Surface & Coatings Technology 202 (2008) 5603–5606

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Surface & Coatings Technology

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Influence of interlayers on corrosion resistance of ion-plated Mg thin films

Myeong-Hoon Lee a,⁎, Kyung-Man Moon b, Ki-Joon Kim a, Il Young Bae a, Sang-Min Baek a

a Divison of Maritime System Engineering, Korea Maritime University, #1 Dongsam-Dong, Youngdo-gu, Busan, South Koreab Divison of Material Engineering, Korea Maritime University, #1 Dongsam-Dong, Youngdo-gu, Busan, South Korea

⁎ Corresponding author. Tel.: +82 51 410 4264; fax: +E-mail address: leemh@hhu.ac.kr (M.-H. Lee).

0257-8972/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2008.06.150

A B S T R A C T

A R T I C L E I N F O

Available online 21 June 2008

Keywords:

It is well known that the filconditions. In this experimonto electroplated Zn, Al an

Ion-plated Mg filmCorrosion resistanceInterlayers

d Sn steel substrates at different Ar gas pressure conditions is studied. From theexperimental results, the corrosion resistance of Mg film is obviously correlated with the morphology of filmand a formation of galvanic couple with interlayer. The film of granular structure which deposited incondition of high gas pressure has certainly good corrosion resistance. Especially Mg film on Zn interlayer, asacrificial anode protection to substrate is continuously sustained by relatively low galvanic current in

m formed by PVD process has different characteristics depending on the processent, therefore, the corrosion characteristic of Mg film prepared by PVD method

galvanic couple between deposited Mg film and Zn interlayer during immersion in 3% NaCl solution.© 2008 Elsevier B.V. All rights reserved.

1. Introduction

The conventional coating processes as hot dip and electrochemicaldeposition coating on steel substrates are commonly used to enhancethe hardness, decoration as well as corrosion resistance for increasingits life-time. However, although conventional processes have rela-tively low cost and high deposition rate, these processes have beenconstantly pointed out to entail fundamental environment pollutionproblems. For this reason, PVD method which produces theenvironmentally friendly film is beginning to receive great attentionas an alternative to the electroplating and electroless plating [1].Accordingly corrosion resistance film by PVD method has been tried,however some of them concluded that the PVD coatings (as TiN, CrN,Cu, Au, etc.) showed a limited corrosion resistance due to theirintrinsic porosity which locally expose the portion of substrate. Also,deposition of thick coating film which is enough to isolate fromcorrosion environment usually resulted in high internal stress and lowadhesion [2–6]. Therefore, coating film for corrosion protection needsto have a good insulation from corrosion environment with thinthickness.

Mg coating film by PVD method can support sacrificial protectionto substrate if damage occurs to the coating and be effective inisolating the substrate from the corrosion environment by producingprotective corrosion products as MgO or Mg(OH)2 [7].

In this experiment, therefore, the corrosion characteristic of Mgfilm prepared by PVD method onto electroplated Zn, Al and Sn steelsubstrates at different Ar gas pressure conditions is studied.

82 51 403 0150.

l rights reserved.

2. Experimental procedure

Mg thin films were deposited by the thermo-electron activatedion-plating system [Fig. 1]. Substrates were electroplated Zn, Al and Snsteel. Evaporation material was magnesium with a purity of 99.99%.Prior to deposition, all of the specimens are cleaned with trichlor-oethylene in an ultrasonic bath and further cleaned by argon ion-bombardment at 5×10−1 Torr (bias voltage: −500 V). The deposition isthen carried out by supplying argon gas and applying −200 V ofsubstrate bias for 20 min as shown in Table 1.

Fig. 1. Schematic diagram of ion-plating apparatus.

Table 1Deposition conditions

Substrate (electroplated steel, 7–8 µm) Argon gas pressure Bias voltage

Zn interlayer 5×10−1 Torr −200 VAl interlayer 5×10−2 TorrSn interlayer 5×10−4 TorrEvaporation metal 99.99% magnesium

Fig. 3. X-ray diffraction patterns of Mg films deposited on different interlayers. (a) Zninterlayer. (b) Al interlayer. (c) Sn interlayer.

5604 M.-H. Lee et al. / Surface & Coatings Technology 202 (2008) 5603–5606

After deposition, the top surface and cross-section of themagnesium film was observed by scanning electron microscopy(SEM: JSM-540, JEOL, Japan). X-ray diffraction was performed by D/Max-2000 diffractometer (Rigaku Co., Japan) with Cu Kα radiation toobtain the diffraction peak of Mg thin film. For evaluation of corrosionresistance, anodic polarization measurements were carried out byPotentio-stat of CMS 100 system (Gamry Ins., America). Prior to thepolarization measurement, open-circuit potential (Eopen-circuit) ofspecimens was measured for 20 min. And the polarization scan wasthen begun at Eopen-circuit and scanned to anodic polarization at a scanrate of 1 mV/s in 3% NaCl solution.

3. Experimental results and discussion

3.1. Morphology and crystal orientation of magnesium film

Fig. 2 shows the top surface and cross cross-section of Mg filmswhich are deposited by ion-plating method. As gas pressure increases,regardless of interlayer, it can be seen that the morphologies of Mgthin film are changed from columnar structure into granular structurewith dense and homogeneous morphologies. It is considered that anucleus of films is becoming relatively faster than a nucleus growth bythe effect of absorption and occlusion according to the increase ofargon gas pressure.

X-ray diffraction patterns as a function of argon gas pressure atdifferent interlayers for the deposited Mg film are shown in Fig. 3. Ascan be seen, two main Bragg diffraction peaks are clearly observed at2θ=34.39° and 36.62°, corresponding to (002) and (101) plane ofmagnesium. The diffraction peak of Mg film has very low relativeintensity which indicates the small grain size of Mg film according tothe increase of argon gas pressure. Also, the preferred crystalorientations of Mg film which deposited on the Zn and Sn interlayersubstrate are heavily correlated with the argon gas pressure [Fig. 3(a)and (c)]. And X-ray diffraction patterns of Mg film which was formedon the Al interlayer exhibited (002) preferred orientation at alldeposition conditions as shown in Fig. 3(b). The formation of Mg filmwith different crystal orientation at the same process conditions is

Fig. 2. SEM photographs of Mg films (−200 V bias voltage).

Fig. 4. The result of open-circuit potentialmeasurement in 3%NaCl solution (5×10−1 Torr).SSCE: Saturated silver chloride electrode.

Fig. 6. Schematic diagram of the galvanic potential and current in galvanic couplebetween Mg film and interlayers.

5605M.-H. Lee et al. / Surface & Coatings Technology 202 (2008) 5603–5606

thought to depend on different re-oxidation properties of eachsubstrate which induces a different surface activity of evaporationparticle during coating process. In this experiment, however, it is notverified yet.

Fig. 5. Anodic polarization curves of Mg thin films measured in 3% NaCl solution. (a) Zn interinterlayer (5×10−1 Torr).

3.2. Electrochemical corrosion resistance

The corrosion resistance of Mg film was evaluated by measuringnatural potential and anodic polarization curves in 3% NaCl solution.

layer. (b) Al interlayer. (c) Sn interlayer. (d) The result of anodic polarization at different

Fig. 8. Natural potential of Mg film in 3% NaCl solution.

5606 M.-H. Lee et al. / Surface & Coatings Technology 202 (2008) 5603–5606

From the experimental results, the corrosion resistance of Mg film isobviously correlated with interlayer between substrate and Mg film. Allthe deposited Mg films are shown to have the lower natural potentialthan interlayers as can be seen in Fig. 4 and this provides the evidencethat sacrificial protection of Mg film is possible. Also, although thesame evaporation source is used in this experiment, natural potentialof Mg film is changed according to the kind of interlayer. Namely,because galvanic corrosion potential (Ecorr) is attributed to the potentialdifference in galvanic couple between interlayer and Mg film, it couldbe understood that the Mg film which deposited onto electroplatedSn substrate has the highest Eopen-circuit [Fig. 4].

Fig. 5 shows the results of anodic polarization measurement. Thecorrosion resistance of all specimens is enhanced by deposited Mgfilm. The film of granular structure which deposited in condition ofhigh gas pressure clearly showed good corrosion resistance at allinterlayers. It is, possibly, thought that the smaller and denser grainstructure than the columnar structure of Mg films may improve theformation of passive layer in corrosion environment. In Fig. 5(d),passive current density of Mg film is changed according to kinds ofinterlayer at the same process condition, too. Especially, Mg filmdeposited on Sn electroplated substrate showed lower passive currentdensity than that of electroplated Al and Zn substrate. It is consideredthat the galvanic corrosion current which is proportional to formationrate of passivity layer is influenced by galvanic corrosion potentialbetween Mg film and interlayer [Fig. 6]. It is considered that Mg filmwith relatively high galvanic current could be quickly reached atcritical current for passivation and it behaves to enhance theformation of passive layer during anodic polarization [Fig. 7]. Howeverthe increased galvanic corrosion current which is induced by a highcorrosion potential difference could be resulted in early exhaustion ofMg film. As an evidence of that, Mg film deposited on Zn interlayer,which thought to have a lowest galvanic current density, is continuallykeeping up a stable condition compared to the other specimens duringan immersion in 3% NaCl solution as can be seen in Fig. 8. Therefore asacrificial protection effect to substrate could be prolonged by arelatively low galvanic corrosion current between Mg film and interlayer.

4. Conclusion

From the experimental results, it can be conclude that:

(a) Dense and uniform passive layer could be formed in granularstructure of Mg film which is deposited at high gas pressure.And it resulted in good corrosion resistance.

Fig. 7. Schematic diagram of anodic polarization. Ic=Critical current density forpassivation. Ip=Passive current density.

(b) Also, corrosion properties of ion-plated Mg film can be changedaccording to different interlayer. Especially Mg film on Zninterlayer, which has the lowest galvanic current, continues asacrificial anodic protection for the longest period compared toMg film on Al and Sn interlayer.

(c) By this experiment, fundamental guideline for corrosionresistance coating film could be offered.

References

[1] B. Navinšek, P. Panjan, I. Milošev, Sur. Coat. Technol. 116–119 (1999) 476.[2] L. Guzman, G.K. Wolf, G.M. Davies, Sur. Coat. Technol. 174–175 (1999) 665.[3] D. Sansom, F. Alonso, J.J. Ugarte, F. Zapirain, J.I. Ofiate, Sur. Coat. Technol. 84 (1996)

480.[4] C. Bowden, A. Matthews, Sur. Coat. Technol. 76–77 (1995) 508.[5] Guosong Wu ⁎, Xiaoqin Zeng, Wenbin Ding, Xingwu Guo, Shoushan Yao, Applied

Surface Science 252 (2006) 7422.[6] P.K. Vencovskya⁎, R. Sanchez, J.R.T. Branco, M. Galvano 108–109 (1998) 599–603.[7] M.H. Lee, I.Y. Bae, K.J. Kim, K.M. Moon, T. Oki, Sur. Coat. Technol. 169–170 (2003) 670.