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.

Keywords:

• Ni base alloy
• Internal oxidation
• Chromia
• Alumina precipitation
• Diffusion modeling

Acknowledgements

The authors are grateful to Mr H. Cosler and Ms A. Kick for carrying out the high temperature exposures and the TG measurements. Mr V. Gutzeit is acknowledged for optical microscopy. Prof. G. H. Meier from Pittsburgh University is kindly acknowledged for discussing the results and giving very valuable comments.