Oxidation of aluminum was carried out with anodizing current of various electric wave forms in 250g/l maleic acid containing 3-4g/l H₂SO₄.
The coatings obtained with low ripple percent wave forms such as direct current and three-phase full-wave were uniform in thickness and color. The higher ripple percent wave forms were apt to cause the abnormally grown multiple layers and the burning. In the case of single-phase half-wave having the highest ripple percent, the abnormally grown multiple layers were easily formed at the early stage of anodizing and the uniform coating was never obtained.
The deepness of color and coating ratio were affected by H₂SO₄ concentration and current density, and the color became deeper and the coating ratio increased with the increase of ripple percentage. The hardness of the uniform coating obtained was in the range of Hv(0.05)521 to 537 which was much higher than Hv(0.05)449 of the hard anodic coating obtained in 0°C 15% H₂SO₄.