Composition and method for chromating treatment of metal

Abstract

A chromate conversion coating with excellent cold workability and protection against corrosion after subsequent painting can be formed on metal surfaces, particularly on galvanized steel sheet, by drying in place on the metal an amount of an acidic aqueous composition comprising (A) from 4.0 to 51.0 g/L of hexavalent chromium; (B) from 6.0 to 38.0 g/L of trivalent chromium; (C) from 0.5 to 97.0 g/L of phosphate ions; (D) a component selected from the group consisting of sulfate ions, nitrate ions, fluoride ions, and mixtures thereof; and (E) a component selected from the group consisting of cations of Cu, Co, Ni, Sn, Fe, and Pb and mixtures thereof, wherein the trivalent chromium/hexavalent chromium weight ratio is in the range from 0.2 to 1.4. Preferably the amount of aqueous composition used is such as to produce a chromium add-on of from 20-160 mg/m.sup.2.

Dependencies are inferred from the claim text.