Metal coating

Abstract

Abstract A coloured chromate coating is formed by treating a substrate with an aqueous dispersion containing a water soluble dichromate of a metal having a valence greater than one, a pigment and a sufficient concentration of a reducing agent to reduce 70-95% of the hexavalent chromium to the trivalent state and heating the coated substrate to 280-500 DEG F. The dispersion also preferably contains dispersed resin particles in a weight concentration not greater than one part of resin to three parts of the dichromate. The preferred dichromate is that of zinc but magnesium or strontium dichromate may also be used. The pigment which is present in a weight concentration of 10-75% of the dichromate may be titania, carbon, iron oxide, molybdate orange, ferrite yellow, chromic oxide, chrome yellow, the phthalocyanine pigments or metallic zinc. The following substrates may be coated: paper, glass, iron, steel, stainless steel, aluminium, beryllium, cadmium, chromium, cobalt, niobium, copper, lead, manganese magnesium, molybdenum, nickel, silver, tantalum, tin, titanium, vanadium, zinc, zirconium and their alloys and also surfaces which have been oxidized, phosphated, sulphided, chromated or oxalated. The dispersion which is applied to the substrate by immersion, flooding, spraying, roller coating or electrostatic spraying may also contain a wetting agent such as anionic, cationic or non-ionic compounds, e.g. quaternary ammonium compounds of long chain alkyl or aralkylamines, long chain alkyl sulphates and tertiary butyl alcohol. Acrylic resins such as polymethyl acrylate, polyethyl acrylate and polymethyl methacrylate resins are the preferred type for the aqueous dispersion but other resins such as methyl methacrylate homopolymers, the alkyds, butadiene styrene copolymers, polystyrene, polyvinyl chloride, and P.T.F.E. can be used. The proportion of resin to reducing agent should be between 1:2 and 6:1 by weight and the resin may contain up to 25% of a plasticizer. The reducing agents should be such that at temperature below the curing range they are not rapidly oxidized by the hexavalent chromium, e.g. sucrose, citric acid, glycerine, and phosphoric acid. The metal substrate may be cleaned cathodically and anodically in KOH, rinsed with water, H2SO4 and etched with 2% HNO3 prior to applying the coating dispersion. Several examples are given of different coating dispersions and methods of application and an electrolytic coating of zinc or tin may be applied to a ferrous base prior to coating with the dispersion. Lithographic coatings, varnishes, lacquers, enamels and other organic coatings may be applied to the chromate coated substrate. Specifications 793,242 and 885,675, and U.S.A. Specifications 2,188,864, 2,562,965, 2,634,245, 2,683,698 and 2,768,104 are referred to.ALSO:A coloured chromate coating is formed by treating a substrate with an aqueous dispersion containing a water soluble dichromate of a metal having a valence greater than one, a pigment and a sufficient concentration of a reducing agent to reduce 70-95% of the hexavalent chromium to the trivalent state and heating the coated substrate to 280-500 DEG F. The dispersion preferably contains dispersed resin particles in a weight concentration not greater than one part of resin to three parts of dichromate. The preferred dichromate is that of zinc but magnesium or strontium dichromate may also be used. The pigment which is present in a weight concentration of 10-75% of the dichromate may be titania, carbon, iron oxide, molybdate orange, ferrite yellow, chromic oxide, chrome yellow, the phthalocyanine pigments or metallic zinc. The following substrates may be coated: paper, glass, iron, steel strip or panels, stainless steel, aluminium, beryllium, cadmium, chromium, cobalt, niobium, copper, lead, manganese, magnesium, molybdenum, nickel, silver, tantalum, tin, titanium, vanadium, zinc, zirconium and their alloys and also surfaces which have been oxidized, phosphated, sulphided, chromated or oxalated. The dispersion may also contain an anionic, cationic or non-ionic wetting agent and the reducing agents may be such as sucrose, glycerine, citric acid or phosphoric acid. Acrylic resins such as polymethyl acrylate, polyethyl acrylate and polymethyl methacrylate are the preferred type for the aqueous dispersion but other resins such as methyl methacrylate homopolymers, polystyrene, polyvinyl chloride and P.T.F.E. can be used. The proportion of resin to reducing agent should be between 1 : 2 and 6 : 1 by weight and the resin may contain up to 25% of a plasticizer. Lithographic coatings, varnishes, lacquers, enamels and other organic coatings may be applied to the chromate coated substrate. The compositions may be applied by roller-coating, dipping, flooding, or spraying (including electrostatic spraying). Specifications 793,242, 885,675, and U.S.A. Specifications 2,188,864, 2,562,965, 2,634,245, 2,683,698 and 2,768,104 are referred to.

Inventors

• 👤 Ludwig K Schuster
• 👤 Jr Alfonso L Baldi

Bibliographic Details