Steels continued stabilisation

Increased Si content at the surface of steel leads to a beneficial effect improving the anti-oxidation behaviour. The surface protection is achieved by several mechanisms that take place at the same time, including the formation of silicon oxide films, increased Cr diffusion from the bulk to the surface, formation of phases such as y-Fe, Cr203, Si02, Si FCy, passivation of the surface by oxidation and surface diffusion. As mentioned in references [1-4], silicon seems to retard breakaway in the presence of water vapour in the environment and may facilitate Cr rediffusion from the bulk which would help repassivation observed after breakaway. As a consequence the level of the Cr reservoir possibly may be kept lower than for Si-free steels. At least, a continuous silica layer is not the reason for the improved oxidation behaviour. The positive effect of silicon seems to stabilise at values above 0.5% Si. A possible reason for the influence of silicon seems to be that silicon enhances the diffusion of Cr in the metal matrix. 

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