Abstract: The spontaneous passivation of 316# stainless steel in simulated cooling water is studied by electrochemical impedance spectroscopy, and a suitable model is chosen through fitting. It is shown that the film resistance R2 increases while the value of Y02 , which reflects the film capacitance decreases continuously in the course of immersion of the electrode, but R2 decreases sharply with addition of sulfide. The Mott-Schottky plots show that the donor and accepter densities of the passive films on stainless steel immersed in simulated cooling water for 65 days have the values of 1.47×1020 cm－3 and 2.20×1020 cm－3, respectively, but the values increase to 4.52×1020 cm－3 and 7.02×1020 cm－3 after the addition of 9 mg•L－1 sulfide for 1 h. The polarization curves indicate that the passive current rises in the presence of sulfide.

Key words: Mott-Schottky plot, Electrochemical impedance spectroscopy, Passive film, Stainless steel, Sulfide