Modelling of CO2 corrosion and FeCO3 formation in NaCl solutions

The corrosivity of carbon dioxide (CO2) corrosion and iron carbonate (FeCO3) layer formation in sodium chloride (NaCl) solutions (1-12 % w/v) were investigated through electrochemical experiments and modelling. Relying on electrochemical measurements (Potentiodynamic polarisation) and simplified current density expressions (employing only H+ activity), reaction enthalpies (Delta H) and rate constants (K-r) for Fe dissolution, H-2 evolution and H2O reduction reactions were estimated over a temperature range of 40-80 degrees C. Additionally, a revised FeCO3 precipitation rate expression was developed based on a newly derived FeCO3 solubility product (K-sp), integrating the effects of temperature and ionic strength (using activity coefficients). Collectively, this yielded a new CO2 corrosion prediction model accounting for the presence of a developing layer of FeCO3 in NaCl solutions. The model was validated over a broad range of conditions (pH, temperature, pressure and NaCl concentrations) by employing the corrosion rate and FeCO3 characteristics as metrics. Notably, it was shown that the activities of dissolved CO2 and Cl were not essential to predict the electrochemical response of anodic processes. Further-more, it was demonstrated that increasing NaCl concentration resulted in a complexly evolving environment where porous, less protective FeCO3 layers were formed.