Cr GRAIN BOUNDARY DIFFUSION IN Cr2O3

La diffusion intergranulaire du chrome dans des polycristaux denses de Cr2O3 a été mesurée en utilisant l'isotope stable 54Cr. Les profils de pénétration ont été établis par SIMS. A 1200 et 1300°C, sous une pression de 5.10-5 atm. (5 Pa), les valeurs trouvées pour Dj sont respectivement de 1,03.10-15 et 4,72.10-14 cm2.s-1 en prenant 1nm comme largeur de joint et en utilisant les valeurs de coefficient de diffusion en volume mesurées dans les monocristaux (1,0.10-18 cm2.s-1 à 1200°C et 4,8.10-18 cm2.s-1 à 1300°C). Ces valeurs sont plus faibles que celles publiées précédemment dans le domaine de températures et de pression d'oxygène.Cr grain boundary diffusion has been measured in dense polycrystals of Cr2O3 using 54Cr as a stable tracer and depth profiling by SIMS. At 1200 and 1300°C, in an oxygen pressure of 5.10-5 atm, (5 Pa), it was found 1.03.10-15 and 4.72.10-14 cm2.s-1 respectively for the grain boundary diffusion, considering the width of the grain boundary equal to 1 nm and using lattice diffusion values obtained on single crystals (1.0.10-18 cm2.s-1 at 1200°C and 4.8.10-18 cm2.s-1 at 1300°C). These values are smaller than those previously published in the same range of temperatures and oxygen pressure

Oxidation of AISI 304 and AISI 439 stainless steels

International audienceThe oxidation behaviour of AISI 304 and AISI 439 stainless steels was studied at high temperatures, under various oxygen pressures and in the presence or not of water vapour. Thermogravimetric analyses were conducted in isothermal conditions from 850 to 950 ◦C for 50 h and microstructural and chemical analyses of the oxide films grown by oxidation were performed by SEM and EDX. The oxide films were also analysed by grazing X-ray diffraction and by X photoelectron spectroscopy (XPS). The AISI 439 steel has higher oxidation resistance than AISI 304, above 850 ◦C, under high oxygen pressures. On the other hand, the AISI 304 steel has higher oxidation resistance under low oxygen pressures in the whole temperature range. In order to check whether the growth kinetics of Cr2O3 formed by the oxidation of stainless steels was controlled by oxygen or/and chromium diffusion through the oxide film, the oxidation constants were theoretically calculated on the basis of diffusion data using Wagner's Theory. The calculated values of the oxidation constants agree reasonably well with experimental values, therefore confirming the role of diffusion processes on the growth of chromia on stainless steels