Comparison of spark and soft regimes in the PEO process in terms of corrosion and wear performance of AZ31 alloy for biomedical applications

In order to improve the corrosion behaviour of AZ31 alloy, PEO coatings in a silicon-fluorine electrolyte were developed using two different regimes, spark and soft regimes, and a bipolar pulse wave to compare them. The surface morphology and chemical composition of the PEO coatings were evaluated by scanning electron microscopy, roughness meter and X-ray diffraction. Corrosion and wear performance was evaluated by electrochemical impedance spectroscopy and immersion test in conditions similar to human body (Simulated Body Fluid and 37°C), and reciprocating dry sliding at room temperature, respectively. The coatings formed in both regimes exhibited similar morphologies and compositions, although it was slightly more compact in the case of soft regime due to the reduction of microdischarges intensity. Regarding to the results of the mechanical properties, the soft regime coating showed higher wear resistance, due to the low percentage of internal porosity, than the PEO coating in the spark regime. At short immersion times, both PEO coatings showed a significant improvement in corrosion resistance compared to the substrate due to the strong influence of the protective barrier layer after 24 h of immersion. After 48 h of immersion, both PEO coatings showed localised corrosion in some areas of the sample due to the presence of crevice at the interface between PEO and substrate. It is worth mentioning that the corrosion progress could be interrupted by the precipitation of Ca-P rich compounds in the pores and cracks of the PEO coatings by the hydrolysed silica.