NOVEL COMPOSITE CHEMICAL-CONVERSION COATING, MULTILAYERED COATING FILM INCLUDING THE SAME, AND METHOD OF FORMING MULTILAYERED COATING FILM

摘要

A composite chemical-conversion coating formed on a metallic base and comprising a crystalline continuous coating film. Also provided is a method of forming a multilayered coating film which comprises (A) a first step in which a metallic base to be treated is immersed in an aqueous solution containing a nitric acid salt of a rare-earth metal and is subjected to cathodic electrolysis to deposit a crystalline continuous coating film comprising a rare-earth metal compound in an amount of 1-110 mg/m² and (B) a second step in which this metallic base is subjected to cathodic electrodeposition with an electrodeposition composition containing an (in)organic acid salt of a rare-earth metal. Compared to conventional multilayered coating films formed by pretreatment and cationic electrodeposition steps with a chemical treating liquid and an electrodeposition composition, this multilayered coating film comprises an extremely small amount of a composite chemical-conversion coating and an electrodeposition film successively formed in this order. This multilayered coating film has excellent coating adhesion and corrosion resistance which are equal or superior to those of the coating films formed by the conventional steps. The composite chemical-conversion coating is a novel coating having high profitability and highly contributing to environmental preservation.