Chemical and electrochemical surface treatments to grow magnesium hydroxystannate/manganese stearate coating for tuning biodegradation rate of AZ31 for bioresorbable implants

• Electrochemical process for tuning AZ31 corrosion rate in biological environment. • MgSn(OH) 6 spheres were grown by chemical conversion on AZ31 alloy. • Mn stearate hydrophobic coating was electrodeposited on MgSn(OH) 6 covered alloy. • HER accurately measured by gas chromatography resulted below tolerated limit. • Cellular vitality was over the accepted threshold. A double layered coating was deposited on AZ31 to tune biodegradation rate of this alloy in physiological solution at 37°C. A first chemical conversion process in a stannate ions containing alkaline solution was carried out on AZ31 to induce the growth of a MgSn(OH) 6 layer constituted by closely packed spheres. A manganese stearate ( Mn [ C H 3 ( C H 2 ) 16 COO ] 2 ) layer was electrodeposited on the magnesium hydroxystannate to make the surface of the alloy hydrophobic. The resulting coating allowed to increase the corrosion resistance in Dulbecco's Modified Eagle Medium (DMEM) at 37°C, as assessed by recording polarization curves and electrochemical impedance spectra. Hydrogen evolution rate in DMEM, directly measured using gas chromatography to avoid misleading information due to negative difference effect, was reduced by the coating since the early stage of immersion. According to cytotoxicity tests with MC3T3 pre-osteoblastic cells, the coating resulted biocompatible.