PURPOSE: To form a chromate film extremely excellent in corrosion resistance and adhesion with an organic film on the surface of various metals.

CONSTITUTION: A chromate bath contg. 5 to 30 pts.wt. chromic acid, 1 to 20 pts.wt. long-chained colloidal silica having 2 to 20nm thickness and 25 to 250nm length, 10 to 35 pts.wt. phosphoric acid and 3 to 10 pts.wt. MgO is applied to a various metal or alloy plated steel sheet to remarkably improve its corrosion resistance. These effects are found to be obtd. only when the synergistic effect of the four components is given, and if only one component is lacked, excellent corrosion resistance can not be obtd. Since the chromate film obtd. by this treatment is remarkably excellent in adhesion with an organic film, it is suitable also as surface treatment for an organic composite plated steel sheet coated with an extremely thin organic film of 1 to 2μm. Moreover, it is suitable as a base steel sheet for coating in which a plated steel sheet is subjected to this treatment and the surface is directly applied with coating.