Refractory metals sulphidation

On the other hand, refractory-metal sulphides are both very stable and slow growing. This has been addressed by Douglass and coworkers, who demonstrated that the addition of Mo and Mo plus A1 to nickel substantially reduced the parabolic rate constant for sulphidation, by about five orders of magnitude for the composition Ni-30 wt% Mo-8 wt% Al and, in the case of iron alloys, by six orders of magnitude for the composition Fe-30 wt% Mo- 5 wt% Al. In these cases the scales formed were AI0.5M02S4, which gave excellent resistance to sulphidation, but would form molybdenum oxides in oxidizing atmospheres. ... 

Hydrotreatments several studies have made it possible to detect some interesting potential among certain metal sulphides dispersed in the zeolites to desulphurize and denitrogenate certain refractory heteroatomic molecules (certain alkyldibenzothiphenes for example). One of the difficulties that has to be solved is to avoid excessive craeking of the hydrocarbons to be purified. 

The subject of the sulphidation of metals was reviewed earlier by Strafford, who has also carried out substantial research into the subject, with particular emphasis on the sulphidation of refractory metals. In the case of the refractory metals, the sulphides are very stable and form scales of tight stoichiometry and, consequently, low rates of growth. 

Figure 6.1 Comparison of typical sulphidation and oxidation rates of several transition and refractory metals. (Most of the data are from Mrowec. )...

In contrast to the sulphides of most of the transition metals, sulphides of the refractory metals have quite tight stoichiometry, similar to Cr203, although, in the cases of the refractory-metal sulfides and oxides, the defects appear on the anion sub-lattice. Figure 6.1 compares the rates of oxidation and sulphidation for several of the transition and refractory metals. The low rates of sulphidation of the refractory metals are thought to be due to the low concentrations of defects in the sulphide structures. 

Thoria has been used for crucibles for the melting of refractory metals but it has poor thermal-shock resistance. Thorium Sulphides. Three sulphides have been reported Th4S7, ThjSg and ThS. Crucibles made of these sulphides have been used as containers for molten Ce. 

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