Initial Structural Changes of Porous Alumina Film via High-Resolution Microscopy Observations

Abstract

The initial growth of a porous alumina film with a large-scale cell structure formed by galvanostatic anodizing in etidronic acid was investigated in detail by high-resolution microscopy. High-purity aluminum plates were galvanostatically anodized in etidronic acid at 2.5–20.0 Am−2. The formation of an anodic oxide and the subsequent instability of the outer oxide simultaneously occurred at the early stage of the linear voltage increase during the anodizing process. Accordingly, a wavy interface boundary between the aluminum oxide that contained incorporated anions and the nearly pure aluminum oxide formed in the anodic oxide. The surviving pores grew as the thickness of the oxide film increased, and a clear porous alumina film with a pore at the center of each cell formed until the voltage reached its maximum value. Finally, steady-state growth of the porous alumina film occurred at the plateau voltage region after a slight voltage decrease. Eggplant-like anion distributions were measured at the head of the pores due to the viscous flow of the anodic oxide. The nanomorphology of the porous alumina film strongly depended on the current density due to the difference in the degree of oxide formation and localized oxide dissolution.