Short- and Long-Term Corrosion Behavior of Carbonate Apatite-Coated Mg-4mass% Y-3mass% RE Alloy in Cell Culture Medium

Osteoclast responsive carbonate apatite (CAp) coatings with different carbonate content rates and non-bioabsorbable hydroxyapatite (HAp) coating were formed on Mg-4mass% Y-3mass% rare earth (WE43) alloy by chemical solution deposition, namely chemical conversion. Electrochemical impedance spectroscopy measurement in a cell culture medium was performed for 6 h and 10 weeks, and the corrosion morphology was observed using a scanning electron microscope. For 6 h, polarization resistance of the CAp-coated specimens were 10 times higher than that of the uncoated specimen, while coating layer resistance of the CAp-coated specimens was 10⁴ Ω•cm² which was higher than 2×10³ Ω•cm² of the HAp-coated specimen. Mean polarization resistance for the initial 4 weeks for the former was 4×10⁴–7.5×10⁴ Ω•cm² which was higher than 2×10⁴ Ω•cm² of the latter. The corrosion of the CAp-coated specimens was localized; whereas the corrosion of the HAp-coated specimen initiated in many places and progressed to corrode almost the entire surface underneath the coating, indicating the less solution permeability of the CAp coatings than the HAp coating. These results demonstrate that the CAp coatings can show higher corrosion suppression ability than the HAp coating during the several weeks. The CAp coatings are a highly potential corrosion suppression coating for biodegradable magnesium alloys.