The role of cathodic current in plasma electrolytic oxidation of aluminium: current density 'scanning waves' on complex-shape substrates

This paper focuses on the factors that influence the surface distribution of the current density during plasma electrolytic oxidation of 2024 Al alloy under alternating polarisation. It was found that employing the combined current mode, including relatively long (100-2000 ms) pulse trains, unproved coating uniformity even when the electrolyser provided a severe nonuniform primary electric field. An experimental investigation employing a sectioned sample showed that the nonuniform distribution of the primary electric field could be compensated by the changes in the coating properties induced by previous cathodic polarisation. Temporary changes in the secondary distribution of current density across the sample surface (attributed to the coating properties) caused dynamic redistribution of the anodic current density during the following AC pulse train, resulting in the so-called 'scanning wave' effect, i.e. migration of the maximum current density along the sample surface. Factorial experimental design, finite element modelling and analysis of the transient current-voltage curves were applied. A mechanistic explanation underlying the considered phenomenon has been suggested. In addition, the increase in plasma electrolytic oxidation efficiency under soft sparking conditions is considered as 'electrocatalysis' of anodic oxidation by previous cathodic treatment.