| 摘要 |
PURPOSE:To improve the corrosion resistance and weldability of a galvanized or Zn alloy plated steel sheet by subjecting the steel sheet to AC electrolysis in a bath contg. sexivalent Cr and phosphate ions under specified conditions to form a P-Cr surface film having a proper composition. CONSTITUTION:A galvanized steel sheet or a steel sheet plated with a Zn alloy contg. >=60wt% Zn is subjected to AC electrolysis in an electrolytic bath contg. sexivalent Cr and phosphate ions. At this time, the wave-form of AC is regulated so as to prevent underwater sparking. By the electrolysis, a surface film having a composition consisting of 30-500mg/m<2> Cr and 50-1,000mg/m<2> P is formed to provide high corrosion resistance and workability to the steel sheet as well as to improve the weldability of the steel sheet.
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