A review of the oxidation behaviour of structural alloys in steam

The focus of this review is the state of knowledge of the oxidation behaviour in steam of alloys with potential for use as pressure parts in steam boilers. The rate of oxide growth on steam-touched surfaces and the characteristics of that oxide are of increasing interest as the quest for improvements in cycle efficiency leads to progressively higher operating temperatures and pressures. The consequences of increased rate of growth of these oxides are of concern because of implications for tube overheating and oxide exfoliation. Mitigation of such problems requires a mechanistic understanding of the influences of alloy composition and microstructure, and especially of the evolution with time of specific scale structures. Similarly, the relative effects of factors such as time, temperature and operating parameters must be understood. The oxidation behaviour of the class of ferritic steels that forms the bulk of the heat transfer surface in steam boilers is of particular importance since alloys in the range 9-12% Cr (% in alloy compositions signifies weight percentage, unless indicated otherwise) are close to a transition from oxidation behaviour based on relatively thick Fe-based scales to the formation of much thinner, Cr-rich oxides. For austenitic steels protective behaviour in steam depends critically on the rapid development of a continuous Cr-rich oxide layer, otherwise oxide growth rates similar to the ferritic steels may result. Understanding the interplay among compositional and microstructural requirements for strengthening and oxidation resistance, and their influence on the rate and mode of scale evolution is key to the most effective application of these alloys. The oxidation behaviour of high-temperature Ni-based alloys in steam has received relatively little attention, but the broad range of alloying additions considered, compared to austenitic steels, has the potential to contribute in different ways to the scale morphologies and oxidation behaviour. Underlying these interests is the apparently significant contribution to oxide growth in steam from inward transport of oxidant species that likely involve hydrogen. The particular species involved and their roles in the oxidation process are expected to exert a large influence on the oxide morphologies developed, while the fate of any hydrogen released in the alloy is a further topic of particular interest.