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 KeiserOak 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

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Common Coolant Salts

D.F. Williams (2006)

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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

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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 -50.9WF6 -59.1 -56.8NiF2 -58.9 -55.3HF -66.3 -66.5

FeF2 -68.6 -66.6NbF5 -74.6 -72.4CrF2 -77.3 -75.2VF3 -82.2 -80.4TiF4 -86.9 -85.4BeF2 -109.9 -106.9LiF -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)

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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

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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 transferis an important issue for nuclearsystems

• Modeling of the mass transfer isstraightforward if solubilities,reaction pathways, and kineticsare known or can be accurately calculated or estimated

• Devil is in the details of the chemistry, especially for fuel salts whereembrittlement is also a concern

HotLeg

ColdLegMass 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 SaltTemp 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 29metal processing, no attempt to purify salt, open to air

Mishra NaCl 845 100 1905 167metal processing, no attempt to purify salt, open to air

Indacochea NaCl 650 144 30.48 1.8crucible 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 FLiNaKshowed 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