Electrochemistry for Investigation of Material

Compatibility in Molten 2LiF-BeF2 (FLiBe) Salt William Doniger, Thomas Chrobak, Brian Kelleher, Kieran Dolan, Karl Britsch,

Cody Falconer, Mohamed Elbakhshwan, Mark Anderson*, Dr. Kumar Sridharan*

University of Wisconsin-Madison, *PIs

Motivation Fluoride Salt Redox Potential & Dissimilar Materials

Corrosion in molten fluoride salts is driven strongly by the presenceof impurities such as moisture, oxygen, and metals. These impuritiespromote the formation of highly unstable fluorides with thetendency to corrode vessel materials [1]. Development of accurateand responsive chemistry controls could enable MSR constructionwith readily available, code certified, and economically viablestructural materials. Electrochemistry is a practical method suitedfor the investigation of molten salt chemistry. The fluoride salt redoxpotential is considered a metric to gauge a salt’s relativecorrosiveness. The dynamic beryllium reference electrode (DBRE) isa quasi-reference electrode used to gauge the salt’s chemicalpotential energy and as a reference point for identifying commoncorrosion products in FLiBe. Cyclic Voltammetry and the fluoride saltredox potential are used to correlate the corrosion of 316 stainlesssteel with salt chemistry.

Impurity Driven Corrosion

Experimental Methods

Moisture Impurity Reactions:𝑥

2𝐻2𝑂 +𝑀𝑠𝑎𝑙𝑡𝐹𝑥 → 𝑀𝑠𝑎𝑙𝑡𝑂𝑥

2+ 𝑥𝐻𝐹

𝑥𝐻2𝑂 +𝑀𝑠𝑎𝑙𝑡𝐹𝑥 → 𝑀𝑠𝑎𝑙𝑡(𝑂𝐻)𝑥+ 𝑥𝐻𝐹

2𝐻𝐹 + 𝐶𝑟 ↔ 𝐶𝑟𝐹2 + 𝐻2

Impurities in the Melt:

𝐹𝑒𝐹2 𝑜𝑟 𝑁𝑖𝐹2 + 𝐶𝑟 ↔ 𝐶𝑟𝐹2 + 𝐹𝑒 𝑜𝑟 𝑁𝑖

Electrochemical Probe for FLiBe Salt

Schematic of an electrochemical system

2: 𝐶𝑟2+ + 2𝑒− ↔ 𝐶𝑟0

4: 𝐹𝑒2+ + 2𝑒− ↔ 𝐹𝑒0

Dissolved Metal Fluoride Impurities

-0.350

-0.300

-0.250

-0.200

-0.150

-0.100

-0.050

0.000

Wei

ght

Chan

ge

(mg/c

m^2)

Pure FLiBeFLiBe + CrF2FLiBe + FeF2 (99 ppm)FLiBe + FeF2 (392 ppm)

Unstable impurity fluorides oxidize important alloying elements.

Chromium difluorides, CrF2, is one of the most stable fluorides and is preferentially attacked.

AcknowledgmentsThe U.S. Department of Energy Integrated Research Project Nuclear Energy University Program funded this research under Contract No. DE-NE0008285. In collaboration with: MIT,

University of California-Berkeley, University of New Mexico

References[1] J. W. Koger, “Effect of FeF₂ addition on mass transfer in a Hastelloy N: LiF-- BeF₂--UF₄ thermal convectionloop system - ORNL-TM--4188.” Oak Ridge National Lab, TennUSA, 1972

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

[3] K. Dolan, “Redox Potential Measurement and Control for the Fluoride-Salt-Cooled High-TemperatureReactor,” MS Thesis, Nuclear Engineering and Engineering Physics, University of Wisconsin - Madison, 2015.

[4] T. Chrobak, “Corrosion of Candidate Materials in Molten FLiBe Salt for Application in Fluoride-salt CooledReactors,” Thesis, University of Wisconsin - Madison, Madison, WI, 2018.

[5] W. H. Doniger, “Electrochemistry for Corrosion Prevention in Molten Li2BeF4 (FLiBe) Salt for Fluoride Salt-cooled Reactors,” Thesis, University of Wisconsin - Madison, 2018.

• ICP-OES and CV confirm that the concentration of dissolved Cr increased after corrosion. • The fluoride salt redox potential is lowered after corrosion in all cases.• Salts containing dissolved FeF2 underwent complete cation exchange by the likely reaction 𝐹𝑒𝐹2 + 𝐶𝑟 ↔ 𝐶𝑟𝐹2 + 𝐹𝑒 [5].

• Different salt impurities lead to variation in corroded sample surface roughness.• Cross section EDS compositional mapping reveals:

• Depletion of chromium at near surface grain boundaries. FeF2 accelerates depletion.• Deposition of an iron rich surface layer in salts with FeF2 as a result of cation exchange.

• Interpretation of weight change is complicated by cation exchange processes (Fe with Cr) [5].

How can changes in salt chemistry be detected? What effect doescorrosion have on the impurity content of FLiBe salt?

?

What is the influence of metal fluoride impurities on 316H StainlessSteel corrosion?

?

What is the influence of salt redox potential on 316L StainlessSteel? How do dissimilar materials behave in these salt chemistries?

?

• Samples in both capsules and salt chemistries experienced similar depth of grain boundary Cr depletion.

• Beryllium salt reduction led to a 10% reduction in weigh loss on average.• Samples exposed to graphite showed lower weight loss due to infiltration of carbon into

the bulk (carbide formation) [4].

• Corrosion causes deep cleavage of grain boundaries.• Surface recession reveals small Mo rich particles.

Weight Change of 316H Stainless Steel (700˚C/1000hrs)

316H Stainless Steel was exposed to four salt chemistries in 316L Stainless Steel Capsules.

(a) 50 g As-received FLiBe

(b) 50 g FLiBe + 0.5 mg Be metal

(c) Graphite capsules & liners

(c)

• 316L Stainless Steel was exposed to two variants of purified UW-made FLiBe: as-received and FLiBe + Be metal.

• The redox potential of as received and Be reduced FLiBewere 1.69 V and 1.41 V vs. Be/Be2+, respectively.

• Samples were placed in graphite and stainless steel lined capsules to investigate the effect of dissimilar materials.

Cyclic voltammograms of molten FLiBe salt

samples with various impurity additions at

650˚C. Scan rate was 80 mV/sec [5].

FLiBe + NiF2

FLiBe + FeF2

1.2

1.4

1.6

1.8

2

0 200 400

E vs

. Be|

BeF

2

FeF2 Concentration [ppm]

1.2

1.4

1.6

1.8

2

0 100 200 300

E vs

. Be|

BeF

2

NiF2 Concentration [ppm]

1.2

1.4

1.6

1.8

2

0 200 400 600

E vs

. Be|

BeF

2

CrF2 Concentration [ppm]

Initially Oxidizing Salt

Initially Reducing Salt

FLiBe + CrF2

A versatile electrochemical probe has been developed for studying FLiBe salt at temperatures between 460˚C and 700˚C [2,3].

Fluoride Salt Redox Potential• A measure of the salt’s inherent chemical potential

energy• The voltage difference between an inert electrode at

the salt’s potential and the dynamic beryllium reference electrode (Be/Be2+ redox couple).

Cyclic Voltammetry• Relates electric potential and current to the

concentration of different species in the salt.• Can detect the difference between dissolved Cr2+ and

Fe2+, two species with different reduction potentials.• Peak current is proportional to concentration [5].

• The redox potential is sensitive to dissolved FeF2

and NiF2 but not CrF2 [3,5].

(Doniger)

(Dolan, Doniger)

(Kelleher)

(Doniger)(Doniger)

(Doniger)

(Doniger)

(Chrobak)

(Chrobak)

(Chrobak)

Redox potential response to impurities in FLiBe [3,5].

Inductively Coupled Plasma – Optical Emission Spectroscopy

• A measure of FLiBe salt metallic content.• Used to bench mark salt chemistries used

in electrochemistry experiments

Reduced

Average

As-received

Average

Standard deviation of

3 identical samplesLOQ LOD

Aluminum 42.98 28.62 4.3 5.92 1.97

Chromium 5.479 3.669 1.4 3.16 0.986

Nickel 2.679 16.76 2.0 3.16 0.986

Iron 33.04 2.268 - 63.1 19.7

ICP-OES of common FLiBe chemistries [4].

Weight Change of 316L Stainless Steel (700˚C/2000hrs)