Aluminium depletion in FeCrAl alloys during oxidation

Oxidation tests of ODS alloy MA956 containing 4,4 wt % Al were carried out between 1000 degreesC and 1100 degreesC under oxygen in order to study the depletion of the aluminium content in the substrate during the oxidation process, due to the formation and growth of an alpha-alumina scale. An Al concentration gradient was observed only for temperatures below 1080 degreesC. Above 1080 degreesC, the aluminium concentration profile becomes flat. By using Wagner's formula corresponding to the loss of an alloy element due to the formation of an oxide film made of this element, the values of the aluminium diffusion coefficients in the substrate can be calculated. The diffusion of aluminium in MA956 during oxidation is expressed by the equation : D(cm(2)s(-1))=0.37 exp(-298 kJ.mol(-1)/RT). Our values are in agreement with determinations of aluminium diffusion coefficients in iron-based alloys carried out by Akimova et al. by X-ray diffraction method and by Heesemann et al. by interdiffusion method. An attempt was made to relate these diffusion coefficients of aluminium in the substrate to the oxidation rate constants and determine the diffusion coefficients at temperatures above 1080 degreesC, when the aluminium concentration profile becomes flat.