BEHAVIOR OF SULFATE-IONS DURING FORMATION OF ANODIC OXIDE FILM ON ALUMINUM

The incorporation of sulfate into anodic film formed on aluminum in an H//2SO//4-MgSO//4 solution was investigated by infrared spectroanalysis using the compensation method. The sulfate content of the film was very small at the initial stage of anodizing, but increased with time to a nearly constant value. This result is explained in terms of the variation of strength of electric field and the mobility of each anion in the barrier layer. The distribution of sulfate across the cell wall was also examined. The sulfate content was relatively small near the wall/solution interface, but increased with the distance across the wall from the surface, reached a maximum value on the intermediate layer between the wall/solution interface and the cell boundary and decreased gradually. Such distribution is considered to be similar to that across the barrier layer.