A Thermodynamic Model for the Prediction of Mild Steel Corrosion Products in an Aqueous Hydrogen Sulfide Environment

In the present study, a comprehensive thermodynamic model, depicted by Pourbaix diagrams, was developed with the relatively narrow focus on corrosion of mild steel in oil and gas field conditions. This thermodynamic model focuses on predicting the formation of metastable or stable corrosion products in sour environments at elevated temperature up to 250°C, which includes mackinawite (FeS), greigite (Fe3S4), the pyrrhotite group (Fe1–xS, x = 0 to 0.17), and pyrite (FeS2). The model is based on theoretical thermodynamic calculations and data found in the open literature. The appearance of Pourbaix diagram is significantly affected by temperature. Long-term corrosion experiments at two different temperature (25°C and 80°C) were conducted to investigate the corrosion product stability predictions made by the Pourbaix diagrams. The equilibrium state predicted in the Pourbaix diagrams was compared with the quasi-equilibrium state attained in the long-term experiments. To this end, the surface pH, bulk pH, ferrous ion concentration in solution, and corrosion potential were all monitored throughout the experiments. The morphology and composition of corrosion products formed on the mild steel sample surface was analyzed using scanning electron microscopy and x-ray diffraction. It was observed that the experimental results generally agreed with the predictions made by the Pourbaix diagrams.