The role of Cr atom in the early steam oxidation of Fe‐based alloys: An atomistic simulation

This study employed the density functional theory to capture the atomic-level dissociation processes of steam and investigate the ions migration on Fe(001) and FeCr(001) surfaces, revealing the role of Cr atom in the early oxidation. Various coadsorption structures with different steam-derived species have been systematically examined to find the most energetically favored surface site. The results showed that the steam dissociation on the alloy surface underwent two steps. First, H2O molecule on the top site was dissociated into OH group and H atom, which further combined with metal atoms on the bridge site and hollow site. Second, the OH group was decomposed into O and H atoms, which moved to two adjacent hollow sites and generated an oxide. On further oxidation, the Fe atom migrated outward and formed an outer Fe oxide, whereas the Cr oxide could only grow inward as O atom passed through oxide. It was found that the presence of Cr atom on the surface was thermodynamically beneficial, which could promote steam oxidation. The Cr atom could effectively block ion diffusion across the oxide scale and protect the underlying substrate from further oxidation. These results were in good agreement with experimental observation.