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

Despite extensive research and proposed theories on formation and breakdown of passive layers, several questions remain unanswered. These include the reasons behind the bi-layer nature of the passive layer, the decrease in hydrogen and oxygen diffusivity upon entering the passive layer, and the influence of microstructure on passive layer formation and function in stainless steels. In this study, we employed ReaxFF molecular dynamics simulations to investigate passivation and depassivation of ferritic stainless steels in a polycrystalline structure. Through static and dynamic calculations, we elucidated the underlying 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 the depassivation process. We have identified several physical phenomena involved in the processes of passivation and depassivation, which can provide explanations for the questions posed above.