Abstract: In order to improve the low corrosion resistance and surface finish of AZ91D magnesium alloy castings,the chromate conversion film was prepared by Cr₂O₇^2-solutions with different concentration on the surface of magnesium alloys.The microstructure of the surface conversion film was observed by scanning electron microscope,and the X-ray diffraction and the energy dispersive instrument were used to analyze the phase composition and elements distribution.The influence mechanism of chromate concentration on the film forming behavior was revealed.In addition,the corrosion resistance of AZ91D magnesium alloy was studied based on electrochemical impedance spectroscopy.The results indicate that the chromite conversion film of magnesium alloy forms based on the redox reaction,which is accompanied by α-Mg dissolving,intragranular etching,island exposing of grain boundary and transformation film coating.The Cr-containing compounds MgCrO₄ and Cr （OH）₃ constantly fill the etching gaps with the increase of Cr₂O₇^2-,which can lead to the reduce of surface microcracks and the improvement of the film finishing.The maximum film thickness is 2.87μm at the concentration of 50g/L,meanwhile,and the self-corrosion potential is positively shifted to -1.299 V,and the fitting impedance value increases obviously compared with that of the untreated sample.There is an obviously enhancement in corrosion resistance for AZ91D magnesium alloys after chromate treatment due to the decrease of corrosion current density.