Formation behavior of anodizing films on various aluminum alloys in oxalic acid solution

The formation behavior of anodic aluminum oxide (AAO) films on various aluminum alloys, was studied in 6 wt% oxalic acid solution. There were observed two different voltage-time behaviors with a sharp peak in the initial stage of galvanostatic anodizing for pure Al, AA1050 and AA5052 (group 1), and with voltage plateaus instead of a peak for AA7075, AA6061 and AA2024 (group 2). AAO film growth rate was found to be higher for the group 1 specimens than the group 2 alloys. Two different anodizing films appeared, with regularly ordered cylindrical nano-pores for the group 1 specimens and irregularly distorted nano-pores along with numerous nano-cavities for the group 2 alloys. As the anodizing time increased, the AAO films on the group 1 specimens became darkened, while those on the group 2 alloys were not darkened but only changed in color. In addition, the surface hardness of the AAO films was much higher on the group 1 specimens than the group 2 alloys.