Role of diffusion-induced grain boundary migration during molten salt corrosion of a Ni-30Cr alloy

This study demonstrates that diffusion-induced grain boundary migration (DIGM) is a critical mechanism driving rapid chromium depletion and subsurface porosity in Ni-30Cr alloys exposed to molten salts, with the severity of corrosion being decisively influenced by the material's initial surface microstructure.

The Gist

Imagine you have a sturdy, two-ingredient cake made of Nickel (the strong, tough part) and Chromium (the part that keeps it from rusting). Scientists are testing how this cake holds up when dipped in a pot of super-hot, liquid "salt soup" (molten salt), which is used in next-generation nuclear power plants.

The big question was: How does the Chromium disappear so quickly from deep inside the metal, even though it shouldn't be able to move that far in that amount of time?

To solve this mystery, the researchers took two identical cakes but prepared their surfaces differently:

1. The "Smooth" Cake (Electropolished): This surface was polished until it was perfectly smooth and flat, like a calm lake.
2. The "Rough" Cake (Sanded): This surface was scratched up with sandpaper, creating a messy, deformed top layer full of tiny cracks and stress, like a road full of potholes.

Here is what happened when they dipped them in the hot salt soup for 96 hours, explained through simple analogies:

1. The Smooth Cake: The "Slow, Layer-by-Layer" Erosion

On the smooth surface, the salt ate away the cake very slowly and evenly.

• What happened: The Chromium dissolved a tiny bit, and the Nickel dissolved a tiny bit, layer by layer. It was like sandpaper slowly wearing down a block of wood.
• The Grain Boundaries: The only place where things got weird was at the "seams" where the metal crystals met (called grain boundaries). Here, the Chromium ran away super fast, leaving behind little islands of pure, shiny Nickel.
• The Result: A mostly intact surface with a few specific spots where the Chromium was gone.

2. The Rough Cake: The "Tornado" Effect

On the sanded surface, the result was a disaster. The salt ate deep tunnels and holes all the way down, leaving a sponge-like structure.

• The Secret Weapon (Recrystallization): When the rough surface was heated, the metal tried to "heal" itself. The tiny, stressed grains rearranged themselves into new, small grains. This created a massive network of new "seams" (grain boundaries) just under the surface.
• The Magic Trick (DIGM): This is where the paper's main discovery comes in. They found a process called Diffusion-Induced Grain Boundary Migration (DIGM).
  • The Analogy: Imagine a crowd of people (the Chromium atoms) trying to escape a burning building.
  • In the Smooth Cake: The people have to walk through the rooms (the metal lattice). It's slow, and they can't get far.
  • In the Rough Cake: The "seams" between the new grains act like moving escalators. As the seams move (migrate), they sweep the Chromium atoms up and out of the metal incredibly fast, dumping them into the salt soup.
  • Because the seams are moving, they don't just carry the Chromium; they leave behind a "ghost town" of pure Nickel. The Chromium is swept away so efficiently that the metal becomes a porous, weak sponge.

The Big Takeaway

The scientists realized that how you finish a metal part matters more than we thought.

• If you make the surface smooth, the metal resists the salt well.
• If you scratch or deform the surface (like sanding it), the heat causes the metal to reorganize, creating "moving escalators" that strip away the protective Chromium instantly.

In simple terms: The paper proves that the "scars" left on a metal surface during manufacturing can turn into highways for corrosion. By understanding this "moving escalator" effect (DIGM), engineers can design better nuclear reactors by ensuring metal parts are finished in a way that prevents these highways from forming, keeping the fuel and structures safe for longer.

Technical Summary

1. Problem Statement

Molten chloride salts are promising candidates for next-generation nuclear reactors (Gen IV) and concentrated solar power due to their excellent thermophysical properties. However, they pose a severe corrosion challenge to structural materials like Ni-Cr alloys.

• The Phenomenon: Exposure typically leads to Cr dealloying, where Chromium selectively dissolves, leaving behind a porous, Ni-rich subsurface layer.
• The Knowledge Gap: While lattice diffusion explains surface corrosion, it cannot account for Cr depletion occurring over distances (micrometers) far exceeding the predicted diffusion limits (nanometers) at intermediate temperatures (e.g., 500°C).
• The Hypothesis: Previous studies suggested fast diffusion pathways (grain boundaries) transport solutes to the surface. However, the specific mechanism allowing solutes to reach these boundaries in large-grained systems, and the role of Diffusion-Induced Grain Boundary Migration (DIGM), remained debated. Furthermore, the impact of surface preparation (deformation history) on these mechanisms was not fully understood.

2. Methodology

The researchers investigated a model Ni-30 at.% Cr alloy exposed to a eutectic LiCl–KCl–2 wt% EuCl3 salt at 500°C for 96 hours. To isolate the effect of microstructure and surface defects, two distinct surface finishes were prepared:

1. Electropolished (EP): Smooth surface with minimal subsurface deformation and large, equiaxed grains (~240 µm).
2. Sanded: Surface with parallel scratches, introducing significant plastic deformation (dislocations) and subsurface damage.

Experimental Workflow:

• Corrosion Setup: Salt pellets were placed on the alloy surfaces in an inert argon atmosphere.
• Chemical Analysis: Post-corrosion salt was analyzed via ICP-MS to quantify dissolved Ni and Cr concentrations.
• Microstructural Characterization:
  • SEM/EDS: Surface morphology and elemental mapping.
  • FIB/STEM: Cross-sectional analysis to observe subsurface microstructures, grain boundaries, and compositional gradients.
  • Diffraction: Used to identify grain boundary types (High Angle vs. Low Angle) and lattice orientations.

3. Key Results

A. Chemical Dissolution (ICP-MS)

• Sanded vs. EP: The sanded samples exhibited ~20% higher Cr dissolution compared to EP samples.
• Ni Dissolution: Ni concentrations were similar in both cases, indicating that while Cr loss varied significantly, the overall dissolution kinetics were influenced by the surface condition.

B. Electropolished (EP) Surface Response

• Grain Interiors: Showed no selective Cr dissolution. The surface retained the original alloy composition. Dissolution occurred via a "layer-by-layer" mechanism where both Ni and Cr dissolved, but Ni dissolution was the rate-limiting step.
• Grain Boundaries: Distinct pure Ni islands formed above the grain boundaries.
  • Mechanism: DIGM occurred. The grain boundary migrated, leaving behind a Cr-depleted (pure Ni) region.
  • Evidence: STEM revealed a chemically abrupt interface between the alloy and the pure Ni region, accompanied by a shift in the grain boundary position. Low-angle grain boundaries (LAGBs) migrated downward to facilitate lateral Cr transport to the surface.

C. Sanded Surface Response

• Microstructure: The surface exhibited a bi-continuous porous network extending 3–10 µm deep, with complete Cr depletion.
• Mechanism: The initial deformation caused recrystallization during the first few hours of exposure at 500°C, creating a fine-grained layer with a high density of grain boundaries.
• DIGM Dominance: These newly formed, moving grain boundaries swept through the subsurface layer, continuously transporting Cr to the surface/salt interface. This resulted in massive Cr depletion and pore formation far deeper than lattice diffusion could explain.
• Contrast: Unlike the EP sample where DIGM was localized to specific boundaries, the sanded sample experienced widespread DIGM due to the high density of migrating boundaries created by recrystallization.

4. Key Contributions

1. Identification of DIGM as a Primary Mechanism: The study unequivocally establishes Diffusion-Induced Grain Boundary Migration (DIGM) as a critical mechanism for long-range Cr transport in molten salt corrosion, resolving the discrepancy between observed depletion depths and lattice diffusion limits.
2. Role of Surface Deformation History: The paper demonstrates that surface preparation (e.g., sanding vs. electropolishing) drastically alters corrosion behavior. Deformation triggers recrystallization, which in turn activates DIGM, leading to severe, deep-seated corrosion.
3. Reinterpretation of Previous Literature: The authors suggest that many previous observations of "bicontinuous" porous structures in heavily deformed alloys (cold-rolled or drawn wires) were likely driven by DIGM rather than just static grain boundary diffusion or percolation dissolution.
4. Mechanistic Schematics: The study provides clear schematic models (Figures 8 and 10) distinguishing between the localized DIGM on smooth surfaces and the pervasive DIGM on deformed/recrystallized surfaces.

5. Significance and Implications

• Material Design: The findings highlight that microstructure is a decisive contributor to corrosion resistance. Simply selecting an alloy composition is insufficient; the manufacturing history and final surface finish are critical.
• Mitigation Strategies: To enhance material lifetime in molten salt environments, surface processing should aim to minimize subsurface deformation or promote the formation of stable, Cr-enriched layers that resist DIGM.
• Predictive Modeling: Future corrosion models for molten salts must account for the coupling of grain boundary motion and solute diffusion (DIGM), particularly in components that undergo cold working or have rough surface finishes.

In conclusion, this work shifts the paradigm of understanding molten salt corrosion from a purely diffusion-limited process to one heavily influenced by dynamic microstructural evolution (recrystallization and boundary migration) driven by the initial surface state.