Surface-treated steel sheet excellent in coating suitability and its production

PURPOSE:To produce a surface-treated steel sheet excellent in coating suitability over a long period by cathodizing a galvanized steel sheet in a soln. contg. chromic anhydride and an anion and then treating the steel sheet with a colloidal silica-contg. soln. CONSTITUTION:A galvanized steel sheet is cathodized in a soln. contg. chromic anhydride or a dichromic acid and an anion with the sheet as the cathode. Sulfate, chlorine, nitrate and fluorine ions are preferably used as the anion. Consequently, a metallic chromium layer is formed on the plating layer and a layer consisting of a chromium oxide contg. Cr and consisting essentially of Cr and the hydrous oxide thereon. The treated steel sheet is not dried but treated with a colloidal silica-contg. soln. A silica layer is formed on a chromate layer in this way. An electrolytically chromated galvanized steel sheet is obtained with the coating suitability not deteriorated even when worked under severe conditions or even if stored under various conditions for a long period.