The corrosion behavior of 3Cr steel in atmospheres composed of only O2 or CO2 or a combination of O2 and CO2 was investigated using an autoclave. The characteristics of the corrosion scale of the 3Cr steel were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), and electrochemical methods. The corrosion scale developed in the combined O2 and CO2 atmosphere was composed of FeCO3, Fe2O3, and Fe3O4, and had a loose texture with a large number of pores. The surface and subsurface corrosion film resistance (R f1, R f2) and charge transfer resistance (R t) were all lower than those found in samples treated in only CO2 or O2 atmospheres. The double-layer capacitance (C dl) and corrosion film capacitance (C f1, C f2) were higher for the sample treated in the combined O2 and CO2 atmosphere than for those treated in only CO2 or O2 atmospheres. The resistance to formation of a corrosion film on the 3Cr steel in the combined O2and CO2 atmosphere was significantly lower than in the CO2 only atmosphere. The corrosion mechanism of 3Cr steel is proposed: In the O2 and CO2 environment the corrosion is proceed by the formation of several corrosion products causing a loose film to develop. The Cr(OH)3 layer which can greatly improve the protection of the corrosion film formed in a CO2 only corrosion environment is not found in the O2 and CO2 environment, thus promoted the corrosion process of hydrogen evolutional and oxidation corrosion in acid medium.