Effect of specimen thickness on chromia scaling of Ni25Cr in N₂–O₂–H₂O test gases at 1000°C

Abstract

The isothermal oxidation behaviour of a Ni25Cr model alloy was studied using specimens of different thicknesses at 1000°C in dry and wet N₂–1%O₂. The oxidation mechanisms were evaluated using thermogravimetry and SEM/electron backscatter diffraction analyses of oxide scale cross-sections. The oxidation rates decreased with increasing specimen thickness and increasing water vapour additions in the gas. The findings can be explained by considering the effect of H-defects and in-scale stress state on point defect concentrations in the chromia lattice.

Keywords:

• Ni25Cr
• Chromia growth
• Water vapour
• Oxygen partial pressure
• Point defects
• Deformation

Acknowledgements

Support for this work by the Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged. The authors are grateful to Mr H. Cosler for carrying out the TG experiments, and to Dr E. Wessel, who performed the ion beam polishing of the samples and carried out the BSD analyses. Mrs. Lippert from ZEA-3 is acknowledged for carrying out the inductively coupled plasma–optical emission spectroscopy and infrared analyses.