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Long time oxidation at 1000�?�?�?�?�?�?�?°c of a cast ni-25cr-0.5c alloy: determination of the kinetic constants and metallographic characterization of the oxidized state for the deduction of the chromium diffusion coefficient in subsurface

Elodie Conrath, Patrice Berthod

The diffusion of chromium in alloys, the oxidation resistance of which depends on their chromia-forming behaviour, is of great importance. After long times of oxidation such alloys are modified in subsurface by notably the development of a carbide-free zone which may significantly influence the diffusion of chromium. Diffusion coefficients red in handbooks for bulk alloys can thus be not efficient to correctly represent diffusion in these subsurfaces. In this work a method associating thermogravimetry measurements themselves and chromium concentration profiles acquired on cross sections prepared from the corresponding oxidized samples, was used to try specifying the chromium diffusion coefficient in this special alloy environment which is the subsurface of the oxidizing alloy itself. This led to a value of almost 3× 10-11 cm2 s-1 for 1000°C in the subsurface of a Ni- 25Cr-0.5C alloy.