Effects of temperature and applied potential on the microstructure and electrochemical behavior of manganese phosphate coating

The effects of solution temperature and applied potential on the formation of manganese phosphate on CrMoV steel surface were investigated. Material characteristics and corresponding corrosion resistances were also evaluated. The experimental results showed that the manganese phosphate can be successfully formed at 90 °C but not at 70 or 80 °C in the solution used in this study. At 90 °C, the phosphate consisted of two distinct layers, with a Fe-rich inner layer and a Mn-rich outer layer. The phosphating treatment was also conducted under applied potential conditions at 90 °C. The results showed that the phosphate layer formed under anodic applied potential was thicker than that formed under open circuit potential (OCP) and cathodic polarization conditions. Electrochemical impedance spectroscopy (EIS) measurements in a 3.5 wt.% NaCl solution indicated that the phosphate coating formed under cathodic polarization conditions exhibited superior polarization resistance to that formed under anodic applied potential condition. The formation of a dense phosphate layer resulting from recrystallization under cathodic phosphating conditions was responsible for the improved polarization resistance.