Journal of Alloys and Compounds 364 (2004) 193198

XPS study of cerium conversion coating on theanodized 2024 aluminum alloy

Xingwen Yu a,, Guoqiang Li ba Department of Interface Chemistry and Surface Engineering, Max Plank Institute for Iron Research, D-40237 Dusseldorf, Germany

b Department of Materials Science and Engineering, Beijing University of Science and Technology, 100083 Beijing, ChinaReceived 3 February 2003; received in revised form 17 March 2003; accepted 7 April 2003

Abstract

Cerium-rich conversion coating was deposited on anodized aluminum alloy 2024 in a solution containing Ce(NO3)3. X-ray photoelectronspectroscopy (XPS) was used as the analysis method. The composition of the Ce conversion coating deposited on the anodized 2024 alloywas investigated using this method. It was revealed that the coating predominately consisted of three-valent state cerium compound. Some ofthe CeIII was oxidized to CeIV in the outer layer coating. 2003 Elsevier B.V. All rights reserved.

Keywords: Ce-rich conversion coating; Aluminum alloy; X-ray photoelectron spectroscopy (XPS)

1. Introduction

For almost 100 years, chromate compounds (Cr6+) havebeen used as very effective and inexpensive corrosion in-hibitors for many alloys systems, including aluminum, zincand steel, in a wide range of aqueous environments. How-ever, the recent recognition that chromate are both highlytoxic and carcinogenic has led to extensive worldwide re-search to develop effective alternative inhibitors.

The idea of cerium conversion coatings as a protectivecoating on aluminum alloys was proposed by Hinton et al.[1] in the mid-1980s. They discovered that additions ofcerium ions to sodium chloride solutions significantly re-duced the corrosion rate of 7075 aluminum alloy [1]. Theresult inspired a great interest for them and other experts ininvestigating the formation processes, formation mechanismand characterization of cerium conversion coatings for alu-minum alloys. These preliminary studies revealed that corro-sion protection was attributed to the formation of a hydratedcerium oxide film on the alloy surface and film depositionproceeded in terms of a cathodic mechanism [28]. So far,cerium conversion coating have been developed to meet thedemand of non-toxic coatings processes [919]. It should

Corresponding author. Tel.: +49-211-679-2547;fax: +49-211-679-2218.

E-mail address: xingwenyu@yahoo.com (X. Yu).

be noted, however, that among these processes almost allcerium conversion coatings have to be deposited directly onthe matrix of aluminum alloys and little attention was paidto its application in anodizing of aluminum alloys. Becauseanodizing is one of the most widely used techniques for cor-rosion protection and decoration of aluminum alloys, it isinteresting to explore the possibility of forming cerium con-version coatings on porous film of anodized aluminum.

In our recent work, the cerium conversion coating with ayellow appearance was deposited successfully as a porousfilm on anodized aluminum alloy 2024. The corrosion re-sistance of the coating was found to be comparable to thatof chromate-sealed anodic coatings, as discussed elsewhere[20]. The purpose of this paper is to investigate the compo-sition of cerium conversion coating by X-ray photoelectronspectroscopy (XPS).

2. Experimental

The material studied was 2024 Al alloy and the majoralloying elements are listed in Table 1.

General procedures of degreasing, alkaline etching, andacid neutralization were used as pretreatment of aluminumanodizing. The specimens were anodized galvanostaticallyin a stirred aqueous solution of 18% (wt%) H2SO4 at acurrent density of 1.5 A dm2 and room temperature. Pb

0925-8388/$ see front matter 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0925-8388(03)00502-4

194 X. Yu, G. Li / Journal of Alloys and Compounds 364 (2004) 193198

Table 1Major alloying elements of 2024 alloy (wt%)Cu Mg Mn Fe Si Zn Ni Ti Others Al

3.84.9 1.21.8 0.30.9 0.5 0.5 0.3 0.1 0.15 0.1 Balance

sheets served as a cathode. After being anodized, the spec-imens were rinsed fully in distilled water for subsequentuse.

The anodized samples were immersed in the solution con-taining Ce(NO3)3 and then rinsed with distilled water, anddried with air. The parameters of the cerium conversion coat-ing process were described in Table 2.

Cerium conversion coating obtained by this process wasabout 35 m (examined by eddy-current thickness indica-tor) with excellent adhesion (examined with bending test)and yellow colorization. The corrosion resistance of the coat-ing was examined to be comparable to that of chromate seal-ing anodized coatings for 2024 alloy by salt solution immer-sion test and electrochemical methods [20].

Surface analysis was performed by X-ray photoelectronspectroscopy (XPS), using a vacuum generator PHI 5700ESCA. The X-ray source was unmonochromatized Al Kradiation. The accelerating voltage was 12.5 kV. The targetpower was 50 kW. A vacuum generator argon ion gun wasused for depth profiling. The sputter rate was 3 nm min1in an argon pressure of 0.6 106 at 3 keV energy. Thesoftware packages MULTIPAK was used for the analysis ofXPS data.

3. XPS analysis

The ASF (atomic sensitivity factors) method was used forelement quantitative analysis. Relative atom concentrationCX was calculated using the equation [21]:

CX = nXi ni

= IX/SXi Ii/Si

(1)

where subscript X and i represent calculated and componentatom, respectively, n is the number of atoms, I is the integralintensity of photoelectron signal, and S is the relative atomicsensitivity factor.

XPS spectra of cerium compounds exhibited complex fea-tures owing to hybridization with ligand orbitals and partialoccupancy of the valence 4f orbital. Fig. 1 showed the Ce3dspectra of CeO2 [22]. The peak, which arose from atransition from the 4f initial state to the 4f final state, wasa satellite line of the Ce3d3/2 line. It had been reported inthe literature that the peak arose exclusively from Ce4+

Table 2The cerium conversion coating process

Ce(NO3)3 (g l1) H2O2 (g l1) H3BO3 (g l1) pH Temperature (C) Time (h)3.0 0.3 0.5 5.0 30 2

Fig. 1. Ce3d spectrum of CeO2.

and was absent from the Ce3d spectra of pure Ce3+ species[22,23]. The major reason for this was thought to be the lackof the 4f configuration in the case of Ce3+. Hence the peak could be used as valence state analysis of cerium com-pounds and quantitative measure of the amount of Ce4+.Shyu et al. [22] have demonstrated that the integral area ofthe peak with respect to the total Ce3d area could betranslated into percentage of Ce4+ with the relative error ofbeing in the range of 10%. In the case of pure Ce4+, the peak should constitute around 14% of total integral inten-sity. According to the linear dependence of percentage on percentage Ce4+ reported in the literature, percentage ofCe4+ was calculated by:

Ce4+ % = %14

100% (2)

where % is percentage of peak area with respect tothe total Ce3d area.

4. Results and discussion

Fig. 2 shows the XPS survey spectra of the cerium con-version coating at different sputter times. Besides Al, O, Ceand S, significant amounts of C were present in the outerlayer of the conversion coating. It was reasonable that thepresence of C and partial O may be due to the accumula-

X. Yu, G. Li / Journal of Alloys and Compounds 364 (2004) 193198 195

Fig. 2. Survey spectra of the Ce conversion coating at different sputter times. (a) Without sputter; (b) 12; (c) 36; (d) 51; (e) 60 min.

tion of contaminants during exposure to air [22]. The ele-ment depth profile corresponding to the survey spectra inFig. 2 was shown in Fig. 3. It shows that the depth of con-tamination of C is less than 30 nm. If changes in elements

Fig. 3. XPS depth profile corresponding to the survey spectra in Fig. 2.

proportion due to the adsorption of adventitious C and Owere considered to be neglected, it was clear that the amountof Ce atoms decreased with increasing depth, which sug-gested that deposition rate of the cerium conversion coatingincreased with immersion time.

The XPS high-resolution spectra of Ce3d obtained fromdifferent sputter times, which correspond to various depthsin Fig. 2, are shown in Fig. 4. When compared with Fig. 1,it was found that only surface Ce3d spectra of cerium con-version coating was substantially similar to that of CeO2 instructure. This indicates that surface cerium was predomi-nantly in the four-valent state and enjoyed a similar chemicalenvironment to that in CeO2. On the other hand, all insideCe3d spectra were quite different to that of surface Ce andhad almost identical peak structure except for the small dif-ferences in spectral intensity of the peak. According toEq. (2), Table 3 shows the percentage of Ce4+ to total Ce atdifferent depth (sputter time). (It should be noted here thatthere might be some errors of the data in Table 3 due to the

196 X. Yu, G. Li / Journal of Alloys and Compounds 364 (2004) 193198

Fig. 4. High resolution Ce3d spectra of Ce conversion coating at different sputter times. (a) Without sputter; (b) 12; (c) 36; (d) 51; (e) 60 min.

possibility of the reduction of Ce4+ to Ce3+ during argonsputtering. But the tendency of decreasing Ce4+ with depthof the coating is correct. This conclusion can be drawn fromthe fact that the increase in Ce3+ is continuous. If the Ce3+was totally from the reduction of Ce4+ by sputtering, theCe3+ content should remain the same after sputtering). FromTable 3, it can be seen that cerium of bulk conversion coatingwas predominantly in the three-valent state. The presence ofCe4+ compounds in the inner Ce conversion coating layersuggested that some Ce3+ was oxidized to Ce4+ during theformation of the film. The majority of Ce4+ compounds in

Table 3Changes in Ce4+ concentration at different sputter times

Sputter time (min)0 12 36 51 60

Ce4+ concentration (%) 81.36 20.17 17.68 15.15 9.54

the outer coating layer indicates that some of the remnantCe3+ were also oxidized to Ce4+ during the course of ex-posing the specimens in the open air.

The high-resolution O1s spectra at different sputter timesmeasured on the sample with Ce conversion coating areshown in Fig. 5. From Fig. 3, it could be seen that there wasno Al in the survey spectra of the un-sputtered surface. Forthe samples after sputtering at different times, the coatingconsisted of aluminum oxide and cerium oxides/hydroxides.The assumption being that the O1s spectrum was attributedto the AlO group (531.6 eV), CeO group (529.5 eV) andthe CeOH group (532 eV) (no AlO on the surface with-out sputter). Based on the three groups above, O1s spec-tra were fixed as in Fig. 5. According to area of the peaks,[OCeOH]/[OCeO] and [Al]/[Ce] were obtained as shownin Table 4. Apparently, cerium oxide was the predominantcomposition of the coating. The cerium hydroxide contentdecreased with increasing depth of the film while the alu-minum content increased with increasing depth of the film.

X. Yu, G. Li / Journal of Alloys and Compounds 364 (2004) 193198 197

Fig. 5. High resolution O1s spectra of Ce conversion coating at different sputter times. (a) Without sputter; (b) 12; (c) 36; (d) 51; (e) 60 min.

Table 4Changes in [OCeOH]/[ OCeO] and [Al]/[Ce] at different sputter times

Sputter time (min)0 12 36 51 60

[OCeOH]/[ OCeO] 0.910 0.284 0.225 0.196 0.129[Al]/[Ce] 0.439 0.453 0.579 1.003

5. Conclusion

Cerium conversion coating on anodized 2024 aluminumalloy consisted of Al oxide, Ce oxide, and Ce hydroxide. TheCe state exhibited a mixture of Ce3+ and Ce4+. The outerlayer of the coating consisted of mostly Ce4+ compounds.This indicates that some of the Ce3+ was oxidized to Ce4+during coating formation in the solution containing Ce3+

ions. During exposure of the specimens to open air, someCe3+ on the surface of the coating were oxidized to Ce4+.

Acknowledgements

This work has been carried out with the support of TheChinese Postdoctoral Science Fund and The Special Fundsfor the Major State Basic Research Projects G19990650.

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XPS study of cerium conversion coating on the anodized 2024 aluminum alloyIntroductionExperimentalXPS analysisResults and discussionConclusionAcknowledgementsReferences