Effect of pH value and preparation temperature on the formation of magnesium phosphate conversion coatings on AZ31 magnesium alloy

Magnesium phosphate (MgP) conversion coatings were prepared on AZ31 magnesium (Mg) alloy to improve its corrosion resistance. The effect of pH value (2.5, 3.0, 3.5 and 4.0) and preparation temperature (40 °C, 60 °C and 80 °C) on the formation of conversion coatings was investigated in this study. The formation mechanism of MgP conversion coating is first proposed and investigated by predominance area diagram of Mg phosphates. The morphologies, compositions and cross-section morphologies of coated samples were analyzed by scanning-electron microscopy (SEM), energy-dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS) and X-ray diffractometry (XRD). Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) measurements were conducted to evaluate the short-term corrosion resistance of coated samples in Hanks' solution at 37 °C and pH 7.4. In addition, a 5-day immersion test was carried out to evaluate their long-term corrosion resistance. It was found that temperature affected the corrosion performance of coated samples more significantly than pH value, and the coating formed at 80 °C-pH 3.0 offered the best corrosion resistance in Hanks' solution at 37 °C and pH 7.4. The formation mechanism of MgP coating was elucidated with reference to the pH value and Mg2+ ion concentration at different regions in the conversion solution according to the predominance area of magnesium phosphates. The microstructure of conversion coating suggests that it consisted of a precipitated outer layer and an in-situ grown inner layer. Compared with the crystallized outer layer, the dense inner layer contributed more to the corrosion resistance of the coated samples in Hanks' solution. Moreover, the corrosion mechanism of different samples including bare AZ31 Mg alloy and coated samples was discussed.