High purity aluminum sheets were anodized in typical electrolytes of sulfuric acid, oxalic acid, and phosphoric acid at different voltage. After detachment of the porous alumina films from the substrate in saturated mercuric chloride solution, the specimens were immersed in phosphoric acid to dissolve the barrier layer from the backside. By SEM observation of structural change of the cells during chemical etching, the homogeneity of the barrier layer thickness could be evaluated. Although the thickness of the barrier layer of porous alumina formed at constant voltage has been believed to be uniform, we confirmed that the thickness of the barrier layer of each cell was not uniform; the thickness of the barrier layer of the smaller cells was thinner than that of the larger cells. Concerning anion incorporation behavior, we evaluated the depth of anion incorporation into the cell wall by the difference in the strength of contrast of the SEM image of rear surface of porous alumina films because the dissolution rate of the anion-free layer was slower than that of the anion-incorporated layer. The details of the thickness inhomogeneity of the barrier layer and anion incorporation behavior will be discussed from a standpoint of the high field theory.
 S. Ono, M. Saito, M. Ishiguro and H. Asoh; J.Electrochem. Soc., 151, B473 (2004).
 S. Ono, M. Saito and H. Asoh; Electrochim. Acta, 51, 827 (2005).