This paper studied the appearance transition of microdischarges, the phase composition and the morphology evolution of the oxide film formed by microarc oxidation on AZ91D magnesium alloy. The appearance of microdischarges population experienced apparent changes in size, spatial density and color, which was related with the changes of the type and quantity of the disintegrated gas bubbles generated at the interface between the electrolyte and substrate. Correspondingly, the diameter of micropores together with net-like fine microcracks increased when a higher voltage was employed. The coating was composed of MgO, MgAl2O4 and there existed a fluoride-enriched zone of about 3-5μm at the film/substrate interface.
The phase composition and electrochemical performances of microarc oxidation（MAO） films prepared on AZ91D alloy by using step-down current method in a phosphate electrolyte（P-film） and silicate electrolyte（Si-film） were studied. The results show that P-film is mainly composed of Mg, MgAl2O4 and MgO, and Si-film is composed of Mg2SiO4 and MgO. There clearly exists a fluoride-enriched zone with the thickness of about 12μm for P-film and 0.71μm for Si-film at the MAO coating/substrate interface. The electrochemical tests show that both P-film and Si-film can enhance the corrosion resistance of AZ91D magnesium alloy significantly. The corrosion failure process of the two films in 5%（mass fraction） NaCl solution is quite different.
