Abstract
The properties of anodic coatings on high strength and free cutting aluminum are often not adequate for their intended uses, due to Cu, Pb and Bi in the aluminum alloys. In this study aluminum alloys, 2014, 2017, 2024 and KS21 were anodized with an alternating current to investigate the characteristics of the anodic coatings. It was found that the hardness of the coatings increased with decreasing temperature and electrolyte concentration. At temperatures below 15°C, the hardest and most uniform coatings were obtained with 35% sulfuric acid bath. The surface roughness of coatings formed with the alternating currents was smaller than that obtained with direct currents. The bath voltages for alternating currents were lower than those for direct currents. The addition of oxalic acid to sulfuric acid baths improves the coating properties, the improvement is remarkable when the concentration of oxalic acid is higher than 20g/l. Both hardness and thickness of the coatings on free cutting alloys were lower than those on high strength alloys, and this may be attributed to Cu, Pb and Bi.
