Fabrication and corrosion resistance of calcium phosphate glass-ceramic coated Mg alloy via a PEG assisted sol–gel method

To slow down the initial biodegradation rate of magnesium (Mg) alloy, crack-free CaO–P2O5–SrO–Na2O glass-ceramic coatings (CaPs) were synthesized using polyethylene glycol (PEG) assisted sol–gel method followed by a heat-treatment at 400 °C. The thermal behaviors of the synthesized glass-ceramics were characterized by differential scanning calorimetry and thermogravimetric (DSC/TG). The glass transition temperature (Tg) of calcium phosphate system shifted to a lower temperature (~226 °C) due to the addition of PEG, which promoted the crystal nuclei formation, thereafter induced a great amount of Ca2P2O7 at a relatively low temperature. The effects of PEG addition on the corrosion behaviors of the coated Mg alloys were investigated by the electrochemical measurements and immersion tests in simulated body fluid (SBF). It was found that the coatings comprised of massive crystalline phases could slow down the degradation rate and decrease the mass loss of the Mg alloy substrates from 78.04% to 3.81% in the 7th day test, showing a better anti-corrosion property than that of the calcium phosphate coatings without the addition of PEG. Moreover, protection effect of PEG assisted synthesis of calcium phosphate coating gradually declined after the initial soaking time, which was confirmed by electrochemical impedance spectroscopy (EIS) analysis. These results implied that crystallization induced by PEG addition would affect the corrosion resistance of the coated Mg alloy and subsequently influence the degradation rate.