Corrosion Science 52 (2010) 1326–1332

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Corrosion Science

journal homepage: www.elsevier .com/locate /corsc i

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 YehDepartment of Mechanical Engineering, Chang Gung University, Taoyuan 333, Taiwan

a r t i c l e i n f o a b s t r a c t

Article history:Received 31 August 2009Accepted 14 September 2009Available online 11 December 2009

Keywords:A. Mg alloyA. Cr depositC. Corrosion resistanceC. Blister

0010-938X/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.corsci.2009.12.010

* Corresponding author. Tel.: +886 3 2118800x565E-mail address: gfehu@mail.cgu.edu.tw (C.A. Huan

An electroplating process was proposed for obtaining a protective Cr/Cu deposit on the two-phase Mgalloy AZ91D. The corrosion behavior of Cu-covered and Cr/Cu-covered AZ91D specimens was studiedelectrochemically in 0.1 M H2SO4 with different NaCl concentrations. Experimental results showed thatthe 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 in0.1 M H2SO4 solution. After immersion in a 0.1 M H2SO4 with a NaCl-content above 3.5 wt.%, the surfaceof Cr/Cu-covered AZ91D suffered a few blisters. Cracks through the Cr deposit provided active pathwaysfor corrosion of the Cu and the AZ91D substrate. Formation of blisters on the Cr/Cu-covered AZ91D sur-face was confirmed based on the results of an open-circuit potential test, which detected an obviouspotential drop from noble to active potentials.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Magnesium alloys are widely used as components in the elec-tronic, automobile and aerospace engineering industries, due totheir low density and high specific strength, as well as their excel-lent heat dissipation and electromagnetic interference (EMI)shielding properties. However, many potential applications of Mgalloys are prohibited or severely limited, due to their high chemicalactivity and corrosion susceptibility. In wet environments or evenin air, magnesium oxide or hydroxide films develop easily on Mgalloys [1]. These Mg oxide or hydroxide films are loose and porous,and therefore do not provide corrosion protection [1]. Thus, forma-tion of a protective coating or film on a component made of Mg al-loy has been an important point of study in recent years.

Several surface treatments have been examined for the protec-tion of Mg alloys, including chemical conversion [2], anodization[3], micro-arc oxidation [4], electroless plating [5], and electroplat-ing [6]. Among these, the electroplating process has some distinctadvantages, such as coating reliability, convenience, and relativelylow cost. Therefore, development of a protective coating processfor Mg alloys using electroplating would be both useful andpractical.

Due to its relatively high strength, amenability to casting, andcorrosion resistance, AZ91D is commonly die-cast to form compo-nents for automobiles, computers, communication hardware, andconsumer electronics (3C). Generally, two phases, eutectic a and

ll rights reserved.

5.g).

intermetallic b (Mg17Al12), are identifiable in AZ91D. Together,these two phases act as a galvanic couple and they have differentdischarging capacities in aqueous solution. As a result, a non-uni-form surface morphology is typically obtained when AZ91D is im-mersed in an aqueous solution, or in an electroplating bath [6].Therefore, it is relatively difficult to use electroplating to deposita protective coating for AZ91D.

Many researchers [7,8] have proposed that a Zn or Ni could beelectroplated directly onto a Mg or Mg alloy and then used as anundercoat for subsequent protective coating or surface treatment.However, in this electroplating method, some complicated pre-treatments are required; moreover, these pretreatments are nor-mally carried out in baths with highly toxic chromate orhydrofluoric acid [9,10]. Recently, we developed an electroplatingprocess to obtain a protective Ni/Cu coating for pure Mg and Mg al-loys using more environmentally benign baths [11,12]. In our elec-troplating procedure, an alkaline Cu-plating bath was used notonly as an activation agent, but also as an electroplating bath. AMg alloy specimen was activated by galvanostatic etching, andthen Cu was electroplated directly on the specimen in the samebath. The Cu-coated Mg alloy could then be further electroplatedin acidic Cu and Ni baths to achieve a Ni/Cu protective coating.

Building on our previous results, in this work, we electroplatedCu onto AZ91D using an environmentally benign electroplatingprocess. We then electrodeposited Cr using a trivalent Cr bath ontoCu-coated AZ91D, to improve its corrosion resistance. The corro-sion behavior of the Cu-coated and Cr/Cu-coated AZ91D specimenswas evaluated in 0.1 M H2SO4 containing different concentrationsof NaCl. The morphologies of Cr/Cu-coated AZ91D specimens cor-

C.A. Huang et al. / Corrosion Science 52 (2010) 1326–1332 1327

roded in 0.1 M H2SO4 containing 3.5 wt.% NaCl were investigatedparticularly.

2. Experimental procedure

Commercial Mg–Al alloy (AZ91D) in bar form was shaped into adisc with a diameter of 12 mm and a thickness of 2.5 mm. TheAZ91D disc specimen was used as the substrate for Cu and Cr elec-troplating. The chemical composition of the AZ91D specimen is gi-ven in Table 1. All electroplating processes and corrosion testswere conducted in an electrochemical three-electrode cell. TheAZ91D disc specimen was used as the working electrode, whileplantized Ti-mesh and Ag/AgCl in saturated KCl solution were usedas the counter and the reference electrodes, respectively. Before Cuelectroplating, the surface of the AZ91D substrate was mechani-cally ground with 600-grit emery paper, cleaned with de-ionizedwater, and dried with an air blaster. To obtain a uniform and denseCu film on the AZ91D disc substrate, the latter was etched galvano-statically with a current density of 2 A dm�2 for 120 s and thenelectroplated in an alkaline Cu-sulfate solution bath [11]. The Cu-coated AZ91D specimen was then further electroplated in an acidCu bath with a current density of 4 A dm�2, to increase the thick-ness of the Cu layer. Finally, an anti-corrosion Cr coating was car-ried out by electroplating onto the Cu-coated AZ91D using anenvironmentally friendly trivalent Cr bath.

Analytical grade H2SO4, NaCl, and de-ionized water were usedto make 0.1 M H2SO4 solutions containing different NaCl concen-trations for the corrosion tests. The responses of AZ91D, Cu-coatedAZ91D and Cr/Cu-coated AZ91D to these solutions were evaluatedusing anodic polarization and open-circuit potential (OCP) tests at25 �C. The anodic polarization behavior of a specimen was poten-tiodynamically 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-platedCr/Cu-coated, and flame-heated Cr/Cu-coated AZ91D specimenswere tested in 0.1 M H2SO4 with different NaCl concentrationsfor 2 h.

The surface morphologies of Cr/Cu-coated specimens wereexamined with a scanning electron microscope (SEM, HITACH S-3000 N). The hardnesses of AZ91D substrates and of as-platedand flame-heated Cr-coated specimens were measured via a mi-cro-hardness tester (Matsuzawa Digital, Model MXT-a7e) with a50 g load. Mean hardness and standard deviation for each speci-

Table 1Chemical composition of AZ91D used in this study.

Element Al Zn Mn Mg

Weight percent 8.88 0.66 0.13 Balance

Fig. 1. The (a) surface morphology and (b) cross sect

men were calculated using ten measurements taken on cross sec-tion of the specimen mounted in epoxy.

3. Results and discussion

3.1. Surface morphologies and interfacial structure

Figs. 1(a) and (b) show the surface morphology and cross sec-tion of Cr/Cu-coated AZ91D that was electroplated in alkaline fol-lowed by acidic Cu-plating baths and then in a trivalent Crplating bath. The Cr coating typically had a nodular surface mor-phology, which is shown in Fig. 1(a). It is visible in the cross-sec-tional SEM micrograph shown in Fig. 1(b), the AZ91D substratewas uniformly covered by a dense 4 lm-thick Cu layer and thena 20 lm-thick Cr layer. No interfacial defects were found, eitherat the boundary between the Cu deposit and AZ91D, or betweenthe Cr and Cu. This finding demonstrates that Cu can be success-fully electroplated onto an AZ91D specimen using our proposedelectroplating process, in spite of the known difficulties in achiev-ing uniform metal deposits on AZ91D by electroplating. As men-tioned, the Cr electroplating process was preceded by atreatment of galvanostatic etching using a constant anodic currentdensity of 2 A cm�2, followed by Cu electroplating in the samealkaline Cu-plating bath. We found the oxides that had formedon the Mg alloy surfaces were nearly removed using the galvano-static etching, leading to an active surface for Cu electroplating[11,12].

As is clear from the SEM imaging, relatively few cracks arefound in the 20 lm thick Cr deposit. Many researchers have dem-onstrated [13] that the Cr film obtained from electroplating in a tri-valent bath has an amorphous structure with a few cracks that runcompletely through the deposit. These cracks cause Cr-coatedspecimens to have comparatively bad corrosion resistance. In thisstudy, cracks in the Cr deposit were found to be relatively fewand narrow when Cr deposited on a Cu-deposited AZ91D substrate.The pattern of crack information could be attributed to the soft Cuundercoat between the Cr deposit and the AZ91D substrate. Ni hadthe same effect on the morphology of the cracks in Cr deposits, asevidenced by our previous work [14]. Therefore, it may be ex-pected that the Cr/Cu deposit should have relatively high corrosionresistance, due to minimization of the density and width of crackscompletely spanning the electroplated layer.

Figs. 2(a) and (b) show the surface morphology and cross sec-tion, respectively, of a Cr/Cu deposit on AZ91D after flame heatingfor 0.5 s. Flame heating was found to widen both superficial andpenetrating cracks in the Cr/Cu coating. Therefore, corrosion resis-tance of as-plated Cr/Cu-coated AZ91D specimens may be expectedto worsen after flame heating, due to the broadening of thesecracks.

ion of as-plated Cr/Cu-coated AZ91D specimen.

Fig. 2. The (a) surface morphology and (b) cross section of flame-heated Cr/Cu-coated AZ91D specimen.

1328 C.A. Huang et al. / Corrosion Science 52 (2010) 1326–1332

3.2. Hardness test

The hardness values of AZ91D and of as-plated and flame-heated Cr/Cu-coated AZ91D, are presented in Fig. 3. The hardnessof 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 AZ91Dgreatly increased after Cr electrodeposition and flame heating. Inour previous study [15], we found that the noticeable increase inthe hardness of flame-heated Cr deposits could be attributed toprecipitation of diamond membranes in amorphous Cr deposits.This may also be the case here. The diamond membranes are trans-formed from amorphous carbon during crystallization of amor-phous Cr deposits using flame heating. Based on the results ofthe 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 isan 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 speci-mens in 0.1 M H2SO4 solution. A low corrosion potential of

AZ 91D ACr/

0

250

500

750

1000

1250

Har

dnes

s (H

v)

Sp

Fig. 3. Hardness values of AZ91D substrate,

�1.8 V (vs. Ag/AgClsat.) and a high corrosion current density of2.3 � 10�3 A cm�2 were detected for the AZ91D substrate. The cor-rosion behavior indicates that the AZ91D substrate had a high cor-rosion rate and high chemical reactivity. After Cu electroplating inan acidic Cu-plating bath, the corrosion potential of AZ91D in-creased to �0.15 V. The Cu-coated AZ91D had a low corrosion cur-rent density of 4.2 � 10�6 A cm�2. However, Cu coatings are easilyoxidized in atmosphere [16]. In this work, Cr electrodeposition wasused as a protective coating for AZ91D because of its high oxida-tion resistance. As-expected, the corrosion current density of Cr/Cu-coated AZ91D was only a few 10�6 A cm�2. Similar results fordifferent protective coatings on Mg alloy specimens have been re-ported by several researchers, who also used an anti-corrosioncoating as the final electroplating process to increase the corrosionresistance of a metal substrate [17,18]. That is, the Cr layer couldact as an anti-corrosion coating for AZ91D. The corrosion potentialof 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 densi-ties of as-plated and flame-heated Cr/Cu-coated AZ91D were al-most equal.

Fig. 5 illustrates open-circuit potentials (OCP) observed in 0.1 MH2SO4 for the AZ91D substrate, the Cu-coated AZ91D, and both as-plated and flame-hardened Cr/Cu-coated AZ91D specimens. Allspecimens exhibited stable OCPs during 2-h immersion. As ex-pected, the AZ91D substrate has the lowest OCP of ca. �1.8 V. This

s-plated Cu-coated

Flame-heatedCr/Cu-coated

ecimen

as-plated and flame-heated Cr coatings.

1E-6 1E-5 1E-4 1E-3 0.01 0.1-2.0

-1.6

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

Pote

ntia

l (V

) v.s

. Ag/

AgC

l (sa

t.)

Current density (A cm ) -2

AZ91D substrate As-plated Cu coated As-plated Cr/Cu coated Flame-hardened Cr/Cu coated

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.

0 1200 2400 3600 4800 6000 7200-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Pot

enti

al (

V)

vs. A

g/A

gCl(

sat.

)

Immersion time (s)

AZ91D substrate

As-plated Cu-coated

As-plated Cr/Cu-coated

Flame-hardened Cr/Cu-coated

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.

C.A. Huang et al. / Corrosion Science 52 (2010) 1326–1332 1329

indicates that the AZ91D substrate was very active in the solution.As shown in Fig. 5, OCP values rose noticeably after Cu or Cr elec-troplating. The OCPs of as-plated and flame-heated Cr/Cu-coatedAZ91D specimens were nearly identical and ca. �0.2 V. A slightlyhigh OCP of 0.1 V was observed for Cu-deposited AZ91D.

For applications, an understanding of the corrosion behavior ofas-plated Cr/Cu-coated AZ91D is necessary, because such materialsmay be used directly following Cr electroplating. We studiedmainly the corrosion behavior of as-plated Cr/Cu-coated AZ91Din 0.1 M H2SO4 solution, varying the NaCl content up to 7 wt.%.Variations of OCP values of as-plated Cr/Cu-coated AZ91D speci-mens under these conditions are shown in Fig. 6. Evidently, theOCP of as-plated Cr/Cu-coated AZ91D decreased with increasingNaCl content, and a marked potential drop was observed whenthe NaCl content was raised above 3.5 wt.%. The potential droppedfrom approximately �0.2 V to �0.6 V, and this drop occurred more

early with increasing NaCl content. The potential drop observed inthe solution with 7 wt.% NaCl took place near the beginning ofimmersion, indicating that the as-plated Cr/Cu-coated AZ91Dwas corroded immediately in this solution.

3.4. Corrosive morphology study

As illustrated in Fig. 7, a few blisters were observed on the sur-face of as-plated Cr/Cu-coated AZ91D, after a potential drop wasinduced by immersion in 0.1 M H2SO4 with 3.5 wt.% NaCl. Sincethe AZ91D substrate has a low corrosion potential, it may be ex-pected that the AZ91D substrate was exposed directly to the solu-tion after the potential drop. However, the SEM data showed only asurface with a few blisters. This implies that the corrosion of as-plated Cr/Cu-coated AZ91D occurred primarily within blisters,leading to a relatively low corrosion potential.

0 1200 2400 3600 4800 6000 7200-0.8

-0.6

-0.4

-0.2

0.0

0.2

Pot

enti

al (

V)

vs. A

g/A

gCl(

sat.

)

Immersion time (s)

0.1 M H2SO

4 solution +

Without NaCl 1 wt% NaCl 3.5 wt% NaCl 5 wt% NaCl

7 wt% NaCl

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 afterobservation of a potential drop in 0.1 M H2SO4 with 3.5 wt.% NaCl.

0 1200 2400 3-0.8

-0.6

-0.4

-0.2

0.0

Pot

enti

al (

V)

vs. A

g/A

gCl(

sat.

)

Immersi

Fig. 8. Immersion periods I, II and III corresponding to those before, during, an

1330 C.A. Huang et al. / Corrosion Science 52 (2010) 1326–1332

To study the corrosion behavior of as-plated Cr/Cu-coatedAZ91D immersed in 0.1 M H2SO4 with 3.5 wt.% NaCl, surface mor-phologies and cross sections were examined after immersion forperiods I, II and III, corresponding to those before, during and afterthe potential drop as indicated in Fig. 8.

Figs. 9(a) and (b) show the surface morphology and cross-sec-tional view of as-plated Cr/Cu-coated AZ91D immersed in 0.1 MH2SO4 with 3.5 wt.% NaCl during period I. A slightly etched surfacewas observed, as in Fig. 9(a), but Fig. 9(b) shows that the Cu layeradjacent to the Cr layer was preferentially etched, particularly atsites directly under cracks penetrating through the Cr layer. Theseetching results indicate that such penetrating cracks provided cor-rosive pathways for the solution to attack the Cu deposit. Interest-ingly, the corrosion potential of Cu-coated AZ91D was a littlehigher than that of Cr/Cu-coated AZ91D, as illustrated in Fig. 5.Nevertheless, the Cu layer was etched preferentially when galvan-ically coupled to the Cr layer. This implies the Cr deposit becamemore passive than the Cu deposit when immersed in 0.1 MH2SO4 with 3.5 wt.% NaCl.

600 4800 6000 7200

on time (s)

II

I

III

d after a potential drop was observed in 0.1 M H2SO4 with 3.5 wt.% NaCl.

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 with3.5 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 with3.5 wt.% NaCl.

C.A. Huang et al. / Corrosion Science 52 (2010) 1326–1332 1331

When immersed for period II, many blisters could be seen onthe surface of as-plated Cr/Cu-coated AZ91D. As shown inFig. 10(a), in views of the surface morphology, the blisters seemunbroken. 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 en-graved Cu layer, which itself was preferentially corroded near pe-netrating cracks in the Cr layer. As the AZ91D was etched, thenearby Cr/Cu bilayer swelled and lifted off of the AZ91D surface,resulting in formation of a blister on the Cr/Cu surface. When theAZ91D was exposed in 0.1 M H2SO4 with 3.5 wt.% NaCl, the speci-

Fig. 11. The surface morphology of as-plated Cr/Cu-coated AZ91D specimen afterimmersion for period III (after potential drop) in 0.1 M H2SO4 with 3.5 wt.% NaCl.

men was dissolved substantially, and the corrosion potential of as-plated Cr/Cu-coated AZ91D decreased rapidly at the period II. Asshown in Fig. 11, a broken Cr/Cu layer was seen in the surface mor-phology of as-plated Cr/Cu-coated AZ91D after immersion for per-iod III. Broken sites were above blisters. Due to the relatively largeexposed area of AZ91D under the broken sites, low corrosionpotentials were detected after immersion for longer than periodIII (as defined in Fig. 8).

The results of the OCP tests in 0.1 M H2SO4 with 3.5 wt.% NaClfor as-plated and flame-hardened Cr/Cu-coated AZ91D specimensare shown in Fig. 12. A potential drop is clearly observable. Theblistered morphology of corroded, flame-hardened Cr/Cu-coatedAZ91D was similar to that of as-plated samples after observationof a potential drop. However, the immersion period required forobservation of the potential drop was relatively short for theflame-hardened Cr/Cu-coated AZ91D specimen, as shown inFig. 8. The potential data indicate that the corrosion resistance ofas-plated Cr/Cu-coated AZ91D decreased after flame heating for0.5 s. irrespective of its high hardness. This can be explained bythe increased presence and width of penetrating cracks in theflame-hardened samples (see Fig. 2), since corrosion occurredmainly through these cracks.

4. Conclusions

An environmentally benign electroplating process was pro-posed to obtain a protective Cr/Cu deposit on an AZ91D substrate.The corrosion behavior of Cu-coated, as-plated Cr/Cu-coated, andflame-heated Cr/Cu-coated AZ91D specimens were electrochemi-cally evaluated in 0.1 M H2SO4 solution with different NaCl concen-trations. The corrosion resistance of an AZ91D specimen could besignificantly improved after electrodeposition of Cr layer. Cracks

0 1000 2000 3000 4000 5000 6000 7000-0.8

-0.6

-0.4

-0.2

0.0 As-plated Cr/Cu-coated

Flame-hardened Cr/Cu-coated

Pot

enti

al (

V)

vs. A

g/A

gCl(

sat.

)

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.

1332 C.A. Huang et al. / Corrosion Science 52 (2010) 1326–1332

penetrating the Cr layer provided active pathways for corrosion ofthe Cu deposit, and subsequently the AZ91D substrate, in 0.1 MH2SO4 solutions with NaCl content higher than 3.5 wt.%. A few blis-ters were visible on the surface of Cr/Cu-coated AZ91D afterimmersion in 0.1 M H2SO4 solution with a NaCl-content above3.5 wt.%. Formation of blisters on the surfaces was confirmed bythe results of the open-circuit potential test, in which an obviousdrop of open-circuit potential from noble to active potentials wasdetected.

Acknowledgements

The author, Ching An Huang, would like to thank the NationalScience Council ROC, for financial support for this research undercontract number: NSC 97-2221-E-182-007.

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