Preparation Of Micro-Arc Oxidation Ceramic Coating On Mg Alloy And The Research Of Galvanic Corrosion

Ceramic coatings including ZrO2 were prepared on surface of AZ91D magnesium alloy by micro-arc oxidation (MAO) with single-polar anode pulse MPO power source. The thickness, composition, structure and galvanic corrosion property of the coatings were investigated.The composition and microstructures of the ceramic coatings were characterized by X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (EDS) and scanning electron microscope (SEM). The thickness and corrosion resistance of the coatings were evaluated by eddy current thickness meter, and electrochemical instrument respectively.The results of EDS and XRD analyses revealed that the zirconium and magnesium were brought into the ceramic coatings prepared on the surface of AZ91D magnesium alloy by micro-arc oxidation (MAO) with different electric parameters and electrolyte. Zirconium is crystalline in the coating. The results of SEM showed that the coating became cruder as the concentration of electrolyte increasing. The coating prepared by low current density and low frequency is of flatness, crackles, and compactness. The ceramic coating was representatively double-layer structure. It included compact barrier layer and pores layer. The double-layer structure was unconspicuous in early period. In the later period of MAO, the growth of the coating developed towards to Mg substrate direction. The thickness of barrier layer and pores layer increased rapidly. The ceramic coating appeared double-layer structure obviously.The corrosion resistance of ceramic coating was greatly affected by the structure of coating. The study showed that the thickness of the ceramic coatings first decreased and later increased with the Na5P3O10 concentration increasing, but the corrosion resistance of coatings changed contrary. As the K2ZrF6 concentration increasing, the coating was thicker and much more compact, the corrosion resistance of coating enhanced. With the current density increasing, the crackle of the coating was larger, and the corrosion resistance of coatings became worse. The low frequency could help enhancing the corrosion resistance of coatings.Comparing with the mg alloy substrate, the galvanic corrosion current of ceramic coating was lower, the property of galvanic corrosion enhanced. Between two coupled materials, the corrosion tendency of coating coupled with M40 carbon fiber was larger, but the corrosion current was lower, so the ceramic coating could couple with M40 carbon fiber in practical application. Coupling with LY12Al alloy, the galvanic corrosion current was greatly high, and the destruction of coating was serious. Ceramic coating was impossible to couple with LY12Al alloy in practical application.