The development of corrosion scale of 3Cr steel in an aqueous environment containing 95% CO2-5% O-2 was characterized and analyzed by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), X-ray diffraction (XRD) and in situ electrochemical impedance spectroscopy (EIS). The results show that the corrosion of 3Cr steel was promoted by dissolved oxygen. Fe(OH)(3) precipitation formed at local areas, leading to the nonuniform distribution of Cr element and pitting corrosion. With the increase of corrosion time, Cr-rich area formed at the bottom of pits, where the pitting corrosion process was inhibited. The importance of Fe(OH)(3) and Cr-rich area on the formation of corrosion pits and pits development was emphasized. (C) 2014 Elsevier B.V. All rights reserved.