It refers to the process of forming a trivalent or hexavalent chromium layer on the surface of a metal by acting on it with a chromate solution. Also known as chromatization treatment is used for the treatment of aluminum, magnesium and its alloys, can also form a chromatization layer on steel, but rarely used alone, often used with phosphating to close the pores of the phosphating layer, so that the bare steel in the phosphating layer passivation, in order to inhibit the corrosion of the residual phosphating accelerator, further increase the protection ability.