Abstract
The internal corrosion of pipeline steel in the presence of hydrogen sulfide (H2S) represents a significant problem in oil and gas industry. Its prediction and control pose a challenge for the corrosion engineers. In previously published research by the same authors, an electrochemical model of H2S corrosion was developed in both pure H2S and H2S/CO2 aqueous systems. An additional electrochemical cathodic reaction, direct H2S reduction, was uncovered based upon the carbon steel corrosion experimental results. However, in the carbon steel corrosion experiments, the cathodic sweeps experienced interference by the anodic iron dissolution reaction, making the kinetics of cathodic reactions unclear. In the present study, experimentation was conducted to better resolve the direct reduction of H2S while minimizing the effect of the anodic reaction by using a passive stainless steel working electrode. The electrochemical kinetics parameters for H2S reduction (i.e. Tafel slope, exchange current density, and reaction order with H2S concentration) were determined. Moreover, the electrochemical kinetics parameters for H+ reduction were also revisited.
>Introduction
The internal corrosion of pipeline steel in the presence of hydrogen sulfide (H2S) represents a significant problem in oil and gas industry. Its prediction and control pose a challenge for the corrosion community. The aqueous H2S corrosion of carbon steel is an electrochemical process occurring at the steel surface. The overall reaction is dependent on the kinetics of different electrochemical reactions, which are composed of two simultaneous electrochemical half-reactions: anodic (oxidation) and cathodic (reduction). The present study is focused on cathodic reactions in H2S corrosion of carbon steel.
The best known cathodic reaction in aqueous solution is hydrogen evolution or hydrogen ion (H?) reduction, Reaction (1), which has been intensely investigated in strong acid solutions with different substrates. The same kinetics has been assumed to hold in both CO2 and H2S corrosion models.