Predicting Minimum Aluminum Concentrations for Protective Scale Formation on Ni‐Base Alloys: II . Cyclic Oxidation

Abstract

Two criteria that predict the minimum bulk solute concentration for repeated formation of a protective oxide scale during cyclic oxidation were applied to Ni‐Cr‐Al(Zr) alloys. Both criteria assume total oxide spallation each cycle but only one criterion accounts for recovery in the solute concentration at the alloy surface which occurs between cycles. Both criteria were shown to be inadequate to predict the minimum Al concentration necessary for repeated formation of an scale on Ni‐Cr‐Al(Zr) alloys irregardless of the adherence of the scale. A diffusion model, developed to simulate diffusional transport in ternary alloys, was also applied to the cylic oxidation of Ni‐Cr‐Al(Zr) alloys and used to predict the minimum Al concentration for repeated formation. The diffusion model accounts for recovery in the solute concentration at the alloy surface and, by incorporating an oxide spalling model, also accounts for partial oxide spallation each cycle. The diffusion model underpredicted by a factor of two and a half the Al concentrations for Ni‐Al alloys but showed good agreement for alloys with Cr concentrations exceeding 20%. Limits of applicability of these criteria and the diffusion model are discussed.