Corrosion resistance of cerium-based conversion coatings on alumina borate whisker reinforced AA6061 composite

doi:10.1016/j.apsusc.2007.04.085

Applied Surface Science

Volume 253, Issue 22, 15 September 2007, Pages 8879-8884

https://doi.org/10.1016/j.apsusc.2007.04.085 Get rights and content

Abstract

Cerium-based conversion coatings on Al₁₈B₄O₃₃w/6061Al composite surface were obtained by immersing the composite into a solution containing various concentrations of CeCl₃. Results indicate that the susceptibility to pitting for the conversion-coated composites was much lower than that of the untreated composite, and the corrosion resistance of the coated composites was improved markedly; moreover, the concentration of CeCl₃ in the cerium solution affects significantly the corrosion behaviors of the coated composites. The coating obtained from a solution containing 7.5 g CeCl₃ into 1000 ml produced better corrosion resistance on the composite due to the surface being almost covered by conversion coating. EDX and XPS experimental results indicated that the coatings were made up of oxygen, cerium, and aluminum.

Introduction

Aluminum and its alloys are widely used as a structural material due to its low cost, excellent strength-to-weight ratio. The demand for materials of superior mechanical, thermal and electrical properties has focused attention on aluminum metal matrix composites (MMCs). However, MMCs have a more inhomogeneous structure than alloys due to presence of reinforcement, which will change the corrosion behavior significantly [1], [2].

Various surface modification procedures are being evaluated for Al alloys and their composites to improve the corrosion resistance. Chromate conversion coatings have been widely applied for corrosion protection of aluminum alloys due to their high electrical conductivity and high efficiency/cost ratio [3], [4]. However, the recent recognition that chromates are both highly toxic and carcinogenic has led to extensive worldwide research to develop effective alternative methods. Recent patents cover a wide variety of a new coating formulation [5], [6], [7], [8]. One of the most promising is based on rare earth elements. Some researches demonstrated that treatment with aqueous solutions of rare earth salts (cerium, lanthanum, etc) effectively inhibit the corrosion of aluminum alloys [9], [10], [11].

It has been found that chemical passivation by immersion in rare earth metal chlorides can produce exceptional resistance to localized corrosion [12]. Corrosion inhibition by cerium salts is generally associated with the formation and precipitation of cerium oxides or hydroxides over cathodic sites on the metal surface [13].

Relatively little is known about the effectiveness of corrosion protection of aluminum borate whisker reinforced aluminum composites (Al₁₈B₄O_33w/Al), which has been considered for a wide range application because of their high specific strength, high modulus and low cost [14], [15], [16].

The purpose of this paper is to report on characterization results for cerium conversion coating formed by immersion process that resulted in improved salt corrosion performance. The effect of the concentrations of CeCl₃ in the Ce-salt solution was investigated using electrochemical method and weight loss test.

Section snippets

Material

The Al₁₈B₄O_33w/6061Al composite was fabricated by squeeze casting. The volume fraction of the whiskers in the composite was about 20%. The whisker had a 0.5–1 μm in diameter and 10–30 μm in length (Shikoku Chemical Co. Ltd). The strength of the whisker is 8 GPa and the module is 400 GPa. The composition of the 6061 alloy: 0.2–0.6 wt.% Cu, 0.5–0.9 wt.% Mg, 0.5–0.9 wt.% Si, 0.2 wt.% Mn and balance Al. The whisker preform and the mould were preheated at 480–520 °C, and then 6061Al alloy at 780–800 °C was

Surface morphology

Fig. 1 shows the surface morphologies for the cerium conversion coated samples. Some whiskers existed on the surface as shown in Fig. 1(a), which indicated that the coating did not cover the whole surface of the sample. As a matter of fact, the coating formed slowly as observed visible which led to a majority of the surface exposed. For 5 g/L CeCl₃ concentration, the combination of Ce³⁺ and OH⁻ is limited on the surface owing to the presence of the lower Ce ³⁺ concentration in the coating

Conclusions

The cerium-based chemical conversion coating is an effective method to improve the resistance to localized corrosion of the composite. The concentration of CeCl₃ in the test solution had a significantly influence on the corrosion behavior and surface morphologies of the coated samples. The entire substrate surfaces were coated with Ce-coatings when the concentration of CeCl₃ was 7.5 and 10 g/L in the conversion solution, which exhibited resistance to localized corrosion because the composite is

Acknowledgement

The authors would like to thank the support of the Postdoctoral Science-Research Development Foundation of Heilongjiang Province, No. LBH-Q05051

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Corrosion resistance of cerium-based conversion coatings on alumina borate whisker reinforced AA6061 composite

Abstract

Introduction

Section snippets

Material

Surface morphology

Conclusions

Acknowledgement

Corros. Sci.

Surf. Coat. Technol.

J. Alloys Compd.

Electrochim. Acta

Surf. Coat. Technol.

Corros. Sci.

Scripta Mater.

Surf. Coat. Technol.

Surf. Coat. Technol.

J. Electron. Spectrosc. Relat. Phenom.