HIGHLY CORROSION REISTANT SURFACE TREATED STEEL AND PREPARATION THEREOF

PURPOSE:To enhance corrosion resistance of a corrion resistant Zn-Ni alloy type plated film without increasing a Ni content by containing a specified small amount of Cr in sald film applied to a surface of a steel plate. CONSTITUTION:In forming a corrosion resistant Zn alloy type plated film on a surface of a substrate plate made of steel such as a steel plate or the like, as a plating bath, an electroplating bath having a composition as described below is used. That is, Zn is contained in a concn. of above 20g/l and Ni in a concn. of above 20g/l and a concn. ratio of Zn/Ni is adjusted to 0.25-4 and, further, 0.1-10g/l of trivalent Cr or 0.05-0.5g/l of hexavalent Cr is contained to form said plating bath and electroplating is carried out by using the resultant plating bath under such a condition that a pH is 1.5-3.5, a temp. is 40-70 deg.C and current density is 5-40A/ dm . A Zn-Ni-Cr alloy type plated film containing 2-20% Ni and 0.005-0.5% Cr which is excellent in appearance, film close adhesiveness, especially, corrosion resistance is formed.