Action Mechanism Of Solute Elements During The Formation And Growth Of Ceramic Coatings On Aluminum Alloy By Microarc Oxidation

Solute elements play a very important role during microarc oxidation treatment on aluminum alloy surface. The effects of the variety and concentration of solute elements and pH value of solution on the course of the initial film formation and ceramic coating growth during microarc oxidation treatment were investigated. The action mechanism of solute elements was analyzed. According to the mechanism the black ceramic coating was prepared on aluminum alloy surface. The surface and cross-section morphologies of initial films and ceramic coatings were observed by SEM. The element composition of initial films and ceramic coatings and the changing trend of elements in ceramic coating were investigated by EDS. The chemical valence of solute elements in ceramic coating was investigated by XPS. The thickness of ceramic coating was measured by eddy current thickness gauge. The electrochemical properties of aluminum alloy in different microarc oxidation solution system were investigated by electrochemical analysis.The results indicate that oxide films are formed by micorarc oxidation at the initial film formation stage and the solute elements only exist in film surface with very small quantity. The ceramic coatings obtained at the later coating growth satge during microarc oxidation are mainly composed of Al and O as well, but the content of solute elements in ceramic coatings present distinct difference. There is not solute element Mo in ceramic coating fabricated in Na₂MoO₄ solution. The content of solute element Si in ceramic coating fabricated in Na₂SiO₃ solution is as high as 10.75%. It is benefit to forming high-impedance film on aluminum alloy surface which would provide conditions for the proceeding of later disruptive discharge in the solution with pH value of 7.50—10.00. As the increasing of solution conductivity, the growing rate of ceramic coating increases. But the growing rate presents some difference in different solute system. The course of initial film formation presents a close relation with electrochemical behavior of aluminum alloy in microarc oxidation solute system. At the initial film formation stage a high-impedance film is formed on the aluminum alloy surface which is immersed in solution. As the thichness of film gtadually increased the applied voltage elevates so as to reaching striking voltage and the phenomenon of microarc discharge is presented. The film formation rate and striking voltage are influenced by the variety of solute elements. At the ceramic coating growth stage solute anions with adsorptive characteristic and so on are adsorbed and deposited on coating surface during microarc discharge and coating growth. As a result, the composition and microstructure are changed. The black ceramic coating is prepared by adding 6g/L NH₄VO₃ into microarc oxidation solution according to the adsorptive characteristic of solute elements.