The effect of fly-ash particulates on the atmospheric corrosion of zinc and mild steel

Abstract

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 SO 2 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 μm. 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 SO 2 pollutant and fly-ash contamination, indicating that effects due to catalytic oxidation of SO 2 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.