Parameters affecting oxide adherence in nickel based alloys and techniques allowing estimation of adherence

It is currently accepted that oxide/substrate adherence is related to the stresses developed in both parts of the oxidised system and many parameters have been suggested as being responsible for the oxidation stresses and for scale breakdown. Among these parameters, it is shown, on nickel based alloys, that predictions using the Pilling–Bedworth ratio are not always verified. However, the difference between the oxide and substrate expansion coefficients seems to be a very important factor contributing to the stress level in both oxide and substrate and the lifetime of oxidised Ni–Co–Cr–Al alloys. Furthermore, it is shown that various other parameters (strain rate differences between the substrate and its oxide, internal oxidation, impurities, etc.) can also contribute to adherence loss. The techniques most frequently used to estimate adherence and oxidation stresses and strains are described and illustrated by examples on oxidised nickel based alloys.MST/861