Sulphur penetration through oxides

In the case where B is present at concentrations sufficiently high to form a protective scale of B O, low reaction rates would be expected until the scale is penetrated by sulphur, as discussed above. If penetration occurs through a scale of Bj O, which is assumed to be the most stable oxide, then the sulphur species will arrive at a metal-scale interface of low oxygen potential, maybe sufficiently low to generate a sulphur potential high enough to sulphidize A or B from the alloy, or both. 

Of course, if the protective scale of chromia or alumina is not penetrated by SO2, sulphide cannot form at the scale-metal interface. This was found for Ni-20 wt% Cr, Co-35 wt% Cr and Fe-35 wt% Cr alloys exposed to pure SO2 at 900 °C and emphasizes the resistance of a chromia scale to permeation. On the other hand, alloys in the Fe-Cr-Al, Ni-Cr-Al and Co-Cr-Al systems were exposed to atmospheres in the H2-H2S-H2O system. These atmospheres had compositions that supported the formation of chromia or alumina together with the sulphides of Fe, Ni and Co at the scale-metal interface. In these cases, a protective layer of chromia or alumina that formed initially was penetrated by sulphur to form iron, nickel, and cobalt sulphides at the scale-metal interface. Furthermore, iron, nickel, and cobalt ions apparently diffused through the oxide layer to form their sulphides on the outside of the protective scale. Thus the original protective scale was sandwiched between base-metal sulphides. 

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