1

Corrosion phenomena induced by coolant, blanket and fuel salts:

focus on stainless steels and high nickel alloys

Surenkov Aleksandr, Ignatiev Victor, Uglov Vadim

National Research Center “Kurchatov Institute”

2

• For all molten salt reactor (MSR) designs, materials selection is a very important

issue This report summarizes results, which led to selection of materials for

MSRs in the Russian Federation.

• Different forced and natural loops operated in reactor and laboratory conditions

with following molten salt mixtures: LiF-NaF-BeF2+PuF3, LiF-BeF2-UF4, LiF-

BeF2-ThF4-UF4 were tested in Russia. These non-isothermal corrosion tests

were done with stainless steels (H18N10T, EP 164) and high nickel alloys

developed for MSR in Russia (HN80М-VI, HN80МТY, HN80МТW, etc.) including

those developed in USA (Hastelloy–N), Czech Republic (MONICR) and France

(EM-721,) in the temperature range from 600 up to 800oC and mechanical loads

on specimens up to 80 MPa

• The main effort in these studies was focused on the study of selective chromium

corrosion and tellurium intergranular corrosion of the alloys and methods of their

suppression. With that purpose, have been developed devices and methods to

control the redox potential of the melts for different compositions of fluoride salts

containing beryllium and uranium, as well as ways to maintain it at a

predetermined level.

• This report presented and summarized earlier obtained and new data of

corrosion-mechanical tests of Ni alloys and stainless steels on the corrosion loops

Introduction

3

In terms of the decrease in the energy of the formation of fluorides, the metals are

arranged in the series:

Li>Na>K>Be>Th>U3+>Zr>U4+>Al>V>Ti>Mn>Cr>Nb>Fe>Co>Ni>Mo>W

Therefore, the fluorides of the melt do not enter into the oxidation reaction of metals

contained in the alloy.

When there are VF2, TiF2, CrF2, FeF2, NiF2, HF in the melt, there are reactions :

nMe1(s) + mMe2Fn(d) nМе1Fm(d) + mMe2(s), (1),

Where Me1(s) are the metals included in the structural material; Me2(d) -metal, the fluoride of

which is present in the melt as a dissolved impurity. The presence of moisture and oxides in the

structural material (oxide films of nickel, copper, molybdenum, etc.) on its surface in contact with

the fluoride salt is extremely undesirable. In this case, the reactions presented below will occur,

followed by oxidation of chromium and iron in the alloy:

• 2NiO + ZrF4 → 2NiF2 + ZrO2 (5)

• NiO + BeF2 → NiF2 + BeO (6)

• 2NiO + UF4 → NiF2 + UO2 (7)

Corrosion chemistry

• UF4 + 2H2O ↔ UO2 + 4HF (2)

• ZrF4 + 2H2O ↔ ZrO2 + 4HF (3)

• BeF2 + H2O ↔ BeO + 2HF (4)

• The development of effective methods for purification of melts of fluoride salts

from impurities of water and its accompanying oxygen (hydrofluorination), as well as

from metal fluoride impurities (reduction by hydrogen, beryllium or electrolysis)

were successfully implemented on loops in ORNL and then at NRC KI

4

• By the relation (9), the equilibrium concentration of CrF2 in the salt melt increases in

proportion to the square of the ratio of the molar concentration UF4/UF3.

• By changing the uranium ratio [U(IV)]/ [U(III)] downward and maintaining at an

acceptable level, it is possible to minimize the rate of corrosion of the alloy.

• Reduction of diffusion activity of Cr in solid alloys also increases its corrosion

resistance.

In the melt with the fuel component of UF4, the corrosion of the alloys proceeds according

to the reaction:

2UF4(d) + Me(s) ↔ UF3(d) + MeF2(d) (8), where chromium is most vulnerable than Fe, Ni or

Mo. Reaction (8) is reversible, therefore in the fuel circuit with a constant temperature

gradient, chrome from the hot zone will be transported and deposited in the cold zone,

forming a mechanism of continuous corrosion. The initiating factor of mass transfer is the

dependence of the equilibrium constant of the reaction (8) Cr with UF4 on the temperature,

the value of which increases with the temperature in the exponential dependence and is

determined by the relation:

Kp = (XUF3 / XUF4) 2 × (XCrF2 / ACr) (9)

Chromium of the structural material in salts recommended as coolants (LiF-NaF-BeF2, LiF-

BeF2, LiF-NaF-KF, Na-NaBF4) will be oxidized by reactions with impurities of metal fluorides

and complex oxygen ions (CrF3, FeF3, FeF2, NiF2, MoF3, SO42-, PO4

2-, NO3-) or as a result of

reactions with impurities in the surrounding shielding gas.

Chromic corrosion of structural materials

5

Control of the oxidation-reduction state of fuel salts

Electroreduction of U(IV) to U(III)

The voltammetric methodis based on measuring of the

difference between the redox potential of the melt EEQ and E1/2,

the voltammetric equivalent of the standard redox potential E0

of the U(IV)/U(III) couple, at [U(IV)] » [U(III)]. In conditions of

linear voltammetry, at a stationary electrode and a reversible

charge transfer of the melt-soluble oxidized and reduced

forms of uranium, E0 is approximately equal to the

polarographic half-wave potential E1/2 and corresponds to the

potential in the voltammogram, at which the current accounts

for 85.2% of the peak current. If the potential EEQ at

nonpolarizable molybdenum wire, which serves as the quasi-

reference electrode, is equated to zero, [U(IV)]/[U(III)] ratio is

given by the Nernst equation:

-E1/2=RT/nF ln[U(IV)]/[U(III) ] (10)

The melt CVA in corrosion tests with redox condition

([U(IV)]/[U(III)])=20 in fuel melt 70LiF-6.9 BeF2-21ThF4-2.1UF4 at

T=735 0C

Ecp[ (U(IV)]/[U(III)]

=–0.32V

Ecp=0

Dynamic beryllium reference electrode

A diaphragm-free three-electrode meter with a nonstationary

(dynamic) beryllium reference electrode was used to

measure the redox potential of the BeF2 and PuF3 containing

salts. Operation the electrode relies on deposition of a

short-lived beryllium coating on the molybdenum cathode

half-immersed into the melt in a three-electrode

electrochemical cell. After the polarization current is cut off,

the time variation of the emf is measured between the

prepared dynamic beryllium electrode and the molybdenum

indicator electrode, which is irreversible relative to the melt

ions and has a potential equal to the redox potential of the

medium.

Curve 1

Experimental e.m.f. relaxation curves at the DRPM recorded

consecutively during the corrosion experiment in the 15LiF -

58NaF - 27BeF2 melt (mole %) at 650 0С.

Curve 1- in the initial solvent melt, Curve 2- in the solvent melt

treated with metallic beryllium

Curve 2

6

Preparation of salt and purification of loop for corrosion tests

• Interaction of the melt with metal oxides and other corrosion products present on loop surfaces leads to salt contamination by nickel, iron, and chromium compounds.

• Another reason for appearance of these compounds in the melt are traces of moisture on loop surfaces and in powders of initial solvent components (ВeF2 is particularly hygroscopic). In the process of salt heating and melting, water interacts with the material of tank and loop walls and lithium and beryllium fluorides.

Sources of impurities in melt:

1. Content of impurities in initial salt components

2. Adsorbing water at powders of initial

components at conservation and transportation

3. Present of structural materials oxides at inlying

surface of corrosion loop designing after

fabrication and annealing.

The following reactions leads to formation

oxidants of structural materials :

BeF2 + H2O BeO + 2HF ( 12 )

NiO + 2HF NiF2 + H2O ( 13 )

Ni + 2HF NiF2 + H2 ( 14 )

Fe + 2HF FeF2 + H2 ( 15)

Technology clearing of melt salt and

corrosion loop from oxidants by following

processes now are realized step by step:

1. Removal the structural materials oxides from

inlying surface of corrosion loop designing by

washing molten salt solvent 15LiF-58NaF-27BeF2

2. Removal the structural materials oxides and

beryllium oxide from molten salt solvent 15LiF-

58NaF-27BeF2 by clearing mixture of gases HF+

He on reactions of the type:

O2– +2НF 2F– + H2O ( 16 )

3 Electrolysis of the salt melt aimed at removing

the main amount of dissolved nickel.

4. Removal the Ni and Fe fluorides from molten

salt solvent 15LiF-58NaF-27BeF by metallic

beryllium on reduction reactions of the type:

NiF2 + Be BeF2 + Ni ( 17 )

FeF2 + Be BeF2 + Fe ( 18)

7

Development of Hastelloy-N in US ORNL

In the early sixties, a new Ni-Mo-Cr structural alloy (INOR-8 or Hastelloy-N) was developed in ORNL

(USA), with increased corrosion and mechanical stability when working with various compositions of

fluoride salts in reactor conditions in the temperature range up to 700 C and a long service life of up to 30

years. The development of the fourth generation MSR concepts required a creating alloys of increased

heat resistance, radiation and corrosion-mechanical durability when operating at temperatures up to 800 °

C. The nickel alloys with improved corrosion-mechanical properties were developed in Russia(HN80М-VI,

HN80МТY, HN80МТW), Czech Republic(MONICR) , France(EM-721) and China(GH3535).

Element Hasteloy

N

US

Hasteloy

NM

US

Hasteloy-N

UNS10003

US

Hastelloy

-NM,

1976

HN80-

MT

Russia

HN80М-

VI

Russia

HN80МTY

(EK-50)

Russia

HN80М

TW

Russia

MONICR

Czech Rep

GH3535

China

EM-721

France

Ni base base base base base 82 82 77 base base 68.8

Cr 7.52 7.3 7 6-8 7.02 7.61 6,81 7 6,85 7.0 5.7

Mo 16.28 13.6 16 11-13 12.1 12.2 13,2 10 15,8 17.3 0.07

Ti 0.26 0.5─2.0 Ti+Al=

0.5

- 1.72 0.001 0,93 1.7 0,026 0.002 0.13

Al 0.26 - - - 0.038 1,12 - 0,02 0.021 0.08

Fe 3.97 < 0.1 4.0 max 0.1 < 0.33 0.28 0,15 < 0.33 2,27 3.9 <0.05

Mn 0.52 0.14 0.8 max 0.15-0.25 <0.1 0.22 0,013 <0.1 0,037 0.6 0.086

Nb - - - 1 - 2 - 1.48 0,01 - < 0,01 - -

Si 0.5 < 0.01 0.5 0.1 <0.05 0.040 0,040 <0.05 0,13 0.45 0.065

W 0.06 - - - - 0.21 0,072 6 0,16 - 25.2

Cu 0.02 - 0.35 max - <0.1 0.12 0.02 < 0.1 0.016 0.007 -

C 0.05 0.05 0.06 max 0.05 0.02 0.02 <0.025

<0.0032

0.014 0.055 <0.002

8

Corrosion resistance of Hastelloy-N alloy

* - Calculated value for the diffusion-kinetic model

Full-scale resource lab tests on loops with natural and forced circulation in various fuel compositions at temperatures up to

700 0С and reactor tests of the Hastelloy-N alloy in MSRE showed its high resistance to selective corrosion of chromium.

Test loop Structural

material Molten salt,

% mole

Fluid test conditions Talloy o C

Corr.

rate

μm/yr Circulation mode T max

о С Δ Тmах

о С Exposure

hrs NCL-1258 Stainless steel

S-304L 70LiF-23BeF2-

5ZrF4-1UF4 Natural convection 688 100 6100

79400 688

688

53

26

NCL-22 Stainless steel

S-316 71.7LiF-16BeF2-

12UF4-0.3ThF4 Natural convection 650 110 4298 650

23

NCL-16 Hastelloy–N

Hastelloy–N,

mod. Ti≤0.5

66.5LiF–34BeF2-

0.5UF4

Natural convection

V=2.5cm/s 704 170 28000 660

675

700

1.0(1.0*)

0.5

0.9 MSRE Hastelloy –N 65LiF-29.1BeF2-

5.0Zr F4-0.9UF4

Fuel circuit

circuit 654 22 21800 654 8.0(1,6*)

66LiF–34BeF2 Coolant circuit 580 35 26100 580 no NCL-15A Hastelloy –N 73LiF-2BeF2-5ThF4 Natural convection

V=0.7cm/s 677 55 35400 677 1.5

NCL-21A Hastelloy –N

Hastelloy–N,

mod. 1%Nb

71.7LiF-16BeF2-

12ThF4-0.3UF4

Natural convection

V=1 cm/s U4+/U3+≈104 704 138 10009

1004

704

704 3.5 (3.1*)

3.7

NCL-23 Inconel 601 71.7LiF-16BeF2-

12ThF4-0.3UF4

Natural convection

V=1 cm/s, U4+/U3+≈40 704 138 721 704 ≥34

NCL-24 Hastelloy–N,

mod. 3.4%Nb 68LiF-20 BeF-

11.7ThF-0.3UF4 Natural convection 704 138 1500 704 2.5

FCL-2b Hastelloy –N

Hastelloy–N mod. 1%Nb

71.7LiF-16BeF2-

12ThF4-0.3UF4

Forced convection

V=2.5–5 m/s U4+/U3+≈100

704 138 4309

2242 704

704 2.6(2.5*)

0.4

FCL-2 Hastelloy –N

92NaBF4-8NaF V=2.3 m/s

V=6.2 m/s 620 170 5100

5100 620

620 12

16

9

Development status of Ni-based alloys at ORNL

•Hastelloy N alloy was the sole structural material used in the Li,Be,Zr,U/F MSRE and contributed significantly

to the success of the experiment. Less severe corrosion attack (<1mil per year) was seen for the Hastelloy N in

contact with the MSRE fuel salt at temperatures up to 704oC for three years (26,000 hours).

•Hastelloy N used for the MSRE was subject to a kind of "radiation hardening," due to accumulation of helium at

grain boundaries.

•In MSRE was found tiny cracks on the inside surface of the Hastelloy N piping. These cracks were caused by

the fission product tellurium.

•The research toward finding a material for constructing a MSR that has adequate resistance to irradiation

embrittlement and intergranular cracking by tellurium has progressed.

•ORNL findings suggest that MSR could be constructed of 1-2%-Nb-modified Hastelloy N and operated

satisfactorily at 650oC.

Variations of severity of cracking with Nb content. Samples

was exposed for indicated times to salt containing Cr3Te4 and

Cr5 Te6 at 700C.

Cracking behavior of Hastelloy-n exposed 260 hr of 700 C to MSBR fuel

salt containing CrTe1.266 .

10

Development of HN80MTY alloy(EK-50) in NRC KI

•The development of Russian domestic structural material for MSR was substantiated by available

experience accumulated in ORNL MSR program on nickel -base alloys for UF4 -containing salts.

Ampoule corrosion tests were conducted at temperatures of 650 and 800 ° C. The alloy HN80MT was

chosen as a base. Its composition (in wt.%) is Ni(base), Mo(12.2), Cr(6.9), Ti(1.7), C(0.02). The

development and optimization of HN80MT alloy was envisaged to be performed in two directions:

improvement of the alloy resistance to a selective chromium corrosion,

increase of the alloy resistance to tellurium intergranular corrosion and cracking.

•About 70 differently alloyed specimens of the HN80MT were tested. Among alloying elements there

were W, Nb, Re, Y, V, Al and Cu. The main finding is that alloying by aluminum at a decrease of

titanium down to 0.5% revealed the significant improvement of both the corrosion and mechanical

properties of the alloy. Irradiation effect on a corrosion activity of fuels was also studied. It was shown

that at least up to the power density 10 W/cm3 in fuel composition LiF–BeF2–ThF4–UF4 there is no

radiation induced corrosion.

•Then the radiation study of 13 alloy modifications were carried out. Specimens (in nitrogen

atmosphere) were exposed to the reactor neutron field up to the fluency of 3·1020 n/cm2. Experimental

results of alloy mechanical properties at temperatures of 20, 400 and 650°C for nonirradiated and

irradiated specimens permits to rest only four modifications. These alloys modified by Ti, Al and V

have shown the best postirradiation properties.

•The results of combined investigation of mechanical, corrosion and radiation properties various

alloys of HN80MT permitted us to suggest the Ti and Al-modified alloy as an optimum container

material for the MSR. This alloy named HN80MTY (or EK-50).

11

Results of corrosion tests at NRC KI with molten fluoride salts

Loop NRC KI Salt,

in mole %

Specimens

material

Тmax , С Т, С Duration

hr

Corrosion

rate, μm/yr

Solaris 46.5LiF-11.5NaF-42KF 12H18N10T

HN80МТ

620 20 3500 250

22

KI C1

КI С2

КI С3

92NaBF4-8NaF 12X18H10T

AP - 164

HN80МТ

630

630

630

100

100

100

1000

1000

1000

250

50

12

КI M1 66LiF- 34BeF2+UF4 12H18N10T 630 100 500 20

KURS-2 66LiF -34BeF2+UF4 12H18N10T 750 250 750 25

КI F1

КI F2

71.7LiF-16BeF2-12ThF4-

0.3UF4+Te

HN80МТ

HN80МТY

750

750

70

70

1000

1000

3.0

6.0

NCL-1

15LiF-58NaF-27BeF2 +PuF3

HN80M-VI

HN80МТY

MONICR

700

100

1600

5

5

19

КI Тe1

15LiF-58NaF-27BeF2 +Cr3Te4

HN80M-VI

HN80МТY

MONICR

700

10

400

3

3

15

KI Te2

75LiF-5BeF2-20ThF4 +(0.27-

2.1)UF4 + Cr3Te4

Average value [U(IV)/U(III)] = 5

HN80M-VI

HN80МТY

HN80MTW

EM 721

730

735

735

735

40

250

29

57

28

10

KI Te3

73LiF -27BeF2+ 2.0UF4

+Te(metal)

Average value [U(IV)/U(III)]= 45

Hastelloy N

HN80МТY

HN80MTW

EM- 721

760

40

256

21(20*)

8

12

22

73LiF -27BeF2+ 2.0UF4

+Te(metal)

Average value [U(IV)/U(III)]=85

Hastelloy

NHN80МТY

HN80MTW

EM 721

800

780

800

800

40

248

45(56*)

52

63

55

* - Calculated value for the diffusion-kinetic model

12

Corrosion resistance of alloys in 8NaF-92NaBF4

Structural

material

Fluid test conditions Content impurities before test,

10-4 mass %

Pres

ence

IGC

Corr.

rate

μm/yr Circulation

mode

T max о С

Δ Тmах о С

Exposur

e

hrs

Fe Ni Cr H2O for

oxygen

12H18N10T

EP-160

Isothermal

condition

420 0 500 11 1.0 0.5 <103

Yes

Yes

-91

-10

12H18N10T

EP-160

Isothermal

condition

630 0 500 11 1.0 0.5 <103

Yes

Yes

-150

-78

S-304

(ORNL)

Isothermal

condition

600 0 100 - - - - -134

XN80MT

XN80MTY

Isothermal

condition

630 0 500 11 1.0 0.5 <103

No

No

-12

-12

12H18N10T

EP-160

Natural

convection

V=2 cm/s

630

630

95 500 11/35

0

1.0/1.2 0.5/650 <103 Yes

Yes

-250

-50

12H18N10T

EP-160

535 95 500 11/35

0

1.0/1.2 0.5/650 <103 No

No

+70

+20

Hastelloy-N Isothermal

condition

600 0 100 - - - - -7

Hastelloy-N

mod. 0.5Ti

Natural

convection

V=3.6 cm/s

607

460

147 500 223/

650

28/

95

19/348 800/

2800

No

No

-18

+18

Stainless steel EP-164: base – Fe, 22-25Ni, 14-16Cr, 0.5-1.0 Mn, 1.4-1.8Ti, 4.0-5.0W, 0.6Si, 0.025 Ce, 0.12C (%mass)

Corrosion rate of alloys in coolant 8NaF-92NaBF4 fluoride salt

• Corrosion of 12H18N10Т and EP-164 in the 8NaF-92NaBF4 (mol %) was studied in the RRC KI in natural circulation corrosion loop

with a maximum temperature of 630oC for 500 hrs exposure time.

• Under these conditions, the uniform corrosion rate for EP-164 was 5 times less than that of steel 12H18N10T. The depth of the

surface defects for both alloys is 60-70μm. The data obtained in ORNL for similar test conditions of SS-304L are in agreement with

our data for 12H18N10T. The maximum corrosion rate for these steels in a NaF-NaBF4 natural convection loop was 250 μm/yr.

• The ORNL data for natural convection loops where the NaF-NaBF4 salt was heated from 400 °C to 600 °C, the corrosion of the

Hastelloy-N alloy lies in the range of 5-20 μm/yr and this value was usually determined by the degree of coolant purification

13

Li,Be,Na,(Pu)/F Loop Corrosion Studies

Results of 1200 hrs loop corrosion experiment with on-line redox potential measurement demonstrated that high temperature operations with molten 15LiF-58NaF-27BeF2 salt are feasible using carefully purified molten salts and loop internals. In established interval of salt redox potential 1.25-1.33 V relative to Be reference electrode, corrosion is characterized by uniform loss of weight from a surface of samples with low rate (2-5μm/yr for HN80М-VI and HN80МTY alloys and 9-19 μm/yr for MONICR).

It was not found any significant change in corrosion behavior of materials samples in melt due to the presence of 0.5 mole % PuF3 addition in Li,Be,Na/F salt. Specimens of HN80M-VI from the loop exposed during 400 hrs at temperature 650C showed uniform corrosion rate <6 μm/yr.

No intergranular corrosion of alloys is observed in the loop.

MONICR

HN80MTY

HN80M-VI

Cr,

mas

s.%

μm

μm

μm

Cr,

mas

s.%

Cr,

mas

s.%

Main parameters of the corrosion loop: • Salt composition 58NaF-15LiF-27BeF2 +PuF3

• Temperature salt melt in zone of exposition of specimens- 620-700oC

• Difference of salt temperature on circuit of circulation–in limits 80-100oC

• Salt velocity in circuit of circulation - in limits 1-5 cm/sec

• Structural material of corrosion loop - nickel NP-2 (99.5 Ni)

• Material of investigated alloys:HN80M-VI (Hastelloy N modified 1.48%Nb),

HN80MTY (1.12 %, Al- 0.93 %Ti), MONICR (Hastelloy N modified 2.27%Fe)

14

The alloy resistance to intergranular cracking is estimated by parameter “K”, representing number of cracks at

length of 1 cm longitudinal section of sample subjected to deformation of a stretching, multiplied by their average

depth in micrometers.The second criterion was maximum depth of cracks (in μm). Data NRC-KI ampule and

71.7LiF-16BeF2-12ThF4-0.3UF4 loop tests containing metallic Te permit us to make the following conclusions:

Te damage of Ni-Mo alloys (NRC КI, 1976 -1986)

1. Alloying by niobium to 1% without titanium reduces significantly the value "K" to 190 pcm/cm (for Hastelloy N-mod. K=1090 pcm/cm). 2. Alloying by niobium with titanium which is necessary in the alloy for ensuring post -radiation properties, resulted in the increase of “K" parameter at 650°C to 6850 pcm/cm. Thus, admitting the niobium as an element capable of increasing the alloy resistance to intergranular cracking, it should be also noted to reduce content of Ti to an acceptable level, which ensures the required radiation resistance. Such a method of alloying, while permits to improve significantly corrosion and radiation characteristics of material, but is not the radical solution. 3. Alloying a standard Hastelloy N modified by 0.5% of aluminum did not improve its corrosion properties, though enhanced markedly its mechanical characteristics. 4. Alloying by aluminum at a decrease of titanium up to 0.5% revealed the significant improvement of both the corrosion and mechanical properties of the alloy.

Structure subsurface layer of HN80МТ, HN80МТY and Standard

Hastelloy-N specimens after 500 hrs exposure in 71.7LiF-16BeF2-

12ThF4-0.3UF4 containing tellurium :

HN80МТ, isothermal test at Т=600С

HN80МТ, isothermal test at Т=750С

HN80МТ, nonisothermal loop test at

Т=750С, Т = 70С

Standard Hastelloy-N, isothermal test

at Т=700С

HN80МТY, isothermal test at Т=750С

HN80МТY, nonisothermal loop test at

Т=750С, Т = 70С

К=1300 pc x mμ/cm

К=2100 pc x mμ/cm

К=5440 pc x mμ/cm

К=9130 pc x mμ/cm

15

F

Without load

With load 80 MPa

MONICR

MONICR

ХН80М-ВИ ХН80М-ВИ

ХН80МТЮ

ХН80МТЮ

Т=700С

Е=1,2В К=690 pc / cm × μm

К= 1560 pc / cm × μm

К=380 pc / cm × μm

К=680 pc / cm × μm

К>10000 pc / cm × μm

К=3590 pc / cm × μm

Resistance of Ni-Mo alloys to Te corrosion depend on:

(1) melt redox potential, (2) thermal- mechanical loads,

(3) exposure time, (4) doping elements in alloy

К= 0 pc / cm × μm

Studies on alloy’s resistance to IGC, should

be continued in corrosion loops with thermal

gradient and longer exposure t=250hr, Cr3Te4 as a

source of tellurium

0 50 100 150 200 250

,hous

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

E, V

t

Te corrosion of Ni-Mo alloys in 15LiF-58NaF-27BeF2 melt without and under

mechanical load in dynamic conditions at 700 ºC and redox potential control

ХН80МТ alloyed by Mn

ХН80МТ alloyed by 0.56Nb1-1.0Ti

К= 3370 pc / cm × μm

16

Te intergranular corrosion in fuel salts

Elements of facility : tank lid (1), device for measuring redox potential (2), beryllium reducer

3), sampler salt (4); the dosing device of chromium telluride (5), cassettes of specimens of

alloys (6)

F

Ar

T

Ar

Ep

Cr3Te4

1

4 6

5

2 3

Ep=0

Ecp [U(IV)/U(III)]

=+0.03V

Test 1 after processing by beryllium –

[U(IV)] /[U(III)] = 0.7, fuel

75,36LiF-4BeF2-20ThF4-0,27UF4-0.37UF3 salt

Ecp [U(IV)/U(III)]

=–0.32V Ep=0

Ecp [U(IV)/U(III)]

=–0.70 V

Ep=0

Test 3 after fist addition 120 g NiF2 -

[U(IV)] /[U(III)] =20, fuel

70LiF–6,9BeF2–21ThF4–2,1UF4 salt

Test 4 after second addition 120 g NiF2 -

[U(IV)] /[U(III)] = 500, fuel

70LiF–6,9BeF2–21ThF4–2,1UF4 salt

The present study tellurium corrosion envisages five compatibility testing of nickel- based alloys, with the fuel salt with high content

of ThF4 (up to 20 mol.%) and UF4 (up to 2 mol.%) additives in the melt containing Cr3Te4, with a duration of 250 hours each, with the

change uranium ratio [U(IV)]/[(III) ] from ~ 1 to 500, in the temperature range from 720 to 750 C and mechanical stress on the samples

from 0 to 50 Mpa. Melt composition 75LiF-5BeF2-20ThF4 (%mol.) with the addition UF4 to 2 mol.% was selected for research

17

Alloy HN80M-VI 1.5%Nb K=8300 pc/cmx μm T=7500C

Alloy HN80MTY 1.1%Al, 0.9%Ti K=1850 pc/cmx μm T=750 0C

Alloy HN80MTW 1.7%Ti, 5.5%W K=8400 pc/cmx μm K=540 pc/cmx μm,T=7450C

Alloy EM721: 69Ni-25W-6Cr K=9200 pc/cmx μm T=750 0C

Te corrosion of Ni alloys in Li,Be,U/F melt

Test conditions: 735oC after 250 hrs with mechanical loading 25 MPa U(IV)/(UIII) ≈ 0.7 U(IV)/(UIII) ≈ 4.0. U(IV)/(UIII) ≈ 20 U(IV)/(UIII) ≈ 500 U(IV)/(UIII) ≈ 100

18

The reaction

2UF3 + СrF2 + Te0 → 2UF4 +

СrTe

at U(IV)/(UIII) <100 blocks

the transfer of free Te to the

structural material and

prevents the corrosion

Resistance of Ni-Mo alloys to Te IGC in Li,Be,U/F melt

Test 1

Average U(IV)/U(III)=45

760oC

Metallographic studies of alloy samples show a clearly pronounced dependence of the intensity of Te IGC on the oxidation-reduction potential and the temperature of the melt.

0

50

100

150

200

250

300

350

0 25 50 75 100 125 150 175 200 225 250

[U(I

V)]

/[U

(III

)]

K=530pc×μm/cm

l=26μm

K=5820pc×μm/cm

l=126μm

K=3500pc×μm/cm

l=69μm

K=5830pc×μm/cm

l=286μm K=4490pc×μm/cm

l=148μm

K=3490pc×μm/cm

l=290μm

K=3380pc×μm/cm

l=178μm

Test 2

Average

U(IV)/U(III)=60

750oC

Test 3

Average

U(IV)/U(III)=85,

800oC

Redox potential of the fuel salt 71LiF-27BeF2-2UF4 during the tests

Test 1. Test 3

EM-721 HN80MTY HN80MTW Ni-12Mo-7Cr-1.0Nb Hastelloy-N

UNS10003

19

• Structural materials recommended for fuel and intermediate circuit MSR are special

Ni-Mo alloys with low chromium concentration. The composition of the reference

Hastelloy-N alloy was optimized by researchers in ORNL (USA) for corrosion

resistance (in the gaseous atmosphere and in molten fluorides), for radiation

resistance and mechanical properties at high temperature.

• It is shown, that the redox potential of salt is a key parameter in the corrosion of

MSR constructional materials. A redox-buffer pair (for example in a fuel salt ratio

(UF4/UF3) can control chemical corrosion. For individual cooling salt compositions

(46.5LiF-11.5NaF-42KF), such redox-buffer pair should be matched.

• New materials are developed and tested in the Russian Federation, China (Ni-Mo-Cr

alloys), France (Ni-W-Cr alloys). The Russian nickel alloy HN80MTY, doped with Ti

and Al, is the most resistant to intergranual tellurium cracking when tested with

molten salts of Li,Th,U/F and Li,Be,Th,U/ F up to a temperature of 800 oC and with a

redox-potential salt [U(IV)]/[U(III)] having a value of ≤100.

• Detailed studies of the kinetics of the boundary diffusion of tellurium in candidate

alloys and the mechanism of their tellurium intergranular embrittlement should be

carried out under nonisothermal conditions simulating the operation mode of the fuel

circuit of the MSR. The metallurgy and properties of these alloys should be studied

in more detail in the future and especially with regard to the resistance to irradiation

and the possibility of manufacturing the required assortment of materials.

Summary