ES272010A1 - Processes of producing tin-nickelphosphorus coatings - Google Patents

Abstract

The outer skin of a nickel-phosphorus coating carried by a workpiece, such as the lining of a tank or other hollow container fabricated from one or more steel sheets joined together, is converted to a Sn-Ni-P layer containing 1-50% Sn, 46-93% Ni and 3-12% P by simultaneously depositing metallic tin upon the coating surface, preferably by reducing a compound of tin such as a stannous or stannic chloride or other halide on the surface heated to a temperature between the melting points of tin, e.g. 332 DEG C., and the Ni-P coating, e.g. 880 DEG C., and diffusing the tin into the outer skin, the tin being deposited at a rate not higher than that of diffusing the tin. The reduction may be carried out with reducing gases containing 25-40% by volume hydrogen such as produced by cracking ammonia gas, e.g. at 930 DEG C., or mixing hydrogen and nitrogen, the gases first passing over the tin compound, e.g. SnCl2 heated to 480-500 DEG C., then on to the surface of the heated coating, e.g. at 630 DEG C. Alternatively, cracked natural gas or heated anhydrous ammonia may be used as the gases. With a Ni-P coating of composition 85-97% Ni and 3-15% P, the Sn-Ni-P alloy formed at a reaction temperature of 630 DEG C. during a time of less than 4 hours shows a single layer with a reaction time of 4 hours or longer, three separate intimately bonded layers are formed, namely a first layer bonded to the inner Ni-P alloy portion and consisting of tin distributed in Ni-P alloy, a second layer consisting of Ni-P alloy distributed in tin, and a third layer of tin. The coating is preferably at least 2 mils thick. The initial Ni-P coating may be chemically deposited from a bath of the nickel cation-hypophosphite anion type, or electrodeposited and subsequently heat-hardened, e.g. at 400-630 DEG C. to produce microcrystals of Ni3P dispersed in a matrix of nickel. For chemical deposition the workpiece must be formed of or have growth nuclei of catalytic material such as iron, copper, aluminium or alloys thereof, e.g. steels containing one or more C, Cr, Co, Si, Mn, Ni, Mo, Cu, brass, bronze including Si or P bronze, Be-Cu, Cd-Cu, Ni-Cu, and aluminium brass or bronze. If the workpiece is formed of Mg, Ti, Zr, Hf, it must be first treated by the method disclosed in U.S.A. Specification 2,928,757. U.S.A. Specifications 2,690,401, 2,690,402, 2,717,218, 2,822,294 and 2,908,419 also are referred to.