PURPOSE: To produce the highly corrosion resistant surface treated steel sheet having excellent corrosion resistance, adhesive property of coating materials and elutability of Cr by subjecting the surface of the steel sheet plated with Zn or Zn alloy to a chromate treatment and further, forming an org. film on the upper layer thereof.
CONSTITUTION: The surface of the steel sheet having the plating layer of the Zn plating or Zn alloy is treated with a chromate treating liquid having 5 to 150g/l chromic acid concn. in terms of metal Cr, 0.3 to 0.6 ratio of Cr3+/(Cr6++ Cr3+) Cr reduction rate, and contg. a silane coupling agent at ≥0.01 weight ratio to the Cr to form such chromate film as to have 10 to 150mg/m2 as Cr deposition. The org. film consisting of resin of an epoxy system, urethane system, acrylic system, etc. is formed at 0.1 to 3.0μm thickness on this chromate film.
TAIRA TAKETOSHI
SHINDO YOSHIO
