Computational chemistry analysis of passive layer formation and breakdown mechanisms in ferritic stainless steels

Despite extensive research on passive layer formation and breakdown, several questions still remain unanswered. These include the reasons behind the bilayer nature of the passive layer, the decrease in hydrogen and oxygen diffusivity upon entering the passive layer, the underlying mechanisms of passive layer formation and breakdown, and the influence of microstructure on passive layer formation and function in stainless steels. In this study, we employed ReaxFF molecular dynamics to investigate passivation and depassivation of stainless steel in a polycrystalline structure. Through static and dynamic calculations, we elucidated the mechanisms of passive layer formation, which were primarily governed by clustering. Our analysis also highlighted the significant role of hydrogen diffusion and its reaction with metallic compounds in depassivation. We have identified several physical phenomena involved in the processes of passivation and depassivation, which can provide explanations for the aforementioned unresolved points.