H2S corrosion of mild steel: A quantitative analysis of the mechanism of the cathodic reaction

Abstract

In the context of H2S corrosion of mild steel, the direct electrochemical reduction of H2S is currently believed to be the main contribution of this species to cathodic currents. That is perhaps due to the distinct behavior of the cathodic polarization curves observed in the presence of H2S, as compared to those obtained in strong acids solutions or in the presence of other weak acids such as carboxylic acids and carbonic acid. In the presence of aqueous H2S, the cathodic polarization curves show a “double wave” shape, that is widely considered to be the result of the direct reduction of H2S. In the present study, the mechanism of H2S corrosion of mild steel is theoretically investigated with the focus on the buffering ability of H2S. It is shown that all characteristic behaviors of cathodic currents that were previously associated with the direct reduction of H2S, including the “double wave”, can be fully explained in terms of the H2S dissociation reaction and its buffering effect. In order to further evaluate this mechanistic argument, a comprehensive mathematical model for the H2S system was developed and the calculated cathodic polarization curves were compared with the existing experimental data in the open literature. The results showed that the model, built with H+ reduction as the sole cathodic reaction, is able to reasonably capture all characteristic behavior of cathodic currents, further supporting this mechanistic argument.