Inner layer, grains

We can suppose that the fine grain inner layer grows by inward diffusion of oxygen, while the coarse grain outer layer grows by outward cation diffusion. 

As a consequence of this type of interaction, the formation of an inner, fine-grained, porous partial layer starts, which fills the gap between the metal and the compact, outward-growing FeS scale. The steps described so far are schematically illustrated by Figs. 2-38 and 2-39. In later steps, even on the porous inner partial layer, a new, out-... 

The processing of grains. As with other essential minerals, there is much more chromium in the outer layers of grains— the branny layers which are usually removed— than there is in the inner layers. 

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