Fluorine-Induced Corrosion in NiCr Alloys: A First-Principles Study on the Role of Grain Boundaries and Chromium Dissolution Mechanisms

Core Concepts

Grain boundaries (GBs) in NiCr alloys act as preferential sites for fluorine adsorption and accelerate chromium dissolution, particularly in the form of CrF3, playing a crucial role in the early stages of molten salt corrosion.

Abstract

• Bibliographic Information: Arkouba, H., & Jin, M. (2024). First-Principles Investigation of Grain Boundary Effects on Fluorine-Induced Initial Corrosion of NiCr Alloys. arXiv preprint arXiv:2411.00673v1.
• Research Objective: To investigate the atomic interactions and Cr dissolution mechanisms at grain boundaries in NiCr alloys under molten fluoride salt environments using Density Functional Theory (DFT).
• Methodology: A Σ5(210)/(001) symmetrical tilt GB model was constructed and DFT calculations were performed to investigate fluorine adsorption energies on Ni(100) and Cr-doped Ni(100) surfaces, as well as Cr dissolution energy barriers under different fluorine bonding scenarios.
• Key Findings:
  • Fluorine exhibits a strong preference for binding at GB sites compared to bulk Ni surfaces.
  • Cr doping amplifies fluorine adsorption at GBs.
  • Fluorine bonding with Cr weakens the interaction between Cr and the Ni substrate.
  • The formation of CrF3 significantly reduces the energy barrier for Cr dissolution, particularly at GBs.
• Main Conclusions: GBs play an essential role in enhancing fluorine adsorption and accelerating Cr depletion in NiCr alloys, contributing to early-stage corrosion. The formation of CrF3 is identified as a critical precursor to Cr dissolution.
• Significance: This study provides atomic-level insights into the mechanisms of fluorine-induced corrosion in NiCr alloys, particularly highlighting the crucial role of GBs. This knowledge is essential for developing corrosion-resistant alloys for molten salt reactors and other applications involving harsh environments.
• Limitations and Future Research: The study focuses on a specific GB structure and does not explicitly consider the influence of a real molten fluoride salt environment. Future research could explore the impact of different GB types, salt compositions, and temperature effects on the corrosion process.

Stats

The adsorption energy of fluorine at GB sites ranges from 3.145 eV to 3.276 eV.
Cr doping increases the absolute adsorption energies at GB sites to 3.200 eV and 3.259 eV.
The dissolution barrier for a Cr adatom in the bulk is 2.650 eV, while at the GB it is 2.945 eV.
The binding energy of CrF3 at the GB is lower than at the bulk surface.
The dissolution barrier for CrF3 is significantly reduced to around 1 eV.

Quotes

Key Insights Distilled From

First-Principles Investigation of Grain Boundary Effects on Fluorine-Induced Initial Corrosion of NiCr Alloys

by Hamdy Arkoub... at arxiv.org 11-04-2024

https://arxiv.org/pdf/2411.00673.pdf

Deeper Inquiries

How would the presence of other alloying elements, such as Mo or Fe, commonly found in NiCr alloys, affect the fluorine adsorption and Cr dissolution behavior at GBs?

Adding Mo or Fe to NiCr alloys would introduce complexities in the fluorine adsorption and Cr dissolution at grain boundaries (GBs) due to their distinct chemical properties and interactions with both fluorine and the primary alloying elements.
Molybdenum (Mo):

Fluorine Adsorption: Mo exhibits a strong affinity for fluorine, potentially even higher than Cr. Therefore, Mo segregation to GBs could lead to competitive fluorine adsorption. This could either exacerbate corrosion by accelerating Mo dissolution alongside Cr or potentially mitigate Cr dissolution by acting as a fluorine sink, protecting Cr. The outcome would depend on the relative concentrations of Mo and Cr at the GB, their respective dissolution kinetics, and the formation energies of their fluorides.
Cr Dissolution: Mo additions are known to influence Cr diffusion in Ni-based alloys. Depending on the Mo concentration and the resulting microstructural changes, Mo could either hinder or promote Cr diffusion to the GB, thereby indirectly affecting Cr dissolution. Additionally, the presence of Mo-fluorides could alter the local chemical environment at the GB, influencing the stability and dissolution behavior of CrF3.
Iron (Fe):

Fluorine Adsorption: Fe generally has a lower affinity for fluorine compared to Cr and Ni. Therefore, Fe segregation to GBs might not significantly alter fluorine adsorption compared to a binary NiCr alloy. However, Fe could indirectly influence fluorine adsorption by modifying the GB structure or the segregation behavior of Cr and Mo.
Cr Dissolution: Fe additions can impact the passivation behavior of NiCr alloys. A higher Fe content might lead to the formation of less protective oxide films, potentially increasing the overall corrosion susceptibility, including Cr dissolution at GBs. However, similar to Mo, the impact of Fe on Cr dissolution would be influenced by its effect on Cr diffusion and the local chemistry at the GB.
In summary, the presence of Mo or Fe in NiCr alloys would introduce a complex interplay of factors affecting fluorine adsorption and Cr dissolution at GBs. Computational models, such as those based on DFT, could be employed to systematically investigate the impact of these alloying elements by considering their segregation energies, binding affinities for fluorine, dissolution kinetics, and the stability of their fluorides. These simulations could provide valuable insights into the corrosion behavior of more complex NiCrMo and NiCrFe alloys in molten fluoride salt environments.

Could the application of surface coatings or grain boundary engineering techniques effectively mitigate fluorine-induced corrosion in NiCr alloys by altering the GB structure and chemistry?

Yes, employing surface coatings or grain boundary engineering techniques holds significant potential for mitigating fluorine-induced corrosion in NiCr alloys by modifying the GB structure and chemistry.
Surface Coatings:

Protective Barriers: Applying a continuous and chemically inert coating on the NiCr alloy surface can physically isolate the alloy from the corrosive molten fluoride salt. This barrier can prevent fluorine from reaching the GBs, effectively hindering both fluorine adsorption and subsequent Cr dissolution. Suitable coating materials should exhibit high thermodynamic stability in the molten salt environment, strong adhesion to the alloy substrate, and resistance to degradation or dissolution.
Compositional Gradients: Coatings with compositional gradients, such as those enriched in elements with a high affinity for fluorine (e.g., Mo or Al), can act as sacrificial layers. These layers preferentially react with fluorine, forming stable fluorides that limit further fluorine penetration and protect the underlying NiCr alloy.
Grain Boundary Engineering:

Grain Size Reduction: Decreasing the grain size increases the total GB area. While this might seem counterintuitive, finer grains can actually improve corrosion resistance by promoting more uniform corrosion attack, delaying the onset of localized intergranular corrosion. This is because the increased density of GBs provides more pathways for stress relaxation and accommodates the volume changes associated with corrosion product formation, reducing the driving force for localized attack.
GB Segregation Engineering: Controlling the segregation of beneficial alloying elements to GBs can enhance corrosion resistance. For instance, enriching GBs with elements that form stable and protective oxides or fluorides can create a local barrier against fluorine attack. This can be achieved through controlled heat treatments or by adding trace amounts of specific elements to the alloy.
GB Character Distribution: Manipulating the GB character distribution, such as increasing the fraction of special GBs (e.g., twin boundaries) known for their low energies and reduced reactivity, can enhance corrosion resistance. Special GBs are less prone to preferential fluorine adsorption and exhibit slower diffusion rates, hindering Cr depletion.
In conclusion, surface coatings and grain boundary engineering offer promising avenues for mitigating fluorine-induced corrosion in NiCr alloys. The effectiveness of these approaches depends on carefully selecting appropriate coating materials, optimizing coating processes, and tailoring the GB structure and chemistry to create a more resistant microstructure.

How can the insights gained from this study be applied to develop more sophisticated computational models that can predict the long-term corrosion behavior of NiCr alloys in complex, real-world environments?

The insights from this DFT study provide a valuable foundation for developing more sophisticated computational models to predict the long-term corrosion behavior of NiCr alloys in complex environments. Here's how:
1. Multiscale Modeling:

Bridging Scales: Integrate DFT calculations into higher-level simulation techniques, such as kinetic Monte Carlo (kMC) or phase-field modeling. DFT can provide accurate parameters for these models, including adsorption energies, diffusion barriers, and reaction rates, enabling simulations of larger-scale corrosion processes over extended time scales.
Microstructure Evolution: Incorporate the influence of GB structure, grain size, and orientation distribution on corrosion behavior. This can be achieved by coupling DFT calculations with crystal plasticity models or phase-field simulations to capture the evolution of the microstructure during corrosion.
2. Complex Environment Representation:

Molten Salt Effects: Go beyond vacuum conditions and explicitly include the molten salt environment in the simulations. This can involve using molecular dynamics (MD) simulations with realistic force fields for the molten salt or incorporating continuum models to describe the salt's chemical potential and transport properties.
Temperature and Stress Effects: Account for the influence of temperature and stress on corrosion behavior. DFT calculations can be performed at different temperatures to determine the temperature dependence of key parameters. Stress effects can be incorporated by coupling DFT with continuum mechanics models.
3. Alloying Element Effects:

Multicomponent Systems: Extend the DFT calculations to investigate the impact of additional alloying elements, such as Mo, Fe, and others commonly found in commercial NiCr alloys. This will require considering the complex interplay of these elements on fluorine adsorption, Cr dissolution, and the overall corrosion resistance.
Synergistic Effects: Explore the synergistic effects of multiple degradation mechanisms, such as corrosion, oxidation, and irradiation damage, which often occur simultaneously in real-world environments. This can be achieved by coupling DFT calculations with models describing these individual degradation processes.
4. Machine Learning Integration:

Data-Driven Approaches: Utilize machine learning algorithms to analyze large datasets generated from DFT calculations and experimental corrosion studies. This can help identify complex correlations and develop predictive models for corrosion behavior under various conditions.
Accelerated Material Design: Employ machine learning to guide the design of new NiCr alloys with improved corrosion resistance. By training on existing data, machine learning models can predict the corrosion performance of novel alloy compositions and microstructures, accelerating the material development process.
By combining these approaches, researchers can develop more sophisticated and predictive computational models that bridge the gap between atomistic insights from DFT and the macroscopic behavior of NiCr alloys in complex, real-world environments. These models will be invaluable for designing corrosion-resistant alloys and predicting the long-term performance of materials in challenging applications, such as molten salt reactors.