At the “cold-end” of biomass and coal-fired plants, with decreasing temperature and increasing relative humidity (RH) of the flue gas, moisture absorption by the hygroscopic deposits may electrochemically cause corrosion of the metal parts, especially during the downtime. In this paper, 10 deposit samples were collected from different sections of biomass-fired and coal-fired plants. The hygroscopicity of the deposit samples was studied by electrochemical impedance and gravimetric water vapor absorption techniques, and representative salt powders and their binary mixtures were also tested for reference. The critical relative humidity (CRH) level in the water absorption process and water absorption capacity at varying RH values were investigated, which were two aspects of the hygroscopicity of the tested salt powders. Deliquescence was the determining factor in dissolution of the soluble salt powders, including CaCl2, MgCl2, KCl, and K2SO4. Meanwhile, CaSO4 and CaCO3 powders formed a small amount of solution mainly through the crystal hydration and capillary condensation, respectively. The deposit samples, as mixtures of various components, usually did not show a specific CRH but rather absorb moisture gradually. Dissolution of the deposits is typically initiated from 15%RH to 20%RH at 25 °C or from an even lower RH at higher temperatures, which could be greatly mitigated by some highly hygroscopic components, like CaCl2 and MgCl2. Corrosion resulting from the hygroscopic deposits could be alleviated by reducing the chloride content in the flue gas and raising the metal surface temperature.