A process for the direct or indirect electro-deposition of a highly corrosion resisting crack-tree technical hard chromium plating layer

摘要

A process for the direct or indirect deposition of a highly corrosion resisting technical hard chromium plating layer on the surface of a metallic workpiece from an aqueous working electrolyte containing chromic acid and sulphate ions is described. In a process wherein the current yield of the working electrolyte is optimised by the addition of at least one saturated aliphatic sulphonic acid having at the most two carbon atoms and at the most six sulphonic acid groups and or by the addition of salts or halogen derivatives thereof and the deposit has a thickness of at least 2 mu m and a hardness exceeding 9oo HV o.1, the deposition is effected with direct current pulses, the pulse frequency lying in the range between a lower critical pulse frequency dependent on the cathode current density setting and an upper critical pulse frequency point dependent on the optimised current yield at the same current density. The procedures for determining the lower critical pulse frequency and the upper critical pulse frequency point are described. The on-off ratio in the range between the lower critical pulse frequency and the upper critical pulse frequency point is kept sufficiently low for the deposit to remain bright and substantially crack-free.