GB1042108A - Processes for coating metal surfaces - Google Patents

Abstract

A zinc or zinc alloy surface is coated by dipping, spraying or brushing it with an aqueous alkaline solution containing complexed metal ions e.g. magnesium, cadmium, aluminium, tin, titanium, antimony, molybdenum, chromium, cerium, tungsten, manganese, cobalt, iron or nickel. The complexing agent may include an inorganic sequestering agent e.g. cyanides, phosphates or ammonia or an organic chelating agent e.g. dicarboxylic acids such as tartaric, malonic and fumaric acids, amino acids such as glycerine, hydroxy carboxylic acids such as citric, glucoric and lactic acids, hydroxy aldehydes such as acetyl acetone, polyhydroxy aliphatic compounds such as sorbitol and 1,2 ethanediol, phenolic carboxylic acids such as phthalic and salicyclic acid, amine carboxylic acids such as E.D.T.A., polyamines acids such as diethanolamino methane phosphonate, and sodium ligno-sulphonate. The alkaline solution may be sodium hydroxide, and a surface active agent may be used. The treated surface may be rinsed with chromic acid (0.01% to 0.5% by weight CrO3). Were part of the surface is iron, e.g. in a partly galvanized article, the alkali solution may act as a cleaning treatment on the iron surface prior to phosphating with a zinc or alkali metal phosphate coating solution. In an example a solution containing (per cent by weight) 0.76 Na OH, 0.1 sodium hexahydroxy heptoate (complexing agent), 0.0037 iron, added as ferric nitrate nonahydrate, and 0.0024 cobalt nitrate hexahydrate is sprayed at 160 DEG F. on steel panels for 1 minute. The panels are then rinsed in hot water for 30 secs and then in an aqueous rinse containing 0.04 trivalent chromium and 0.1 hexavalent chromium, pH 4.5 for 30 secs. The panels are then dried and painted with melamine-alkyd paint.