Diffusion, coefficients grain boundary

Fig. 7.37 Diffusion coefficients for some impurities in NiO grain boundaries compared with the corresponding lattice diffusivities (the grain boundary width is assumed to be I nm) (after...

During low temperature oxidation, oxide films grow by high-field conduction (Section 8.1) rather than by solid-state diffusion because the value of the diffusion coefficients is too small. Under these conditions the thickness of the oxide layer does not exceed a few nanometers. In contrast, in high temperature corrosion, volume diffusion and grain-boundary diffusion are the principle transport mechanisms by which oxide layers grow. As a consequence their thickness can reach much larger values. 

It has often been stated that the diffusion coefficients for lattice diffusion Di, grain boundary diffusion and surface diffusion Ds increase in the order Di < Dgb < Ds, and that the corresponding activation energies vary as Qi > Qgb > Qs. However, these relations may not always be correct. 

Dgb grain boundary diffusivity 5 grain boundary width A dimensionless constant D diffusion coefficient... 

A parabolic rate law will also be obtained if part or even all, of the diffusion through the product layer is by grain boundary diffusion rather than diffusion through the volume of each grain. The volume diffusion coefficient is quite simply defined as the phenomenological coefficient in Fick s laws. The grain boundary diffusion must be described by a product, DbS, where S is the grain... 

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