The electrochemical characteristics of pitting for two steels in phosphate buffer solution with chloride

Metastable pitting and stable pitting behaviors of two carbon steels in 0.05 M Na3PO4 + 0.03 M Na2B4O7 + 0.03 M Na2SO4 solution with 0.05 M NaCl or without NaCl (pH = 9.4) were studied by polarization measurements, current fluctuation measurements, EIS tests and analysis of Mott–Schottky plots. Compared with A3 steel, 16Mn steel shows higher passive film impedance and lower pitting susceptibility, which is characterized by higher pitting breakdown potential Eb, metastable pitting potential Em and flat band potential Ufb values, and lower electron concentration in the passive film. When the potential is above Em, diffusion tails occur on the resistance plots, indicating the beginning of local damage to the surface.