Towards improved prediction of scale exfoliation from steam tubes

Abstract

Available mechanistic understanding suggests that thermally-grown oxide scales will detach (exfoliate) from the metal surfaces on which they were grown when they are strained beyond a critical limit. Such strain accumulation occurs largely on cooling from service temperature, due to the mismatch in coefficients of thermal expansion between the oxide and the alloy substrate, and increases with increasing scale thickness and increasing rate of temperature change. Also important is the amount of scale that detaches in an exfoliation event, as well as the size and shape of the oxide flakes, since these influence the ultimate destination of the debris in the steam circuit. Mechanistic understanding of the oxidation process allows these issues to be addressed analytically, and this approach has been used to map the regimes where various forms of exfoliation are expected, as a function of oxide thickness (time at temperature) and accumulated strain (oxide type, oxide and alloy properties). Limited use of this approach has provided quite accurate (post-event) predictions, albeit for only two classes of alloys for which sufficient data are available. The program from which this paper was drawn is aimed at extension of this approach to develop a tool for predicting the conditions under which an exfoliation event will occur, and its consequences. Progress depends on the ability to accurately predict oxide thicknesses and to develop mechanistic descriptions of the evolution of scale morphologies for a wider range of alloys and at temperatures and steam pressures higher that those accommodated in the original mapping approach. Of particular concern are the 9–12%Cr ferritic–martensitic steels which do not appear to follow the behavior established for the lower-Cr steels, since the oxidation behavior of these newer steels appears to be significantly affected by small changes in, for instance, alloy composition, making mechanistic description of the evolution of scale morphologies very difficult.