The article discusses the effects of molybdenum (Mo) on the stability of passive films at the nanometer and atomic scales, using advanced surface analysis techniques and density functional theory (DFT) modeling. The study focuses on model FeCrNi(Mo), 316 L stainless steel, and FeCrNiCo(Mo) passivated surfaces. Key findings include: 1. **Compositional and Structural Defects**: The passive film's stability is influenced by compositional and structural nanoscale defects, which originate from pre-passivation mechanisms. These defects are crucial for corrosion protection. 2. **Mo's Role in Passivity Enhancement**: - **Mo(VI) Species**: Enriched in the outer exchange layer of the passive film, Mo(VI) species impede the deep penetration of Cl− ions, limiting their access to the inner barrier layer. - **Mo(IV) Species**: Dispersed in the inner layer, Mo(IV) protects against Cl− ion entry into defect sites of the Cr(III) oxide barrier. - **Mo(IV + δ)**: In Fe-rich compositional defects, Mo enhances the selective dissolution of iron and its replacement by chromium and molybdenum, improving resistance to Cl−-induced passivity breakdown. 3. **DFT Insights**: - **Substitutional Mo**: DFT simulations show that substitutional Mo in the inner barrier layer increases the resistance to Cl− entry, curing Fe-rich weak sites. - **Vacancy Formation**: Substitutional Mo disfavors the formation of O vacancies, further enhancing resistance to passivity breakdown. 4. **Accelerated Testing**: - **316 L SS**: Vibratory finishing increases Cr and Mo enrichments, enhancing passivity and resistance to Cl-induced passivity breakdown. - **Multi-Principal Element Alloys (MPEAs)**: Higher Mo content in MPEAs significantly reduces Cl penetration, preventing passivity breakdown and localized corrosion. 5. **Conclusion**: - Mo's multiple effects, including its enrichment in the outer and inner layers of the passive film, contribute to enhancing the stability of passive films against Cl−-induced passivity breakdown. These findings provide valuable insights into the role of Mo in corrosion protection. The article highlights the importance of Mo in stabilizing passive films and offers a comprehensive understanding of its mechanisms at the nanometer and atomic scales.The article discusses the effects of molybdenum (Mo) on the stability of passive films at the nanometer and atomic scales, using advanced surface analysis techniques and density functional theory (DFT) modeling. The study focuses on model FeCrNi(Mo), 316 L stainless steel, and FeCrNiCo(Mo) passivated surfaces. Key findings include: 1. **Compositional and Structural Defects**: The passive film's stability is influenced by compositional and structural nanoscale defects, which originate from pre-passivation mechanisms. These defects are crucial for corrosion protection. 2. **Mo's Role in Passivity Enhancement**: - **Mo(VI) Species**: Enriched in the outer exchange layer of the passive film, Mo(VI) species impede the deep penetration of Cl− ions, limiting their access to the inner barrier layer. - **Mo(IV) Species**: Dispersed in the inner layer, Mo(IV) protects against Cl− ion entry into defect sites of the Cr(III) oxide barrier. - **Mo(IV + δ)**: In Fe-rich compositional defects, Mo enhances the selective dissolution of iron and its replacement by chromium and molybdenum, improving resistance to Cl−-induced passivity breakdown. 3. **DFT Insights**: - **Substitutional Mo**: DFT simulations show that substitutional Mo in the inner barrier layer increases the resistance to Cl− entry, curing Fe-rich weak sites. - **Vacancy Formation**: Substitutional Mo disfavors the formation of O vacancies, further enhancing resistance to passivity breakdown. 4. **Accelerated Testing**: - **316 L SS**: Vibratory finishing increases Cr and Mo enrichments, enhancing passivity and resistance to Cl-induced passivity breakdown. - **Multi-Principal Element Alloys (MPEAs)**: Higher Mo content in MPEAs significantly reduces Cl penetration, preventing passivity breakdown and localized corrosion. 5. **Conclusion**: - Mo's multiple effects, including its enrichment in the outer and inner layers of the passive film, contribute to enhancing the stability of passive films against Cl−-induced passivity breakdown. These findings provide valuable insights into the role of Mo in corrosion protection. The article highlights the importance of Mo in stabilizing passive films and offers a comprehensive understanding of its mechanisms at the nanometer and atomic scales.