A theoretical carbon dioxide (CO2) corrosion model was used to conduct numerical experiments, which allowed total insight into the underlying physicochemical processes. The focus was on factors influencing protective iron carbonate film formation and the effect that these films have on the CO2 corrosion process. It was confirmed that high bulk pH, high temperature, high partial pressure of CO2, high Fe2+ concentration, and low velocity all lead to favorable conditions for protective iron carbonate film formation. The model can be used to identify threshold values of these parameters. Corrosion rate was not strongly correlated with protective film thickness. The so-called surface film “coverage” effect appeared to be more important. Corrosion rates decreased rapidly as the film density increased. It was shown that in the presence of dense films diffusion of dissolved CO2 through the film is the main mechanism of providing the reactants to the corrosion reaction at the metal surface. It was demonstrated that “detached” films have poor protective properties even when they are very dense. Serious errors in prediction/ reasoning can be made by operating with bulk instead of surface water chemistry conditions. The former is made possible by using advanced models such as the one used in the present study.