METHOD FOR ELECTROLYTICALLY COLORING ALUMINUM OR ALUMINUM ALLOY

PURPOSE:To give efficiently a uniform and fine color to the surface of anodically oxidized Al by electrolytic coloring by applying positive voltage having a specified intermittent wave-form to the Al to pretreat the Al before the electrolytic coloring. CONSTITUTION:Al or an Al alloy subjected to conventional anodic oxidation is electrolytically colored in an electrolytic coloring soln. contg. a metallic salt. Before this electrolytic coloring, the metal is pretreated in the electrolytic coloring soln. or a soln. having the same electric conductivity as the electrolytic coloring soln., e.g., an aqueous sulfate soln. by applying practically positive voltage having an intermittent wave-form in which the practical suspension time is longer than the positive voltage application time. The cycle of the wave- form is about 1-100Hz, the ratio between the positive voltage application time T1 and the suspension time T2 is 1:(1-10), the maximum voltage TP is about 20-120V, and the voltage applied during the suspension time is 0 - about 0.3X1VP1. The barrier layer is made thick and uniform by the pretreatment, so a uniform colored film is formed by the following electrolytic coloring in a short time.