This study investigates the effect of carbon dioxide (CO2) partial pressure on the rate of formation and the protective characteristics of iron carbonate (FeCO3) corrosion product layers on X65 carbon steel in aqueous systems. Autoclave corrosion tests were performed at 80 °C in CO2-saturated 3 wt.% sodium chloride solution at partial pressures from 5.5 to 15.5 bar. Linear polarization resistance and electrochemical impedance spectroscopy electrochemical methods were utilized to provide insight into the stages of corrosion product layer development. Analysis of the electrochemical response under each condition revealed key, distinguishable stages associated with the development of an FeCO3 layer as its role transitions from predominantly blocking steel active sites, to ultimately providing additional corrosion suppression through formation of a diffusion barrier. It was found that at higher CO2 partial pressures, the onset of diffusion control was accelerated, but the overall corrosion rate was greater than at lower pressures.