Anti-corrosion behavior of magnesium phosphate (Mg–P) coatings after exposure to elevated temperatures

In this study, Mg–P coatings, prepared on steel alloy substrates, were exposed to temperature from 25 °C to 900 °C. The effects of high temperature exposure and the salt spray test on the anti-corrosion ability of Mg–P coating were investigated in detailed. The results show that the corrosion resistance of coatings decreases continuously from 25 °C to 700 °C, then increases at 900 °C of exposure. However, the corrosion resistance of coatings after 2000h salt spray test was significantly improved. In particular, the corrosion rate of the coating is only one-ninth that of the coating before the salt spray test when the temperature is 900 °C. Furthermore, the microscopic characterization test revealed that a dense oxide layer form between the steel alloy substrate and the coating when temperature exceed 400 °C. Besides the crystal phase transition and oxide layer , the synergistic effect of the secondary reaction of the raw material is the key factor to ensure that the coating still has good corrosion resistance at a high temperature.