Effects of minor alloying additions on the formation of protective scales under sulfidizing conditions at 700C. Final report.

摘要

Effects of Al, Si, Mn, Y, and Hf on the formation of protective scales have been investigated in environments simulating substoichiometric combustion of high-S coal, and an oxidizing flue gas. Minor additions were made to three clean alloys bases: Fe-25Cr, Fe-25Cr-20Ni, and Fe-25Cr-6Al, by conventional alloying, and by powder metallurgical routes involving rapid solidification or high-energy milling. Scales formed at 700(degree)C were compared with those formed on the unalloyed base alloys, in terms of weight change, scale morphology and composition. Alumina scales on the Fe-25Cr-6Al-base alloys proved the most effective as a group, with excellent protection being provided for more than 6000 h in aggressively sulfidizing conditions. Chromia scales formed on Fe-25Cr-base alloys were markedly more effective than those on equivalent Fe-25Cr-20Ni-base alloys prepared by conventional casting. Observations of initial stages of corrosion of the base alloys indicated that a major difference was that the protective scales formed on the FeCr alloy were of the M(sub 2)O(sub 3)-type, whereas those that led to inferior protection on the FeCrNi alloy were of the M(sub 3)O(sub 4)-type, and led to the inclusion of higher levels of Fe in the nominally protective scale. It is postulated that alloy modifications that can promote the exclusive formation of M(sub 2)O(sub 3)-type scales alloys should also enhance their sulfidation resistance. 23 refs., 107 figs., 16 tabs. (ERA citation 16:000089)