Advanced surface engineering processes based on polarisation-driven electrochemical reactions can be easily programmed and automated using feedback loops and digital models of the process. Being a distinctive feature of Industry 4.0 manufacturing strategy, such intelligent electrochemical approaches require in-depth understanding of the relations between polarisation conditions, electrical response and surface properties, which are of critical importance for process modelling. In this study, we focus our attention on numerical evaluation of an advanced high-voltage anodic technique, known as plasma electrolytic oxidation (PEO) which is often carried out with reverse biasing. In spite of many positive features brought by negative polarisation in PEO treatments of aluminium, the mechanism of reactions induced by the cathodic current is still unclear. We propose a quantitative approach to numerical evaluation of the changes taking place in the oxide layer under the negative polarisation. The analysis of current response to the linear polarisation sweep allows a charge corresponding to the hysteresis in the anodic voltammogram to be quantified. By direct experiments, it was found that effects induced by the negative current are not associated with surface charging, but likely associated with chemical or morphological changes within the coating. A mechanistic explanation to this observation is provided in accordance with previously suggested active zone concept including temporary proton incorporation-neutralisation reactions.