Abstract:The influence of cathodic pulse parameters was evaluated on plasma electrolytic oxidation (PEO) coatings grown on 7075 aluminum alloy in a silicate-based electrolyte containing potassium titanyl oxalate (PTO) using pulsed bipolar waveforms with various cathodic duty cycles and cathodic current densities. The coatings were characterized by SEM, EDS, and XRD. EIS was applied to investigate the electrochemical properties. It was observed that the increase of cathodic duty cycle and cathodic current density from 20% and 6 A/dm² to 40% and 12 A/dm² enhances the growth rate of the inner layer from 0.22 to 0.75 μm/min. Adding PTO into the bath showed a fortifying effect on influence of the cathodic pulse and the mentioned change of cathodic pulse parameters, resulting in an increase of the inner layer growth rate from 0.25 to 1.10 μm/min. Based on EDS analysis, Si and Ti were incorporated dominantly in the upper parts of the coatings. XRD technique merely detected γ-Al₂O₃, and there were no detectable peaks related to Ti and Si compounds. However, the EIS results confirmed that the incorporation of Ti⁴⁺ into alumina changed the electronic properties of the coating. The coatings obtained from the bath containing PTO using the bipolar waveforms with a cathodic duty cycle of 40% and current density values higher than 6 A/dm² showed highly appropriate electrochemical behavior during 240 d of immersion due to an efficient repairing mechanism. Regarding the effects of studied parameters on the coating properties, the roles of cathodic pulse parameters and PTO in the PEO process were highlighted.

Key words: Al₂O₃-TiO₂ coating; plasma electrolytic oxidation; potassium titanyl oxalate; electrochemical impedance spectroscopy