Classification of atmospheric corrosivity in humid tropical climates

Abstract

International Standard ISO 9223-92 permits the determination of atmospheric corrosivity based on either corrosion rates determined from annual tests on several standard metals or on the time of wetness (TOW) and pollution levels of SO₂ and Cl⁻. Experience shows that the corrosion rate is influenced by the exposure angle of plate specimens and specimen form (flat or wire open helix). These two factors, which are not discussed in the standard, give rise to different time of wetness values, amounts of incident sunlight, temperatures, and quantities of deposited corrosive contaminants. Moreover, tropical humid climates have large annual time of wetness values (4500-8400 h), and in marine-coastal environments most of this occurs in the temperature range 25-30°C, which accelerates the corrosion process. Owing to the higher airborne salinity in these atmospheres, a decrease in the critical value of relative humidity used for the calculation of TOW is recommended. As an example, the monthly and annual mass losses for copper and low carbon steel, corroded in the tropical humid climate of the Yucatan Peninsula, Mexico, are discussed as a function of TOW: SO₂ and chloride contaminant concentrations, exposure angle, and specimen shape.