Sequential run of the PEO process with various pulsed bipolar current waveforms

Abstract Oxide coatings were produced on a 2024 aluminium alloy using a pulsed bipolar current plasma electrolytic oxidation (PEO) process in an alkaline electrolyte. The use of sequences including various current waveforms during the same treatment was investigated, mainly by applying various combinations of two different anodic to cathodic charge quantity ratio (RCQ = 0.9 and 6.0). The morphology of the produced PEO coatings was observed using scanning electron microscopy while the behaviour of the micro-discharges was characterized in real time with the process using plasma diagnostic techniques. Compared with conventional treatments conducted in a single sequence, results evidenced that the thickness and the compactness of the PEO coating are enhanced by switching from a high to a low RCQ value in the course of the process. In contrast, reverse sequenced treatments leads to a significant deterioration of the produced PEO coating. A possible explanation exposed in the present study consists in considering the main role played by the mechanism of charge accumulation at the electrolyte/oxide interface on the micro-discharges behaviour with respect to the coating morphology and the applied charge quantity ratio. Finally, by establishing a relationship between process, physical and material quantities, this study demonstrate that the use of sequenced treatments open opportunities for a better energetic management of the PEO process.