Optimization of the PEO process for in-situ synthesis of SiO2 and hydroxyapatite on Mg alloy and assessment of biodegradation and bioactivity

In the current study, a MgO-based bioactive coating consisting of silicon dioxide (SiO2) and hydroxyapatite (HA) was fabricated on the surface of AZ31 magnesium alloy through the plasma electrolytic oxidation (PEO) method. To achieve the goal, the process conditions were optimized in an electrolyte containing 5 g/L Na3PO4.12H2O, 15 g/L Na2SiO3.9H2O, and 10 g/L C3H7O6CaP with a pH of 12.5 and an electrical conductivity of 30 mS.cm−1. Thereafter, silicon dioxide and hydroxyapatite were simultaneously synthesized in-situ over the PEO process for 10min at an operating voltage of 370 V. The percentage of surficial open porosity of the relevant PEO layer was estimated to be approximately 10.30 % using the ImageJ software. Furthermore, in-vitro corrosion and bioactivity assessments of PEO layer were conducted in the simulated body fluid (SBF) at 37 °C with immersion intervals of up to 10 days. The results of electrochemical impedance spectroscopy (EIS) indicated that the PEO coating containing both SiO2 and HA maintained its protective performance against corrosion over the entire 7 days of immersion in the SBF, whereas the corrosion resistance of the PEO layer with only the synthesized SiO2 experienced a significant decline after 3 days of immersion. Assessment of in-vitro bioactivity over 10 days of immersion in the SBF revealed that the sample with synthesized SiO2 and hydroxyapatite in its PEO layer exhibited a higher potential for hydroxyapatite deposition on the surface compared to SiO2-containing PEO coating.