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The interaction of different transition metal oxides such as TiO2, ZrO2, Nb2O5, MoO3 and WO3 with Na2SO4 in the temperature range 900-1200 K in flowing oxygen has been studied. The oxides chosen for the studies are initial scaling products during the oxidation of industrial alloys and are invariably involved in hot corrosion reactions in the presence of a molten salt. Thermogravimetric studies were carried out by measuring weight change as a function of time and molar fraction of Na2SO4 in the reaction mixture. The different constituents in the reaction products were identified by X-ray diffraction analysis and the morphologies of the reaction products were discussed on the basis of metallography, scanning electron microscopy and energy dispersive X-ray analysis. Quantitative estimation of soluble metal has also been carried out with the help of an atomic absorption spectrophotometer. Information regarding the formation of different species in the reaction product is also obtained from the study of relevant thermodynamic phase stability diagrams.
The high temperature reaction product usually contains a three-phase structure, namely sodium metal oxide, metal oxide and metal sulfide. The concentrations of the various constituents vary according to the composition of the interacting species, e.g. metal oxide and Na2SO4.
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