The transport of sulphur through preformed Al 2O 3 scales on iron-base alloys in environments of high sulphur and low oxygen activities pertinent to coal conversion processes at high temperature

Abstract

Preoxidation of Fe-27% Cr-4% Al and similar alloys containing yttrium additions in H 2/H 2O gas mixtures to develop external Al 2O 3-rich scales improves considerably the subsequent sulphidation resistance on exposure to gaseous environments of high sulphur and low oxygen activities at high temperatures. However, breakdown of these oxide scales can occur eventually, particularly in the more aggressive environments, following localized transport of sulphur-containing species to the scale/alloy interface. The rate of such transport is determined by the mechanical integrity of the scale and the relative sulphur and oxygen activities in the environment. Sulphur penetration is relatively rapid through cracks in the scale but is much slower through other localized short-circuit diffusion paths. Scale breakdown then results from the progressive development of less-protective sulphide ducts from the alloy/scale interface to the scale/gas interface which provide easy diffusion paths for metal ions to the scale/gas interface, leading to formation and relatively rapid growth of sulphides at the surface.