GB290901A - An improved process for providing a resistant coating upon the surfaces of aluminiumor aluminium alloys - Google Patents

Abstract

290,901. Gower, C. R. R., and O'Brien & Partners, Ltd., S. Oct. 20, 1927. Aluminium compounds, obtaining. - A resistance coating is produced on the surface of aluminium or an aluminium alloy by anodic treatment in a bath containing a substantial proportion of sulphuric acid or of a substance giving rise to sulphuric acid at the anode. Bichromate of potash, hyposulphite of soda, acetate of lead, barium sulphite, or sulphocyanide of ammonia or of potash or mixtures thereof may be added to the bath to colour the coating. Ammonia or an ammonium salt may be added to darken the colour. The bath may contain 1¢-2 lbs. sulphuric acid per gallon of water, or may contain 10 per cent by volume of acid. The bath should be cold on starting but not below 60‹ F. The treatment may last from 25 to 70 minutes. The object may be first cleaned with an abrasive powder, then immersed in a boiling solution of brown potash and washed in cold or hot water. It is then connected as anode and put into the electrolyte bath with the current on. The object is suspended by a short thick wire of copper or of a metal not attacked by the bath. A large object may first be partially immersed and then reversed to treat the remainder. A voltage of 6 is employed, the current density being initially 10 amps. per sq. ft. to a steady falling value of 2 amps. per sq. ft. The object may be treated successively in different baths. To obtain a thick coating, the object is treated repeatedly in the same bath, being dried and exposed to the air or oxygen between successive treatments. Detailed directions for the preparation of baths to produce coatings of particular colours are given. A final anodic treatment in a bath containing sulphuric and nitric acids may be employed to give the coating a hard white or silvery appearance.