PROCESS OF ELECTROPLATING WITH CHROMIUM AND COATING WITH CHROMIUM OXIDE

1,250,013. Electrolytically coating steel with Cr and chromium oxide; lead alloy. UNITED STATES STEEL CORP. Oct.10, 1968 [Nov.3, 1967], No.48151/68. Heading C7A and C7B. Can stock is produced by electrolytically coating steel with both Cr and overlying chromium oxide. The process comprises (a) providing a first stock-treating zone including an electrolyte containing from 40-100 g/1 of chromium trioxide and also containing sulphate ions (and optionally fluoride ions), the electrolyte being effective, under the influence of an electric current, to deposit metallic Cr and a hydrated, trivalent Cr-containing oxide upon stock as a cathode and capable of chemically dissolving the oxide; (b) continuously moving a cleaned, elongated, flat rolled steel stock article as cathode through the first zone while applying to the article a current of at least 500 amps/sq ft of cathode surface for a time sufficient to electro-chemically deposit a continuous layer of metallic Cr of a weight of at least 1.5 mg/sq ft of article surface together with an overlying chromium oxide film; (c) continuously passing the article from the first zone into and through a second stock-treating zone containing said electrolyte; and (d) applying electric current to the article as cathode in the second zone of zero or less than 212 amps/sq ft of cathode surface to provide on the final, Cr-plated article a continuous oxide film overlying the metallic Cr layer and containing 0.5 to 2.0 mg of trivalent Cr per sq ft of article surface. The sulphate ions may be contributed by H 2 SO 4 or chromium sulphate and the fluoride ions by HF or a silicofluoride. The steel may be strip moving at 200-1000 feet/min. For the electrolysis, the counter electrodes may be Pb-1% Ag alloy. Before the Cr and chromium oxide coating treatment, the steel may be cleaned by electrolytic alkali treatment and be electrolytically pickled. Following the chromium oxide coating treatment, the steel may be coated with a clear epoxy phenolic enamel.