Material Compatibility in

Molten 2LiF-BeF2 (FLiBe) Salt William Donigera, Guiqiu Zhenga,b, Brian Kellehera,c, Kieran Dolana,b, Thomas Chrobaka, Karl Britscha,

Guoping Caoa,d, Mark Andersona,, Dr. Kumar Sridharana,*aUniversity of Wisconsin-Madison, bMassachusetts Institute of Technology, cTerra Power, dIdaho National Laboratory, *PI

Motivation

Materials exposed to FLiBe salts can encounter corrosionmechanisms unique to molten fluoride salts that could limit theirlongevity. To aid ASME Section III code qualification of candidatematerials for construction of Molten Salt Reactors (MSRs), corrosionexperiments in molten FLiBe salt are necessary. MSR componentswill be made from a select group of structural metal alloys, graphiteand silicon carbide. Both the compatibility of the materials with thesalt and with each other will greatly influence MSR design.

Structural Alloys

Isothermal Corrosion in FLiBe

Impurity Driven Corrosion

• Impurity driven corrosion is caused bythermodynamically favorable reactions betweenunstable impurities and alloying elements.

• The final corrosion product is the mostthermodynamically stable dissolved fluoride, CrF2.

Experimental Methods

• Static corrosion tests are conducted in FLiBe salt at 700˚C.

• Corrosion capsules are made of graphite crucibles designed forboth in-reactor and out-of-reactor corrosion experiments.

FLiBe Salt Production & Purification at UW-Madison

Static Corrosion Experiments

• High purity salt constituents (99.9%),inert atmosphere and purificationminimize corrosion.

• Hydrofluorination involves spargingof HF and H2 gas through moltenFLiBe to removes corrosive moisture,sulfur and some metal fluorides fromFLiBe [1].

Handling FLiBe in inert atmosphere gloveboxes

Corrosion Capsules Design

Dissimilar materials in the Fluoride Salt-Cooled High-Temperature

Reactor (FHR) primary loop.

• Graphite and structural metalsfor MSRs inhabit differentregions of the EMF series.

• Steel and nickel could besusceptible to galvanic attackwhile graphite is very inert.

• The combination of thesematerials could lead to long termmass transport.

Dissimilar Materials Corrosion

Production methods influence salt purity

FLiBe hydrofluorination purification

• The capsules can be lined with othermaterials such as nickel or steel tosimulate different environments.

• Samples are hung from either graphite orboron nitride caps depending on whetherelectrical contact is desired [2,3].

Oxide layers that normally provide corrosion protection for metalsare unstable in molten fluoride salts.

316 Stainless Steel & Hastelloy-N, a nickel based alloy, are attractivefor their resistance to fluoride salt corrosion and air-side oxidation.

Do dissimilar materials affect corrosion rate of 316L stainlesssteel and Haselloy-N in purified FLiBe salt?

0 2000 4000 6000 8000 10000

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Corrosion Time (hours)

fitting curve, in 316SS capsule

experimental data, in 316SS capsule

fitting curve, in graphite capsule

experimental data, in graphite capsule

one year

The depth of 316L stainless steel corrosion

attack, in terms of maximum Cr depletion depth,

as a function of corrosion time. (700˚C) [3]

3000 hour

2000 hour

1000 hour

316L SS GraphiteCapsule Material

?

Graphite

• IG-110 was exposed to moltenFLiBe salt for 1000 hours at 700 °C.

• Experienced negligible weightchange and surfaces appearuntouched after exposure [3].

700˚C/1000hrs in Nickel Capsule

700˚C/1000hrs in Graphite Capsule

Hastelloy-N

Capsule MaterialWeight Change

(mg/cm2)Nickel -0.124

Graphite 0.165

316L Stainless Steel

• In graphite capsules, the weight gain andretention of large chromium carbides atthe grain boundaries suggests C migratesfrom graphite into structural alloys.

• Coarsening of Mo- and Si-rich phasesindicate microstructural instability inHaselloy-N.

Hastelloy-N Weight Change (700˚C/1000hrs)

Surface SEM images of polished IG-110

graphite (700˚C/1000hrs) [3]

Silicon Carbide

Chemical vapor deposited siliconcarbide (SiC) is a load bearing coatingand responsible for containing fissionproducts in TRISO particle fuel pelletsfor FHR.

AcknowledgmentsThank you to the U.S. Department of Energy Integrated Research Project Nuclear Energy

University Program for funding this research under Contract No. DE-NE0008285

In collaboration with: MIT, University of California-Berkeley, University of New Mexico

TRISO particle fuel pellets

Nuclear grade graphite moderates neutrons in MSRs.

• 280 TRISO particles with ZrO2surrogate kernels were obtainedfrom Oak Ridge National Laboratoryand exposed to purified FLiBe

• No damage and very little corrosionobserved in 1000hrs at 700˚C [3].

Pre- and post- corrosion images of TRISO particles (700˚C/1000hrs) [3]

References[1] B. C. Kelleher, “Purification and Chemical Control of Molten Li2-BeF4 for a Fluoride Salt Cooled Reactor,” PhD, University of Wisconsin -Madison, 2015.

[2] T. Chrobak, “Corrosion of Candidate Materials in Molten FLiBe Salt for Application in Fluoride-salt Cooled Reactors,” Thesis, University ofWisconsin - Madison, Madison, WI, 2018.

[3] G. Zheng, “Corrosion Behavior of Alloys in Molten Fluoride Salts,” 2015 PhD thesis, April, 2015.

• The depth of corrosion attack is increased when graphite is present [3].

Cr compositional maps of post corrosion

316L stainless steel. (700˚C/1000hrs) [3]

(Kelleher)

(Chrobak)

(Zheng)

(Zheng)

(Zheng)

(Zheng)

(Zheng)

(Zheng)

(Zheng)

• Structural alloy corrosion is characterized by depletion of chromium fromgrain boundaries.