Electrochemical impedance response of a thick and porous calcium carbonate layer deposited by thermal growth on a carbon steel electrode

Thermal calcareous scaling has been studied on a simulated heat-exchange carbon steel cylinder set-up in a CaCO3 -rich CO2-saturated solution (SIcalcite=2) using in situ electrochemical methods and ex situ surface characterization. Open circuit potentiometry (OCP) and electrochemical impedance spectroscopy (EIS) indicate dramatic increases to the impedance of the electrode over the first two days. The response quickly changes from a single capacitive loop in the early hours to an impedance characterized by a 45 degrees slope at low frequency in Nyquist format, suggesting fast precipitation kinetics towards a protective state. At the end of 14-day tests, SEM and mu-Raman spectroscopy detected a distinctly layered aragonite-calcite scale. The bi-layered precipitate film is proposed to limit the diffusion of cathodic reactants across the porous, permeable scale, resulting in the 45 degrees slope linear tail characteristic of the so-called transmission line response. EIS responses are experimentally reevaluated away from open circuit, prior to proposing a bespoke equivalent circuit based on De Levie's porous electrode model to link the electrochemical data to surface observations.