GB374806A - Improvements in the production of oxide coatings on aluminium and its alloys - Google Patents

Abstract

374,806. Electrolytic production of oxide coatings on aluminium and its alloys. VEREINIGTE ALUMINIUMWERKE AKT.-GES., Lautawerk, Lausitz, Germany. Jan. 27, 1932, No. 2472. Convention date, Dec. 18, 1931. [Class 41.] Coatings having special qualities such as hardness, flexibility, and dielectric strength are produced by the use of direct current and alternating current in succession in either order. The procedure may be repeated several times. In an example, an aluminium hot-plate is treated in a 10 per cent solution of oxalic acid first with continuous current of current density 2 amps per square dcm. for 20 minutes, then immediately with alternating current of current density 5 amps per square dcm. for 1 hour. The coating is coarse-grained and elastic but not particularly hard. In another example, an aluminium band of thickness 0À1 mm. is drawn through a bath containing 8 per cent oxalic acid at 35‹ C. and treated by alternating current of density 2 amps per square dcm. ; it is then passed through a similar bath at 20‹ C. under continuous current. The coated band may be curved to a radius of 50 mm. without formation of chinks and is resistant to electric puncture at 1000 volts. In another example, the inside of an aluminium container for receiving hot slightly concentrated hydrochloric acid is first oxidized using alternating current and then treated with continuous current in an electrolyte cooled to 15-20‹ C. The layer is impregnated with melted wax at 150‹ C and dried in a furnace at 100‹ C.