Sulfur dioxide (SO2) is one of the principal impurities in carbon dioxide (CO2) streams captured from coal-fired power plants, and the synergistic effect of CO2/SO2 on the corrosion of steels has attracted significant recent attention. In this work, the corrosion mechanism of valve steel X80CrSiMoW152, in a mixed CO2/sulfurous acid aqueous environment, was investigated. Experiments were conducted by adding different concentrations of sulfurous acid to a CO2-saturated solution at 25 degrees C under different pH conditions. The effect of sulfurous acid on the corrosion mechanism was examined by potentiodynamic scanning. Results revealed that for the anodic reaction, the presence of 100 ppmw sulfurous acid promoted the dissolution of steel in CO2-saturated solutions. In the cathodic polarization curve, a "second-wave" phenomenon was observed before the second limiting current for cases with sulfurous acid at pH 3.0 and for the case with 1,000 ppmw sulfurous acid at pH 2.0. These new "waves" were potentially caused by the direct reduction of sulfurous acid or bisulfite ions on the steel surface. The adsorption of sulfurous acid or bisulfite could also impede the reduction of hydrogen by competitive adsorption, and by a decrease in the cathodic-reaction area for the reduction of hydrogen ions. (C) 2017 Elsevier Ltd. All rights reserved.