Study On Corrosion Resistence Of Al₁₈B₄O_33w/6061Al Composite With Rare Earth Conversion Coating

Technological process of rare earth conversion(REC) coating on Al₁₈B₄O₃₃w/6061Al composite was researched in this paper. The best technical parameter of conversion coating was obtained by orthogonal tests. The effects of technical parameter (concentrations of rare earth salts, concentrations of oxidizer H₂O₂, pH and immersion time) on the formation and corrosion properties of the coating were studied in detail. The microstructures, elements composition, and thickness of the conversion coating have been examined by scanning electron microscopy(SEM) and energy dispersive X-ray, and the corrosion behavior and corrosion mechanism of the composite with conversion coating were evaluated by polarization curves and electrochemical impedance spectroscopy(EIS).Various surface microstructures were observed in the composite with conversion coating. An obvious difference in morphological transformations was observed on the surfaces of the coating composites. The surface of the sample with the optimal conversion parameter treated can be covered by the coating.XPS analysis indicated that the conversion coatings were composed of cerium oxidation and hydroxide. The main phases detected in the conversion coating were Ce3+and Ce4+, according to the characteristic peak.The results of polarization curves indicate that concentration of cerium solution has significant influences on the corrosion resistance of the conversion coating, and the optimal conversion parameter of cerium concentration was 7.5 percent. Other parameters have also significant influences on the corrosion resistance of coating. An increase in Epit was obtained after conversion treatment for the composite. The improvement was considered primarily due to the rare earth conversion coating inhibit the propagation of corrosion.Moreover, the specimen with the optimal conversion parameter treated exhibited a much greater resistance to corrosion than the other specimens with conversion treated. This was attributed to the formation of the homogeneous and compact microstructure in the surface of the composite.