Abstract
With an object of decreasing the bath voltage during hard anodizing of Al (99.99%) in a tartaric acid bath (1mol/l), effects of several additives (0.01 to 0.5mol/l) on the bath voltage, local corrosion of anodes and hardness of films were examined at 40°C and 1.87A/dm2. Grayish brown and hard (Hv585) films were formed with tartaric acid at 190V. By adding 0.5mol/l sulfuric acid, non-colored and uniform film was obtained at 19V but it was not so hard (Hv250). With sodium sulfate and sulfamic acid, the bath voltage decreased but local corrosion took place under every condition. The film formed by adding 0.5mol/l phosphoric acid at 115V was uniform but soft (Hv65). When oxalic acid was added (0.01 to 0.5mol/l), uniform films were formed at 60 to 130V; the hard films more than Hv400 were obtained by adding 0.1mol/l oxalic acid at 70 to 90V. Through these experiments, oxalic acid was found to be the most suitable as additive.
