THE EFFECT OF FLY-ASH PARTICULATES ON THE ATMOSPHERIC CORROSION OF ZINC AND MILD-STEEL

The atmospheric corrosion of zinc and mild steel was investigated in the laboratory at relative humidities of 65%, 80% and 90% in unpolluted atmospheres and at a relative humidity of 80% in atmospheres containing HCl and SO2 pollutant gases. In order to investigate the effect of atmospheric particulates on corrosion rates, metal samples were contaminated with three coal and three oil fly-ashes from different industrial sources. Control specimens were either uncontaminated or contaminated with small glass beads of similar size to the fly-ashes <45 mum. In unpolluted atmospheres, particulate contamination increased corrosion in approximate proportion to the quantity of leachable ionic species present in the fly-ash. Additionally, glass beads slightly increased corrosion rates probably due to differential aeration effects and an increased local time-of-wetness in the vicinity of the beads. In polluted environments, the corrosion rates of the specimens increased and the additional effect of fly-ash contamination on the corrosion rates was consequently decreased in proportion to the presentation rate of pollutant. There was no significant additional increase in corrosion rates with SO2 pollutant and fly-ash contamination, indicating that effects due to catalytic oxidation of SO2 to sulphuric acid or sulphates were not significant. Overall, this study provides strong evidence that the atmospheric corrosion rates of metals are dependent on the conductance of the thin-film surface electrolyte and that the first-order effect of contaminant particles is to increase solution conductance and hence corrosion rates.