Surface-treated steel sheet excellent in resistance to white rust and method for production thereof

摘要

The invention is to provide a surface-treated steel sheet of good white rust resistance with no chromium in its surface treatment film, and it has realized a surface-treated steel sheet of higher-level corrosion resistance by forming a reaction layer of plating metal with a barrier layer effective for anticorrosion not in a two-layered film but in a single-layered film. The surface-treated steel sheet is fabricated by applying a surface-treating composition that contains: (a) a water-dispersible resin and/or a water-soluble resin obtained through reaction of an epoxy group-containing resin (A) with an active hydrogen-containing compound (B) a part or all of which is an active hydrogen-containing hydrazine derivative and preferably with a hydrophilic organic component (C), (b) a silane coupling agent, (c) a phosphoric acid and/or a hexafluoro-metal acid, and (d) preferably an aqueous dispersion of an urethane resin, to a zinc-plated or aluminium-plated steel sheet followed by drying it to form thereon a surface treatment film having a thickness of from 0.02 to 5 mum. Regarding its film structure, the surface treatment film comprises an amorphous compound layer containing P, Zn and/or Al, and O, and an overlying organic resin matrix layer where the matrix is a hydrazine derivative-modified epoxy group-containing resin or the epoxy group-containing resin and an urethane resin.