Thermodynamic Prediction of FeCO_3/FeS Corrosion Product Films

摘要

Oil and gas production facilities transport a wide range of fluids which may contain carbon dioxide (CO_2), hydrogen sulfide (H_2S) and many other species that may lead to a corrosive environment. To ensure safe and cost effective production operations it is important to understand the combined effect of H_2S and CO_2 on corrosion rates and the corrosion products formed in these environments. The formation of iron sulfide (FeS) and iron carbonate (FeCO_3) scale under a corrosive environment is one of the important factors governing corrosion rates in CO_2 and H_2S corrosive systems. This paper presents a simplified thermodynamic model to predict the dominant mineral on the corroding surface exposed to a range of mixed partial pressures CO_2 and H_2S. The approach is based on thermodynamic estimate of equilibrium constants i.e. K_(sp) for FeCO_3 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 CO_2 and H_2S. 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.