High temperature interactions of metal oxides and carbides with ionic salts

Na2SO4 and NaCl are the two important ionic salts which are actively involv ed in inducing hot corrosion reactions. The aggressiveness of these salts t owards superalloys at high temperatures is very well documented. Metallic c arbides and/or oxides form the important constituents of high temperature m aterials; oxides are also integral constituents of protective scales formed on the superalloys. During hot corrosion reaction, these constituents reac t with molten Na2SO4 or molten NaCl to propagate the reaction. However, the chemistry of such hot corrosion reaction is not very well understood. This paper presents the results of important studies carried out on this subjec t in recent years. High temperature interactions of metal oxides, namely Co3O4, NiO, Al2O3, Cr 2O3, Fe2O3, SiO2, TiO2, ZrO2, Nb2O5, Ta2O5, MoO3 and WO3, and carbides, nam ely Cr7C3, Fe3C, TiC, ZrC, NbC, TaC, MoC, WC, VC and HfC, separately with s odium sulphate (Na2SO4) and sodium chloride (NaCl) have been studied in the temperature range 900-1200K. At these temperatures, reaction kinetics (wei ght change vs time) and variation in weight with mole fraction of salt have been measured. The reaction products were identified by X-diffraction (XRD ) and energy dispersive X-ray analysis (EDAX). The formation of products wa s also investigated by thermodynamic computation of free energies of the re actions and the study of relevant equilibrium phase diagrams. The multiphas e structures of the reaction products were studied by metallographic analys is. The solubility measurements of the reaction products were carried out t o determine the soluble species in aqueous solutions. The weight losses dur ing Interaction are interpreted in terms of evolution of gases like CO2/SO2 or Cl-2 or in some cases to volatilisation of oxide or chloride, whereas w eight gains are explained in terms of the formation of sodium metal oxide, metal sulphide or metal chloride. The mechanism(s) of the high temperature interaction of metal oxides and carbides with Na2SO4 and NaCl has been disc ussed, and experimental evidence has been presented to support the proposed mechanism. In general, NaCl appears to be more reactive than Na2SO4.