Experimental Determination of the Deliquescence Relative Humidity and Conductivity of Multicomponent Salt Mixtures

ABSTRACTThe conductivity of hygroscopic salt deposits containing Na+, K+, NO3– and Cl– ions was measured in air as a function of relative humidity at constant temperatures. The deliquescence relative humidity (DRH) of multicomponent salts containing Na+, K+, NO3– and Cl– was also determined experimentally. The results of the conductivity experiments show that the conductivity of initially dry salt deposits start to increase after reaching a relative humidity value that is 15 to 20% lower than the DRH of the salt. When the DRH is reached, the conductivity increases dramatically as the salt dissolves and transforms into a saturated aqueous phase. The increase in conductivity at humidities below the DRH is attributed to the adsorption of water on the surface of the salt particles. Because of the increase in conductivity, the initiation of aqueous corrosion of metals in contact with hygroscopic salts may occur at a relative humidity much lower than the DRH of the salt. Thus, the onset of aqueous corrosion of metallic nuclear waste package and the drip shield may be earlier, the duration may be longer, and the temperature at which it occurs may be higher than assumed based on the DRH of the salt. The results of the DRH experiments show that the DRH of a salt mixture is usually significantly lower than that of any of its component pure salt.