Advanced hard anodic alumina coatings via etidronic acid anodizing

Advanced hard anodic alumina coatings measuring Hv = 610-769 on the Vickers hardness scale were obtained on an aluminum surface via aluminum anodizing using a new electrolyte, etidronic acid. The ordered porous alumina was fabricated by two-step etidronic acid anodizing at 260 V under self-ordering conditions, and pore-widening was carried out to control the porosity of the porous alumina. The Vickers hardness of the ordered porous alumina increased with decreasing diameter of the pores and porosity. Aluminum specimens were also anodized by the constant-current method under various concentrations, temperatures, and current densities. The Vickers hardness increased with decreasing concentration and temperature because chemical dissolution of the anodic oxide during anodizing was suppressed. A hard porous alumina measuring Hv = 610 was obtained by anodizing in a 0.05 M etidronic acid solution at 278 K and 5 Am− 2. Subsequent thermal treatment caused the dehydration and corresponding hardening of the porous alumina, and a higher porous alumina hardness of Hv = 769 was successfully achieved by thermal treatment at 873 K for 12 h.