Influence of heat treatment on crystallization and corrosion behavior of calcium phosphate glass coated AZ31 magnesium alloy by sol–gel method

To improve the corrosion resistance of AZ31 magnesium alloy, crack-free calcium phosphate glass ceramic coatings were successfully prepared on AZ31 substrates using a sol–gel dip-coating technique followed by a heat treatment at the temperatures of 400 °C, 450 °C and 500 °C, respectively. For studying the effects of heat treatment on crystallization of the coatings and the corrosion behavior, thermogravimetric and differential scanning calorimetry (TG–DSC) analysis, scanning electron microscopy (SEM), and X-ray diffraction (XRD) have been used for characterizing the coatings. The corrosion resistance of the coatings was tested by immersion in the simulated body fluid (SBF) and by electrochemical corrosion techniques. The structure of the sample heat treated at 400 °C was amorphous glass and the increase of temperature from 400 °C to 500 °C caused crystallization of Ca2P2O7 and Ca4P6O19 in the glass matrix, and subsequently led to the decrease of the corrosion resistance. Immersion test in SBF also indicated that samples heat treated at 400 °C exhibited the lowest corrosion rate and pH value, showing a good anti-corrosion property. Meanwhile, calcium-deficient hydroxyapatite agglomerates were only found on the surfaces of samples heat treated at 400 °C after immersion in SBF for 7 days, suggesting a better bioactivity than those samples treated at 450 °C and 500 °C. These results implied that crystallization affected both corrosion resistance and apatite forming ability.