GB915034A - Improvements relating to the coating of aluminium and its alloys - Google Patents

Abstract

Aluminium or aluminium based alloys are coated by treating with a solution comprising 5-150 g./l. phosphate (as PO4), 2.5-62 g./l. chromate (as CrO3), 0-55 g./l. aluminium and up to 123 g./l. fluoride as activator, with a pH of 0.5 to 1.5, the phosphate and chromate contents being maintained constant and when the aluminium content of the solution reaches at least 1 g./l. the fluoride content is controlled so that it is correlated to the aluminium content according to Table 1 of the Specification (not shown). The phosphate and chromate contents are maintained by adding phosphoric and chromic acids and the fluoride by addition of hydrofluoric acid. After cleaning the substrate in acid or caustic cleaners coating is effected by spraying, dipping or brushing at a temperature up to 180 DEG F. The coating may be rinsed in chromic acid of a concentration of 0.02 to 0.1% CrO3, then water rinsed and air dried. Anions such as sulphates, chlorides, and nitrates may be present in solution and the acidity may be controlled with mineral acids. Cations other than hydrogen, aluminium and trivalent chromium should be avoided and the F : CrO3 ratio is preferably 0.5 to 3.8. The aluminium content of the solution is controlled by discarding part of the solution and analyzing for aluminium. The initial coating solution may contain 1-55 g./l. aluminium and 2.5 to 123 g./l. fluoride or the initial solution may be free of aluminium which is then supplied by solution from the material being coated.