The Pore-Branching Channels of Porous Anodic Alumina Formed in an Alkaline Solution of Potassium Phosphate

In this study, the anodizing processes of aluminum in oxalic acid solution (pH ≈0.98) and alkaline aqueous solution (pH ≈12.86) were compared. The channels of porous anodic alumina (PAA) obtained from the two electrolytes were characterized by FESEM. The pore-branching channels of PAA were obtained by anodizing aluminum in an alkaline aqueous solution of potassium phosphate. The pore-branching channels of PAA cannot be clarified by the acidic field-assisted dissolution theory (FADT). In this article, the formation mechanism of PAA under alkaline conditions is explained by the oxide viscous flow model around the oxygen bubble model and electronic current theory. The pore diameter of porous anodic alumina formed in potassium phosphate electrolyte (≈90 nm) is larger than that formed in oxalic acid conditions (≈25 nm) because there are two oxygen bubbles at the large bottom, which is conducive to the formation of pore-branching channels.