Patent ID: 7517591

Claim:
A surface-treated steel sheet with excellent corrosion resistance comprising: a zinc-based plated steel sheet or an aluminum-based plated steel sheet; a surface-treatment coating disposed on a surface of the plated steel sheet having a coating thickness ranging from 0.01 to 1 μm, and being formed by applying and drying a surface-treatment coating composition which contains ingredients (a) through (c) described below; and a top coating having a coating thickness ranging from 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 ranging from 6000 to 20000 on the surface-treatment coating: (a) a water-epoxy resin dispersion which is prepared by dispersing in water a resin obtained by a reaction of: an (A) polyalkyleneglycol-modified epoxy resin derived from a reaction of polyalkyleneglycol having a number average molecular weight ranging from 400 to 20000, a bisphenol type epoxy resin, an active hydrogen-containing compound for blocking an isocyanate group, and a polyisocyanate compound; a (B) epoxy group-containing resin other than the (A) polyalkyleneglycol-modified epoxy resin; and an active hydrogen-containing compound which reacts with an epoxy group in the (A) polyalkyleneglycol-modified epoxy resin and the (B) epoxy group-containing resin, a part or all of the active hydrogen-containing compound being a (C) hydrazine derivative having active hydrogen; (b) a silane coupling agent in an amount ranging from 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; and (c) phosphoric acid and/or a hexafluorometal acid in an amount ranging from 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.