We show the temporal change in repassivation mechanism as a time-dependent linear combination of a high-field model of oxide growth (HFM) and the point defect model (PDM). The observed switch in transient repassivation current-decrease under potentiostatic control occurs independently of the active electrode size and effective repassivation time for all applied overpotentials.
For that, in situ depassivation of plasma electrolytically oxidized (PEO) coatings on aluminium was performed with nanosecond laser pulses at 266 nm and the repassivation current transients were recorded as a function of pulse number.
A mathematical model combines the well established theories of oxide-film formation and growth kinetics, giving insight in the non linear transient behaviour of micro-defect passivation.
According to our findings, the repassivation process can be described as a charge consumption via two concurrent channels. While the major current-decay at the very beginning of the fast healing oxide follows a point-defect type exponential damping, the HFM mechanism supersedes gradually, the longer the repassivation evolves. Furthermore, the material seems to reminisce former laser treatments via defects built-in during depassivation, leading to a higher charge contribution of the PDM mechanism at higher pulse numbers.
The authors like to thank Keronite International Ltd and Happy Plating GmbH for providing PEO-Al samples, Stephan Puchegger (Faculty Centre for Nanostructure Research, University of Vienna) for his invaluable assistance with SEM analysis and the Erwin Schrödinger Society for Nanosciences as well as the Austrian Ministry for Transport, Innovation and Technology for their financial support.
©2016 Walter de Gruyter Berlin/Boston