Electrochemical Study and Modeling of H₂S Corrosion of Mild Steel

The internal corrosion of mild steel in the presence of hydrogen sulfide (H₂S) represents a significant challenge in oil production and natural gas treatment facilities, but the underlying mechanisms involved in H₂S corrosion are still not fully understood. This lack of knowledge makes the prediction, prevention, and/or control of aqueous H₂S corrosion of mild steel much more difficult. In the present study, H₂S corrosion mechanisms were experimentally investigated in short-term corrosion tests (lasting 1 h to 2 h), conducted in a 1 wt% sodium chloride (NaCl) solution at different pH (pH 2 to pH 5), at different temperatures (30°C to 80°C), under various H₂S/N₂ gaseous concentration ratios (0 to 10%[v]) and flow rates, using a X65 mild steel rotating cylinder electrode. Corrosion rates were measured by linear polarization resistance (LPR). Corrosion mechanisms were investigated by using potentiodynamic sweeps and by comparison with electrochemical modeling. LPR results showed that corrosion rates increased with increasing temperature, partial pressure of H₂S, flow rate, and decreasing pH. Results of potentiodynamic sweeps show the presence of H₂S could affect both cathodic reactions and the anodic reaction. An electrochemical model was developed and can be used to predict the effect of temperature, pH, pH₂S, and flow on corrosion mechanisms of mild steel in aqueous solutions containing H₂S in the absence of protective iron sulfide layers.