In-situ Impedance Spectroscopy Studies of the Plasma Electrolytic Oxidation Coating Process

Plasma electrolytic oxidation (PEO) is a relatively novel surface modification technique that provides excellent wear- and corrosion-resistant coatings on light-weight metals, in particular on aluminium. Formation of PEO coatings involves complex processes and mechanisms that are difficult to study. In this work, a new method of in-situ impedance spectroscopy is used to provide new insights into fundamental characteristics of PEO processes and coating formation mechanisms as well as to develop new means of process control.
This method is based on application of a variable frequency voltage perturbation signal to obtain impedance characteristics of the electrolyser during the PEO processing. The applied voltage signal and the current response are collected and post-processed to verify the system linearity, refine phase, reduce noise and spline the impedance spectra. The obtained spectra are then fitted with appropriate equivalent circuits to reveal mechanisms underlying the PEO process. Physical meaning of various circuit components is verified using specially designed experiments in which certain system characteristics are set up in such way to obtain controllable processing conditions, such as electric field distribution in the electrolyser, electrolyte resistance or coating thickness. The circuit analysis reveals specific RC and RL loops that can be related to individual processes associated with interfacial charge transfer and transport phenomena. Characteristic time constants corresponding to these processes are evaluated and their evolution with PEO treatment time is considered. Correlations of the process kinetics with phenomena observed during the PEO treatment of Al and coating growth characteristics are discussed.
Several experiments involving complimentary methods and devices are also designed and carried out to assist the main method in investigation of the PEO processes. COMSOL Multiphisics software package is used for modelling the distribution of electric field and electrolyte resistance in the electrolyser; ex-situ EIS analysis- for impedance spectra comparison with ISIS results; fractal analysis- for studying the effects of coating morphology on impedance spectra; specialized imaging multi-channel (SIM) framing camera- for real-time observation of discharge events; and FFT analysis of high-resolution current signals- for studying the information on individual discharge events. Based on these studies, the characteristics of PEO process are discussed from different aspects and these better understanding is eventually achieved.