Effect of frequency of plasma electrolytic oxidation on the microstructure and corrosion resistance of 6061 aluminium alloy

In this study, a hybrid method was used to generate a continuous thin alumina coating layer on the surface of Al6061 alloy at different frequencies of 100 Hz, 400 Hz, and 2000 Hz in pulsed unipolar mode in Step 1. In Step 2, a thick coating layer was formed at 300 Hz in pulsed bipolar mode. The microstructure of the PEO coating layer was observed using both SEM and TEM. The PEO coating layer exhibited the densest cross-sectional microstructure and porosity at a unipolar frequency of 400 Hz. The corrosion resistance of the PEO coating layer at the frequencies used in Step 1 was evaluated through potentiodynamic polarization testing and electrochemical impedance spectroscopy in a 3.5 wt% NaCl solution. Initially, the corrosion resistance of the PEO layer increased with increasing frequency up to 400 Hz, but eventually decreased, indicating a strong correlation between the corrosion resistance and the distribution of microcracks and pores in the coating layer. Comparing the results of the uncoated Al6061 alloy, PEO coated specimen at 400 Hz exhibited a significantly lower corrosion current density (I_corr) of 5.06 × 10⁻¹¹ A/cm² and a higher corrosion potential (E_corr) of −0.088 V, in contrast to the uncoated Al6061 alloy's I_corr of 1.01 × 10⁻⁶ A/cm² and E_corr of −0.673 V.