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 .mu.m.
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.