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

The effect of calcium (Ca2+) on the carbon dioxide (CO2) corrosion of mild steel was investigated in simulated saline aquifer environments (1 wt% sodium chloride [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 evaluate the corrosion behavior. Surface analysis techniques (scanning electron microscopy [SEM], energy-dispersive x-ray spectroscopy [EDS], and x-ray diffraction [XRD]) were used to characterize the morphology and identity the corrosion products. The results showed that with low concentrations of Ca2+ (10 ppm and 100 ppm), the corrosion rate decreased with time as a result of the formation of protective iron carbonate (FeCO3) and/or mixed carbonate (FexCayCO3) (x + y = 1). However, the presence of high concentrations of Ca2+ (1,000 ppm and 10,000 ppm) resulted in the change of corrosion product from protective FeCO3 to non-protective calcium carbonate (CaCO3), and an increasing corrosion rate with time. Results of surface analysis revealed a different steel surface morphology with pitting observed in the presence of 10,000 ppm Ca2+.