~ ---.---

If one starts with a theoretically clean metal surface exposed to an oxidant, the initial step in the interaction is adsorption of the oxidant on the metal surface. On continued exposure to the reacting environment, the initial nuclei of the reaction product are formed at some preferential sites. These often grow laterally to produce a continuous film that ultimately covers the entire surface of the metallic substrate. In certain situations the oxidant may get dissolved in the metal substrate to an extent determined by the solubility and diffusivity of the oxidant in the metal at the temperature under consideration. Once the continuous adherent film is formed, it separates the reactants by creating a physical barrier for which further

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Figure 5.5 Stability diagram for the Fe-Cr-0 system at 1300°C [Ref. 3].