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 t he laboratory at relative humidities of 65%, 80% and 90% in unpolluted atmospheres and at a relative humidity of 80% in atmospheres containi ng HCl and SO2 pollutant gases. In order to investigate the effect of atmospheric particulates on corrosion rates, metal samples were contam inated with three coal and three oil fly-ashes from different industri al sources. Control specimens were either uncontaminated or contaminat ed 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 c orrosion rates probably due to differential aeration effects and an in creased local time-of-wetness in the vicinity of the beads. In pollute d 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 poll utant. 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 th e atmospheric corrosion rates of metals are dependent on the conductan ce of the thin-film surface electrolyte and that the first-order effec t of contaminant particles is to increase solution conductance and hen ce corrosion rates.