Evaluation of pore diameter of anodic porous films formed on aluminum

The porosity of anodic films formed in four major electrolytes, which was measured by pore-filling technique, decreases with increasing voltage. Comparing the porosity at an identical voltage, it increases in the order of the films formed in sulfuric acid, oxalic acid, chromic acid, and phosphoric acid solutions. When the voltage decreases down to lower than 10 V, the porosity increases remarkably, especially in the films formed in phosphoric acid. The pore diameter estimated from the porosity and cell diameter measured by pore-filling technique decreases with decreasing voltage down to 5 V. However, at the voltages lower than 5 V, the pore diameter increases while the cell diameter decreases. Pore and cell diameters of the films formed in the four electrolytes at an identical voltage increase in the order: sulfuric acid, oxalic acid, chromic acid, and phosphoric acid. These results are basically consistent with those reported in our previous TEM study, although the pore sizes are slightly larger than the latterʹs. The reason of the discrepancy between the methods is discussed.