ABSTRACT In certain regimes of atmospheric corrosion, the corrosion rate is limited not by electrochemical reactions but by the rate of mass transfer of pollutants. In these cases, amass transfer model that accounts for the transport of pollutants, such as a marine salt aerosol, provides a theoretical and predictive framework for assessing corrosivity severity. Such a model of the transport of a marine aerosol fairly near the ground and well within the planetary boundary layer was developed. The predicted aerosol concentration as a function of distance for 1500 m from a steady source was consistent with published data on steel corrosion and salinity rates near an ocean. Implications from the model regarding objects that are exposed to aerosol-containing wind include: (i) increasing wind speed increases the aerosol deposition rate and therefore the corrosion rate, (ii) objects that are in the lee of prevailing winds from an aerosol source will corrode faster than objects on the windward side of an aerosol source, and (iii) smaller objects can be expected to corrode faster because of a greater capture efficiency of salt aerosols.
INTRODUCTION Design factors such as proximity to a source of pollution, degree of wind exposure and an object?s size affect the rate of pollution deposition to an object because, in certain regimes of atmospheric corrosion, the corrosion rate is limited by the rate of mass transfer of pollutants and not by the rate of electrochemical reactions. In these cases, a mass transfer model that accounts for the transport of pollutants, such as a marine salt aerosol, provides a theoretical and predictive framework for assessing atmospheric corrosivity. A desire to understand the effects of distance from an aerosol source, object size, and wind speed and direction on the corrosivity of microenvironment prompted the development of a first-generation model of the transport and deposition of marine-type aerosols.
The corrosive effect of salt aerosols that are carried by the wind from salt-water bodies is well recognized. In the Pacer Lime 1algorithm, locations that are within 4.5 km from a sea are given the highest of four corrosivity ratings without any other considerations. In a statistical study, atmospheric damage functions for four metals were developed that each included a term for the chloride deposition rate2. The chloride deposition rate was either measured by a salt candle or calculated as wet deposition from rainfall rates and average chloride concentration in precipitation. Also, the 1S0 atmospheric classification algorithm requires a chloride deposition rate from salt candle measurements as well as time-of-wetness and sulfur dioxide measurements3.
However, there are limitations to the above approaches for characterizing local corrosivities. The Pacer Lime approach does not take into-account graduations in the effects of salt deposition within 4.5 km from a coastline or in the vicinity beyond 4.5 km. Published wet candle measurements are scarce relative to time-of-wetness data which can be fairly readily obtained from meteorological data. The use of wet chloride deposition rates from precipitation data, which is fairly accessible, is limited because it is only a measure of aerosol concentration and not deposition rate which depends on other factors such as wind speed.
Previous work has revealed the relationship between the mass transfer characteristics of pollutants to corrosion severity to some degree. An atmospheric corrosion study found that wire samples corroded at nearly twice the rate as sheet samples in a manner consistent with the turbulent mass transfer of gaseous pollutants. In another atmospheric corrosion study, the effects of dry dep