CHEMICALLY PRODUCING ANODIC OXIDATION COAT ON AL OR AL ALLOY

摘要

A method of enhancing the naturally developed colour of an anodic oxidation coat chemically produced on an Al alloy in which the coat is formed by electrolysis employing a warmed electrolyte of sulphuric acid and an alternating current the polarity of which is periodically reversed to be negative for a period of 10-20% of each cycle while being applied to the Al alloy dipped in the electrolyte as an anode for 20 to 40 minutes. The thus coated Al alloy is subjected to a heat treatment in a boiling sealing liquid for 20 minutes. During each negative polarity period of the current, electrolytic reduction takes place on the Al alloy (which is acting as a cathode), whereby sulphur in the electrolyte reacts with the added alloy element of the Al alloy to form a sulphide of the alloy element. The alloy metal sulphide thus formed is accumulated in the coat and, during the heat treatment, the alloy metal sulphide accumulated is further combined with the alloy element in the form of metallic ion to further develop the colour of the coat.