The initial wet oxidation process on Fe-Cr alloy surface: Insights from ReaxFF molecular dynamic simulations

Abstract The durable use of stainless steels depends on the formation of a protective chromium-rich oxide layer. To better understand the atomistic mechanism of surface oxide film formed on Fe-Cr alloy, the initial wet oxidation process on Fe-Cr alloy surface was investigated using ReaxFF molecular dynamic simulations. The oxidation process initiates from the adsorption and local dissociation of the water molecules near Cr atoms, followed by the segregation of Cr and the formation of cation vacancies on alloy surface. Meanwhile, the dissociated O and H atoms diffuse into the alloy substrate, which consequently facilitates to form a Cr-rich oxide layer. The compositions of oxide film possess distribution from Cr hydrates and hydroxides, Fe hydroxides and oxides to hydrogen ions with negative charge along depth. It was also demonstrated that there is a charge transfer between the metal atoms and hydrogen atoms. The effects of Cr content (0 ∼ 20 at.%) and crystal surface orientation on the initial wet oxidation process were further discussed. These results can provide a microstructural understanding of the wet oxidation process on the surface of stainless steels from an atomistic point of view.