Abstract
Galvanostatic and potentiostatic anodizing processes of aluminum leads to the formation of large-scale porous alumina. It is previously reported that self-ordered porous alumina measuring above 500 nm in cell diameter can be fabricated via two-step anodizing in etidronic acid at higher voltage more than 200 V. On the other hand, extremely little has been reported on the growth behavior and their nanostructure during galvanostatic anodizing in etidronic acid. Herein, we investigate the nanostructural characterization of the porous alumina film formed via galvanostatic anodizing in etidronic acid under various operating conditions. Electropolished aluminum specimens were anodized at a constant current density of 0.005-500 Am-2 in 0.03-3 M etidronic acid solutions at 273-333 K. After anodizing, the porous alumina film was dissolved in a 0.20 M CrO3/0.51 M H3PO4 solution at 353 K, and the aluminum dimple array, which corresponded to the bottom shape of porous alumina, was exposed to the surface. The anodized specimens were examined by field-emission scanning electron microscopy (FE-SEM). The cell size (interpore distance) of the porous alumina was calculated using image analysis software. Typical voltage-time curves for the formation of anodic porous alumina were obtained during galvanostatic anodizing in etidronic acid: The voltage linearly increased, gradually decreased, and then reached plateau value. However, excess current density caused oxide burning with unstable oscillation in the voltage-time curve, and non-uniform porous alumina was formed on the aluminum surface. The plateau voltage greatly increased with the current density at each operating temperature, and various porous alumina films with different nanostructures were successfully fabricated by anodizing at a wide voltage range measuring 7-218 V. Figure 1 shows the changes in the average, maximum, and minimum cell sizes with the anodizing voltage under various operating conditions. The cell sizes are directly proportional to the anodizing voltage with proportionality constants of 2.5 (average), 3.5 (maximum), and 0.7 (minimum), respectively. Figure 1
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