This document summarizes research on corrosion of structural materials in molten fluoride and chloride salts. It describes experiments conducted on the Molten Salt Reactor Experiment from 1965-1969 which observed no corrosion of metallic and graphite components when operating with fluoride fuel salt. Subsequent testing has found corrosion occurs due to reactions between salts and chromium in alloy materials. Current challenges include the lack of data on chloride salt corrosion and developing predictive models of corrosion behavior to aid in materials selection and licensing.

1. ORNL is managed by UT-Battelle
for the US Department of Energy
Corrosion of
Structural
Materials in
Molten Fluoride
and Chloride
Salts
Stephen Raiman, James Keiser
Oak Ridge National Laboratory
USA
1st IAEA workshop on Challenges for Coolants in the
Fast Spectrum: Chemistry and Materials

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Viable Salts for MSR Concept
D.F. Williams

3. 3 Presentation_name
Common Coolant Salts
D.F. Williams (2006)

4. 4 Presentation_name
Molten Salt Reactor Experiment (MSRE)

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Initial MSRE Operations
• Reactor went critical in 1965 operating with fuel salt
containing 235UF4 in LiF, BeF2 and ZrF4
• Sustained operation at full power began in December,
1966
• First phase of operation was completed in March, 1967
• Fluoride fuel salt operated at temperatures >1200°F
(~650°C)
• No corrosive attack observed on metallic and graphite
components
• Reactor operated reliably and radioactive materials were
safely contained

6. 6 Presentation_name
Second Phase Of Operation Began In 1968
• The fuel salt was treated with fluorine gas in a vessel in
the Fuel Processing Cell
• This removed uranium as UF6 but also other elements
like Mo (as MoF6)
• Subsequent examination showed significant corrosion
likely occurred to the fuel processing vessel
• A charge of 233U fluoride was then added to the LiF-
BeF2-ZrF4 carrier salt
• On October 2, 1968, the reactor went critical with the
233U-containing fuel
• Eventually, a small amount of plutonium fluoride (as
PuF3) was added to the salt

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Comparison Of Relevant Alloy Compositions
For Use In Containing Molten Salts
Alloy Ni Mo Cr Fe Co Si Mn C Al Other
Hastelloy N Bal 16 7 4 M 0.02M 1 M 0.8M 0.06 0.5M V 0.5M,
Haynes 242 Bal 25 8 2 M 1 M 0.8M 0.8M 0.03M 0.5M B 0.006M
316L stainless 10.2 2.1 16.4 Bal 0.02
Inconel 625 Bal 9 21.5 5 1.0 0.1 0.5 0.1 0.4 Nb 3.6, Ti 0.4
Mod Hast N* Bal 16 7 4 M 0.02M 1 M 0.8M 0.1 Nb 1-2 or Ti 0.5
Alloy A Bal 19 7 0 0.8 0.05 Ta 0.5
Alloy B Bal 17 7 0 0.8 0.05 Ta 4
M indicates maximum content allowed
* Modified Hastelloy N content from McCoy

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Free Energy Of Formation Will Determine
Which Compounds Will Ultimately Form
Compound Free energy of formation (kcal per gram-atom)
800°K 1000°K
MoF6 -52.7 -50.9
WF6 -59.1 -56.8
NiF2 -58.9 -55.3
HF -66.3 -66.5
FeF2 -68.6 -66.6
NbF5 -74.6 -72.4
CrF2 -77.3 -75.2
VF3 -82.2 -80.4
TiF4 -86.9 -85.4
BeF2 -109.9 -106.9
LiF -128.5 -125.2
From S. Cantor and W. R. Grimes, “Fused-Salt Corrosion and its Control in Fusion Reactors,” Nucl.Tech. 22,
120, (1974).
Similar trend in Cl salts, see: Ambrosek, PhD
Thesis, U. of Wisconsin (2011)

9. 9 Presentation_name
Reactions Of Metallic Materials In Fluoride
Salts Result In Chromium Removal
• Salt impurities like H2O, O2 and HF will react with
metallic materials
2HF(d) + Cr(s) CrF2(d) + H2(d)
• Many metallic fluorides will result in formation of Cr
fluorides that are soluble in the salt
Cr(s) + FeF2(d) CrF2(d) + Fe(s)
• Some constituents of certain salts will react with Cr
and cause removal of Cr from metallic materials
Cr(s) + 2UF4(d) CrF2(d) + 2UF3(d)
All these reactions result in Cr being removed from the metallic
container and going into solution in the molten salt

10. 10 Presentation_name
Chromium Depletion in Salt-Facing
Materials
Cross-sectional images of Hastelloy-N after corrosion testing in FLiNaK at
850°C for 500 h for
L.C. Olson et al. Journal of Fluorine Chemistry 130 (2009) 67–73

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Corrosion Rate is High Initially, Largely
Due to Impurities
Calculated corrosion
rate and cumulative
attack for 800°C
section of Hast-N loop
containing NaF-ZrF4-
UF4 salt. Hot-leg
temperature, 800°C;
cold-leg temperature,
600°C. at 600°C
J. H. Devan, R. B. Evans, ORNL-TM-328

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Corrosion Testing Methods

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Capsules Provide A Quick Check Of Alloy
Compatibility In Isothermal Tests
• Samples are attached to bottom lid of
precleaned (Mo) capsule
• While in a glove box, preprocessed salt
is added to the capsule, then the
capsule is EB welded shut in a vacuum
box
• The Mo capsule is enclosed in a second
capsule (SS) and is ready for exposure
• After exposure, the capsule is removed
from the furnace and inverted to drain
the salt away from the sample
• Once salt has cooled, the sample is
removed and examined
Sample mounted on
capsule lid
Inner and outer
capsules

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Thermal Convection Loops Have Been
Used Extensively To Study Mass Transfer
• Loop design has evolved over
several decades
• Heating of the left vertical leg and
cooling of the right vertical leg
enables development of a
temperature gradient and clockwise
flow of salt around the loop
• Corrosion coupons can be inserted
in the tubes that form the vertical
legs of the loop
• The top left tank provides a site for
electrochemical probes and additions
to the salt
Keiser et al., J. Nucl.
Mater. (1979)

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Such Dissolution Drives Materials
Degradation And Impacts Salt Flow
• For fluoride and chloride salts dissolution is key
Cr + NiF2 = CrF2 (in salt) + Ni
• Under such conditions,
thermal-gradient mass transfer
is an important issue for nuclear
systems
• Modeling of the mass transfer is
straightforward if solubilities,
reaction pathways, and kinetics
are known or can be accurately
calculated or estimated
• Devil is in the details of the chemistry,
especially for fuel salts where
embrittlement is also a concern
Hot
Leg
Cold
Leg
Mass transport
Dissolution Deposition
Liquid Liquid
Containment Containment
Ji = ks,i(Co,i – Ci)
Ji = kp,i(Co,i – Ci)

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Current Challenges

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Current Challenges
• Lack of data on chloride salts
• Understanding conditions at salt/material interface
• Design and testing of new candidate materials
• Lack of predictive modeling

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Limited Information On Chloride Salt
Corrosion Shows Wide Range In Results
Study Salt
Temp
max
(°C)
Time
(h)
Thickness
affected
(μm)
Corrosion
Rate (mm/yr)
Condition notes
Susskind NaCl-MgCl2-KCl 500 1000 5.334 0.044 Loop with purified salt
Susskind NaCl-MgCl2-KCl 500 2500 3.556 0.013 Loop with purified salt
Mishra BaCl-KCl-NaCl 845 720 2387.6 29
metal processing, no attempt to
purify salt, open to air
Mishra NaCl 845 100 1905 167
metal processing, no attempt to
purify salt, open to air
Indacochea NaCl 650 144 30.48 1.8
crucible test with 10% oxygen in
argon cover gas
Shankar LiCl-KCl 400 2 5.08 22 crucible with air cover gas
Shankar LiCl-KCl 500 2 27.432 120 crucible with air cover gas
Shankar LiCl-KCl 600 2 25.4 111 crucible with air cover gas
Results vary wildly with quality of salt and the atmosphere.
Clearly, well controlled studies are needed.

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Impurities in Fl Salt
Corrosion of Hastelloy-N Coupons exposed in FLiNaK
showed strong effect of H2O addition
Ouyang et al. J. Nucl. Mat. 437 (2013) 201–207

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Design of New Materials
• New materials needed for more demanding
conditions
– Higher temperatures, aggressive chemicals, longer
lifetimes, radiation damage
• Path to deployment is long
– Extensive testing
– Licensing

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Computational Modeling
• Lifetime modeling of material behavior in salt to aid
in licensing, component selection, safety, and
economic operation
• Predictive, physics-based modeling to aid in reactor
design, alloy selection and salt selection.

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Path Forward
• Targeted corrosion experiments to understand
corrosion phenomena
• Focus on effect of salt chemistry on alloy behavior
• Advanced alloys with improved properties
• Efforts toward computational modeling of corrosion,
and integration with multiphysics reactor codes