SOME EFFECTS OF MINOR ALLOYING ADDITIONS ON THE DEVELOPMENT AND BREAKDOWN OF PROTECTIVE OXIDE SCALES

The development and breakdown of scales formed on Fe-25Cr and Fe-25Cr- 20Ni alloys have been studied in O-S environments, mainly at 700 degre es C. The chromia scales formed on Fe-25Cr were markedly more effectiv e than the scales formed on Fe-25Cr-20Ni. A major difference was that the protective scales formed on the FeCr alloy were predominantly M(2) O(3) type, with low levels of Fe, whereas those that led to inferior p rotection on the FeCrNi alloy were predominantly M(3)O(4) type, with h igher levels of Fe in the nominally protective scale. The influence of small additions of Y, Hf, Ce, Nb, Mn, Pt, Al and Si on the formation of protective scales on these alloy bases was investigated. Only Si, a nd possibly small additions (2%) of Al, as well as Al2O3 or Y2O3 dispe rsions appeared to exert any obvious beneficial effect. Additions made through ion implantation did not result in improved protective behavi our in any of the alloys. It was postulated that alloy modifications t hat can minimize the Fe content of the scale, probably by promoting th e exclusive formation of M(2)O(3)-type scales, should enhance the sulf idation resistance.