The electrochemical impedance spectroscopy (EIS) was used to study the characteristics Of CO2 corrosion of N80 and 4Cr steels with corrosion scales. The results indicated that CO2 corrosion scale on tube steel could prevent the rate of mass transfer remarkably, corrosion rate was controlled by ions diffusion in corrosion scale, which led to finite length diffusion impedance occurred in electrochemical impedance spectra. Additionally, pitting of N80 steel could lead to additional capacitive reactance in impedance spectrum. The ion diffusion coefficient in corrosion scale and porosity of corrosion scale could be calculated by Warburg impedance coefficient, the results shown that the value of H+ diffusion coefficient in N80 and 4Cr corrosion scale is (3.46 and 1.76) x 10(-10) m(2) s(-1), respectively. The protective ability of 4Cr corrosion scale was better than that of N80 corrosion scale. (c) 2005 Elsevier B.V. All rights reserved.