Patent ID: 8012599

Claim:
A surface-treated steel sheet with excellent corrosion resistance comprising: (a) a plated steel sheet selected from the group consisting of a zinc-based plated steel sheet and an aluminum-based plated steel sheet; (b) a surface-treatment coating having a coating thickness of 0.01 to 1 μm, and being formed by applying and drying on a surface of the plated steel sheet a surface treatment coating composition which contains the following ingredients (α) to (δ): (α) a water-epoxy resin dispersion which is prepared by dispersing in water a (Z) modified epoxy resin obtained by a reaction of a (W) epoxy group-containing resin, an (X) primary amine compound and/or secondary amine compound, and an active hydrogen-containing compound in which a part or the entirety of the compound is structured by a (Y) hydrazine derivative having active hydrogen; (δ) a water dispersed polyurethane resin having a mass ratio of resin solid matter in the water-epoxy resin dispersion to resin solid matter in the water dispersed polyurethane resin in the surface treatment coating composition, expressed as (water-epoxy resin dispersion)/(water dispersed polyurethane resin), being 95/5 to 5/95; (γ) a silane coupling agent in an amount of 1 to 300 parts by mass of solid matter therein to 100 parts by mass of the resin solid matter in the water-epoxy resin dispersion in the surface treatment coating composition; and (δ) phosphoric acid and/or a hexafluorometal acid in an amount of 0.1 to 80 parts by mass of solid matter therein to 100 parts by mass of the resin solid matter in the water-epoxy resin dispersion in the surface treatment coating composition; and (c) a top coating having a coating thickness of 0.3 to 2 μm, and being formed by applying and drying a coating composition for top coating, containing an (E) high molecular weight epoxy group-containing resin having a number average molecular weight of 6000 to 20000, wherein said high molecular weight epoxy group-containing resin is a hydrazine derivative-modified epoxy resin, the top coating being disposed on the surface-treatment coating.