To estimate the concentrations of the electrolyte layers formed on metal surfaces in an atmospheric environment, relative humidity (RH) in equilibrium with various concentrations of strong electrolyte solutions were calculated through available thermodynamic data. The activity coefficient of water (f_w(X)) for the solution with molar fraction of water (X) can be given as a function of ionic strength of the solution, which was determined by using the mean activity coefficients data found in the literature for electrolytes in solution. RH values for solutions with various concentrations of sea salt were also calculated as those for solutions with NaCl-MgC'U mixed electrolytes. In this case, calculated RH values also fitted well with measured ones. Thicknesses of water films containing those strong electrolytes were also estimated by using specific gravities of the solutions. When the amount of sea salt, IFs, is lg/m2, NaCl is precipitated and the thickness of water film, d, remains at 0.3-0.7 urn, the RH is lower than 75%. On the other hand, NaCl is dissolved completely and the d value reaches 3~3Oum when Rl-I >75%. It was confirmed that the d value was in proportion to W_s when W_s≥10~(-2)g/m~2, Based on exposure test results with carbon steels in the constant relative humidity chamber, corrosion rate of carbon steel, CR, increases with increasing d when d<56μm. The value of CR reaches up to the maximum of CR=0.28mm/y at d=56p.m, and it takes the constant value of CR=0.16mm/y when d≥170μm.
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