Anodizing aluminum is very important for protecting the aluminum substrate, and anodizing in a sulfuric acid solution at low temperature causes the formation of hard porous alumina with a high Vickers hardness of more than Hv = 300. However, harder porous alumina is required to expand the applicability of porous alumina in various industrial applications. In the present investigation, we fabricated a hard porous alumina by aluminum anodizing in a new electrolyte, etidronic acid (1-hydroxyethane-1,1-diphosphonic acid, C₂H₈O₇P₂).

Highly pure aluminum plates (99.999 wt%) were electropolished in a 78 vol% CH₃COOH/22 vol% 70%-HClO₄ solution. The electropolished specimens were immersed in a 0.05–1.0 M etidronic acid solution (150 mL) at 278–308 K and then anodized at a constant voltage of 260 V or a constant-current density of 2.5–50 Am⁻² for up to 64 h. Two-step constant-voltage anodizing was also carried out for the fabrication of highly ordered porous alumina.

The electropolished specimens were anodized in etidronic acid solutions with various concentrations at various temperatures under the maximum current density condition without oxide burning. Figure 1 shows the changes in the Vickers hardness of the porous alumina, Hv, with changing concentration of the etidronic acid solution, c, at T_a = 278–308 K and i_a = 5–40 Am⁻². Here, the thickness was adjusted to approximately 55 µm in each anodized specimen. The Vickers hardness decreased with increasing solution temperature for all concentrations. In addition, the Vickers hardness also decreased with increasing concentration at each solution temperature. This decrease is due to the chemical dissolution of porous alumina during anodizing. A hard porous alumina film measuring Hv = 610 can be fabricated by anodizing in a 0.05 M etidronic acid solution at 278 K and 5 Am⁻².