Morphologies of oxide growth and exfoliation in superheater and reheater tubing of steam boiler

The intent of this paper is to present the current state of knowledge concerning the development of the oxide morphologies associated with scales grown in steam on alloys used in the superheater and reheater circuits of conventional fossil and heat recovery steam generator plants, and the influence of those morphologies on the scale exfoliation behaviour. Scale development is described for three classes of alloy: standard 1 – 9 wt%Cr ferritic steels, using T22 as an example; the ferritic-martensitic steel T91; and the 300-series austenitic steels, represented by TP304 and TP347, for which coarse-and fine-grained versions are examined. Detailed information on oxidation mechanisms is combined with extensive knowledge of alloy behaviour in boiler service in an effort to allow engineers and materials scientists to gain an understanding of similarities and differences, as well as to appreciate the likely contributions of major parameters that influence oxide growth and exfoliation behaviour.