Synthesis and growth mechanism of ceramic coatings on an Al-Cu alloy using plasma electrolytic oxidation in molten salt

Plasma electrolytic oxidation (PEO) is innovative approach for fabricating ceramic coatings on metallic substrates. Traditionally, an aqueous electrolyte is used in PEO; however, the size of the treated sample is limited because of the system heating, and production of the undesired components in the coating. To overcome these drawbacks, a PEO process in molten salt was designed and implemented. The growth mechanism of the coating formation has not yet been studied. In the present work, an aluminum oxide coating was created on an Al-Cu 2024 alloy using PEO at various current frequencies in molten salt. Chemical and phase compositions, coatings morphology, X-ray photoelectron spectroscopy, and X-ray diffraction were studied. Potentiodynamic polarization method and electrochemical impedance spectroscopy were used to analyze a corrosion behavior of the coated alloys. The results revealed the formation of a double-layered aluminum oxide coating free of any undesired components. To achieve the highest corrosion resistance, the current frequency should be intermediate; high frequencies produce a non-uniform oxide inner layer, and low frequencies produce a thin oxide inner layer. Finally, based on the observations, a coating growth mechanism for PEO at high and low current frequencies was proposed.