Mechanism of Breakaway Oxidation on Fe/10% Cr

Abstract

In the breakaway oxidation of Fe/10% Cr in air at 600°c, the rate switches between two values differing by an order of magnitude. High-resolution microstructural analysis shows that the large and fluctuating rate is not due to gross cracking of the scale. The only observable feature that can be correlated-with the rate of oxidation is the grain size in the layer of chromium-containing spinel immediately outside the oxide/alloy interface, but the overall rate cannot be controlled simply by the rate of grain-boundary diffusion.

A mechanism is suggested involving two coupled cyclic processes: (1) Periodic precipitation of voids at the alloy/oxide interface followed by filling of the voids by inward diffusion of oxidant. (2) Advance and retreat of the Fe₂O₃/Fe₃O₄ interface, leading to opening and sealing of fine cracks in the outer layer of scale.