GB636880A - Improvements in or relating to surface treatments for aluminium and aluminium alloys - Google Patents

Abstract

A method of treating the surface of a metal consisting substantially wholly of, or comprising a major proportion of, aluminium, comprises the step of pickling the metal in a mixture of more than 90 per cent by the volume of concentrated nitric acid and less than 10 per cent by volume of concentrated hydrofluoric acid, and ascertained by test upon the particular metal to be treated, that chemical attack of the mixture upon the pure aluminium is inhibited but attack on constituents of the metal other than aluminium is not inhibited. Thus, the metal may be immersed in a quantity of concentrated nitric acid, concentrated hydrofluoric acid being added with mixing until evolution of gas commences and concentrated nitric acid then added until evolution ceases. The acids referred to are approximately 71 to 72 per cent nitric acid and 50 per cent hydrofluoric acid. Preferably a mixture is used which is approximately 95 per cent of 67 per cent nitric acid and 5 per cent of 48 per cent hydrofluoric acid. After treating with the acid mixture the metal may then be anodized, e.g. in an aqueous solution of sodium carbonate through which alternating current may be passed, and may then be electroplated, e.g. with copper in a bath containing copper pyrophosphate and a conducting salt, or with cadmium. On a coating of copper another metal component may be bonded by fusion, e.g. by soldering or brazing, the unbonded copper being then stripped, e.g. by dipping in concentrated nitric acid, and cadmium plating applied generally.