Patent GB-1199657-A (GB)
Title: Improvements in or relating to the Coating of Metal Surfaces
Inventor: PALM BERT EDWARD
IPC: C23C22/74
Filed: 1968-05-27
Abstract: 1,199,657. Welding by pressure. DIAMOND SHAMROCK CORP. 27 May, 1968 [29 May, 1967; 29 March, 1968], No. 25286/68. Heading B3R. [Also in Division B2] Prior to subsequent welding, a metal substrate is coated with an adherent, corrosionresistant composite coating which comprises an undercoating comprising a residue obtained by applying a hexavalent-chromium-containing coating composition to the surface and heating the substrate at a temperature and for a time sufficient to vaporize volatile substituents from the coating composition and deposit the residue on the surface, and on the undercoating a top coating comprising the residue obtained by applying and baking a top coat composition comprising a particulate, electrically-conductive pigment in a vehicle, the pigment being aluminium, copper, cadmium, magnetide, carbon or zinc. The coating permits extended electrical resistance welding free from film pickup on the electrodes. Metal substrates mentioned are aluminium and its alloys, zinc and its alloys, copper, brass and bronze, cadmium, titanium, nickel and its alloys, tin, lead, chromium, magnesium and alloys thereof, iron, stainless steel, cold rolled or hot rolled or pickled steel. The hexavalent-chromium-containing coating composition may contain chromic acid or a salt thereof such as ammonium dichromate or sodium or potassium salts or calcium, barium, magnesium, zinc, cadmium and strontium dichromate. The hexavalent-chromium-providing substance may be a mixed chromium compound and may include trivalent chromium compounds. These compositions may also contain a reducing inorganic or organic compound; alternatively the reducing agents may be applied say during drying of the chromic acid solution of a metal surface. Other compounds which may be present in the coating compositions include phosphoric acid or a compound phosphorous such as a metallic dihydrogen phosphate, manganese compounds, pigments, resinous materials, organic and inorganic acids, surface activations. The metal surface can be preheated, electrolytically, may be etched and/or cleaned. The binder for the top coating composition may comprise with zinc pigment polyamide, epoxy resins, polystyrene, chlorinated or isomerized rubber, polyvinyl acetate, vinyl chloride-vinyl acetate copolymers. The top coat composition can also contain flow control agents, e.g. urea formaldehyde resins, thixotroping agents such as silica, anti-settling agents such as hydrogenated castor oil and aluminium stearate. After baking, the resulting composite coatings can be further coated with electro-coating primers, enamel, varnish or lacquer which may contain pigment, e.g. cellulose lacquers, rosin varnishes, tung oil varnish, oil paints including phenolic resins paints, solvent-reduced alkyds, epoxy, acrylics, vinyl including polyvinyl butyral and oil-waxtype coatings such as linseed oil-paraffin wax paints. The paints can be solvent reduced or water reduced or can have reactive solvents such as in the polyesters or polyurethanes. The subsequent welding can be spot-welding, i.e. localized electrical resistance welding, or seam welding such as with roller electrodes. When the weldable substrate is susceptible to coating on more than one surface, e.g. a sheet or strip of ferruginous material, the undercoat composition can be applied on all surfaces and the top coat composition may be applied only to those surfaces which will be in close proximity or direct contact with the welding electrodes.