Bio-corrosion of PEO-modified titanium and magnesium alloys

The present work reports the characteristics of Ca- and P-enriched PEO coatings in the thickness range of 5-100 urn generated on titanium of commercial grades (I and V) and on Mg alloys (Mg 99.99%, AM50 and Mg0.8Ca) and their corrosion resistance in simulated body fluid (SBF) at 37oC. The corrosion mechanism of the coated systems was investigated by DC polarization and electrochemical impedance (EIS). Additionally, metal ion release from Ti/PEO systems was evaluated by ICP-MS and hydrogen evolution rate was determined for Mg/PEO systems during in vitro immersion. The protective mechanism of PEO coatings on Ti and Mg alloys is determined by the resistance of their innermost dense layer adjacent to the substrate; diffusion resistance of the porous part of the coatings has significant contribution. PEO coatings on Mg alloys dramatically reduced the H2 evolution rate compared with untreated materials (10-15 mL cm^-2 week^-1) even though an increasing tendency of H₂ generation (from <0.2 mL cm^-2 week^-1 to 2 mL cm^-2 week^-1) was observed over 2 month immersion period. PEO coatings on Ti alloys reduced the Ti⁴⁺ liberation by 5 times compared with non-coated substrates over the period of 1 month. However, the liberation of Al³⁺ and V⁵⁺ were higher from PEO coated Ti grade V than from noncoated alloy. The coating itself, that incorporated the substrate elements, was found to be the main source of these ions. This indicates the necessity of further improvement of the PEO coating engineering for Ti alloys that contain potentially toxic elements.