Chromate passivation treatment of galvanized sheet can form passivation film, its chemical reaction formula is as follows: Zn+H2GrO4-ZnGrO2=H2

The trivalent chromium in solution passivation group is insoluble in water, and its chemical properties are not active, which plays the skeleton role, while the hexavalent chromium is easily soluble in water, which can play the role of re-passivation when the passivation film scratches, and has the healing effect of passivation film. Therefore, within a certain limit, passivation film can prevent steam or moist air directly erosion galvanized sheet, zinc layer plays a protective role.