Study Of Rare-earth Conversion Coatings On Zinc Alloy

Rare-earth conversion film was obtained on surface of zinc alloy by immersion process. Several factors to affect the anti-corrosion efficiency of the conversion film were studied. Corrosion resistance of conversion film was examined by means of weight loss test and electrochemical test. And the composition, structure and morphologies of coating were identified by means of x-ray diffraction analysis ( XRD) ,electron microprobe analysis (EMP). Cathodic polarization was also studied. The weight loss test indicated that the pure cerium passivation treatment without additives appeared to be as superior as the chromium treatment in increasing the corrosion resistance, while the cerium plus additives treatment were more superior than the pure cerium treatment.. The polarization curves indicated that the presence of the hydrated cerium oxide film on the zinc in NaCl solution inhibits corrosion process of the zinc, while additives HF and BA in teatment solutions have played a very beneficial role in inhibiting the cathodic reactions of corrosion, leading to a better corrosion prevention effect for zinc. The scanning electron microscopic examination indicated that the conversion coatings formed on this zinc alloy are' inhomogeneous and structurally complex. They consisted of a background film of non-uniform thickness upon which particles of different size and shape are attached. Electron microprobe analysis(EMP) of these coatings on the surface of specimens indicated that the proportion of cerium in the total amounts of cerium and zinc increased with the increase of the exposure time. And it also indicated a strong cerium concentration in the particles and a weak cerium concentrationinthe background film. The particles or grains of the coatings on surface of the specimen treated in cerium salt + additives solutions appear to be denser and finer than that of treated in pure cerium salt solutions. The cerium +additives conversion coating mainly consists of cerium fluoride(CeF3), Ce6O11 and BA, while the single cerium conversion coating is basically composed of ZnO, CeO2 and Ce2O3 .