A family of creep-resistant, Al2O3-forming austenitic (AFA) stainless steels was recently
developed. The alloys exhibit excellent oxidation resistance up to ∼800°C, but are susceptible to
internal attack of Al at higher temperatures. In the present work, higher levels of Ni, Cr, Al, and Nb
additions were found to correlate with improved oxidation behavior at 900°C in air. The alloys
generally appeared to be initially capable of external Al2O3 scale formation, with a subsequent
transition to internal attack of Al (internal oxidation and internal nitridation) that is dependent on
alloy composition. Compositional profiles at the alloy/scale interface suggest that the transition to
internal oxidation is preceded by subsurface depletion of Al in the lower-Al compositions. In higher
Al-containing compositions, NiAl second-phase precipitates act as an Al reservoir, and Al depletion
may not be a key factor. Alloy design directions to increase the upper-temperature limit of
protective Al2O3 scale formation in these alloys are discussed.
Investigation on control in oxide scale formation and prevention of peeling off in ultra-supercritical unit