Effect of Fe_xCa_yCO₃ and CaCO₃ Scales on the CO₂ Corrosion of Mild Steel

Calcium ions are usually present at high concentrations in brines produced with oil and gas. Such brines are typically saturated with respect to CaCO₃. Consequently, precipitation of CaCO₃ as scale on the internal wall of the pipeline can readily occur due to changes in operational and environmental parameters as produced fluids are transferred from downhole to surface facilities. Despite its importance, there is minimal research in the literature addressing the effect of calcium ions, and specifically CaCO₃ scale, on the CO₂ corrosion mechanism. The main objectives of this research are to further broaden the mechanistic understanding of CO₂ corrosion of mild steel in the presence of high concentrations of calcium ions and evaluate the protectiveness that Fe_xCa_yCO₃ and CaCO₃ scale confers against further corrosion. The corrosion behavior was studied in situ by electrochemical methods, including linear polarization resistance and open-circuit potential, along with weight loss using a UNS G10180 steel with ferritic-pearlitic microstructure. Surface characterization of the scale and corrosion product was performed using scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction. A descriptive model is proposed for the CO₂ corrosion mechanism of mild steel in the presence of high concentrations of calcium ions. Unprotective Fe_xCa_yCO₃ (y>x) and CaCO₃ scales were observed to act as a mass transfer barrier that could promote surface conditions favoring FeCO₃ precipitation. The presence of uniform CaCO₃ scale did not result in the onset of localized corrosion at the conducted experimental conditions.