A. Chyrkin

Modelling compositional changes in nickel base alloy 602 CA during high temperature oxidation

Abstract The high chromium nickel base alloy 602 CA is used as construction material for high temperature components in aggressive environments such as gas burners or heat exchangers. The high temperature oxidation behaviour of this alloy is characterised by the formation of an external Cr2O3 scale and internal precipitation of alumina. The main goal of the present investigation was to elucidate the effect of aluminum on the subscale chromium depletion phenomena. The chromium concentration profiles were found to be flat and sometimes even increasing towards the oxide-alloy interface. The experimental observations were interpreted in terms of alloy thermodynamics, i.e. effect of oxidation induced compositional changes on chemical potentials of the alloy constituents. The concentration profiles in alloy 602 CA observed during high temperature exposure were modelled using the CALPHAD approach for thermodynamic and kinetic calculations. Thermodynamic calculations revealed that the Al depletion in the alloy subsurface due to internal alumina precipitation leads to enhanced Cr diffusion towards the alloy surface.

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

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.