A Simple Expression for Predicting the Oxidation Limited Life of Thin Components Manufactured from FCC High Temperature Alloys

Abstract

Chromia and alumina forming high temperature alloys suffer from breakaway oxidation if the concentration of the preferred scale forming element in the alloy decreases below the level required to sustain growth of the protective oxide scale. In thin components, the breakaway may occur even before oxide spallation starts to contribute to alloy depletion. In the present paper a simplified method is developed to predict the time to breakaway as a function of oxidation rate, initial concentration and diffusivity of the scale forming element in the alloy as well as component thickness. The first approach used is an approximation of the analytical solution previously derived by Whittle. The second method is based on a numerical solution and an exploration of the way in which the time to breakaway varies with the above mentioned parameters. Comparison with literature data reveals that for a number of applications good agreement between calculated and measured lifetimes can be achieved using both approaches. The lifetime equation derived using the numerical approach has the advantage that it allows prediction of breakaway oxidation in a larger range of experimental and alloy composition related parameters. It not only describes the behaviour of materials with a face centered cubic lattice but also includes the limiting case in which solute diffusion is fast compared to surface recession rate, as in, for example, the oxidation of ferritic alumina forming FeCrAl alloys at high temperatures.