Fabrication of anodic porous alumina via galvanostatic anodizing in alkaline sodium tetraborate solution and their morphology

The anodizing of aluminum in an alkaline sodium tetraborate (Na₂B₄O₇)solution was investigated with respect to the nanostructural characterization of anodic porous alumina. Electropolished aluminum specimens were galvanostatically anodized under various conditions in 0.1–0.5 M sodium tetraborate solutions at 293–353 K and a current density of 2.5–400 A m⁻². Anodic oxide with numerous flower-like defects was formed by anodizing in a 0.1 M Na₂B₄O₇ solution due to the film breakdown with continuous visible sparking. On the other hand, a uniform porous alumina film without any breakdown was successfully obtained by anodizing a more concentrated solution than 0.3 M at 333 K. The anodic oxide was almost pure alumina and consisted of a thin outer layer with numerous small pits and a thick inner layer with typical porous alumina cells. The pore walls possessed continuous bumpy surfaces measuring 10–20 nm in roughness. As the temperature further increased to 353 K, the regularity of the porous alumina improved due to the high current density of more than 150 A m⁻² during anodizing. Slippery superhydrophobic and sticky superoleophobic aluminum surfaces could be easily fabricated via high temperature anodizing and subsequent self-assembled monolayer modification.