Improved corrosion protection of magnesium by hydrothermally deposited biodegradable calcium phosphate coating

• The corrosion protection of hydrothermally deposited Ca–P coatings was studied.
• The Ca–P coatings improved the corrosion performance of Mg substrate up to 10,000-fold.
• The corrosion resistance improved by increasing deposition temperature.
• Deposition mechanism of coating changed by increasing deposition temperature.

The corrosion protection of hydrothermally deposited calcium phosphate coatings on AZ31 magnesium alloy were studied for their potential use in biocompatible and bioresorbable temporary implants. The coatings mainly consisted of calcium phosphate phases (monetite and tricalcium phosphate). Potentiodynamic and electrochemical impedance spectroscopy (EIS) confirmed that the coatings provided varying levels of corrosion protection depending on the coating deposition temperature and duration. EIS results showed that the size of capacitance loops and the absolute impedance value (|Z|) increases by increasing the deposition temperature and corresponding growth in coating thickness. In agreement with the electrochemical experiments, immersion tests in simulated body fluid also indicated large improvement in corrosion protection as the mass loss was significantly reduced when coating was applied as compared to the bare metal. Using the thickest coating obtained at 190 °C deposition temperature, the corrosion current density of the coated magnesium was 10,000 fold lower compared to the bare metal. This result confirmed that the new hydrothermal coating is suitable to protect Mg implant against corrosion with further advantage of being bioactive and biodegradable.