm− 2 was applied to pure Mg, AZ31 and AZ61 specimens in an alkaline Cuplating bath. We found that the oxides that formed on Mg and Mg
Fig. 3. SEM micrographs of (a) surface morphology and (b) cross section of a Cr/Cu-coated AZ91D specimen.
Fig. 4. Surface morphologies of (a) Cu- and (b) Cr/Cu-coated AZ91D specimens after the ASTM D3359-02 Standard Test Method for Measuring Adhesion by Tape Test.
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Fig. 6. Weight-loss values of as-plated Cu-, as-plated Cr/Cu- and flame-heated Cr/Cu-coated AZ91D specimens as well as AZ91D after the wear test.
alloy surfaces could be removed using galvanostatic etching, leading to an active surface for Cu electroplating. The effect of galvanic etching on the surfaces of Mg and Mg alloys can be recognized by their potential variation during etching. The potential variation of AZ91D during galvanostatic etching and Cu electroplating in the alkaline Cuplating bath is presented in Fig. 2. The potential variation from period I to III has the same trend as those of Mg, AZ31 and AZ61 [14,15], which implies that an activated surface of AZ91D could be achieved by galvanostatic etching. However, the bonding strength, wear and
corrosion resistance of Cu-coated AZ91D specimen still needed to be tested. To obtain an anti-wear Cr deposit, the Cu-coated AZ91D was electroplated with a current density of 30 A dm− 2 in a plating bath with trivalent chromium ions. Fig. 3(a) and (b) show the surface morphology and cross section of the Cr/Cu-coated AZ91D. It can be clearly seen that a 20-μm-thick Cr deposit with relatively few cracks and a typical nodular surface morphology was distributed uniformly on the Cu deposit. Many researchers [21] have shown that the Cr deposit obtained from a trivalent bath has an amorphous structure with some through-deposit cracks. Because of the through-deposit cracks, trivalent Cr-coated specimens have comparatively bad corrosion resistance. However, we have demonstrated that a Ni undercoat between the Cr deposit and the steel substrate significantly reduces the crack density and even narrows the cracks developed in the Cr deposit [22]. This could be probably attributed to removal of internal stress in the Cr deposit to a degree. In this study, we found the same effect for Cu as that of a Ni undercoat in reducing the crack density in Cr deposit. That is, relatively few and narrow cracks were found in the Cr deposit when it was electroplated on a Cu-coated AZ91D substrate. Therefore, it can be expected that the Cr/Cu deposit has relatively high corrosion resistance because no through-deposit cracks were observed in the Cr deposit. 3.2. Bonding strength and hardness tests As expected, a very good bonding strength was detected between the Cu deposit and the AZ91D substrate. As can be seen in Fig. 4(a) in which the Cu-coated AZ91D is estimated to be of class 5 according to the ASTM D3359-02 standard. However, a slight decrease in bonding strength for the Cr/Cu-coated AZ91D specimen was observed, as presented in Fig. 4(b). There were a few positions with detached
Fig. 7. Surface morphology of Cu-coated AZ91D specimen observed with (a) OM and (b) SEM; (c) and (d) are EDS-spectra of the as-plated area A and ground area B, respectively, on the SEM micrograph shown in (b).
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deposits along cut marks after peeling the adhesive tape from the specimen. According to the ASTM D3359-02 standard [23], the bonding strength of Cr/Cu-coated AZ91D lay between class 3 and class 4. The bonding strength between the Cr/Cu deposit and the steel substrate will be further evaluated with ball-on-disc wear tester in following section. Fig. 5 shows the hardness values of AZ91D, the Cu deposit, and the as-plated and flame-heated Cr deposits. The hardness of AZ91D was 90 Hv and that of the Cu deposit was 180 Hv. A hardness of 740 Hv was detected with the as-plated Cr deposit; furthermore, the Cr deposit obtained a very high hardness of 1000 Hv after reduction-flame heating for 0.5 s. The experimental results show that the surface hardness of Cu-coated AZ91D was greatly increased after trivalent Cr electrodeposition. Moreover, a further increase in hardness of asplated Cr/Cu deposits could be achieved by using reduction-flame heating for 0.5 s. In our previous study [24], we found that the obvious increase in hardness of flame-heated Cr deposits could be attributed to precipitation of diamond membranes in amorphous Cr deposits. That is, the diamond membranes are transformed from amorphous carbon membranes during crystallization of the amorphous Cr deposits using flame heating. Thus, we could expect high wear resistance of the Cr/Cucoated AZ91D specimen and, moreover, superior wear resistance of the flame-heated Cr/Cu-coated specimen. 3.3. Wear and corrosion tests The weight-loss values of AZ91D, Cu-coated AZ91D, and as-plated and flame-hardened Cr/Cu-coated AZ91D specimens are given in Fig. 6. As expected, low weight-loss values for the Cr/Cu-coated AZ91D and high weight-loss values for AZ91D and Cu-coated AZ91D were detected, which implies that the Cr/Cu-coated AZ91D specimen has
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high wear resistance. These results are in full agreement with those from the hardness test. As the specimens with higher the hardness values had higher wear resistance. The weight-loss values of AZ91D and Cu-coated AZ91D were roughly evaluated to be 3 mg. An order reduction in the weight-loss value to 0.37 mg was detected when the Cu-coated AZ91D specimen was electroplated with a Cr deposit. Furthermore, an additional order reduction of the weight-loss value to 0.05 mg resulted when the Cr/Cu-coated AZ91D specimen was flameheated for 0.5 s. That is, Cr/Cu-electrodeposition is helpful for improving the wear resistance of AZ91D. Superior wear resistance could be obtained when an as-plated Cr/Cu-coated AZ91D specimen was flame-heated for 0.5 s. The surface morphologies of Cu-, as-plated Cr/Cu-, and flamehardened Cr/Cu-coated AZ91D specimens after the wear test are shown in Figs. 7–9. Fig. 7(a) and (b) show the unground and ground surfaces of Cu-coated AZ91D in optical and SEM images. Unsurprisingly, deeply ground marks in a circular track can be clearly seen on the surface of the Cu-coated AZ91D specimen after the wear test. Fig. 7 (c) and (d) show the results of EDS analyses on areas A and B, which correspond to the unground and ground portions of Cu-coated AZ91D, respectively. In the analysis of the unground area A, the only element detected was Cu; in contrast, in the analysis of ground area B only Mg was observed, which means that the Cu deposit was fully worn away from the AZ91D surface after the wear test. That is, Cu-coated AZ91D has low wear resistance. Fig. 8(a) and (b) show the ground surfaces of the as-plated Cr/Cucoated AZ91D specimen after the wear test. As presented in the optical image in Fig. 8(a), more than half of the Cr-coated surface had a nodular morphology, similar to the as-plated Cr deposit. There were no observable deeply ground surfaces and the ground tracks did not be continuous. An EDS analysis at position A in Fig. 8(b) on the nodular
Fig. 8. Surface morphology of Cr/Cu-coated AZ91D specimen observed with OM (a) and SEM (b). (c) and (d) are EDS-spectra of the positions A and B, respectively, on the SEM micrograph shown in (b).
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Fig. 9. Surface morphologies observed with (a) OM and (b) SEM of flame-heated Cr/Cu-coated AZ91D specimen after the wear test.
surface showed a chemical composition consisting mainly of Cr with a minor amount of Fe (see Fig. 8(c)). In contrast, the result of EDS analysis on the ground track at position B in Fig. 8(b) show that more abundant Fe was detected, as shown in Fig. 8(d). Because the hardness of the asplated Cr deposit was higher than that of steel, the Fe detected on the surface of Cr/Cu-coated AZ91D must be from the steel counterpart. That is, the ground track in the Cr/Cu-coated AZ91D is the cold-welded steel ball. Consequently, the as-plated Cr/Cu-coated AZ91D specimen had a high wear resistance. As presented in the optical image in Fig. 8(a), more than half of the surface of the flame-hardened Cr after the wear test was ground tracks, which means much more cold-welded or worn steel ball was observed. The cold-welded areas on the flame-heated Cr/ Cu-coated surface were larger than those found on the as-plated one, as shown in Fig. 9(a) and (b), which implies that the flame-heated Cr/Cucoated AZ91D has high hardness and superior wear resistance. As shown in Figs. 8 and 9, the cold-welded surfaces of as-plated and flameheated Cr/Cu-coated AZ91D could possibly pick up a few weight gains from the steel counterpart. However, the wear resistance of all tested specimens can be evaluated from their weight-loss values after the wear test (see Fig. 6). Zeng et al. [25] have demonstrated that better wear resistance of a trivalent Cr-coated specimen can be achieved by annealing at 200 °C for 1 h. However, the wear resistance of the trivalent Cr-coated specimen was worse than that of a hexavalent Cr-coated specimen, owing to relatively low hardness after 200 °C annealing. In this study, a very high hardness of ca. 1000 Hv, which is much higher than that of a hexavalent Cr deposit, could be obtained when the as-plated trivalent Cr deposit was flame-heated for 0.5 s. Moreover, the flameheated Cr/Cu-coated AZ91D had a superior wear resistance compared to a steel counterpart. That is, the flame-heated Cr/Cu-coated AZ91D had a suitable bonding strength, high hardness and superior wear resistance.
Fig. 10 shows the anodic polarization behavior of the AZ91D substrate, Cu-, as-plated Cr/Cu-, 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 high corrosion current density of 2.3×10− 3 A cm− 2 were detected for the AZ91D substrate. Its logarithmic anodic current density increased with increasing anodic overpotential, which indicates that the AZ91D substrate had a high corrosion rate and chemical reactivity. The corrosion potential of AZ91D increased to −1.12 V after Cu electroplating in an alkaline Cu-plating bath. However, from anodic polarization behavior, we can clearly see that the Cu-coated AZ91D obtained from the alkaline Cu-plating bath had poor corrosion resistance, which could be possibly attributed to a too-thin Cu deposit on AZ91D. The Cu-coated AZ91D had a high corrosion potential of −0.15 V and a corrosion current density of 4.2×10− 6 A cm− 2 when it was further electroplated in an acidic Cu-plating bath. Although the Cu-coated AZ91D specimen had a low corrosion current density, it could be easily oxidized in the atmosphere [26]. Therefore, trivalent Cr electrodeposition was used as a protective coating for AZ91D owing to its high oxidation resistance. The corrosion current density of Cr/Cu-coated AZ91D was also in the range of a few 10− 6 A cm− 2. Similar results for an anti-corrosion coating for Mg alloy specimens have been reported by several researchers [27,28], who also used an anti-corrosion coating as the final electroplating process to increase the corrosion resistance of the Mg alloy. The corrosion potential of as-plated Cr/Cu-coated AZ91D was −0.27 V and shifted to −0.15 V after flame heating. That is, trivalent Cr electrodeposition could act as an anti-corrosion coating for AZ91D. 4. Conclusions An eco-friendly electroplating process to obtain an anti-wear and anti-corrosion Cr/Cu coating on Mg alloy AZ91D was proposed in this work. Specifically, an AZ91D specimen was successfully electroplated
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Acknowledgement The authors would like to thank the National Science Council of Republic of China (Taiwan) for the financial support for this research under contract number NSC 97-2221-E-182-007.
References
Fig. 10. Anodic polarization behavior of Cu-, as-plated Cr/Cu-, and flame-heated Cr/Cudeposited AZ91D specimens as well as an AZ91D substrate in 0.1 M H2SO4 solution.
in an alkaline Cu-plating bath by using a pretreatment of galvanostatic etching. Then, the Cu-coated AZ91D specimen was further electroplated in an acidic Cu bath followed by a trivalent Cr bath. Based on the results of the wear test, the wear resistance of the AZ91D specimen was significantly improved after trivalent Cr electrodeposition. Moreover, the hardness of the Cr deposit was markedly increased from 760 Hv to 1000 Hv using reduction-flame heating for 0.5 s. A superior wear resistance was also achieved when the Cu-coated AZ 91D specimen was electroplated with a trivalent Cr deposit and then heated with reduction-flame for 0.5 s. Furthermore, an anodic polarization test showed that the AZ91D specimen had high corrosion resistance when electroplated with a protective Cr/Cu coating.
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