Oxidation-Limited Lifetime of ODS-FeCrAl Alloys: Observations on Influence of Surface Shape

Oxide dispersion-strengthened (ODS) FeCrAl ferritic steels constitute a class of alloys with a combination of mechanical strength capabilities and environmental resistance that permits operation at temperatures significantly higher than available classes of wrought alloy. A significant issue for these ODS alloys is that, because of the extreme operating temperatures, unexpected failure of the protective scale in service could lead to catastrophic degradation of high-temperature, high-pressure components. In the absence of handbook data for quantification of their high-temperature environmental degradation, significant efforts have been made to develop approaches for predictive oxidation lifetime modeling. The main basis for such models is an aspect of oxidation behavior peculiar to these alloys at very high temperatures, specifically, their ability to maintain a protective alumina scale until practically all available Al in the alloy has been consumed. At the high temperatures involved the concentration profile for Al throughout the component remains flat while the alumina scale is intact, so that knowledge of the oxidation kinetics provides a basis for calculating the time to exhaustion of the alloy's Al reservoir, hence service life. Difficulties arise when unanticipated gradients in the Al concentration profile are introduced following, for instance, premature local mechanical failure of the protective scale. The intent of this work was to consider if and how scale failures change with differences in surface shape, and how related mechanistic understanding of the actual oxidation behavior may be used to extend the utility of the modeling approaches. Experimental results for coupons of different shapes were very largely in agreement with the basic tenets of the lifetime modeling approaches, but a notable finding was that cylindrical shapes reproducibly exhibited a reduction in unexpected early scale failures, leading to more consistent oxidation-limited lifetimes.