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

Abstract

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, improved 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.

A Novel Self-Adaptive Control Method for Plasma Electrolytic Oxidation Processing of Aluminum Alloys

“…When a low V u value is chosen, the discharges occur in the soft-sparking regime and this prevents the formation of cracks and other destructive effects on the coating, but this influences the choice of the coating thickness. The solution of such a compromise lies in the use of a novel soft-sparking regime based on the bipolar pulse mode, which occurs only when the ratio between the anodic and cathodic charge is lower than one [19,20,21,29].…”

“…Most of the research focuses on processing conditions [3,4,5,6,7,8], discharge characteristics [1,9,10,11,12,13], coating microstructure [14,15,16], mechanical properties [17], environmental performance [16,18], and functional characteristics [2,18]. As a result, the correlations [3,4,5,13,14,15,19] between the electrical conditions, the electrolyte compositions, the coating microstructure, and the growth rate of coating, which are linked via the characteristics of the discharges, have become clearer over recent years. Moreover, progress has been made [20,21,22,23,24,25] in unraveling the inter-relation between the energetics of individual discharges, the pulse energy, the soft-sparking regime, and the energy consumption.…”

Smart Arbitrary Waveform Generator with Digital Feedback Control for High-Voltage Electrochemistry

“…Under soft sparking conditions (R > 1 for Al), there is a "critical point" in the process, after which coating growth is accompanied by a decrease in anodic voltage. The latter is probably counterintuitive, since breakdown voltage is expected to increase with coating thickness; however, phenomena underlying the role of cathodic (negative) current are quite complex, and few attempts of its analysis were performed [56][57][58].…”

Dissolution Behavior of Different Alumina Phases within Plasma Electrolytic Oxidation Coatings

“…In this area, thermally induced phase changes of the oxides take place. According to Rogov et al, the conversion proceeds in the direction of the Al 2 O 3 modification with the lowest Gibbs free energy [3]:…”