GB716554A - Improvements in anodising aluminium and its alloys - Google Patents

Abstract

716,554. Anodising aluminium and aluminium alloys. CAMPBELL, W. J. Sept. 2, 1952, [June 25, 1951; Aug. 20, 1951; Aug. 20, 1951; Oct. 16, 1951; March 3, 1952.] Nos. 14975/51; 19540/51; 19541/51; 24165/51 and 5451/52. Class 41. A process of anodising aluminium and aluminium alloys to form ultra-hard surfaces thereon consists in increasing the applied voltage continuously so as to maintain a steady current density at the surface being treated during the major part of the process period, maintaining the temperature of the electrolyte below 10‹C. and causing relative movement between the electrolyte and the treated surface at a rate greater than 10 feet per minute for each foot of the surface measured in the direction of movement. Preferably combined alternating and direct currents are used. For pure aluminium and alloys containing less than i per cent of any other constituent a combination of A.C. and D.C. in the proportion of 1 volt A.C. to 0.5-6.0 volts D.C. may be used. For alloys with a copper or nickel or manganese content of from 1 to 15 per cent or silicon from i to 25 per cent A.C. alone may be used throughout or A.C. initially followed by combined pressures in -the proportion of i volt C.D. to 0.5 to 6 volts A.C. Alloys containing 1 to 15 per cent magnesium may be treated with pressures in the. proportion of 6 volts D.C. to i to 24 volts A.C., while alloys containing i to 15 per cent zinc may be treated with pressures in the proportion of o to 1 volts A.C. to 1 volt D.C.; or these alloys may be treated with D.C. only. The current density may be over 50 amps per square foot, and may be reduced toward the end of the process. The voltages may be regulated automatically to preselected values over the time of treatment by cams arranged to rotate and operate voltage regulators. The electrolyte may be sulphuric acid, preferably 10 to 20 per cent, or oxalic acid, preferably 3 to 8 per cent, or a 1/1 mixture of both acids; 2 to 5 per cent magnesium sulphate or oxalate may be added. The temperature is preferably at 0‹C. to 2‹C. The relative movement between the surface and the electrolyte is preferably greater than 70 feet per minute per foot of surface measured in the direction of movement. A rapid reciprocatory or rotatory movement may be imparted to the article being anodised, or the electrolyte may be rapidly agitated, or both. A reciprocating movement of 15 to 8o feet per minute, with a stroke greater than half of the dimension across the direction of flow, may be given to the article. The electrolyte may be agitated by injected air, paddles, propellers, turbines or pumps. The electrolyte may be cooled by pumping it through a refrigerating unit, and mixed at the same time. Specification 701,390 is referred to.