AZ91D magnesium alloys were processed by micro-arc oxidation（MAO） in silicate solution by means of a self-designed AC（alternative current） power suppler.The optimized processing parameters of MAO for AZ91D magnesium alloy in silicate solution were determined on the basis of an orthogonal testing with four factors and three levels used to systematically investigate the coating thickness

surface roughness and corrosion resistant.The results show that the MAO film is characterized by porous structure and smaller apertures and compact film layer as well as uniform distribution in the surface of the AZ91D magnesium alloy

consisting of MgO

Mg2SiO4

AgAl2O4 and minor SiO2

in the condition of the optimized processing parameters.The absence of apparent pit-erosion in the film surface can be observed after immersing for 168 h in 3.5% NaCl solution at room temperature

demonstrating the corrosion resistance greatly superior to that of AZ91D magnesium alloy substrates.