Anodizing Aluminum and Its Alloys in Etidronic Acid to Enhance Their Corrosion Resistance in a Sodium Chloride Solution

The formation behaviors of the anodic oxide film that forms on various commercially available aluminum plates were investigated by galvanostatic anodizing in etidronic acid, and their corrosion resistances were examined in a 3.5 wt% NaCl solution. High-purity aluminum and A1050, A5052, and A7075 aluminum alloys were galvanostatically anodized in an etidronic acid solution. An amorphous porous oxide film formed on the pure aluminum, the A1050 alloy, and the A5052 alloy, whereas a plasma electrolytic oxidation (PEO) film formed on the A7075 alloy. The nanopores formed in the porous oxide film could be sealed with hydroxides by immersion in boiling water. Corrosion-resistant pure aluminum with a measured corrosion current density of i_corr = 10^-9 Am^-2 could be formed by anodizing and subsequent pore-sealing. Although the corrosion current increased due to the presence of alloying elements in the matrix, the fabrication of the corrosion-resistant A1050 and A5052 alloys with i_corr = 10^-6 Am^-2 could also be achieved by anodizing. Alternately, the corrosion resistance of the A7075 alloy covered with the PEO film was measured to be i_corr = 10^-4 Am^-2, which is not as small as the i_corr of the A1050 and A5052 alloys due to its open microporous structure.