GB495190A - An improved electrolytic method of coating surfaces of iron or steel - Google Patents

Abstract

495,190. Electrolytic production of phosphate coatings on iron and steel. AMERICAN CHEMICAL PAINT CO. April 6, 1937, No. 9729. Convention date, May 12, 1936. [Class 41] A coating of a water-insoluble metal phosphate is produced on iron or steel by treatment as an electrode in an electrolyte containing phosphate and metal ions, the polarity of the current being reversed at least once during the treatment, the concentration of ion in the electrolyte being maintained below 0À6 per cent and preferably below 0À25 per cent by the addition of a precipitating agent. The iron content of the phosphate coating may be thereby maintained below 15 per cent. The precipitating agent may be an oxidizing agent such as a permanganate, a dichromate or hydrogen peroxide whereby the iron is precipitated as ferric phosphate, or may be sodium ferricyanide. A cathodic depolarizing agent such as nitrate of an alkali metal, magnesium, cerium or zinc is preferably also present in the solution. Nitrous acid or a nitrite may be employed as a precipitating agent in association with a different substance as depolarizing agent or as depolarizing agent in association with a different precipitating agent. Nitrous acid or nitrite should not be present in excess of 0À03 lb. per U.S.A. gallon of electrolyte. The metal ions present in the solution may be those of zinc, cadmium, calcium, cobalt, nickel or manganese. Alternating current of 25-60 cycles may be employed. The current density may be 35 amps. per sq. ft. The temperature of the bath is preferably 120‹ F. The surface to be coated should be gently rubbed in the dry condition prior to immersion in the electrolyte. Oil, paint or lacquer may be applied to the coatings. Specification 435,773 is referred to.