Abstract: Apparatus for electrolytically coloring anodized aluminum comprises a power transformer whose secondary winding has one phase connected directly to the charge and its other phase connected to the charge through two anti-parallel thyristors. One of the thyristors is triggered in accordance with a predetermined program, and the other thyristor is triggered in dependence upon unbalance in the average values of the voltages applied to the charge in consecutive half cycles of the alternating current applied to the charge, in such a manner as to tend to eliminate the unbalance.

Abstract: A process for coloring an aluminum or aluminum alloy piece previously subjected to anodic oxidation, in which codeposition of at least one metal selected from the group consisting of nickel, cobalt, and cadmium with at least one auxiliary metal is carried out by stirring, and applying an alternating voltage between such piece and a counterelectrode immersed in a bath containing the salts of the mentioned metals. The alternating voltage, for each period, has a zero value for at least a certain period of time, and the bath has a pH below or equal to 1.8 and contains at least one auxiliary metal salt selected from the group consisting of copper and tin salts. The concentration of the auxiliary metal salt in the bath is lower than 2.5 g/l and the concentration in the bath of the salt selected from the Ni, Co and Cd salt is preferably lower than 200 g/l.