Chemical conversion method and aqueous chemical conversion solution used therefor

A method for forming a chemical conversion coating on a member made of aluminum or its alloy comprising the steps of:

• (a) etching a surface of said member with an aqueous solution of acid or alkali;
• (b) immersing the etched member in a phosphate-based chemical conversion treatment solution which is substantially free from fluoride ions;
• and
• (c) applying a negative voltage, V.sub.t, to said member during at least a part of said immersion step, so that the potential, E.sub.t, of said member defined by the formula:
• E.sub.t =E.sub.0t +V.sub.t reaches a minimum potential, E.sub.m, satisfying the following formula:
• E.sub.0 -1.5V.ltoreq.E.sub.m .ltoreq.E.sub.0 -0.8V,
• wherein E.sub.0 represents an initial value of said natural electrode potential of said member, and E.sub.0t is said natural electrode potential of said member at a time when a certain period of time, "t", has passed.