Calcium phosphate glass/MgF2 double layered composite coating for improving the corrosion resistance of magnesium alloy

To improve the corrosion resistance of AZ31 magnesium alloy in simulated body fluid (SBF), a double layered composite coating of calcium phosphate (CaP) glass/MgF2 is synthesized on AZ31 substrate via a pre-treatment of hydrofluoric acid to form fluoride conversion coating, and then a sol-gel dip-coating method to develop an outer layer of calcium phosphate glass. The resulting composite coating exhibits uniform, dense, and smooth surface morphology, with excellent bonding to the magnesium substrate. Electrochemical tests reveal that CaP glass/MgF2 double layered composite coating improves the corrosion potential and the charge transfer resistance of magnesium substrate. Meanwhile, corrosion current density of CaP glass/MgF2 coated sample is significantly decreased, indicating an effective anti-corrosion property of the composite coating. Immersion test of the coated sample in simulated body fluid demonstrates that the composite coating is biodegradable, and can simultaneously reduce the corrosion rate of the magnesium alloy substrate during an 18-day immersion. Therefore, the CaP glass/MgF2 coated magnesium alloy possesses suitable corrosion behavior for the application as biodegradable implant material.