Electrochemical impedance spectroscopy during the process of plasma electrolytic oxidation

The growth process of plasma electrolytic oxidation (PEO) coating conducted for Ti6Al4V was studied by electrochemical impedance spectroscopy (EIS). The sample was treated to different voltage in sodium aluminate solution with DC power at current density of 2 A/dm² and 8 A/dm², and then it was transferred with the solution to the electrolytic cell to perform the EIS test. SEM, EDS and XRD were also adopted to characterize the porous structure, element and phase composition of the PEO coating, respectively. The results show that the PEO coating is composed of titanium, aluminum and oxygen. In the spark region, no crystalline phase is detected, and the growth process of PEO coating, especially the loose outer layer, is strongly affected by the treatment voltage, but nearly independent on the current density. In the micro-arc region, the coating consists of aluminum titanate phase, and the current density has strong effects on the growth process of the loose outer layer, but has little effects on the growth process of the dense inner layer.