Atmospheric Corrosion Initiation by Sulphur Dioxide and Particulate Matter: II. Characterisation and corrosivity of individual particulate atmospheric pollutants

Abstract

This research has involved investigation of the simulated atmospheric corrosion of zinc, aluminium and especially iron in situations where particulate pollutants initiate attack at various relative humidities, in the presence and absence of sulphur dioxide. Metal surfaces holding small population densities of particulate material. collected at sites near well-defined sources in rural. industrial and marine locations. have been examined directly and after exposure in simple humidity cabinets. or in a specially designed cellfor in situ observation. Additionally. SEM/EDAX examination has assisted in identifiying the nature and corrosivity of the individual particles. The four most aggressive species were salt and salt/sand from marine or de-iced locations. the emitted ash from iron smelters. plume ashfrom municipal incinerators and coal mine dusts. In each case the corrosiveness was associated with the presence of chloride ions and/or sulphur compounds. Other particulate types. such as fly ashes. had more limited corrosivity. Thus. corrosion was promoted in the vicinity of coal ash in the presence of sulphur dioxide. whilst oil fly ash was relatively non-corrosive even in the presence of sulphur dioxide. Considering the much higher inorganic content of coal fly ash, and the greater porosity of oil fly ash. this suggests that the corrosion mechanism depends upon the catalytic promotion of sulphate formation from sulphur dioxide by inorganic species, such as Fe²⁺ and Mn²⁺ ions, which then promotes the anodic reaction, and not simply the absorbed presence of sulphur dioxide per see Chalk particles were non-corrosive even in the presence of sulphur dioxide, due perhaps to the removal of sulphur dioxide as insoluble sulphite/sulphate. The desirability of controlling the emission or distribution of particulates from certain plants is indicated.