Specification of parasitic electrochemical subprocesses during plasma electrolytic oxidation of magnesium

Depassivation processes known by literature detected by electrochemical methods during polarization experiments of Mg substrates in alkaline silicate PEO electrolytes below the ignition voltage were investigated in more detail by SEM images of the generated reaction layers. Remarkable crater structures were discovered on the Mg alloy AZ31 that do not form on the aluminum AlMgSi1 substrate under identical experimental conditions. The formations of these structures were described using the concepts of the high-field model. Through systematic comparative investigations of PEO processes on the two studied substrates by means of in situ process diagnostics, electrochemical subprocesses in the plasma electrolytic oxidation of magnesium were identified by analyzing electrical and photoelectric process signals. A negative influence on the morphology of PEO layers on magnesium was attributed to subprocesses in the context of the depassivation mechanism below the ignition voltage as well as microstructure images of the generated PEO layers.