Effect of heat treatment on the microstructure and electrochemical behavior of manganese phosphate coating

The effect of heat treatment on the microstructure and electrochemical behavior of manganese phosphate coating formed on the surface of CrMoV alloyed steel was investigated. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed to investigate the dehydration and decomposition behaviors. The surface morphologies and crystal structures of various heat-treated phosphate coatings were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The associated changes of the electrochemical properties were evaluated by measuring the open circuit potential (OCP) and conducting electrochemical impedance spectroscopy (EIS). The results showed that the as-deposited phosphate coatings mainly consisted of (Fe,Mn)5H2(PO4)4·4H2O (Mn,Fe-hureaulite). The coating underwent a two-step dehydration process if it was heat treated below 325 °C. Decomposition of the phosphate coating occurred at 340 °C resulted in the formation of Fe0.5Mn2.5(PO4)2 with poor crystallinity. The polarization resistance of the phosphate coating was greatly affected by the loss of structural water, the heat treatment-induced crack and the formation of Fe0.5Mn2.5(PO4)2.