Passivation model of 316 stainless steel in simulated cooling water and the effect of sulfide on the passive film

Through the measurement and analysis of electrochemical impedance spectroscopy (EIS) data from the 316 stainless steel/simulated cooling water system, among the four selected passivation models, the equivalent circuit of Model D is most consistent with the conditions of this system. In this model, the charge-transfer resistance R1 is relatively small at the beginning of the immersion. When the immersion time increases, the values of R1 and Y01 stabilize. Over the time period of the immersion, the film resistance R2 increases while the value of Y02 decreases continuously, indicating that the passive film is getting thicker and more compact. With the addition of sulfide, R2 decreases rapidly. The polarization curve shows that the passivity current of the stainless steel increases with addition of the sulfide. The Mott–Schottky plots indicate that as the sulfide concentration increases, the slope of the straight segment which reflects the properties of p-type semiconductor (chromium oxide) decreases or even almost disappears. The slope of the straight segment which reflects the properties of n-type semiconductor (iron oxide) also decreases, and the acceptor density NA and the donor density ND increase to a certain extent. This shows that the sulfide changes the composition and structure of the passive film of the stainless steel and thereby decreases the corrosion resistance of the film.