GB1331844A - Coated metal product and process for electrolytically coating metal surfaces - Google Patents

Abstract

1331844 Electrolytically applying a coating of metallic chromium plus chromium oxide BROKEN HELL Pty CO Ltd 19 April 1971 [24 March 1970] 25046/71 Heading C7B An article (such as strip, wire, rod or strapping) of or coated with Zn or Zn alloy (e.g. electro -, hot dipped - or pad wiped - galvanized wire or electrogalvanized steel strapping) has an electrolytically applied coating comprising both a film of metallic Cr having a thickness of at least 0À02 mg/sq ft and, over the metallic Cr film, a film of chromium oxide having a thickness as hereinafter defined of at least 0À01 mg/sq ft. Chromium oxide thickness is recorded as mgs of Cr/sq ft. The Cr plus chromium oxide coating may be produced in two-step or a one-step process. The two-step process comprises (1) cathodically producing the metallic Cr film in an aqueous solution of CrO 3 and SO 4 ions (optionally rinsing in hot water) and (2) cathodically producing the film of chromium oxide in an aqueous solution of chromium trioxide with a negligible amount of SO 4 ions. The one-step process comprises cathodic treatment in an aqueous solution of CrO 3 and SO 4 ions and optionally a reducing agent to produce Cr<SP>3+</SP> ions, e.g. alcohol, glycerine, sugar or H 2 O 2 . In both the two-step and one-step processes concentrations of CrO 3 , weight ratios of CrO 3 to SO 4 ion, current densities and electrolyte temperatures are specified. The anodes may be Pb-Sn, Pb-Sb or mild steel. Before treatment, the galvanized surface may be cleaned by a) degreasing in trichlorethylene and b) etching either (i) in H 2 SO 4 (optionally electrolytically as cathode or (ii) electrolytically as cathode in NaOH solution. Colours produced by the Cr plus chromium oxide coating on galvanized wire are tabulated. The Cr plus chromium oxide coating is suitable for subsequent coating with polyvinyl chloride or acrylic polymers or for electrophoretic painting.