Chemical stability of porous anodic aluminum oxide in both acidic and alkaline solutions

A porous anodic aluminum oxide (AAO) which has a higher chemical resistance in both acidic and alkaline solutions, was fabricated by anodizing Al in an alkaline sodium tetraborate solution. The high-purity Al plates were anodized in typical acidic solutions (sulfuric, oxalic, phosphoric, chromic, and etidronic acids) and in alkaline sodium tetraborate solution for the formation of AAOs. The anodized specimens were then immersed in a phosphoric acid solution (pH = 1.3) and a sodium hydroxide solution (pH = 13.8). The electrochemical impedance measurements and morphological characterizations of the AAOs before and after immersion were examined. The time to fully dissolve the AAOs formed in typical acidic electrolytes, including sulfuric, oxalic, phosphoric, and etidronic acids, increased linearly with the anodizing voltage. The AAO formed in chromic acid exhibited a higher chemical resistance than this linear relationship. In addition, the AAO formed in sodium tetraborate exhibited the highest chemical resistance in both acidic and alkaline solutions – approximately two times higher than that formed in etidronic and chromic acids. This is due to their higher anodizing ratio and the formation of a high-purity alumina layer.