Corrosion protection is crucial for metallic components, and passivity, the formation of a protective oxide film, is a primary defense mechanism. However, aggressive anions like chloride ions can initiate localized corrosion, compromising this protection. In stainless steels and nickel-based alloys, chromium is the main element responsible for passivity due to the formation of a Cr(III) oxide layer with low dissolution kinetics. The passive film typically exhibits a bilayer structure: a dense, Cr(III) oxide-rich inner barrier layer and an outer exchange layer containing various metal hydroxides and oxides. Molybdenum is another key alloying element improving corrosion resistance, particularly against pitting, crevice, and stress corrosion. Despite extensive research, the precise role of molybdenum in stabilizing passivity remains debated, with suggestions ranging from preventing passive film breakdown to promoting self-repair after breakdown. This study aims to unravel the complex roles of molybdenum in enhancing the stability of passive films through a combination of experimental surface analysis techniques and DFT modeling, focusing on the key role played by compositional and structural nanoscale defects that originate during the initial surface oxidation.

Numerous studies have explored the impact of molybdenum on stainless steel corrosion resistance. Some propose that molybdenum prevents passive film breakdown, while others suggest it facilitates self-repair after breakdown. The concentration and oxidation state of molybdenum in the passive film are highlighted as critical factors in its protective effect. Existing literature often lacks a detailed, combined nanometer and atomic-scale understanding of molybdenum's influence on passive film stability and its interaction with aggressive chloride ions. This work directly addresses this gap.

This study employed a multi-pronged approach combining advanced surface analysis techniques and DFT modeling. Model FeCrNi(Mo), 316L stainless steel, and FeCrNiCo(Mo) samples were used. Surface analysis involved: 1. **Scanning Tunneling Microscopy (STM):** Provided high-resolution images of the passive film's surface morphology and nanoscale structure. 2. **X-ray Photoelectron Spectroscopy (XPS):** Determined the chemical composition and oxidation states of elements within the passive film at different depths. Angular-resolved XPS was used to probe depth profiles. 3. **Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS):** Offered detailed depth profiling of the passive film and its constituent elements, including the distribution of chloride ions. The experimental data was complemented by: 4. **Density Functional Theory (DFT) modeling:** Provided atomic-scale insights into the interactions of molybdenum with the passive film, including its incorporation into the oxide matrix, its influence on the formation of defects (metal vacancies and oxygen vacancies), and its impact on the energy landscape of defect formation. DFT calculations, specifically DFT+U, were performed on periodic models of Cr2O3 and Fe2O3 surfaces with substituted Mo atoms to investigate charge distribution, oxidation states and defect formation energies. Electrochemical measurements (potentiostatic polarization) were conducted in various environments to assess the corrosion resistance and the stability of the passive film (with and without chloride ions). The combination of experimental techniques and DFT simulations allowed for a detailed understanding of Mo’s effects at different length scales.

The research revealed several key effects of molybdenum on passive film stability: 1. **Outer Layer Barrier:** Mo(VI) species concentrated in the outer exchange layer effectively impede the penetration of chloride ions, preventing them from reaching the inner barrier layer. This effect was further supported by the observation that chloride penetration was significantly reduced in samples pre-passivated in chloride-free electrolytes. 2. **Inner Layer Protection:** Mo(IV) species dispersed at the interface between the outer and inner layers protect against chloride ion entry into the defect sites of the Cr(III) oxide barrier layer. 3. **Defect Site Healing:** In Fe-rich nanoscale defects, which originate during the initial oxidation process from local Cr depletion, Mo(IV+δ) species enhance the selective dissolution of iron and its replacement by chromium and molybdenum. This healing mechanism stabilizes these inherently weaker regions of the passive film, mitigating the initiation of localized corrosion. DFT simulations confirmed that Mo substitution is energetically favored in Fe2O3 over Cr2O3, aligning with the experimental observation of increased Mo presence in Fe-rich defect sites. DFT also showed that Mo substitution disfavors the formation of oxygen vacancies, further increasing the resistance of the oxide matrix to chloride attack. 4. **Improved Passivity:** Accelerated corrosion tests using both 316L stainless steel and Cr- and Mo-bearing multi-principal element alloys (MPEAs) confirmed that Mo enrichment significantly enhances the resistance to chloride-induced passivity breakdown and localized corrosion. MPEAs with higher Mo content exhibited superior corrosion resistance, even at high chloride concentrations, demonstrating a synergistic effect of Cr and Mo in creating extremely stable passive films. In some cases, even at high chloride concentrations, the passive current density decreased over time, indicating no degradation of the passive state. 5. **Atomic-Scale Insights:** DFT calculations revealed that the substitutional Mo in the oxide matrix has a charge slightly higher than the replaced metal atoms, suggesting a +3 oxidation state in neutral systems. However, under charged conditions, the Mo oxidation state can reach +4 or even an intermediate value between +4 and +6, consistent with the experimental observation of Mo(IV+δ) in the inner layer. Furthermore, Mo substitution decreases the formation energy of metal vacancies, promoting selective dissolution of iron and its subsequent replacement with chromium and molybdenum, further contributing to the healing of Fe-rich defect sites. The suppression of oxygen vacancies by Mo also explains the observed increased resistance to chloride penetration.

The findings address the long-standing debate on the role of molybdenum in passivity by providing a comprehensive model encompassing multiple mechanisms at the nanometer and atomic scales. The synergistic effects of molybdenum at different locations and oxidation states within the passive film contribute significantly to its enhanced stability and corrosion resistance. The ability of Mo(VI) to act as a barrier against chloride penetration, Mo(IV) to protect defect sites, and Mo(IV+δ) to promote the selective dissolution of iron and its replacement by more stable species, create a robust defense system against chloride-induced corrosion. This study highlights the importance of considering the pre-passivation processes and the resulting nanoscale defects in understanding the overall corrosion behavior of alloys. The insights gained are crucial for the design and development of novel corrosion-resistant alloys.

This research provides a comprehensive understanding of the multiple roles of molybdenum in enhancing passive film stability on stainless steels and MPEAs. The combined experimental and DFT results demonstrate how Mo, through various mechanisms acting synergistically at different locations and oxidation states within the passive film, significantly improves resistance to chloride-induced corrosion. This detailed understanding has far-reaching implications for the design of advanced corrosion-resistant materials and optimizing alloy compositions for enhanced durability in corrosive environments. Future work could focus on investigating the influence of other alloying elements and environmental factors on these mechanisms.

The study primarily focuses on specific alloy compositions and corrosive environments. The generalizability of the findings to other alloys and conditions requires further investigation. While DFT calculations provide valuable atomic-scale insights, limitations inherent to DFT modeling, such as approximations made in the computational approach, need to be considered. The nanoscale heterogeneity observed in the passive films presents challenges in quantifying the precise contribution of different Mo species to the overall corrosion resistance.