Effect of static and dynamic tensile stress changes on carbon steel passivation in alkaline environment

Abstract

This work investigated the influence of static and dynamic tensile stress variations on the passivation of steel in chloride-free alkaline solution. Two types of loading with different stress levels were considered, including the constant tension which was applied on the steel in the beginning of immersion (i.e., without pre-passivation in stress-free condition), and the stepwise increasing tension ranging from 0 to 90 %σ_0.2. In-situ electrochemical measurements including electrochemical noise, electrochemical impedance spectroscopy, Mott-Schottky test, potentio-static and -dynamic polarization were performed during loading, in addition to the microstructural characterization of steel and passive film. Results indicate that, elastic tensile stress facilitates the passivation of carbon steel in chloride-free alkaline solution by enhancing the diffusion of oxygen vacancies, forming a thicker passive film with a lower donor density. Nevertheless, in the case of stepwise increasing tension, stress introduces mechanical damage or even fractures on passive film, depending on the level of stress/strain. These two aspects of stress-induced effect on the passivation and the properties of passive film need to be interpreted with caution.