Effect of positive and negative pulse voltages on surface properties and equivalent circuit of the plasma electrolytic oxidation process

The paper discusses a methodology for the equivalent circuit design for the plasma electrolytic oxidation (PEO) process from the voltage and current waveform transients recorded during pulsed bipolar PEO treatment. Joint analysis of the coating morphology and the electric transients enabled the proposal of an equivalent circuit with elements corresponding to characteristic parts of the system - electrolyte, porous and dense coating layers, showing different behaviours at anodic and cathodic polarisations of PEO. Two thresholds based on the sparking voltage for a positive pulse and on the minimal resolved current for a negative pulse were introduced to draw the boundaries for the application of the circuit structures. The simplest equivalent circuit corresponds to lower positive and negative pulse voltages (U_p< 526. V, U_n< 20. V); it comprises a linear parallel RC branch corresponding to the thinnest coating in series with the electrolyte resistance. For higher negative voltages (U_p< 526. V, U_n> 20. V), the anodic and cathodic branches become separated due to different coating behaviour under positive and negative polarisations. The RC branch represents the anodic behaviour of a thinner porous single layer coating, and the RL branch describes the cathodic response. For higher positive and negative voltages (U_p> 526. V, U_n> 20. V), the PEO microdischarges become stronger, and the anodic branch evolves into two RC loops connected in series in order to represent heterogeneity developed in the coatings containing porous outer and dense inner regions. The fact that the equivalent circuit elements evolve with the coating growth enables assessment of the coating thickness in-situ during the PEO treatment. Finally, the equivalent circuit modelling methodology can contribute towards design of advanced PEO equipment with power supplies optimised to the electrolyser properties depending on the treatment conditions and with diagnostic instruments providing in-situ estimates of the coating properties.