Analysis of electrical response, gas evolution and coating morphology during transition to soft sparking PEO of Al

The soft sparking regime of Plasma Electrolytic Oxidation (PEO) of valve metals attracts increasing attention as it enables a better process control with improved energy efficiency and coatings with enhanced performance to be obtained. Here, the discharges softening phenomenon was characterised by evaluation of changes in electrical signal, composition of gaseous products and morphology of the metal-oxide interface in PEO coatings on Al. The coatings were formed using two electrical regimes retarding or promoting soft sparking conditions. Harmonic analysis of PEO process has been performed using independent evaluation of the positive and negative half-waves of polarisation signal. The composition of gas evolved during the PEO process was evaluated by a quantitative gas analyser. Coatings were characterised by scanning and transmission electron microscopy with energy dispersive spectroscopy and small area electron diffraction. It was found that discharge softening is accompanied with a decrease in oxygen evolution rate, an increase in equivalent capacitance of anodic process as well as structural and compositional changes near the metal-oxide interface. The experimental findings provide further support to the explanation of discharge softening phenomenon by narrowing the active zone at the metal-oxide interface due to intercalation of hydrogen species into anodic alumina under cathodic polarisation. Such mechanistic understanding allows rational regime selection, in-situ process control and optimisation of the coating properties to be performed that paves the way to intellectual manufacturing approach based on careful process monitoring in operando.