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 E-b, metastable pitting potential E-m and flat band potential U-fb values, and lower electron concentration in the passive film. When the potential is above E-m, diffusion tails occur on the resistance plots, indicating the beginning of local damage to the surface. (C) 2009 Elsevier B.V. All rights reserved.