PATTERNS OF ATMOSPHERIC CORROSIVITY

Abstract

The corrosion of metals exposed to the atmosphere is expensive to our societies in terms of aesthetics, safety and functionality. The factors that influence atmospheric corrosivity are relative humidity, pollutants and particulates such as salt aerosols. Corrosivity measurements near a de-iced highway and a marine coast revealed that corrosion rates can vary dramatically between locations that are only meters apart. In a study of the effects of wind sheltering, there was a 34-fold difference in corrosivity between the most wind-protected and the least wind-protected site even though each set was exposed to the same relative humidity. De-iced highways in the winter produce zones where the corrosivity is as high as that near a salt-water body and extend beyond 150 m from the road edge. Corrosivity measurements near hills and a radar tower revealed the effects of elevation above the ground and topography on local corrosivity. The airflow pattern around objects that significantly obstruct wind is very complicated but one conclusion is that the wind flow speed and turbulence are highest near the top. Thus aerosol deposition, and therefore corrosion, rates are higher near the top of obstructions to wind such as hills and buildings. The corrosivity pattern around two buildings near a marine coast was quite nonuniform due to differences in wind speeds. The principles of mass transfer and deposition of pollutants and aerosols, namely convection and turbulent diffusion, can form a theoretical framework for interpreting and predicting atmospheric corrosivity. The implications for the design of structures is that local corrosivity rates can be predicted based on simulated airflow patterns.