Metals and alloys may experience accelerated oxidation when their surfaces are coated by a thin film of fused salt in an oxidizing gas. This mode of attack is called hot corrosion, and the most usual or dominant salt involved is sodium sulfate because of its high thermodynamic stability. The corrosive oxyanion fused salts exhibit an acid-base chemistry and are usually ionically conducting electrolytes, so that the corrosion attack must exhibit an electrochemical mechanism with certain characteristics analogous to aqueous atmospheric corrosion.
Hot corrosion may involve fluxing of the protective oxides as either acidic or basic solutes in the fused salt. The thermodynamic phase stability can be described by high-temperature Pourbaix-type diagrams, and these can be used to interpret the basicity-dependent solubilities. Measured solubilities for NiO, Co3O4, Al2O3, Fe oxides, SiO2, and Cr2O3 in fused Na2SO4 at 1200 K exhibit remarkable agreement with the expected behavior and permit the calculation of activity coefficients for the acidic and basic solutes.
Various transient electrochemical measurements have identified S2O72– (dissolved SO3) as the oxidizing agent that is reduced in hot corrosion attack. Electrochemical polarization studies of alloys in deep salt melts are discussed and compared with the local chemical changes observed for the polarization of a nonreactive Pt electrode. Mechanistic models for Types 1 and 2 hot corrosion are discussed.
