Method for producing chromate conversion coatings

In a non-electrolytic process for providing a chromate conversion coating on a metal surface selected from the group consisting of zinc, cadmium and zinc-cobalt alloy, wherein said metal surface is contacted with a composition for producing said chromate conversion coating for a time effective to produce said chromate conversion coating on said metal surface, and wherein said composition comprises an aqueous solution comprised of a source of hexavalent and/or trivalent chromium, acid, and a source of at least one activator anion, each in amounts effective to produce on said metal surface a protective chromate conversion coating when said metal surface is contacted with said composition, the improvement comprising including in said composition from about 1 to about 50 g/l of alkane sulfonic acid to increase the corrosion resistance afforded by said chromate conversion coating as compared to a chromate conversion coating formed on said metal surfaces from a composition without said alkane sulfonic acid.

In a non-electrolytic process for providing a chromate conversion coating on a metal surface selected from the group consisting of zinc, cadmium and zinc-cobalt alloy, wherein said metal surface is contacted with a composition for producing said chromate conversion coating for a time effective to produce said chromate conversion coating on said metal surface, and wherein said composition comprises an aqueous solution comprised of a source of hexavalent and/or trivalent chromium, acid, and a source of at least one activator anion selected from the group consisting of acetate, formate, sulfate, chloride, fluoride, nitrate, phosphate and sulfamate anions, and mixtures thereof, each in amounts effective to produce on said metal surface a protective chromate conversion coating when said metal surface is contacted with said composition, the improvement comprising including in said composition from about 1 to about 50 g/l of methane sulfonic acid to increase the corrosion resistance afforded by said chromate conversion coating as compared to a chromate conversion coating formed on said metal surface from a composition without said methane sulfonic acid.