Predicting the transition from internal to external oxidation of alloys using an extended Wagner model

Résumé : Leblond [Oxid. Metals 75 (2011) 93–101] recently estimated the conditions governing the transition from internal to external oxidation of alloys using a variant of Wagnerʼs [Z. Elektrochem. 63 (1959) 772–782] model incorporating the possible role of oxides as diffusion barriers, through a heuristic dependence of the diffusion coefficient of oxygen upon their local volume fraction. But the crudeness of the formula adopted made the prediction of the onset of external oxidation only qualitative. A more accurate formula is derived here by using a thermal analogy and finite element computations of the reduction of the conductivity generated by nonconducting, more or less flat obstacles. The extended Wagner model incorporating this formula leads to a prediction of the “critical” local fraction of oxides corresponding to the transition from internal to external oxidation, depending on the “aspect ratio” of the oxides. The predicted value is in acceptable agreement with that measured by Rapp [Acta Metall. 9 (1961) 730–741] for the Ag–In system, for a reasonable postulated value of this aspect ratio.
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Comptes Rendus Mécanique, Elsevier Masson, 2013, 341 (3), pp.314 - 322. 〈10.1016/j.crme.2013.01.003〉
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Jean-Baptiste Leblond, Moïse Pignol, Didier Huin. Predicting the transition from internal to external oxidation of alloys using an extended Wagner model. Comptes Rendus Mécanique, Elsevier Masson, 2013, 341 (3), pp.314 - 322. 〈10.1016/j.crme.2013.01.003〉. 〈hal-01434008〉

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