Formation of the unique bead-like pore wall morphology in porous anodic alumina

Porous anodic oxides have been extensively studied. However, the field-assisted dissolution theory and field-assisted flow model cannot explain the increasing or decreasing trend of the current-time curve in the anodizing process. In this paper, porous anodic alumina (PAA) was obtained by constant voltage anodization of aluminum in the same phosphoric acid electrolyte for only 600 s. SEM characterization showed that PAA of different lengths was obtained at three constant voltages (120 V, 130 V, and 140 V), and the current-time curves presented three interesting stages that were different from the conventional curves. PAA with bead-like pore wall was obtained in an electrolyte of H₃PO₄and polyethylene glycol under 130 V anodizing. This phenomenon can be well explained by the oxygen bubble mold in conjunction with the electronic current theory. The appearance of the bead-like morphology is attributed to the formation of oxygen bubbles within the pore wall. Oxides grow around the oxygen bubbles within the pore wall, causing the pore wall to bend inward, thereby forming the bead-like pore wall of PAA.