Correlation between the transient variation in positive/negative pulse voltages and the growth of PEO coating on 7075 aluminum alloy

A bipolar pulse mode is usually used in the plasma electrolytic oxidation (PEO) process of aluminum alloys. The negative pulse can eliminate the cavity and improve the compactness of the PEO coating. However, the effect mechanism of negative pulse on the electrical signal of PEO is not clearly understood. In this study, a method of improving the compactness of PEO coating and reduce energy consumption under different transient voltages and current waveforms is established and discussed. An equivalent circuit of RC series corresponding to the coating and the electric double layer during the PEO process was proposed to analyze the effect of negative pulse on the compactness of the PEO coating. Results showed that the voltage in the T+off stage can be rapidly reduced to less than the breakdown voltage by introducing negative pulses, and the discharge of unextinguished sparks on the sample surface was inhibited, resulting in a relatively compact PEO coating. The time constants RC decreases with increased negative currents. The decline of the time constant RC indicates the rapid response of the oxidation reaction to the electric signal, which can improve the coating's compactness and reduce the energy consumption. In actual production, the energy consumption can be reduced by setting up a reasonable ratio of positive and negative pulse voltages in the PEO.