Corrosion-Resistant Porous Alumina Formed via Anodizing Aluminum in Etidronic Acid and Its Pore-Sealing Behavior in Boiling Water

Alkaline corrosion-resistant porous alumina was fabricated by anodizing aluminum in etidronic acid, and its hydration behavior during pore sealing in boiling water was investigated. High-purity aluminum plates were anodized in sulfuric, oxalic, citric, and etidronic acid solutions. Anodizing with etidronic acid caused stable growth of a uniform, porous alumina layer at a high voltage of approximately 200 V. This porous alumina possessed a thick barrier layer with an inner layer of pure aluminum oxide and exhibited a 10-fold increase in the corrosion resistance in a 2.5 M sodium hydroxide solution. When the porous alumina film formed by sulfuric acid anodizing was immersed in boiling water, plate-like hydroxide scales rapidly formed on the whole surface, and the pores were sealed within 10 min. In the case of etidronic acid, the hydroxides formed at the bottom of the pores in the initial stage of immersion, and the thickness of the hydroxide layer gradually increased with the immersion time. The porous layer was completely sealed by long-term immersion. Although the barrier layer was reduced to approximately 80% of its original size due to hydration, a thick barrier layer was still maintained at the bottom of the pores after immersion.