The Importance of FeCO3 Supersaturation on the CO2 Corrosion of Carbon Steels

Abstract The amount of dissolved iron carbonate is very important for film formation, film breakdown and film reformation on carbon steel in CO2 containing water. Experiments have shown that the amount of dissolved FeCO3 does not fit with published solubility data when corroding steel is present in the solution. The influence of temperature and steel to water ratio on the precipitation kinetics and on the saturation and supersaturation of iron carbonate have been studied. It is shown that the supersaturation of FeCO3 is very temperature dependent and that the concentration when corroding steel is present can be 5-10 times that calculated from solubility data. An increased precipitation rate at high temperature (>60°C) can explain why the corrosion rate goes through a maximum in the temperature range 60-90°C. Detailed information on supersaturation at various temperatures gives important input to CO2 corrosion prediction models. Variation in supersaturation level can possibly also explain initiation of mesa attack.