Mechanisms of Breakaway Oxidation and Application to a Chromia-Forming Steel

Breakaway oxidation or chemical failure has beendescribed in this paper in terms of two possiblemechanisms and applied to the behavior of achromia-forming 20Cr-25Ni austenitic steel. Bothmechanisms relate to the depletion of chromium arisingfrom its selective oxidation and quantitative modelingof the depletion profile is used to identify thedominant chemical-failure mechanism as a function oftemperature. Intrinsic Chemical Failure (InCF) develops whenthe chromium concentration within the alloy at theoxide-metal interface is less than that in equilibriumwith chromia. This occurs at high temperatures, typically above 1390 K, but the temperature atwhich the alloy becomes susceptible to this form offailure increases as the alloy grain size decreases. Atlower temperatures, chemical failure is associated with the general depletion of chromium acrossthe specimen section to a level below which reformationof a healing chromia layer will not occur, should thesurface layer become damaged. In this regime, failure is termed Mechanically Induced ChemicalFailure (MICF).