Morphology, sulfide scales

On the other hand, morphological changes can occur on the minute scale [8], or transformations during activation of a catalyst (temperature-programmed reaction/ reduction/sulfidation), ignition of a reaction, or oscillations can even occur on the subsecond timescale [11, 14, 15],... 

There has been relatively little work published on the reaction of titanium aluminides in atmospheres other than air or oxygen. Niu et al. [96] studied the reaction of Ti-25Al-llNb in a simulated combustion atmosphere (N2+1%02+ 0.5%SO2) with and without surface deposits of Na2S04-t- NaCl at temperatures between 600 and 800°C. Exposures in the absence of surface deposits resulted in reaction rates similar to those described above for simple oxidation. The rates in the presence of the deposits at 600 and 700 °C were initially rapid and then slowed markedly after 25 to 50 hours exposure. The rate at 800°C remained rapid with the kinetics being essentially linear. The major difference in the corrosion morphology at 800 °C was the presence of copious amounts of sulfides below the oxide scales. The authors postulate a mechanism of attack involving a combination of sulfidation-oxidation and scale-fluxing. 

In the oxidation, sulfidation, and so on of metals and alloys, solid reaction products are growing as film, scale, crystals, or in other morphologies, on the metal phase. A frequent case is the formation of a dense scale, separating the metal and gas phase. In this case, generally, a parabolic rate law is observed for the increase of film thickness x ... 

In addition to studies focusing exclusively on the catalyst surface, the catalyst support (when employed) can play a major role in enhacing the activity/selectivity via morphologic, electronic, and physico-chemical effects. These factors have been extensively explored in the case of thermochemical heterogeneous reactions where a variety of compounds and structures have been successfully used on an industrial scale as catalyst supports (e.g., oxides, sulfides, meso- and microporous materials (molecular sieves), polymers, carbons [251-256]). In electrocatalysis, on the other hand, the practical choice of support in gas diffusion electrodes has been largely limited thus far to carbon black particles. The high electronic conductivity requirement, combined wifli electrochemical stability and cost effectivness, imposes serious restrictions on the type of materials that could be employed as supports in electrocatalysis. 

Alloys generally rely upon an oxidation reaction for the formation of a protective scale that will improve the corrosion resistance to sulfidation, carburization, and the other forms of high-temjjerature attack. The properties of high-temperature oxide films, such as their thermodynamic stability, ionic defect structure, and detailed morphology, therefore play a crucial role in determining the oxidation resistance of a metal/alloy in a specific environment. 

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