Abstract
Anodizing aluminum in a mixture of maleic acid-sulfuric acid at constant current densities frequently causes pittings in the coatings. However, the initial two-step current control, under which the current density was first raised to 0.5A./dm2 in 1 minute, kept at the current density for another 1 minute and finally raised to the specified range of 1 to 3A/dm2 in 1 minute, was able to prevent pittings and coatings of uniform thickness and color were obtained. This is interpreted that at the low current densities, the structual change from initial barrier layer to porous layer takes place through the slow dissolution of the barrier layer. Thus by anodizing aluminum for 30 minutes in a mixture containing 250g/l maleic acid and 3.0 to 6.0g/l H2SO4, colored coatings of various tones from light yellow to dark brown were obtained and they did not contain any defects. Colors of the coatings so obtained were found to correspond to the content of SO42- incorporated into the films. The more the amount of SO42- increased, the lighter the color of coatings decreased.
