Chemical conversion method and aqueous chemical conversion solution used therefor

摘要

The method for forming a chemical conversion film on a substrate made of aluminum or its alloy which comprises the steps of: (a) etching a surface of the substrate with an aqueous solution of acid or alkali; (b) immersing the etched substrate in an aqueous, phosphate-based chemical conversion solution which is substantially free from fluoride ions; and (c) applying a negative voltage to the substrate during at least a part of the immersion step so that a potential of the substrate reaches a predetermined minimum potential which is lower than a natural electrode potential of the substrate in the aqueous chemical conversion solution.