Some effects of minor alloying additions on the formation of protective scales under sulfidizing conditions at 700 °C

Abstract

Results are presented from an investigation of the effects of minor additions of elements such as aluminum, silicon, manganese, yttrium and hafnium on the formation of protective chromia scales on model alloys in an aggressive, sulfidizing environment. The additions were made to a clean master alloy, Fe25Cr20Ni, by conventional alloying, and by powder metallurgical routes involving rapid solidification or high-energy milling. The scales formed by exposure to an H 2H 2OH 2S mixture at 700 °C, in which the oxygen and sulfur partial pressures were 10 −20 and 10 −8 atm or < 10 −23 and 10 −8 atm, respectively, were compared with those formed on the unalloyed master alloy and on a clean Fe25Cr alloy, in terms of weight change, scale morphology and composition. The chromia scales formed on the Fe25Cr alloy were more effective than those on the Fe25Cr20Ni base alloys prepared by conventional casting. Observations from short term exposures indicated that a major difference between the Fe25Cr and Fe25Cr20Ni alloys was that the scale formed on the former was a chromium rich M 2O 3 type, while that on the latter was a mixture of M 2O 3 and M 3O 4 types in which sulfur appeared at a relatively early stage. The scales formed on Fe25Cr20Ni alloys containing silicon or Y 2O 3, which exhibited superior oxidation resistance, contained a higher proportion of M 2O 3-type scale and had lower Fe:Cr ratios compared with the base alloy.