Iron third-phase formation

Uranium(VI) is present in solution as the uranyl sulfate anion. The extraction of uranium(VI) by amines occurs in the order of tertiary > secondary > primary amine. The extraction of iron(Ill) occurs in the reverse order, so tertiary amines represent an obvious choice of extractant The tertiary alkyl amine sold as Alamine 336 or Armeen 380 is widely nsed, nsnaUy in conjunction with an alcohol phase modifier (such as isodecanol) to prevent the formation of a third phase and inhibit the formation of emulsions. 

A third example is the water-gas-shift reaction catalysed by iron catalysts. The active phase is an intermediate iron oxide (Fe304), distinctly different from the manufactured catalyst (Fe203). So, mild reduction should be carried out. In optimising the pretreatment procedure it should be realized that over-reduction is to be avoided because of the danger of carbon deposition and methane formation (highly exothermic). It has been found that a well-controlled reduction in a H2/H2O mixture is possible, whereas the use of H2 and steam separately should be avoided. 

A third explanation of the development of porosity assumes that selective dissolution of the less noble metal creates a high vacancy concentration near the surface. The coalescence of these vacancies generates stresses, which may become sufficiently high to cause formation of cracks (c). The vacancy concentration near the surface increases with potential due to the higher dissolution rate, and so does the tensile stress. Eventually, at a sufficiently anodic potential this leads to cracking. The cracks expose new alloy surface and permit dissolution to continue. Figure 7.37 shows the cracked surface of a zinc-iron alloy (5-phase, 6.9%Fe) after polarization in an acid sulfate solution under conditions where zinc selectively dissolved [15]. 

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