Relation between the oxidation growth rate of chromia scales and self-diffusion in Cr203.
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1992
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In most cases, chromia scales are assumed to grow by predominant chromium diffusion.
However, results of Atkinson and Taylor indicated that chromium bulk diffusion could not
account for the growth rate of chromia scales. Moreover, recent results of Park et aL showed
that oxygen diffusion in chromia was faster than chromium diffusion. So, at this date, the
controlling process of the growth of chromia scales is not elucidated.
To interpret such a phenomenon, oxygen and chromium self-diffusion coefficients in Cr203
single crystals and polycrystals were determined in the same materials and in the same
experimental conditions, thus allowing a direct comparison. Tracers were introduced by ion
implantation, thick film methods, and isotopic exchange, using the 54Cr, ~~ and 180
isotopes. Depth profiling was made by secondary ion mass spectroscopy (SIMS). The bulk
diffusion coefficients were computed by using a general solution of the Fick's law taking into
account evaporation and exchange at the surface. Grain-boundary diffusion coefficients were
computed by using the Whipple-Le Claire equation for type B intergranular diffusion. Lattice
and grain boundary self-diffusien coefficients were determined as a function of temperature
and oxygen pressure.
The diffusion coefficients are lower than results given in the literature and do not depend on
the oxygen pressure. Moreover, it is found that oxygen diffusion is faster than chromium
diffusion. These results are compared to the oxidation constants of chromia-forming alloys and
it is shown that neither lattice self-diffusion, nor grain-boundary self-diffusion can justify the
growth rate of chromia scales. Such a situation is compared to NiO case, for which authors
found important differences in grain-boundary diffusivity, according to the elaboration mode
of NiO (thermal oxidation or growth from the melt).
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SABIONI, A. C. S. et al. Relation between the oxidation growth rate of chromia scales and self-diffusion in Cr203. Journal of Materials Science, v. 27, n. 17, p. 4782-4790, set. 1992. Disponível em: <https://link.springer.com/article/10.1007/BF01166020>. Acesso em: 08 mar. 2017.