Characteristics of CO2 corrosion scale formed on N80 steel in stratum water with saturated CO2

The component and structure of CO₂ corrosion scale formed on N80 tubing steel were studied by using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM); the electrochemical property of N80 steel covered by corrosion scale was investigated by electrochemical impedance spectroscopy (EIS). The results shows a double-layer structure of the corrosion scale, in which the principal component of the outer layer is FeCO₃ with a limited amount of ∂-FeOOH; while for the inner scale, FeCO₃ is still the major component, but some Fe positions in FeCO₃ lattice are substituted by Ca, and form a composite compound of (Fe,Ca)CO₃ in the inner scale. EIS study shows that the anodic impedance spectrum has three time constants, i.e., the capacitance at high frequency, Warburg impedance at middle frequency and capacitance at low frequency.