Spallation models and their relevance to steam-grown oxides

An overview is given of existing models of oxide spallation with particular emphasis on their relevance to the mechanical integrity of steam-grown oxide layers on alloys used in power generating systems. Although the bulk of the experimental and modelling work over the last decade has been undertaken on alumina-forming high-temperature alloys, the understanding obtained can be used advantageously to identify likely key factors affecting the mechanical response of alloys at lower temperatures. Various aspects are addressed, such as the mechanisms of cracking and spallation, but a central issue is whether creep rates in the multi-layered scale formed on ferritic alloys can ever be high enough to relax possible growth stresses and to permit the Critical Strain Energy Criterion to be used as a method for predicting oxide spallation.