PURPOSE: To improve remarkably the corrosion resistance of a Zn alloy electroplated steel sheet by plating the surface of a steel sheet with a composite Zn alloy, carrying out chromating, and coating the resulting chromate film with a specified resin layer.
CONSTITUTION: A layer of a Zn alloy such a Zn-Ni, Zn-Ni-Co, Zn-Ni-Cr or Zn-Fe is formed on the surface of a steel sheet by electroplating. A chromate film is formed on the Zn alloy layer by a conventional method by 2W60mg/m2 (expressed in terms of Cr). An aqueous dispersion contg. 10W30pts.wt. water soluble melamine resin and 10W60pts.wt. colloidal silica basing on 100pts.wt. carboxylated polyethylene resin having 3W20mol% carboxyl groups is applied to the chromate film, and it is dried by heating the steel sheet to ≥130°C to coat the chromate film with a resin layer. The amount of the resin layer is 0.3W5g/m2.
TANDA TOSHIKUNI
ISHITOBI HIROTAKE
