Ion diffusion study in the oxide layers due to oxidation of AISI 439 ferritic stainless steel.

Abstract

Chromium diffusion coefficients were determined in thermally grown chromia layers on the AISI 439 ferritic stainless steel oxidized at 750, 800, 850 and 900 C in synthetic air. The stable isotope 54Cr was used as chromium tracer and the diffusion profiles were established by SIMS analysis. The chromium ion bulk diffusion coefficients evaluated numerically from experimentation were around five orders of magnitude smaller than the chromium ion diffusion coefficients at grainboundary regions. The values of the chromium ion diffusivities in chromia grown on the AISI steel were lower than the corresponding oxygen ion diffusivities obtained in a previous study. Comparison of experimental and calculated parabolic oxidation constants, for the oxidation of the AISI 439 steel, showed that the chromia scale growth mechanism is controlled not only by inward oxygen ion diffusion from atmosphere, but also by outward chromium diffusion from the metallic substrate. However, the role of the oxygen ion diffusion appears to be more important than that of the chromium ion diffusion.