Plasma electrolytic oxidation of AZ91D magnesium alloy was studied in an optimized solution mainly containing Na2SiO3. The microstructure, phase constituents and elemental distribution of ceramic films were investigated by using scanning electron microscopy （SEM） and X-ray diffraction （XRD）. The corrosion resistance of the magnesium alloy with ceramic films were evaluated by potentiodynamic polarization curves and electrochemical impedance spectroscopy （EIS）. The results indicated that Icorr of the magnesium alloy oxidized in the optimized solution was decreased by three orders of magnitude compared with that oxidized in the conventional solution. The polarization resistance of the magnesium alloy oxidized in the optimized solution was higher than that oxidized in the conventional solution, and the time for appearance of obvious corrosion pits during immersion in 3.5% NaCl solution was increased from 24 h for the sample oxidized in the conventional solution to 240 h for that oxidized in the optimized solution. The better corrosion resistance of the magnesium alloy oxidized in the optimized solution results from fewer defects, fewer pores, as well as higher content of the phase of Mg5 F2 （SiO4 ）2 with lower dissolubility and the MgAl2O4 with better passivity in the oxide film.
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