Design of sulphidation resistant alloys

Abstract

The interaction of metallic materials with sulphur and sulphur bearing gases at elevated temperatures leading to the formation of sulphide corrosion products – sulphidation – is generally an extremely rapid process, much more so than oxidation. Conventionally developed high temperature alloys with adequate oxidation performance generally have poor resistance to sulphidation and the design of alloys or coatings to withstand such aggressive environments represents a major technological challenge. The basic process and mechanisms of sulphidation are reviewed and contrasted with oxidation, leading to an enunciation of the principles underlying the development of sulphidation resistance in model alloys involving ‘barrier layer’ formation of protective sulphide(s) to inhibit the usually extraordinarily fast diffusion processes responsible for rapid wastage. Underlying principles for the development of sulphidation resistant alloys are shown with reference to the behaviour of various experimental model MX type alloys in which the limitations of addition elements (X) such as chromium or aluminium, used successfully to control oxidation, are emphasised. The potential offered by the use of various group IV–VI semirefractory and refractory elements such as Ti, Zr, Hf, V, Nb, W, and Mo is discussed. The actual greatly enhanced performance of various experimental alloys containing such elements is illustrated and interpreted, leading to suggestions for projected future areas of research.MST/1078