Atomistic Mechanisms of Stress-Dependent Molten Salt Corrosion in NiCr Alloys

This study employs reactive molecular dynamics simulations to reveal that tensile strain accelerates intergranular corrosion in NiCr alloys exposed to molten FLiNaK by increasing free volume and atomic mobility, whereas compressive strain suppresses corrosion by promoting a protective ridge-like surface layer.

The Gist

Imagine you have a high-tech metal fence made of Nickel and Chromium. This fence is designed to hold up a very hot, very angry liquid called molten salt (think of it as a super-heated, chemical soup that eats metal).

Usually, scientists know that if you pull on this fence (tension), it gets eaten faster. But what happens if you push on it (compression)? Does it get eaten faster, slower, or the same?

This paper uses a super-powerful computer simulation to zoom in all the way to the level of individual atoms to answer that question. Here is the story of what they found, explained simply:

The Setting: The "Grain Boundary"

Think of the metal fence not as one solid block, but as a mosaic made of two different tiles meeting in the middle. The line where these two tiles meet is called a Grain Boundary.

• The Problem: This meeting line is the weak spot. It's like a crack in a sidewalk where weeds (the corrosive salt) can easily sneak in and start digging.
• The Enemy: The molten salt contains Fluorine, which is like a hungry little monster that loves to grab Chromium atoms from the metal and drag them away into the soup.

The Experiment: Pulling vs. Pushing

The researchers simulated three scenarios for this metal fence at a scorching 800°C:

1. Pulling it apart (Tension): Stretching the metal.
2. Pushing it together (Compression): Squeezing the metal.
3. Leaving it alone (No Stress): Just sitting there.

What Happened?

1. The "Pull" (Tension) = The Open Door

When they pulled on the metal, it was like stretching a rubber band.

• The Analogy: Imagine the gap between the two tiles (the grain boundary) gets stretched wide open, like a door being forced ajar.
• The Result: Because the door is wide open, the hungry Fluorine monsters can easily rush in. They grab the Chromium atoms and drag them out. The metal fence gets eaten deep into the crack very quickly.
• The Takeaway: Pulling stress makes the corrosion much worse because it opens up the cracks for the salt to invade.

2. The "Push" (Compression) = The Self-Repairing Ridge

When they pushed on the metal, something surprising happened.

• The Analogy: Imagine you squeeze a tube of toothpaste. Instead of the toothpaste just getting squished, it pops out the top. In this metal, the pressure forced the metal atoms to push upward out of the crack, forming a little ridge or a "mound" right over the weak spot.
• The Result: This little mound acts like a shield or a roof. It blocks the hungry Fluorine monsters from getting into the crack. The salt can't reach the deep parts of the metal anymore.
• The Takeaway: Pushing stress actually protects the metal. It forces the metal to build a barrier that stops the salt from eating the weak spot.

3. The "No Stress" Case

When the metal was just sitting there, the salt ate it a little bit, but not as fast as when it was pulled, and not as protected as when it was pushed.

Why Does This Matter?

In real life, machines like nuclear reactors or solar power plants have metal parts that are constantly being pulled, pushed, and twisted by heat and pressure.

• Old Thinking: Scientists mostly worried about parts being pulled apart.
• New Insight: This paper shows that pushing parts together might actually be a good thing! It can stop corrosion in its tracks by forcing the metal to "heal" itself with a protective ridge.

The Big Picture

Think of the metal like a sponge.

• If you stretch the sponge, the holes get bigger, and water (salt) soaks in deep and fast.
• If you squeeze the sponge, the holes get blocked or the sponge bulges out, preventing the water from going deep.

Conclusion: To keep our high-tech metal structures safe from molten salt, we need to be careful about where we pull them (which makes them weak) and we might actually want to design them so they are squeezed in certain areas (which makes them strong and resistant to rust).

Technical Summary

1. Problem Statement

Nickel-based structural alloys used in advanced energy systems (e.g., molten salt reactors) operate under extreme conditions involving high temperatures, aggressive halide chemistry (molten FLiNaK), and complex mechanical loading. While the detrimental effects of tensile stress on stress corrosion cracking (SCC) are well-documented, the role of compressive stress remains poorly understood.

• Knowledge Gap: Existing studies rely heavily on macroscopic metrics (mass loss, crack formation) and focus primarily on tensile loading. There is a lack of atomistic understanding regarding how different stress states (tensile vs. compressive) influence early-stage corrosion kinetics, grain boundary (GB) stability, and atomic transport mechanisms in molten salts.
• Challenge: Direct experimental observation of early-stage corrosion under stress in molten salts is extremely difficult due to high temperatures and aggressive environments.

2. Methodology

The authors employed Reactive Molecular Dynamics (RMD) simulations using the ReaxFF force field to investigate the coupled effects of strain and corrosion.

• System Model: A $\Sigma5(210)$ symmetrical tilt grain boundary model in an FCC Ni$_{0.75}$Cr$_{0.25}$ alloy was constructed.
• Environment: The alloy was exposed to molten FLiNaK (LiF-NaF-KF) at 800°C.
• Stress Conditions: Three uniaxial strain states were applied along the x-direction:
  • Tensile strain: +4%
  • Compressive strain: -4%
  • Unstrained (control): 0%
• Simulation Protocol:
  • Simulations were run for 500 ps in the NVT ensemble.
  • The bottom two alloy layers were fixed to simulate bulk constraints.
  • Ten independent simulations were performed for each strain state to ensure statistical reliability.
• Analysis Metrics: The study quantified fluorine adsorption, charge redistribution (Bader charge), atomic mobility (Mean Square Displacement - MSD), free volume (Voronoi analysis), and morphological evolution.

3. Key Contributions

• Mechanistic Insight into Compressive Stress: The study provides the first atomistic evidence that compressive stress can actively suppress intergranular corrosion in molten salts, contrasting with the accelerating effect of tensile stress.
• Decoupling Stress and Corrosion: By isolating strain states, the authors demonstrate that stress does not merely accelerate corrosion uniformly but fundamentally alters the morphology and transport pathways of the degradation process.
• Identification of a Protective Mechanism: The discovery of a "ridge-like" surface layer formed under compression that acts as a physical barrier to salt infiltration.

4. Key Results

A. Morphological Evolution

• Tensile Strain (+4%): Caused significant intergranular penetration. The grain boundary dilated, creating a recessed channel where the molten salt infiltrated deeply.
• Compressive Strain (-4%): Resulted in the formation of a ridge-like surface protrusion along the grain boundary. This feature emerged within ~100 ps of salt contact and acted as a barrier, limiting salt access to the underlying alloy.
• Unstrained: Showed relatively uniform surface roughening with no pronounced recession or protrusion.

B. Fluorine Adsorption and Reactivity

• Tensile: Exhibited the highest fluorine surface coverage, particularly localized at the grain boundary. The elastic dilation of the lattice created low-coordination sites that favored fluorine adsorption and Cr-F bond formation.
• Compressive: Showed significantly reduced fluorine accumulation at the grain boundary. The ridge layer physically blocked salt penetration, reducing localized Cr-F bonding.
• Bulk vs. GB: In all cases, the grain boundary was the primary site for corrosion (nearly twice the dissolution of the bulk), confirming preferential intergranular attack.

C. Atomic Transport and Diffusion

• Mobility: Tensile strain yielded the highest atomic mobility (MSD) for both Ni and Cr, followed by compressive, then unstrained.
• Mechanism of Transport:
  • Tension: Enhanced by elastic lattice dilation and increased excess free volume, which lowered migration barriers and allowed fluorine to penetrate deep into the GB.
  • Compression: Surprisingly, compressive loading also increased GB mobility compared to the unstrained state. However, this mobility was directed outward (vertical mass transport) to form the ridge, rather than inward penetration.
• Charge Redistribution: Under tension, positively charged Cr atoms (indicating oxidation/dissolution) migrated deeply along the GB. Under compression, charged Cr atoms were confined near the surface ridge, preventing deep penetration.

D. Free Volume Analysis

• Tensile: Significantly increased the cavity radius and excess free volume at the GB, facilitating salt ingress.
• Compressive: Did not significantly reduce the intrinsic free volume of the GB core compared to the unstrained state. Instead, the compressive stress was accommodated by outward mass transport (ridge formation) rather than uniform densification.

5. Significance and Implications

• Design Guidelines: The findings suggest that engineering components to maintain compressive residual stresses (e.g., via shot peening or specific thermal treatments) could be a viable strategy to mitigate intergranular corrosion in molten salt reactors.
• Theoretical Advancement: The study clarifies that stress-corrosion coupling is not a simple linear acceleration but a complex interaction where stress state dictates the directionality of mass transport (inward penetration vs. outward extrusion).
• Methodological Validation: The work validates the use of RMD with ReaxFF for capturing dynamic, chemically reactive corrosion processes that are inaccessible to DFT or traditional MD, bridging the gap between thermodynamics and kinetics in harsh environments.

In conclusion, the paper establishes that while tensile stress accelerates intergranular corrosion by dilating the grain boundary and facilitating salt infiltration, compressive stress suppresses this attack by promoting the formation of a protective surface ridge, thereby shifting the degradation mode from localized intergranular cracking to distributed surface degradation.