Diffusion across metal grain boundary

In the case of alumina scales formed on alumina-forming materials, the presence of the garnet phases is explained by the diffusion of RE from the underlying metal to the oxide surface via the oxide grain boundaries. This theory, called dynamic segregation , has been proposed by Pint and is based on ihe oxygen potential gradient across the oxide scale [128], The consequences of the diffusion of the RE towards the gas-oxide interface are, on the one hand, to suppress the diffusion of metal ions, and on the other hand, to form a mixed Al-RE oxide at the oxide surface. 

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