# Microstructural optimization attempt of Hastelloy C-2000 for use in molten salt reactors

A. Lascoste, Olier Patrick, E. Rouesne, R. Malacarne

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A. Lascoste, Olier Patrick, E. Rouesne, R. Malacarne. Microstructural optimization attempt of Hastelloy C-2000 for use in molten salt reactors. EUROCORR 2025 - European Corrosion Congress, Sep 2025, Stavenger, Norway. hal-05515122

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# Microstructural optimization attempt of Hastelloy C-2000 for use in molten salt reactors

A. Lascoste ¹a, P. Olier ¹, E. Rouesne ¹, R. Malacarne ²

1. Université Paris-Saclay, CEA, Service de Recherche en Matériaux et procédés Avancés, France

2. Université Paris-Saclay, CEA, Service de recherche en Corrosion et Comportement des Matériaux dans leur environnement, France

a Student in a work-study program
## Introduction

* Study context and Goal

* Literature survey

## Experimental part

* Alloy selection

* Description of the GBE * treatment used for optimizing the microstructure

* Characterization of the microstructure (EBSD analysis)

* Corrosion tests results

* Tensile tests results before and after corrosion

## Conclusion

*GBE = Grain Boundary Engineering

<figure><img src="image_3.png" /><figcaption>EBSD Analysis</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fume-hood
Corrosion tests</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Tensile tests</figcaption></figure>
**Context:**

* France 2030 recovery plan: fast-spectrum chloride salt MSR

* Projet ISAC: « Innovative System for Actinides Conversion » (2023/2027)

* Partners : CEA, Orano, EDF, CNRS, Framatome

* Objectives: Find innovative solutions for radioactive material and waste management

Design of a prototype reactor: ARAMIS (Advanced Reactor for Actinide Management In Salt)

Main components to be manufactured :

- vessel

- neutron reflectors,

- heat exchanger,

- pumps

- ...

<figure><img src="image_6.png" /><figcaption>Molten salt reactor</figcaption></figure>

Material challenges:

- good mechanical strength

- resistance to irradiation

- corrosion resistance in chloride salts (NaCl-MgCl₂-PuCl₃)

<figure><img src="image_7.png" /></figure>

Nickel base alloys
• **Goal**

➤ Improve the corrosion resistance of a Hastelloy grade (NiMoCr) by optimizing the nature of Grain Boundaries (GBs) by maximizing the proportion of “special” GBs (i.e. low coincident site lattice (CSL) GBs ($\Sigma \le 29$)) compared to general boundaries.

1. How ?

➤ Application of the GBE treatment (GBE: Grain Boundary Engineering) by using thermomechanical cycling (cold rolling with low strain : $\epsilon$ = 5% )

2. Why ?

➤ According to the bibliography, an increase in “special” boundaries improves corrosion resistance by limiting :
- the risks of impurity segregation at the grain boundaries (embrittlement)
- the risks of intergranular cracking
# Bibliography (1/2)

## The GBE treatment results in clusters of grains
promotes the formation of special Σ3n boundaries (with n ≥1)

* Σ : coincidence index

* 1/n : fraction of atoms in coincidence between two grains

<figure><img src="image_8.png" /></figure>

*Grain boundaries network of Hastelloy N after GBE (ε : 3% / cycle)
showing a cluster of grains and the different types of special GBs

[W. Cao et al, 2017]

+38% of special
boundaries

<figure><img src="image_9.png" /></figure>

L : solution annealed
1177°C (grain size : 30µm)

L5: GBE 5 cycles (5%)

*Proportion of the twins based after GBE for Hastelloy N*
Bibliography (2/2)

<figure><img src="image_10.png" /></figure>

Grain boundaries network and mass loss for stainless steel 304 specimens as a function of the proportion of special boundaries (low Σ CSL GB) linked with cold rolling reduction

<figure><img src="image_11.png" /></figure>

Fig. 5. Weight losses vs. exposure time for the specimens with different proportion of low Σ CSL boundaries.

The higher the proportion of special GBs the better the corrosion resistance (red curve compared to black curve))

** [C. Hu et al , 2011)]
Alloy Composition

<table><tr><td>Hastelloy C2000</td><td>Al</td><td>C</td><td>Co</td><td>Cr</td><td>Cu</td><td>Fe</td><td>Mn</td><td>Mo</td><td>Ni</td><td>P</td><td>S</td><td>Si</td></tr><tr><td>Weight %</td><td>0,27</td><td>0,003</td><td>0,22</td><td>22,68</td><td>1,53</td><td>1,47</td><td>0,2</td><td>15,47</td><td>58,1</td><td>0,002</td><td>0,003</td><td>&lt;0,02</td></tr></table>

<table><tr><td>Hastelloy C22</td><td>V</td><td>C</td><td>Co</td><td>Cr</td><td>W</td><td>Fe</td><td>Mn</td><td>Mo</td><td>Ni</td><td>P</td><td>S</td><td>Si</td></tr><tr><td>% massique</td><td>0.35</td><td>0.015</td><td>2.5</td><td>22.5</td><td>3.5</td><td>6</td><td>0.5</td><td>14.5</td><td>50</td><td>0.02</td><td>0.02</td><td>0.08</td></tr></table>

<figure><img src="image_12.png" /></figure>

Nickel-based alloys containing as main addition elements Cr, Mo and W

[H. Sun, 2018]

Depth of dechromisation for different corroded alloys in NaCl-KCl-MgCl2 at 700 °C under N2 atmosphere for 100 h
# Experimental method (GBE)

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>[É. Martin, 2020, Materialia 9]</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Rolling mill - CEA</figcaption></figure>

<table><thead><tr><th>Metallurgical conditions</th><th>Sample reference</th><th>Thickness</th></tr></thead><tbody><tr><td rowspan="2">REF : Solution Annealing<br>1150°C</td><td>MA-t ST</td><td rowspan="2">8,2 mm</td></tr><tr><td>MA-t SL</td></tr><tr><td rowspan="2">GB1 : rolling 5 cycles 5%<br>+ Sol. annealing 1150°C</td><td>5C5% ST</td><td rowspan="2">6,3 mm</td></tr><tr><td>5C5% SL</td></tr><tr><td rowspan="2">GB2 : rolling 50% +<br>rolling 5 cycles 5%<br>+ Sol. annealing 1150°C</td><td>50% 5C5% ST</td><td rowspan="2">3,1 mm</td></tr><tr><td>50% 5C5% SL</td></tr></tbody></table>

<figure><img src="image_16.png" /></figure>

MA-t
REF

5C5%
GB1

50% 5C5%
GB2

<figure><img src="image_17.png" /></figure>
# Grain size measurements

Poor correlation between the results obtained from optical micrographs and EBDS analysis
By EBSD low angle GBs are considered in the same “cluster” => Importance of the disorientation criterion

<figure><img src="image_18.png" /></figure>

Grain
delimitation
considered
in optical
microscopy

<figure><img src="image_19.png" /></figure>

Grain
delimitation
considered
in EBSD
analysis

Images obtained with optical microscopy software (top)
and EBSD analysis software / disorientation criterion 5°
– 5C5% ST (GB1) (bottom)

<figure><img src="image_20.png" /><figcaption>(a) Dis. Criterion GB > 5°</figcaption></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /><figcaption>(b) Dis. Criterion GB > 15°</figcaption></figure>

Mapping of grains (random grain coloration, no orientation indication),
sample C2000 50%-5C5%, excluding twin Σ3 for the estimation of grain size

<table><thead><tr><th rowspan="2">Mean Grain size<br>(µm)<br>EBSD analysis<br>Deviation<br>criterion 5°</th><th colspan="2">MA-t</th><th colspan="2">5C5%</th><th colspan="2">50%5C5%</th></tr><tr><th>ST</th><th>SL</th><th>ST</th><th>SL</th><th>ST</th><th>SL</th></tr></thead><tbody><tr><td>Number of<br>analyzed grains</td><td>1623</td><td>1817</td><td>1565</td><td>1354</td><td>191</td><td>203 9</td></tr></tbody></table>
<figure><img src="image_23.png" /></figure>

MA-t - ST
(REF)

<figure><img src="image_24.png" /></figure>

50%-5C5%-ST
(GB2)

Mapping of the distribution of GBs

<table><thead><tr><th rowspan="2">Length fraction<br>special GB<br>(Σ3 + Σ9)<br>/<br>General GB</th><th colspan="2">MA-t (REF)</th><th colspan="2">5C5% (GB1)</th><th colspan="2">50%5C5% (GB2)</th></tr><tr><th>ST</th><th>SL</th><th>ST</th><th>SL</th><th>ST</th><th>SL</th></tr></thead><tbody><tr><td>Averages<br>Relative<br>increases</td><td>0.68</td><td>0.71</td><td>0.8</td><td>0.77</td><td>0.77</td><td>0.76</td></tr><tr><td></td><td>0.695</td><td>0</td><td>0.785</td><td>+13%</td><td>0.765</td><td>+10%</td></tr></tbody></table>

=> relative increase of special GB compared to general GB limited to +13% for C-2000 while +38% was reported in the literature for Hastelloy N

<figure><img src="image_25.png" /></figure>
Evolution of the grain size and the length ratio of Σ3 + Σ9 / general GBs after GBE

<figure><img src="image_26.png" /></figure>

The GBE treatment leads to a little change in the nature of the grain and twin boundaries :

* the total length proportion of Σ3 + Σ9 boundaries increases slightly in conjunction with the formation of large size highly twinned grain-cluster microstructure.
# Corrosion tests in molten salt tests : NaCl-MgCl2 at 600 °C

## MESCAL - MoltEn Salt Corrosion mAterialS

* Experiment performed under a fumehood

* Salt introduced into the reactor in a glove box

* Purification with electrolyses

<figure><img src="image_27.png" /><figcaption>CEA/S2CM device</figcaption></figure>

Coupon for
micrographic exams :

<figure><img src="image_28.png" /></figure>

Tensile specimen :

<figure><img src="image_29.png" /></figure>

Partially immersed
in the salt

Totally immersed
in the salt

The corrosion sample are more corroded than tensile specimen

Before
corrosion

After 168 hours in
corrosion at 600 °C

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>
<figure><img src="image_33.png" /><figcaption>Coupon
after 168 hours at 600 °C</figcaption></figure>

<figure><img src="image_34.png" /><figcaption>Tensile specimen
after 168 hours at 600 °C</figcaption></figure>

• Uniform corrosion

Electronic micrographs (SEM)

<figure><img src="image_35.png" /></figure>

<figure><img src="image_36.png" /><figcaption>Before GBE</figcaption></figure>

<figure><img src="image_37.png" /></figure>

<figure><img src="image_38.png" /><figcaption>Salt
Alloy</figcaption></figure>

<figure><img src="image_39.png" /></figure>

<figure><img src="image_40.png" /></figure>

<figure><img src="image_41.png" /><figcaption>After GBE</figcaption></figure>

<figure><img src="image_42.png" /></figure>

<figure><img src="image_43.png" /><figcaption>Area
with no
evidence
of
corrosion
pitting</figcaption></figure>

<figure><img src="image_44.png" /></figure>
EDX analysis maps for Hastelloy C2000 GBE (GB2)

<figure><img src="image_45.png" /></figure>

Cr slightly more consumed on the surface
than other elements (48 µm depth)

<figure><img src="image_46.png" /></figure>

<figure><img src="image_47.png" /></figure>

EDS analysis profile for Hastelloy C2000 GBE

In coherence with the formation of unstable Cr2O3 and
dissolution of Cr in the salt [Chmakoff, Ph D, 2023]
## Ambient temperature tensile tests

<figure><img src="image_48.png" /></figure>

<figure><img src="image_49.png" /></figure>

<figure><img src="image_50.png" /></figure>

<table><thead><tr><th colspan="2">Condition</th><th>Longitudinal direction</th><th>Transverse direction</th></tr></thead><tbody><tr><td rowspan="2">Before corrosion</td><td>Brut (REF)</td><td>2</td><td>2</td></tr><tr><td>GBE (GB2)</td><td>2</td><td>2</td></tr><tr><td rowspan="2">After corrosion</td><td>Brut (REF)</td><td>2</td><td>2</td></tr><tr><td>GBE (GB2)</td><td>2</td><td>2</td></tr></tbody></table>

<figure><img src="image_51.png" /></figure>

<figure><img src="image_52.png" /></figure>

CEA/LC2M facility

Deformation rate: 5.10-4 s-1
Conventional traction curve at the initial state (MA-t) and after GBE treatment

<figure><img src="image_53.png" /></figure>

<figure><img src="image_54.png" /></figure>

- No significant difference between SL and ST (isotropic material)

- Large plastic domain (A% ~ 75 %)

- High strain hardening rate

- Low necking before fracture

⇒ After GBE : the properties vary very little
low decrease of Rm and Rp0,2 in agreement
with the larger grain size obtained after GBE
Traction curves of the Hastelloy C2000 before and after corrosion

<figure><img src="image_55.png" /><figcaption>Ductile Fractures</figcaption></figure>

<figure><img src="image_56.png" /></figure>

Comparison after corrosion – **Orange curves**

- MA-t (REF) : significant decrease for At, Rm and Rp0.2 (especially in ST)

- GBE 50%-5C5% (GB2) : little or no decrease in properties after corrosion contrary to REF
# Summary

Improve the corrosion resistance of Hastelloy C2000 by modifying the grain microstructure
Thermomechanical treatment (GBE) : 5 rolling cycles, ε : 5% per cycle

## Conclusions

* Confirmation of the modification of the nature of the GBs and the relevance of the EBSD to characterize their nature (proportion, length)

* After GBE
    1) the grain size increases (grain clusters)
    2) the length fraction of special boundaries (Σ3+Σ9) compared to general boundaries increases slightly

* After corrosion tests (NaCl MgCl2, 600°C, 1 week) : Observation of a surface degradation (uniform corrosion) with a chromium depletion at the surface (stronger after GBE compared to REF)

* After tensile tests at RT :
    1) Degradation of properties (especially A%) for the "solution annealing" condition (REF) after corrosion
    2) Little or no decrease of the properties after GBE treatment (GB2) after corrosion

## Future work

- Improve the statistic (duplicate corrosion tests and tensile tests) to confirm the trends

- Conduct a study on another alloy more sensitive to intergranular cracking and more sensitive to GBE (Inconel625 ?)
<figure><img src="image_57.png" /></figure>

Thanks to the main contributors :

E. Rouesne (CEA-LA2M) : EBSD analysis

R. Malacarne, M. Geneve (CEA-LECNA) : corrosion tests

M. Sarrazin (CEA-LC2M) : tensile tests

D. Soumet, M. Ronfard (CEA-LTMEx) : Cold rolling, heat treatments, machining

Thank you for your attention !

Reference :

Wei Cao et al., "., Effects of initial microstructure on the grain boundary network during grain boundary engineering in Hastelloy N alloy" Journal of Alloys and Compounds 704 (2017) 724-733

C. Hu et al., « Improving the intergranular corrosion resistance of 304 stainless steel by grain boundary network control », Corrosion Science 53 (2011) 1880-1886

H. Sun et al., « Corrosion behavior of 316SS and Ni-based alloys in a ternary NaCl-KCl-MgCl2 molten salt », Sol. Energy. 171 (2018) 320-329. https://doi.org/10.1016/j.solener.2018.06.094.

E. Martin et al., Materiala 9 (2020), 100544

Thèse A.Chmakoff, « Compréhension des mécanismes de corrosion en environnement réacteurs du futur à combustible et caloporteur sels fondus », 2023