Improvement of corrosion resistance of pure magnesium in Hanks’ solution by microarc oxidation with sol–gel TiO2 sealing

A composite coating was prepared on degradable magnesium (Mg) implant for improving its corrosion resistance in Hanks’ solution (a simulated body fluid). The composite coating was fabricated using a two-step process: (i) a thick and porous oxide layer (with pore size of a few μm) was first formed by microarc oxidation (MAO) in a silicate/fluoride containing electrolyte, and (ii) a top TiO2 sealing layer was formed on the porous layer by sol–gel dip coating followed by hydrothermal treatment. SEM imaging and EDS mapping revealed an average overall thickness of about 12 μm. XRD analysis showed that the MAO layer was composed of magnesium oxide and fluoride and the TiO2 was present as an amorphous phase. Both electrochemical impedance spectroscopic (EIS) and anodic polarization measurements consistently recorded an increase in the initial corrosion resistance of about 30 times due to the composite coating. Immersion tests also showed that the corrosion behavior of the coated samples was more stable over time, and the degree of corroded damage was much reduced compared with bare Mg. The porous MAO layer provided anchorage sites for the subsequent TiO2 sol–gel coating, which sealed the pores and hence significantly enhanced the corrosion resistance while direct sol–gel coating on bare Mg was not successful. The present results indicate that the corrosion resistance of Mg implants could be significantly improved by a simple method using non-toxic materials. The increase in corrosion resistance implies the possibility of using less bulky fixation plates, thus enhancing the potential of using Mg as degradable implants.