Interrogating Iron Calcium Carbonate Corrosion Layer Properties Using Electrochemical Impedance Spectroscopy

Abstract An experimental method for analyzing iron-calcium carbonate corrosion layers is developed using a controlled CO2 charging procedure. Corrosion layers of varying composition were produced on X65 carbon steel by modifying the initial bulk Ca2+ concentration and surface area of steel specimens immersed in solution. The selected CO2 charging procedure allowed for repeatable generation of a 5.5 bar pCO2 initial vapor phase for autoclave experiments conducted at 80°C. Corrosion layer formation was monitored using electrochemical techniques, primarily Linear Polarization Resistance (LPR) and Electrochemical Impedance Spectroscopy (EIS). Despite creating thicker layers, high calcium concentrations led to a slower decrease in general corrosion rate, attributed to heterogeneous morphology and non-uniform surface coverage. The change in corrosion layer build up created noticeable differences in impedance response from the various layers, suggesting a change in the mechanism of how the layer is protecting the surface. The results supplement a further investigation into the mechanistic link between calcium incorporation in iron carbonate and the subsequent effect on localized corrosion of the substrate steel.