On AC-DC voltage superposition in the oxide formation and corrosion performance of PEO-treated Al 6061 alloy

The enhancement of wear and corrosion performance of Al and its alloys is essential to meet the growing demand for lightweight materials in energy-efficient vehicles. Plasma electrolytic oxidation (PEO) is an emerging surface treatment utilized for this purpose. However, its high voltage/current requirements limit its broader applications. In this work, we performed PEO treatments on Al 6061-T6 alloy in a phosphate-based electrolyte under a low voltage (< 200 V) by applying a superimposed AC-DC signal. Microstructural observations suggested that the DC component mainly governed the growth kinetics of oxide ceramic coating, while the AC component played a key role in regulating plasma activity, facilitating oxide formation at lower applied voltages. The resulting coatings consisted of a metastable Al₂O₃ phase together with amorphous constituents such as Al₂O₃ and AlPO₄. AC voltage component was found to favor the formation of crystalline structures, while the DC counterpart supports fast coating growth by also promoting the amorphous fraction. Furthermore, XPS analysis demonstrated that the variations in AC and DC voltage components affected the Al-P interactions and the degree of oxide dehydration. The specimen fabricated at 20 V AC + 140 V DC showed the highest surface roughness with hydrophobic characteristics, outperforming specimens treated at higher voltages in corrosion tests via electrochemical polarization and impedance techniques. Overall, this study highlights that even slight changes in DC or AC voltage component can significantly influence the coating structure and properties under low-voltage PEO conditions.