Abstract
The current understanding of the corrosion mechanisms in H2S containing environments is based on the direct electrochemical reduction of H2S as the main contribution of this species to the corrosion process. Such an argument has been developed based on the distinctive behavior of cathodic polarization curves in H2S containing solutions, as compared to the behavior observed in the solutions of strong acids or those in presence of other weak acids such as carboxylic acids and carbonic acid. The direct reduction of H2S is generally associated with the observation of a “double wave” in a cathodic polarization curve. In the present study, the mechanism of cathodic reaction in H2S containing acidic solutions was studied theoretically with the aid of a comprehensive mathematical model. The model includes a mechanistic description of main processes including mass transfer, chemical reactions, and electrochemical reactions. A quantitative analysis based on this model showed that all the characteristic behaviors previously associated with the direct reduction of H2S, including the “double wave” behavior, can be explained based on the kinetics of homogeneous chemical dissociation of H2S and hydrogen ion reduction as the sole cathodic reaction. This analysis suggests that H2S is not a significant electroactive species, and its main contribution to the corrosion process is through its buffering ability as a weak acid, similar to other weak acids such as carboxylic acids and carbonic acid. In order to validate these mechanistic observations, the results from this model were compared to existing experimental data from the open literature. The model was found to be able to capture the main characteristic experimental behavior with reasonable accuracy, further supporting this mechanistic argument.
