Characteristics of non-crystalline thin oxide films formed on aluminum by plasma electrolytic oxidation

Anti-corrosion oxide films were developed on Al substrate by plasma electrolytic oxidation (PEO) in a mix of phosphate, carbonate, silicate, and hydroxide electrolyte. The role of silicate and hydroxide in the PEO process were investigated. PEO was conducted under a DC mode at 400 A·m^–2 at 283 K for 120 s. The resulting films were free of cracks with thicknesses of ~2– 3 μm composed of amorphous aluminum oxide, as confirmed by XRD and EDS analyses. Variation of anion concentration did not vary the incorporation of the anion species in the oxide film significantly. The films enhanced the mechanical hardness ~20 HV. Substantial improvement in corrosion resistance up to two orders of magnitude was demonstrated by the polarization curves and electrochemical impedance spectra. Surface investigation using a scanning electron microscope revealed a pore population reduction with increasing silicate volume in the electrolyte. Synergism effect of silicate and hydroxide in the electrolyte containing a higher fraction of both anions induced the generation of soft sparking, which produced a more protective inner layer. The electrolyte composition is beneficial in obtaining thin oxide films with superior corrosion resistance.