First study of manganese diffusion in Cr2O3 polycrystals and thin films by SIMS

Chromia layers are formed on many industrial alloys and act as a protective barrier against the corrosion of the materials by limiting the diffusion of oxygen and cations. Most of these alloys contain manganese as an impurity, and manganese oxides are often found at the outer surface of the oxide films. In order to clarify the oxidation mechanism and to check if chromia acts as a barrier, manganese diffusion in chromia was studied in both polycrystals and oxide films formed by oxidation of Ni - 30Cr alloy in the temperature range 700 - 1100 degrees C at an oxygen pressure of 10(-4) atm. After deposition of Mn on the chromia surface and a diffusing treatment, the manganese penetration profiles were established by secondary ion mass spectrometry. In all cases, the diffusion profiles showed two domains. For the first domain, using the solution of Fick's law for diffusion from a thick film into a semi-infinite medium, bulk diffusion coefficients were determined. For the second domain, the Le Claire model allowed the grain boundary diffusion parameter (alpha D-gb delta) to be obtained. Manganese diffusion does not vary significantly according to the nature and microstructure of chromia. The activation energy of grain boundary diffusion is not far from that obtained for bulk diffusion, probably on account of segregation phenomena. Manganese diffusion was compared to cationic self-diffusion and iron diffusion, and related to the protective character of chromia.