Stress corrosion cracking has become the critical issue that dominates the lifetime of various metallic materials such as stainless steels used under harsh operating conditions. In the EAC process, chemical reaction of the metal surface at crack tip such as oxidation and/or anodic dissolution, and subsequent formation of an oxide film plays an important role for crack propagation. In this study, tight-binding quantum chemical molecular dynamics simulations were employed to understand the chemical reactions caused by water molecules on Fe-Cr alloy surfaces in nano-scale. The main targeted temperature was boiling water nuclear reactor (BWR) condition which is 561 K. Water molecules were dissociated on the Fe-Cr surfaces at 561 K. The chromium atoms at the top layer segregated from the surface to bond with the dissociated oxygen atoms. Oxygen concentration around the chromium atoms gradually increased to preferentially form Cr-O bonds during the simulation, which resulted in clustering of oxygen and chromium atoms on the surface. The clustering of oxygen and chromium is thought to bring about Cr-based oxide nucleation and therefore, the preferential formation of Cr-O bonds is considered to be the initial process of the formation of oxide films on the Fe-Cr surface.