Characteristics of corrosion scale of 3CR steel at high temperature and pressure in an O2 and CO2 environment

Abstract

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 (Rf1, Rf2) and charge transfer resistance (Rt) were all lower than those found in samples treated in only CO2 or O2 atmospheres. The double- layer capacitance (Cdl) and corrosion film capacitance (Cf1, Cf2) 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 O2 and 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. © Editorial office of Acta Physico-Chimica Sinica.