GB710096A - Improvements relating to the electroplating of aluminium and alloys thereof - Google Patents

Abstract

In preparing articles of aluminium or its alloys for electroplating the oxide film thereon is broken over the area or areas to be plated in such manner that the surface is polished and finely roughened without, however, exhibiting depressions visible to the naked eye, and the area or areas to be plated is or are then treated with a solution of caustic alkali and alkali metal zincate. The mechanical treatment may be effected by vapour blasting, i.e. by directing at the surface a liquid suspension of grit of smaller particle size than is used in sand blasting (see Group XXIII). The grit may be of size 100-1230 mesh and may be obtained from the crushed rock Novaculite. The vapour blasting step must be preceded by degreasing and followed by a treatment with a solution of nitric or sulphuric acids alone, together or in succession. For alloys containing both magnesium and silicon treatment with both acids may be necessary; suitable concentrations of acids are specified. In addition to the final treatment with caustic alkali and zincate solution such a treatment may be interposed between the vapour blasting and the acid treatment; in such case the acid treatment is continued until the layer of zinc has disappeared. After rinsing following vapour blasting or the zincate treatment further treatment may be delayed if desired. Chromium may be directly electroplated on to the prepared surface. Treatment as above described is applicable to aluminium alloys containing viz. (a) 4.5 per cent. copper, 1.5 per cent. magnesium, 0.6 per cent. manganese, (b) 1.6 per cent. copper, 2.15 per cent. magnesium, 5.6 per cent. zinc, 0.3 per cent. chromium, (c) 3.5 per cent. copper, 9 per cent, silicon, (d) 1.3 per cent. copper, cent. silicon, 0.5 per cent. magnesium, (e) 7 per cent. silicon, 0.3 per cent. magnesium.