AN ELECTROCHEMICAL STUDY OF THE ATMOSPHERIC CORROSION OF MILD-STEEL .3. THE EFFECT OF SULFUR-DIOXIDE

The effects of atmospheric SO2 pollutant on increasing the atmospheric corrosion rate of iron are well known. Several mechanisms have been p roposed for this effect, one of which, due to Rozenfeld, suggests that SO2 in solution is cathodically reduced to the S2O42- ion more rapidl y than reduction of oxygen, stimulating the cathodic kinetics. These r esults were valid for concentrations of SO2 (0.1-1%), greatly in exces s of industrial levels of pollution. Inspection of thermodynamic data indicates that sulphite or bisulphite may be reduced to all other sulp hur oxidation states in the potential range -1200 mV to -600 mV(SCE) b ut is stable to oxidation to sulphate. This work investigates the effe ct of SO2 pollutant at a gas concentration of 1 ppm, using cyclic volt ammetry in a thin-film electrolyte on iron. Results show that although the reduction of bisulphite to thionite, and its re-oxidation, occurs during polarisation in an oxygen-free atmosphere between -600 and -12 00 mV(SCE), no evidence for such a redox process was found when the ex periment was performed in air. Only sulphate species were found in the electrolyte film in air, implying relatively rapid oxidation of disso lved SO2 to sulphate in the presence of ferric species as a catalyst. It can be concluded that the Rozenfeld mechanism of the effect Of SO2 is invalid. As an alternative, SO2 may affect the anodic reaction (per haps by delaying passivation) or may increase the surface conductivity and acidity thereby reducing resistive losses in the electrolyte.