Lifetime assessment: Modelling of steamside oxidation

The performance of materials in critical high temperature components of power and process plants is one of the major technical reasons limiting both the component life and process efficiency. New structural materials can be attractive in principle, but it has been challenging to simultaneously achieve good mechanical strength, oxidation resistance and cost-effective composition in the candidate alloys. The optimal material solutions for a given component depend on multiple factors, but one typical limiting factor is the waterside oxidation resistance at highest operating temperatures and pressures that extend to the supercritical (SC) or even ultra-supercritical (USC) range. To facilitate development of improved alloys for the new demanding applications, the methods of materials modelling can be useful to explore the effects of new ranges of chemical composition and fabrication routes. In the current paper oxidation modelling based on the concept of Cr and Ni equivalents is presented and applied to long term operation of superheaters made of nominally similar 11%Cr steel. The model quantifies the impact of the alloying elements on steam side oxidation to facilitate prediction of the oxide growth according to the alloy composition. The model can be used to predict the effect of variation of particular alloying elements, to explore the effects of alloying modification on performance for alloy development, and for life prediction when life is limited by water/steam side oxidation. The model prediction shows generally good agreement with the observed oxide layer thickness and also with prediction from conventional parabolic expressions when available for the alloy.