Thermodynamic Prediction of FeCO3/FeS Corrosion Product Films

Oil and gas production facilities transport a wide range of fluids which may contain Carbon dioxide (CO₂), Hydrogen sulfide (H₂S) and many other species which may lead to a corrosive environment. To ensure safe and cost effective production operations it is important to understand combined effect of H₂S and CO₂ on corrosion rates and the corrosion products formed in these environments. The formation of iron sulfide (FeS) and iron carbonate (FeCO₃) scale under a corrosive environment is one of the important factor governing corrosion rates in CO₂ and H₂S corrosive systems. This paper presents a simplified thermodynamic model to predict the dominant mineral on the corroding surface exposed a range of mixed partial pressures CO₂ and H₂S. The approach is based on thermodynamic estimate of equilibrium constants i.e. K_sp for FeCO₃ and K_sp for FeS to account for the formation of iron sulfide and iron carbonate scales at varying temperature, pH, iron concentrations and partial pressure of CO₂ and H₂S. The application and limitations of this model as a methodology to predict the dominant scale (i.e iron sulfide or iron carbonate) on the corroding surface is discussed.