Predictive Model of Thermodynamic Properties and CO₂ Corrosion of Carbon Steels in CCS Environments

Abstract

A thermodynamic model （U-Cal model） has been developed that predicts fugacities and water content in the CO₂ phase （gas or supercritical fluid）, and CO₂ solubility and pH in aqueous solution in CCS environment, which is a CO₂ environment in a supercritical state. The values predicted by the U-Cal model agreed well with the measured values. The water content in the CO₂ phase increases to 1-10 g/L due to the mutual dissolution of CO₂ and H₂O. The increase in the solubility of CO₂ in aqueous solution and the decrease in pH with increasing pressure are small. The CO₂ corrosion behavior of carbon steel was discussed by using the U-Cal model. In iron dissolution dominant CO₂ corrosion in aqueous solution of carbon steel, the corrosion rate could be understood as a function of pH. In FeCO₃ formation dominant CO₂ corrosion, it was considered that the corrosion progressed as FeCO₃ dissolves to supersaturation and then FeCO₃ precipitated on the surface of the material. The FeCO₃ precipitation behavior was predicted from the crystal growth rate equation. Corrosion of carbon steel in the CO₂ phase involves similar mechanisms to corrosion in aqueous solution, but the corrosion rate is lower.