Mechanisms of breakaway oxidation and application to a chromia-forming steel

Breakaway oxidation or chemical failure has been described in this paper in terms of two possible mechanisms and applied to the behavior of a chromia-forming 20Cr-25Ni austenitic steel. Both mechanisms relate to the depletion of chromium arising from its selective oxidation and quantitative modeling of the depletion profile is used to identify the dominant chemical-failure mechanism as a function of temperature. Intrinsic Chemical Failure (InCF) develops when the chromium concentration within the alloy at the oxide-metal interface is less than that in equilibrium with chromia. This occurs at high temperatures, typically above 1390 K, but the temperature at which the alloy becomes susceptible to this form of failure increases as the alloy grain size decreases. Ar lower temperatures, chemical failure is associated with the general depletion of chromium across the specimen section to a level below which reformation of a healing chromia layer will not occur, should the surface layer become damaged. In this regime, failure is termed Mechanically Induced Chemical Failure (MICF).