Corrosion performance of plasma electrolytic oxidation grown oxide coating on pure aluminum: effect of borax concentration

Plasma electrolytic oxidation (PEO) is a widely applied surface treatment method which can improve the corrosion resistance of valve metals and their alloys. In this study, PEO treatment was conducted on pure Al using a power supply under a bipolar pulsed mode at fixed duty cycle and frequency. The effect of different concentrations of borax in the electrolyte on the corrosion resistance of PEO grown oxide coatings in 3.5 wt% NaCl solution was comprehensively studied by a potentiodynamic polarization test and electrochemical impedance spectroscopy method. We found that the average pore size and porosity on the PEO grown oxide coating decreased with increasing borax concentration. The dense microstructure of oxide coating can be seen when the borax concentration in the electrolyte was high. The corrosion resistance of PEO coating increased with increasing borax content. The corrosion resistance of pure Al was greatly improved up to 23,058 times higher by the formation of a PEO layer treated in the electrolyte containing 6 g L⁻¹ borax due to its ability to grow a dense γ-Al₂O₃ phase with less defects.