A study of the wetting of metal surfaces in order to understand the processes controlling atmospheric corrosion

Abstract

This paper explores the simple principle that a metal surface wets when the surface relative humidity (RH) exceeds the deliquescent RH (DRH) of any salts on the surface. Data from field exposures across 19 sites in China, the Philippines, Indonesia, and Australia is used to determine the conditions under which openly exposed surfaces wet. At each site, surface temperature (of a zinc plate), ambient RH, sensor wetness, airborne salinity, and gaseous SO x and NOx were determined over a one-year period. In conjunction with these microclimate measures, the chemistry of airborne and deposited aerosols, as well as rainwater, were measured. Complimentary data from an environmental scanning electron microscope are presented in which salts derived from the evaporation of sea salt are rewetted. Using all of this data, an assessment of the probable contaminants controlling sensor wetting at each site is made. It is found that sites with similar International Standard Organization, (ISO) 9223, classifications in terms of industrial and marine airborne pollutants show similar surface contaminants and wetting characteristics. It is proposed that dominant contaminates can be identified for each ISO classification that are consistent with the general principle that wetting occurs when surface RH exceeds the DRH of the salts making up the contaminates. These salts can change from day to day due to the continual change in the composition of the contaminates and the ongoing homogenization of previously deposited salts through chemical reaction between salts and with the surface. Rain events usually clean the surface and start the cycle over again. The application of these findings to process models of corrosion is discussed, while generalized rules for predicting surface wetting based on climate data are proposed. It is found that these generalized rules predict total time of wetness to a high degree of accuracy. Show more