Effects of pH and Cl− Content on Degradation Process of Pre-Passivated Stainless Steels in an Alkaline Solution

Abstract

The degradation behavior of four prepassivated stainless steels (SSs) in the alkaline solutions with different Cl− concentrations and pH values were investigated by cyclic potentiodynamic polarization (CPP), electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis and the X-ray photoelectron spectroscopy (XPS). The results indicated that the pH value and Cl− concentration, together with the phase ratio difference in the microstructure of SS, played a critical role in the degradation process of passive film. At low pH and high Cl− concentration, the passive film gradually enriched in Fe3+ and Cr3+ species and depleted in Fe2+ oxides and its self-regeneration process became more difficult. As the pH decreased, the passive film exhibited clear n-type and p-type semiconductor characteristics more clearly, and the decrease in dopant density well reflected the changes in Fe2+/Fe3+ ratio and electrochemical behavior. Based on the viewpoint of adsorption balance between OH− with inhibitory effect and Cl− with promotion effect on the degradation process, a mechanism for the degradation process of prepassivation film was proposed.