Soft sparking can be utilized to di

Soft sparking can be utilized to dismiss the intrinsic porosity problem of plasma electrolytic oxidation (PEO) coatings on aluminum alloy, since this type of surface treatment generates a dense inner layer. Researchers usually recognize soft sparking through the anodic voltage drop, along with the decline in light and acoustic emissions. Analysis of V-I transients in the anodic pulse of bipolar DC current reveals the decline in RC time constant is a forerunner, which reflects the transition in micro arc state prior to the conventional definition. In studying the negative current dependence and the current frequency dependence of soft sparking, the results suggest the time constant may serve as an auxiliary indicator, monitoring growth activity variations and further assuring the dense inner layer. The role of time constant is exemplified in the soft sparking experiments at 500 Hz and duty 40%. Microstructure analysis reveals three relatively dense stacks separated by two layers of pores and loose grains. The porous layer is attributed to the micro arc state of less intense microdischarges, marked as a substantial drop in the R2C2 value. Matching between stratified layers and R2C2 rises-and-falls is verified in two electrolytic solutions with and without chromia inclusions. Thus, we suggest time constant as a probing tool of microdischarges softening to help analyze the events prior to voltage drop.