Corrosion of Structural Materials in Molten Fluoride and Chloride Salts

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1 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 ORNL is managed by UT-Battelle for the US Department of Energy

2 Viable Salts for MSR Concept 2 Presentation_name D.F. Williams

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

4 Molten Salt Reactor Experiment (MSRE) 4 Presentation_name

5 Initial MSRE Operations Reactor went critical in 1965 operating with fuel salt containing 235 UF 4 in LiF, BeF 2 and ZrF 4 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 5 Presentation_name

6 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 UF 6 but also other elements like Mo (as MoF 6 ) Subsequent examination showed significant corrosion likely occurred to the fuel processing vessel A charge of 233 U fluoride was then added to the LiF- BeF 2 -ZrF 4 carrier salt On October 2, 1968, the reactor went critical with the 233 U-containing fuel Eventually, a small amount of plutonium fluoride (as PuF 3 ) was added to the salt 6 Presentation_name

7 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 M 0.02M 1 M 0.8M M V 0.5M, Haynes 242 Bal M 1 M 0.8M 0.8M 0.03M 0.5M B 0.006M 316L stainless Bal 0.02 Inconel 625 Bal Nb 3.6, Ti 0.4 Mod Hast N* Bal M 0.02M 1 M 0.8M 0.1 Nb 1-2 or Ti 0.5 Alloy A Bal Ta 0.5 Alloy B Bal Ta 4 M indicates maximum content allowed * Modified Hastelloy N content from McCoy 7 Presentation_name

8 8 Presentation_name Free Energy Of Formation Will Determine Which Compounds Will Ultimately Form Compound Free energy of formation (kcal per gram-atom) 800 K 1000 K MoF WF NiF HF FeF NbF CrF VF TiF BeF LiF 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 Reactions Of Metallic Materials In Fluoride Salts Result In Chromium Removal Salt impurities like H 2 O, O 2 and HF will react with metallic materials 2HF(d) + Cr(s) CrF 2 (d) + H 2 (d) Many metallic fluorides will result in formation of Cr fluorides that are soluble in the salt Cr(s) + FeF 2 (d) CrF 2 (d) + Fe(s) Some constituents of certain salts will react with Cr and cause removal of Cr from metallic materials Cr(s) + 2UF 4 (d) CrF 2 (d) + 2UF 3 (d) All these reactions result in Cr being removed from the metallic container and going into solution in the molten salt 9 Presentation_name

10 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) Presentation_name

11 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 11 Presentation_name J. H. Devan, R. B. Evans, ORNL-TM-328

12 12 Presentation_name Corrosion Testing Methods

13 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 13 Presentation_name

14 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 14 Presentation_name Keiser et al., J. Nucl. Mater. (1979) The top left tank provides a site for electrochemical probes and additions to the salt

15 Such Dissolution Drives Materials Degradation And Impacts Salt Flow For fluoride and chloride salts dissolution is key Cr + NiF 2 = CrF 2 (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 Mass transport J i = k s,i (C o,i C i ) Dissolution Containment Liquid Deposition Liquid Containment J i = k p,i (C o,i C i ) Cold Leg 15 Presentation_name

16 16 Presentation_name Current Challenges

17 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 17 Presentation_name

18 Study Limited Information On Chloride Salt Corrosion Shows Wide Range In Results Salt Temp max ( C) Time (h) Thickness affected (μm) Corrosion Rate (mm/yr) Condition notes Susskind NaCl-MgCl 2 -KCl Loop with purified salt Susskind NaCl-MgCl 2 -KCl Loop with purified salt Mishra BaCl-KCl-NaCl metal processing, no attempt to purify salt, open to air Mishra NaCl Indacochea NaCl metal processing, no attempt to purify salt, open to air crucible test with 10% oxygen in argon cover gas Shankar LiCl-KCl crucible with air cover gas Shankar LiCl-KCl crucible with air cover gas Shankar LiCl-KCl crucible with air cover gas Results vary wildly with quality of salt and the atmosphere. Clearly, well controlled studies are needed. 18 Presentation_name

19 Impurities in Fl Salt Corrosion of Hastelloy-N Coupons exposed in FLiNaK showed strong effect of H 2 O addition 19 Presentation_name Ouyang et al. J. Nucl. Mat. 437 (2013)

20 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 20 Presentation_name

21 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. 21 Presentation_name

22 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 22 Presentation_name