PRETREATMENT FOR COATING STAINLESS STEEL

PURPOSE:To form a film having excellent adhesion to an org. coating layer by impressing a square wave potential to a stainless steel immersed into an electrolytic bath. CONSTITUTION:The square wave potential having the amplitude between a base potential range EL from a natural potential E1 of Cr to a passivation initiation potential E2 of the Cr and a noble potential range EH from a passivation initiation potential E3 of Fe to a potential E4 at which the overpassivation dissolution current of the Cr is increases drastically is pressed to the stainless steel immersed into the electrolytic bath to form a film contg. chromium hydroxide on the surface of the stainless steel; thereafter, the passivation is subjected to a coating type chromate treatment. As a result, the metal ions eluted from the stainless steel by the electrolysis in the noble potential range EH are deposited on the surface of the stainless steel by the electrolysis in the base potential range E1, by which the film is formed. The Cr contained in the stainless steel migrates as the hydroxide to the film in the process of such elution-precipitation. The surface layer having high affinity to rubber, resin, coating material, etc., is thus obtd.