Effect of Calcium on the Formation and Protectiveness of Iron Carbonate Layer in CO₂ Corrosion

Among the known options in carbon capture and storage (CCS), the injection and storage of CO₂ in deep saline aquifers has the potential to cause casing corrosion due to the direct contact between injected CO₂ and the saline aquifer, containing highly concentrated aqueous salts such as NaCl and CaCl₂. Thus, in the present study, the effect of Ca²⁺ on the CO₂ corrosion behavior of mild steel was investigated in simulated saline aquifer environments (1 wt.% NaCl, 80°C, pH 6.6) with different concentrations of Ca²⁺ (10, 100, 1,000 and 10,000 ppm). Electrochemical methods (open circuit potential (OCP) and linear polarization resistance (LPR) measurements) were used to measure the corrosion rate. Surface analysis techniques (scanning electron microscope (SEM), X-ray spectroscopy (EDS) and X-ray diffraction (XRD)), were used to characterize the morphology and identity of the corrosion products. The results show that with low concentrations of Ca²⁺ (10 and 100 ppm), the corrosion rate decreased with time due to the formation of protective FeCO₃ and/or Fe_xCa_yCO₃ (x + y =1). However, the presence of high concentrations of Ca²⁺ (1,000 and 10,000 ppm) resulted in the change of corrosion product from protective FeCO₃ to non-protective CaCO₃, and an increasing corrosion rate with time. While the general corrosion rate was high for both 1,000 and 10,000 ppm Ca²⁺, surface analysis data revealed a different steel surface morphology with pitting observed in the presence of 10,000 ppm Ca²⁺.