This work utilizes immersion experiments to investigate the CO2 corrosion behaviour of low-alloy martensite steel by analysing its microstructure, corrosion kinetics, corrosion phases, surface morphology, cross-section morphology and elemental distribution. The experimental results show that the designed steel is free of localized corrosion and that its corrosion rate in vapour-saturated CO2 conditions is lower than that in CO2-saturated brine conditions. The inner layer consists of FeCO3 crystals and Cr-rich compounds, which plays a crucial role in the corrosion behaviour of the designed steel immersed in CO2-saturated brine conditions. The corrosion processes of the low-alloy martensite steel immersed in vapour-saturated CO2 and CO2-saturated brine conditions are proposed according to the experimental results. Additionally, the microscopic corrosion process of steel materials with a martensite microstructure is presented. (C) 2016 Elsevier Ltd. All rights reserved.