Abstract:
Previous studies of the adsorption characteristics of iron with respect to sulphur dioxide gas in air at low concentrations have shown that the uptake of this gas is not uniform. Instead, it accumulates at seemingly random sites on the surface. Moreover, the rate of uptake is a function of the relative humidity of the air to which such an iron surface is exposed.
This thesis examines the role of water in this uptake phenomenon in terms of both the atmospheric water vapour concentration and the equilibrium surface water mass found on iron samples. Data is presented which measures the equilibrium surface concentration of water on iron samples. Correlation of regions of high water mass on iron surfaces with regions of high sulphur dioxide uptake is also presented and discussed. Further experimental data concerns sulphur dioxide adsorption and desorption characteristics of synthetically prepared iron oxides and oxyhydroxides likely to be found on iron surfaces. Experiments also include an examination of a variety of iron surfaces using ESCA.
Finally, a description of iron corrosion films is presented which is consistent with the experimental evidence derived from this study and other literature information. It appears that the major surface species of this corrosion film is αFeOOH. This is the species from which most of the sulphur dioxide desorbs when iron with this gas adsorbed to its surface is exposed to uncontaminated air. Sulphur dioxide adsorbs on to γFe2O3 at a much higher rate than the other iron oxides and oxyhydroxides examined. Moreover, it appears that it is the presence of this compound in iron corrosion films which is largely responsible for the dependence of sulphur dioxide uptake by iron surfaces on the relative humidity. It is proposed that the regions of high sulphur dioxide uptake on iron surfaces observed in earlier studies are regions of γFe2O3. A model based on sulphur dioxide/water clusters formed in the atmosphere is presented as being the principal vehicle for the transport of sulphur dioxide on to iron surfaces.