Toward a nearly defect-free coating via high-energy plasma sparks.

A nearly defect-free metal-oxide-based coating structure was made on Al-Mg-Si alloy by plasma electrolytic oxidation at high current density accompanying high-energy plasma sparks. The present coatings were performed at two different current densities of 50 and 125 mA/cm in the alkaline-phosphate-based electrolytes with different concentrations of sodium hexafluoroaluminate (NaAlF). The addition of (NaAlF) to the electrolyte used in this study would result in a decrease in the size of the micropore, and a reasonably defect-free coating structure was achieved in the sample treated at high current density of 125 mA/cm. This was attributed mainly to the hydrolysis of AlF triggered by intense plasma sparks, which resulted in a uniform distribution of fluorine throughout the coating. Accordingly, the corrosion performance of the coating formed in the electrolyte containing 1.5 g/L NaAlF at 125 mA/cm was improved significantly as confirmed by electrochemical impedance analysis. In addition, the formation mechanism of the nearly defect-free coating in the presence of NaAlF was discussed.