Plasma electrolytic oxidation of AZ31 and AZ91 magnesium alloys: Comparison of coatings formation mechanism

The growth kinetics of PEO coatings on AZ31 and AZ91 magnesium alloys were studied and correlated with their structure, compositions (phase and elemental) and corrosion resistance. It was established that the coatings have a two- (outer and anodic) or three-layer structure (outer, inner and anodic) depending on the treatment time. Briefly, at short treatment time only an anodic layer and outer layer exists. Growth of the outer PEO layer takes place due to the micro discharges, which occur in vertical pores and voids with spherical cross-section. If the time is increasing, and electrolyte inside of the pores is heating-up, etching of the Mg substrate and oxide film becomes more dominant and horizontal pores in the interface between coating and metal are formed. In the pores new anodic layer will form and at this time the formation of the third inner layer starts. The growth of the inner layer happens via the anodic film as a result of micro discharge ignition in the horizontal pores, accompanied by formation of plasma in numerous micro-voids of this layer. The coatings formed on AZ91 alloy are denser, than those on AZ31, which is related to the difference in the rates of inner layer growth and dissolving of oxides which are located at the bottom of the horizontal pores. Because of the lower Al content, the AZ31 substrate itself and the also the oxide films are less stable and tend to dissolve at a higher rate compared to AZ91. Thus, it was demonstrated that a good corrosion resistance of the coatings was only obtained on AZ91 and if the average thickness of the coating is around 50 µm, correlating with the formation of a sufficiently dense inner layer. Knowing this mechanism, a new two-step treatment was suggested, combining the standard PEO treatment with a subsequent PEO process in an electrolyte supporting the inner film formation. The concept was successfully applied and a further improved corrosion resistance was obtained compared to the single stage PEO process. This improvement of corrosion resistance was related to the better sealing of porosity and formation of a denser inner layer.