The oxidation mechanism of dilute Ni-Cr alloys

Abstract

Metallographic and electron microprobe analyses of oxide films formed on 90Ni-10Cr revealed duplex NiO scales overlying Cr 2O 3. The Cr 2O 3 existed as continuous thin films when formed above 900 °C and as a band of particles when formed at lower temperatures. The thin Cr 2O 3 film prevented vacancy transfer across the oxide-metal interface and severely restricted plastic flow of NiO, thereby leading to massive voids at the NiO-Cr 2O 3 interface. The outer NiO layer was dense and columnar, containing very little chromium; the inner NiO layer was porous and equiaxed, containing much chromium. Grain boundaries of the alloy were much more oxidation resistant than the grains. The films over the boundaries had no inner NiO layer and were characteristic of films formed on alloys containing more than 20%Cr. The oxidation mechanism is basically ismilar to that proposed by Wood and Hodgkiess, but this study suggests a more important role of vacancy disposition and plasticity of the oxide scales.