Ultra-fast fabrication of porous alumina film with excellent wear and corrosion resistance via hard anodizing in etidronic acid

This paper describes a fast-fabrication strategy for hard-anodized film prepared on 6063T5 aluminum alloy in environmentally friendly etidronic acid (HEDP). The mild HEDP anodizing was first investigated at a temperature range of 15-45 degrees C, just under the burning current densities. The voltage curves and microstructures of the anodized film prepared in HEDP (Eti-film) indicate that 35 degrees C is the most appropriate temperature for the fast fabrication of Eti-film, in contrast to the micro-arc oxidation (MAO) coating and the seriously surface-corroded Eti-film prepared at 15 and 45 degrees C, respectively. Then, the HEDP hard anodizing without burning was successfully conducted via a gradient-increase-current approach, resulting in a high growth rate (similar to 2.1 mu m/min), low porosity ( < 10%), thick barrier layer (similar to 510 nm) and branched nanoholes structure of the hard anodized Eti-film. The Eti-films were systematically investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, and a nanoindentation tester. In addition, the wear and corrosion resistance of the Eti-film and the hard sulfuric acid anodized film (Sul-film) were compared. The results indicate that the hardness of the Eti-film can reach similar to 11 GPa (similar to 1000 HV0.01). The better wear resistance of the Eti-film as compared to the currently widely used Sulfilm is attributed to the low porosity and less hydrated alumina content of the Eti-film. Moreover, the corrosion resistance of the Eti-film has been found to be 10 times higher than that of the Sul-film. In general, our results suggest a possibility of replacing the pollution-carrying anodizing methods currently used in the industry with the etidronic acid anodizing.