Effect of bath pH on microstructure and corrosion behavior of manganese phosphate coating on SCM420H steel

In order to improve the surface quality and prevention of corrosion and reduction of wear in the components like gears and bearings, they can be covered with phosphate coatings. SCM420H alloy steel was coated with manganese phosphate deposition. The microstructure and corrosion resistance of the coating was studied by change of bath pH. In order to investigate the phase analysis and coating microstructure, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used, respectively. XRD analysis from the coated surface revealed the phases of MnFe2(PO4)2(OH)2H2O, Mn(PO)3، Fe(PO)3 and (Mn,Fe)5H2(PO4)4·4H2O. Results showed the obtained coating at pH=2.1 was uniform and continuous and no crack or porosity was observed. The phosphate surface at pH=1.9 and pH=2.4 was non-uniform and included cracks. In order to investigate the corrosion resistance, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests were applied on coated and uncoated specimens in 3.5% NaCl solution. The results of potentiodynamic polarization and electrochemical impedance spectroscopy tests were in agreement with microscopic images. The results of electrochemical impedance spectroscopy demonstrated that the formed manganese phosphate deposition in the bath with pH=2.1 had the highest polarization resistance (28020 Ω) compared to the formed coating in the bath at pH=1.9 (1480 Ω) and the formed coating in a bath with pH=2.4 (3155 Ω).