Impedance Characteristic Of Ceramic Coating On Titianium Alloy During Plasma Electrolytic Oxidation Process

The impedance characteristic of the plasma electrolytic oxidation (PEO) coating depends strongly on the coating structure. By studying the impedance characteristic, the growth process of the PEO coating can be well understood. During the PEO process, the response of the power source is affected by the impedance characteristic of the breakdown coating. By observing the response of the power source, i.e., the wave shape of the current or voltage, the growth process of the PEO coating can be controlled, especially for the transfer of micro-arc stage to arc stage. Doing so is very helpful for the preparation of the PEO coating of high performance.Based on the above purpose, electrochemical impedance spectroscopy, combined by SEM, EDS, XRD analysis, are adopted to study the impedance characteristic of the PEO coating prepared at different stages. The response of the single-pulse power during the PEO progress is recorded by the oscillograph, and analyzing the waves of the current and voltage presents the impedance characteristic of the breakdown coating, which includes the active resistance during the breakdown, Ra and the capacitance of the part where the breakdown does not occur, C. Moreover, the potentiodynamic polarization is also performed in different solutions to testify the fitting results of EIS and to study the corrosion resistance of the coating.For the corrosion system of very low corrosion rate, the potentiodynamic polarization is more readily disturbed by the charging current. In this paper, the corrosion behavior of TC4 in 3.5% NaCl solution is taken as an example to illustrate how the potentiodynamic polarization curve and the obtained kinetics parameters are affected by the charging current. Based on the relation of the charging current with the scan direction, the disturbance of the charging current can be eliminated or reduced greatly by averaging the two current densities measured by positive and negative scan, respectively.The EIS spectra measured in the working solution can reflect well the growth character of the coating during the PEO process. In spark stage, the equivalent resistance and capacitance of the coating increase with the treatment voltage. In micro-arc stage, the equivalent resistance and capacitance decreases and increases, respectively. For the coating prepared by DC power, the outer and inner layers are nearly not affect by the current density in spark and micro-arc stage, respectively. For the coating prepared by single-pulse power source, the arc discharge is easily to happen, which causes great damage to the coating and makes the equivalent resistance and capacitance both decrease. With the increase of the frequency and the peak current density, the voltage where the arc discharge occurs increases, and the decrease of the duty ratio also makes the arc discharge voltage increase.During the PEO process, the real wave shape of the output of the power was very different from its theoretical shape. At the initial stage of the pulse, about within 1μs, the voltage and the current reached a maximal value at the same time, afterward, the voltage fall down to a stable value quickly, but the current decayed exponentially in a long period to the stable value, which was exactly like the behavior of the parallel component of a resistor and a capacitor. The variation of the decay rate of the current with the treatment voltage can be divided into three stages, which corresponded to the spark stage, micro-arc stage and arc stage of the PEO process. The fitting results of the current decay curve showed that at the early stage of the PEO process, the bottom of the discharge channel was very close or contacted directly with the substrate, but at the later stage of the PEO process, the discharge channel was separated from the substrate by coating. With the increase of the treatment voltage, the active resistance Ra increases before arc stage; the capacitance C decreases in spark stage, then increases in micro-arc stage, and it decreases again in arc stage.This paper also deals with the corrosion resistance of the PEO coating. After PEO treatment, the corrosion current density of TC4 decreases by more than one orders. The corrosion resistance of the PEO coating increases with the treatment voltage in spark stage, and gets worse when the PEO process comes into micro-arc stage. For the coating prepared by single-pulse power source, the increase of the frequency and decrease of the duty ratio can lead to the improvement of the corrosion resistance of the PEO coating. The influence of the technical parameters on the corrosion resistance is exactly in the same way as their influence on the resistance value of the inner layer in the equivalent circuit established to the fit the EIS spectra.