The effect of chloride, sulfate, and ammonium ions on the semiconducting behavior and corrosion resistance of AISI 304 stainless steel passive film

Water cooling systems usually receive additional chemical treatment and different features to prevent pitting corrosion on stainless steel which is dependent on medium factors such as species concentration. Chloride is an aggressive ion with which sulfate can act as an inhibitor. By applying cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and Mott-Schottky plots, the behavior of 304 stainless steel (304 SS) was evaluated in solutions containing chloride, sulfate and ammonium ions. The polarization curves indicated the inhibiting effect of sulfate ions against pitting corrosion and the increase of the repassivation potential with increasing ammonium concentration. The most pronounced inhibiting effect occurred with a smaller ratio between chloride and sulfate ions (1:2). By EIS, the values obtained for charge transfer resistance of 304 SS sample corroborated the results of the cyclic voltammetry, ranging between 0.4 and 0.7 MΩcm² . The passive film showed semiconductor behavior, in agreement with the model of the chromium oxide inner layer and iron oxide outer layer. The passive films formed were highly doped, with doping density values on the order of 10^20-21 cm^-1. The difference between the corrosion potential and the flat band potential was found in the solutions where the material was less susceptible to corrosion. (author)