Effect of the Fluoride Species and Content of the PEO Electrolyte on the Corrosion Properties of the Layers Obtained on AZ31 for Biomedical Purposes

This study aims to investigate the effects of different fluoride salts added in the PEO bath on the corrosion resistance and morphology of AZ31 magnesium alloy coatings. The PEO process was performed using a bipolar cycle with varying durations (4 and 30 min) in baths containing different fluoride salts (NaF, LiF, Na2SiF6) and a reference bath without fluoride. The coatings were characterised using SEM-EDS, XRD, and electrochemical impedance spectroscopy (EIS) to assess their morphology, chemical composition, and corrosion resistance. The results indicate that the presence of fluorides significantly influences the coating properties. NaF and Na2SiF6 coatings exhibited better corrosion resistance and more compact microstructures compared to LiF and the fluoride-free reference. The study highlights the importance of the fluoride counter ion in the PEO bath, demonstrating that NaF and Na2SiF6 provide superior protection against corrosion, making them suitable for biomedical applications where both porosity and corrosion resistance are critical.