Carbon steel (CS) was pre-treated in CO2-saturated methyldiethanolamine(aq) to form FeCO3 surface films, the protectiveness of which against corrosion in monoethanolamine was studied using electrochemical impedance spectroscopy, open circuit potential measurements and ex situ surface characterisation. Only pure, dense FeCO3 films were found to provide protection in lab-scale tests. Dissolved O2 in pre-treatment solutions resulted in traces of iron (hydr)oxide within the films, exacerbating non-uniform corrosion. Impure films were compromised in pilot-scale experiments in the CO2 capture plant at Imperial College London. Therefore, pre-treating CS with methyldiethanolamine is not a robust corrosion control strategy for industrial CO2 capture plants.