This work proposes an in situ electrochemical analysis procedure for characterizing iron carbonate (FeCO3) development on carbon steel at elevated pressures. X65 carbon steel was exposed to an unbuffered carbon dioxide (CO2) saturated brine in an electrochemical autoclave at 80°C and 5.5 bar pCO2 until a protective layer of FeCO3 had formed. Open Circuit Potential (OCP) and Linear Polarization Resistance (LPR) were coupled with Electrochemical Impedance Spectroscopy (EIS) and ex situ surface and cross-sectional imaging to identify key stages in corrosion layer development. The impedance response shows clear transitions in the nature of the interface which coincides with trends observed in both LPR and OCP data. Cross-sectional SEM images reveal that FeCO3 forms almost entirely within the exposed Fe3C matrix after ferrite depletion. The result is a thick (100 μm) but highly porous layer that presents a finite length (transmissive) diffusion barrier between the substrate and electrolyte.