Alloy-induced acidic fluxing

Acidic fluxing may be further subdivided into alloy-induced acidic fluxing, in which the acid conditions in the salt are established by dissolution of species from the alloy, which react strongly with Na20, or the ions, and gas-induced acidic fluxing in which the acid conditions are established by interaction with the gas phase. 

In alloy-induced acidic fluxing, elements such as molybdenum, tungsten, or vanadium in the alloy cause deposits to become acidic as oxides of these elements are incorporated into the deposits. Typical reachons considering molybdenum are given... 

Figure 8.22 Comparison of the isothermal hot corrosion of Na2S04-coated Ni-8 wt% Cr-6 wt% Al and Ni-8 wt% Cr-6 wt% Al-6 wt% Mo. Both alloys undergo basic fluxing, but Ni-8 wt% Cr-6 wt% Al-6 wt% Mo eventually undergoes alloy-induced acidic fluxing.

In discussing the specific case of the high-temperature corrosion of alloys with deposits containing vanadium, the corrosion process will depend on the source of the vanadium. If the vanadium comes from the alloy, then the corrosion process will be similar to that described previously for molybdenum under alloy-induced acidic fluxing. When the vanadium arrives via deposition from the combustion of the fuel, the vanadium component of the fuel is oxidized in the combustion... 

Figure 8.40 Schematic diagram to show the relationship of the different hot corrosion mechanisms as a function of temperature and SO3 pressure, (i) Type II, gas-phase induced acidic fluxing, (ii) At high SO3 pressures (PSO3 > 10 atm) pronounced sulphide formation accompanied by oxidation of sulphides and fluxing reactions, (iii) Type I (alloy-induced acidic fluxing basic fluxing, sulphidation).

Birks and Meier, 1983). The first situation, which is also called alloy induced acidic fluxing, is generally due to the dissolution of oxides of the refractoiy metals (Mo, W, V) in the Na2S04 melt. In this case the oxygen ion concentration in the salt melt is reduced, which makes the melt more acidic and the dissolution of oxides can occur according to Reaction (2-48). 

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