Effect of Relative Humidity on the Wettability of Steel Surfaces

Abstract

Abstract Metals and metal oxides are usually classed as high-energy solids since in the ultra-clean state they have surface free energies typically greater than 500mJ/m2. However, Bernett and Zisman1 measured the critical surface tension, yc , of wetting of various hydrophilic, high-energy, metal oxides and pure metals at two extreme humidities namely 0.6% r.h. and 95% r.h., and found that the surfaces were converted to ones of low yc (i.e. about 46 and 38 mJ/m2 respectively) and, further, the yc values were almost independent of the solid surface. They ascribed these observations to the adsorption of water and proposed that the surface free energy of any clean, high-energy surface after exposure to an atmosphere containing water vapour is mainly dependent upon the surface concentration of adsorbed water. Recent studies2,3 using surface-specific analytical techniques have indicated that the surfaces of high-energy solids are not only covered by adsorbed water but also by other contaminants such as nitrogen and organic adsorbates such as hydrocarbons.