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ORNL-TM-2490

C o n t r a c t No. W-7405-eng-26 METALS AM> CERAMICS DIVISION

COMPATIBILITY OF HASTELLOY NaBFk-NaF-KBF4

N AND CROLOY 9 M W I T H

(904-6 m o l e

FLUOROBORATE SALT

J. W. K o g e r and A. P. Litman

LEGAL NOTICE

P 3

Thla mwrt

DovernmMt mpollllored work. Neither the Unlted h i e m , wr the CommlasIon. nor uy perm .CUI.% on behalf of the Commlaslon: A. Makes .epWuMtyor represent8tim,exPressedor Implled.with respect to the sccum y . completenese. or ~ ~ I u l n eof s sthe Wormatlon contained la thla report, or that the w e of my Wormatlo% aprmratus, mstbod. or pmcess disclosed In thls mport m a y not infringe prhtnly owned r i g h or B. h m u n e n m y IllbiUueswith respect to the we of. or for &mZea readfmm the nsa of uy Information. apparatus. method. or +ess dl.cl0sed h thla r e m A. uaed tu the h e . "pernon actbg 011 behal~ of tbe &mmiaaion" includes uy emp l o y ~or conmetor of the Commlsslon. or employee ol much conmetor. to the extent that much employee or contractor of the Commlsaion, or employee of much contractor p p r e s . j dls~mlnatas.or provides sccess to. my informath puauaat to his empiopeut or contract 4 wlth the Commlmdon. or his employment with Neb contractor. Y I prepnred ~ as u1 scEOIult'of

APRIL 1969

OAK RIDGE NATIONAL LABORATORY Oak R i d g e , Tennessee c

operated by UNION CARBIDE CORPORATION for the U. S. ATOMIC ENERGY COMMISSION

iii

CONTENTS

Page

........................... Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental Procedure . . . . . . . . . . . . . . . . . . . . . . Materials Section and Fabrication . . . . . . . . . . . S d t Preparation . . . . . . . . . . . . . . . . . . . . Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Abstract..

Results

............................. NCL-10 (Hastelloy N) . . . . . . . . . . . . . . . . . . NCL-12 (Croloy 9M) . . . . . . . . . . . . . . . . . . . .......................... Thermodynamics of System Corrosion . . . . . . . . . . . Kinetics of System Corrosion . . . . . . . . . . . . . . S a l t Purification . . . . . . . . . . . . . . . . . . .

Discussion..

........................... Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions

1 1

3 5 5

7 9 9

19 26 31 33 36

37 37

COMPATIBILITY OF HASTELLOY N AND CROLOY 9M WITH NaBF4-NaF-KBF4 (90-4-6 mole

FLUOROBORATE SALT

J. W. Kbger and A. P. Litman

ABSTRACT

The compatibility of a relatively impure (> 3000 ppm impurities ) fluoroborate salt, NaBF4-NaF-KBF4 (90-4-6 mole $1, with Hastelloy N and Croloy 9M was evaluated i n natural c i r culation loops operating a t a maximum temgerature of 605째C with a temperature difference of 145째C. The Croloy 9Mloop (EL-12) was completely plugged a f t e r ,1440 h r of operation and the Hastelloy N l o (NCL-10) was three-quarters plugged a f t e r 8760 h r (one year of .operation. All major a3loying elements of the container materials mass transferred during operation as the r e s u l t of nonselective attack by virtue of the i n i t i a l oxygen and water contamination of the salt. Saturation.concentrations of 700 ppm Fe and 470 ppm C r were determined f o r the fluoroborate salt a t 46OOC. The mechanism of corrosion of the system i s as follows. I n i t i a l l y , metal fluoride compounds t h a t a r e soluble i n the salt are formed i n the hot leg. "he reverse reaction occurs i n the cold l e g and causes the metal t o de o s i t and t o diffuse i n t o the cold leg. This continues u n t i l 1) an ecpilibrium concentration of one o r more m e t a l fluorides is reached i n the salt at the coldl e g temperature and these compounds start depositing (e.g., NCL-10) or, (2) the equilibrium constant of the reaction changes so much with temperature t h a t the pure metal i s deposited (e.g., NCL-12).

-4 % c

ODUCTION

Nuclear reactors t h a t use molten fluorides as fhels a r e under development f o r the dual purpose of power production and thorium conversi0n.l

Heat generated i n the core region of such a molten-dalt

L l i

breeder reactor is transferred from the fuel-containing fluoride s a l t t o a secondary cool

rcuit of fluoride salt with

M ' SR Program Semiann. Progr. Rept. Feb. 29, 1968, ORNL-4254.

then dissipates the heat t o stem2 In the h l t e n - S a l t Reactor Experiment

(MSRE), the nickel-base alloy Hastelloy N has proved t o be an effective container m a t e r i a f o r the fluoride salt that contains the fuel and for the LiF-BeF2 diluent.

any potential fissile2 (containing U F ~ f e r t i l e 2 (containing ~ o r combination' (containing both UF4 and ThF4) salts proposed f o r the

h ), ~

MSBR have been or are being tested f o r t h e i r compatibility w i t h

Hastelloy N and other container materials.

"he choice of a salt for

the secondary coolant, however, remains open.

Test programs have only

considered coolant salts l i k e NaF-ZrF4 and, recently, IXF-BeF;!.

While

these salts have demonstrated excellent ccanpatibilitywith Hastelloy N, there is need f o r cheaper fluoride mixtures t h a t m e l t a t lower tempera-

On the basis of l o w cost (approximately 504/lb) and a r e l a t i v e l y l o w melting point (about 4OO0C, necessary f o r transferring heat t o supertures.

c r i t i c a l steam), fluoroborate s a l t s

- especially NaEF4 - w i t h

small

additions of NaF and/or KEP4 have recently become of interest as secondary coolants.

L i t t l e i s known, however, about the corrosion behavior

of these salts. The space diagram of the ternary NaF-NaRFc-KBFk system (Fig. 1 ) shows t h a t the salt mixture under consideration l i e s very near the NaBF4 corner and has a m e l t i n g point close t o 390째C.

Note t h a t oxygen impu-

rities that form BF3OH-c0mpounds significantly lower the melting point of the mixture below t h a t Shawn by the diagram.

The vapor pressure of

fluoroborate salts, especially NaBF4, i s higher than t h a t of other fluoride saltsY3because i n the temperature range of interest the sodium fluoroborate maintains such an equilibriumwith i t s components t h a t a significant amount of boron t r i f l u o r i d e gas i s present i n the system: NaBF'4 'NaF

+ BF3(g)

(1)

2P. R. Kasten, E. S. Bettis, and R. C- Robertson, Design Studies of 1000-W(e) h l t e n Salt Breeder Reactors, ORNL-3996 (August 1966). 3H. S. Booth and D. R. Martin, Boron Trifluoride and Its Derivatives, Wiley, New York, 1949.

3 ORNL-OWG NaF*K6F4

(90-4-6

SE-5560R

mole %I

KBF.

NaF (999)

I ?. 1

/vi

V 720

(6%P,

Fig. 1. Little

Space Diagram of the NaFLNaBFheKBFb-KP System

is known about the corrosion behavior of the BF3 gas.

A few corro-

sfon experiments ha&,been,a, run with very dry BF3 gas,4 and little appreciable attack':was found up to 200°C on metals such as brass, stainless steel, nickei, Monel, mild-steel, and ,manyothers. $his report describes-the first comprehensive study of the compatibility of a relatively impure fluoroborate salt with Rastelloy N and‘Croloy.9M alloys. The experiments were designed to yield information on temperature gradient mass transfer, the major form of corrosion in fluoride salt systeks. ~Twoloops were operated with NaEIlQ-NaF-KBF4 (90-4-6 mole $1 salt at a maximumtemperature of 605°C with a.temperature difference of 145°C to obtain the data presented. These*temperatures match those proposed for fluoroborate salts in a1 molten,,salt breeder '. reactor. I EXPERIMENTALPRCCEDURE !Thenatural-circulation loops for this program were'of the type shown in Hg. 2. Flow results from the difference in density of the 4F. Rudswell, J. S. Nairn, and K. L. Wilkinson, J. Appl. Chem &, 333 (1961).

Fig. 2.

Natural. Circulation Loops in Test Stands.

* \ A rn

salt i n the hot and cold portions of the loop. of salt f l o w i n the

We estimated the velocity

loops t o be 7 f't/min.

Materials Section and Fabrication 1

The Croloy 9M material was from Babcock 8~ Wilcox Company heat 18760 and was vapor blasted before fabrication t o remove oxides. NCL-12,

The loop,

was fabricated fYom 0.750-in. -OD tubing with a w a l l thickness of

0.109 in

The material was TIGwelded and inspected t o meet existing

internal standards.

The welded areas were torch annealed before and

a f t e r welding. The Hastelloy N material was f'rom Union Carbide Corporation, Materials Systems Division heat 5096.

The loop, NCL-10, was fabricated

from 0.672-in. -OD tubing with a w a l l thickness of 0.062 in.

, T I G welded

and inspected t o meet the same standards as those required for the Croloy 9M material. The compositions of both alloys are given i n Table 1. Table 1. Chemical Composition of Alloy Test Materials c

$1 Mns

Composition (wt

Hastello N (NCL-107

.Fe

70.8

7.47

15.59

4.01

0.07

0.54

0.005

89.00

0.09

0.48

0.012

0. a 0.010

0.47

S a l t Preparation

Reactor Chemistry Division a t ORNL. P

with a fluoroborate salt,

This was t h e i r first experience

they prepared it by techniques established

f o r other fluoride salt

raw materials - r e l a t i v e l y impure NaBFi,

ORNL-3626, p. 146.

6 NaF, and

KEP4

(90-4-6 mole

(Table 2) -were loaded into a container

l i n e d w i t h nickel, which w a s then evacuated and purged several times with helium.

Then, the materials were melted and heated t o 400째C under

a helium atmosphere.

N e x t , the l i q u i d was sparged with helium,

Since I

t h i s caused a large increase in pressure, the reactor vessel was vented. Large quantities of BF3, which had caused the increase in pressure, were then released.

Later steps included sparges with a mixture of hydrogen

fluoride and hydrogen for several days at 550째C t o remove oxides and a The salt

sparge with hydrogen t o remove structural metal impurities.

was then transferred t o a f i l l tank made of Hastelloy N i n preparation for f i l l i n g the loop. The chemical analysis (Table 3) of the prepared salt disclosed two important compositional changes germane t o t h i s t e s t series:

(1) a

significant loss of BF3 during preparation, and (2) a high content of oxygen and water. Since l i t t l e was k n m about the corrosive behavior of t h i s salt o r the effect of impurities and because information on i t s compatibility with the container alloys was needed quickly, w e decided t o use it as prepared. Table 2.

Composition of S a l t Mixture Before Purification

&uantities Loaded in Container Compound DF4

NaBF4 NaF

5.88

6.85

90.20

91.61

3.92

1.53

Chemical Analysis Element 2 -10

20.00

9.60

68.30

Table 3. -~

Chemic82 Analysis of Salt Before Fill

Element

Content (wt

2.20

25.80

9.65

60.40

< 5&

54&

28&

6' 3O0Oa

900, 21ooa

H20

8PeLlrts per million. OPERATIONS The hot portion of each loop was heated by s e t s of clamshell heaters, with the input power controlled by silicon controlled r e c t i f i e r s .(SCR units) and the temperature controlled by a Leeds and Northrup Speedomax H Series 60 type C. A. T. (current proportioning) controller.

The loop tem-

peratures were measured by Chromel-P vs Alumel thermocouples t h a t were spot welded to the outside of the tubing, covered by a layer of Quartz

tape, and then covered with stainless s t e e l shim stock. Before each loop and heated t o 150째C unde

a t

i t h s a l t , it was degreased with acetone t o remove any moisture i n the system.

8 We checked for leaks with a helium mass spectrometer leak detector while the interior of the loop was evacuated t o < 5

torr.

All l i n e s

from the f i l l tank t o the loop that were exposed t o the fluoroborate

salt were of the'same material as the loop and were cleaned and tested ~

i n the same manner as the loop.

A l l temporary l i n e connections were

made w i t h stainless s t e e l compression f i t t i n g s . The loops were f i l l e d by heating the loop, the salt pot, and a l l connecting l i n e s t o a minirmlm of 530째C and applying helium pressure t o the salt pot t o force the salt into the loop.

Air was continuously blown on the freeze valves leading t o the dump and flush tanks t o provide a positive salt seal. Tubular e l e c t r i c heaters controlled by variable autotransformers fbrnished the heat t o the cold-leg portions.

Once

the loop was f i l l e d the heaters were turned off and the insulation was

removed t o obtain the proper temperature difference by exposing portions of the cold leg t o ambient a i r . The first charge of salt was dumped a f t e r 24 h r i n the loops a t the

maximum operating temperature with some circulation and l i t t l e temperature gradient. This flush remved surface oxides and other impurities that could have been l e f t in the loops. The loops were then r e f i l l e d with new salt and put into operation. A helium cover gas under slight positive pressure (about 5 psig) was maintained over the salt i n the loops during operation.

l t

Each loop was operated a t a maximum temperature

of 605째C and with a temperature difference of 145째C.

A temperature pro-

f i l e around the loops i s sham around the schematic of the loop i n Fig. 3. During circulation each loop contained about 920 g of salt that contacted 1200 cm2 of surface and traveled 254 cm around the loop. Temperature excursions indicated a f l o w stoppage in NCL-12 (Croloy 9M) after 1440 hr.

A t t e m p t s t o drain the loop were &xsuccess-

f'ul, and the loop was' allowed t o cool w i t h the test salt in place. In NCL-10 (Hastelloy N), a significant increase of temperature i n the hot l e g accompanied by a simultaneous temperature decrease i n the cold leg occurred a f t e r 8335 h r of operation.

A perturbation of t h i s type indi-

cates a disruption in salt flow and can indicate plugging.

This tem-

perature cycle ceased after 1hr, and no f'urther incidents occurred

1 l -

9 ORNL-DWG 68-4t180

SURGE TANK ~

f 10.C

CLAMSHELL HEATERS

i+-INSULATION

I I

jI II

0.C

DUMP TANK

Fig. 3.

Schematic of MSW Natural Circulation Loop.

during the l i f e of the loop.

The loop was shut dawn a f t e r 8760 h r (1 year), and the salt w a s drained'into a dump tank i n a normal manner. RESULTS

Analyses of loop components and salt were made by standard chemich analysis, metallographic examination, and electron microprobe analysis. The results are discussed beluw. c

NCL-10 (Hastelloy N) Visual.

- A p a r t i a l plug i n the lower part of the cold l e g

(Fig. 4 )

closed approximately 75$ of the cross-sectional area of the pipe.

Analysis

10 -.

. TUBE WALL

PLUG

Fig. 4. Plug Formed i n NCL-10 (Hastelloy N) Containing NaBF4-NaF-KBF4 (90-4-6 mole $1 After 8760 h r a t a Maximum Temperature of 605째C and Temperature Difference of 145째C. &. Reduced 40%.

shared this emerald-green plug t o be single crystals of Na3CrF6 (ref. 6 ) . Smaller amounts of two complex iron fluorides, Na3FeF6 and NaFeF4, were a l s o identified in the cold leg. Chemical.

- The analysis of

the salt cake (Fig. 5) collected from

the dump tank of NCL-10 i s given in Table 4. The concentrations of nickel, molybdenum, iron, and chromium i n t h i s s a l t were higher than those of the salt before t e s t .

The average chromium concentrationwas

470 ppm. Due t o cooling, nickel was segregated i n the top and bottom portions

of the cake, reaching 11.15 and 4.47 w t $ i n these portions, respectively. The analysis indicated that the water content of the salt 6MSR Program Semiann. Progr. Rept., AM.

- 400 t o 900 ppm

31, 1967, ORNL-4191, p. 228.

-4

Fig. 5.

Table 4.

Drain S a l t Cake From NCL-10 (Hastelloy N)

Impurity Analysis:

(90-4-6 mole

'$1

NaBFd-NaF-KEiFh Analysis (ppm)

Before t e s t

Ivb

146 1400 3000 4850

H20

400 2,6 900

After test

Top slag

E L -15'

Center layer

Bottom layer

-.-

4.47a

%eight percent.

1.35a 210 160 1500 7300

4200 1200 1800 5 1750 270 3540 1.300 2, < 5 3120 1500 3550 2800 3660

< 5,19

before t e s t -increased t o 1300 t o 2800 ppm.

The oxygen analyses showed

much scatter, but we believe t h a t the oxygen content also increased. Presumably these increases are due not t o air inleakage but t o other factors discussed l a t e r i n t h i s report. The crossover l i n e t o the cold leg, the cold leg, and the crossover

r t

l i n e t o the hot l e g all showed slight increases i n wal+ thickness due t o deposition of complex surface layers (Fig. 6).

Chemical analysis of the

layers disclosed that they were primarily metallic nickel (60 t o 90 w t $) and molybd&mm.

A sma.ll quantity of iron was present i n proximity t o the

base metal, but chromium was conspicuously absent. Metallurgical.

- Micrometer measurements of

the hot leg of the loop

disclosed 1 t o 2 m i l s of metal l o s s and s l i g h t surface roughening. Metallographic examination of an area fromthe hottest section (605OC), however, showed a smooth surface w i t h an occasional penetration along a grain boundary (Fig. 7 ) .

Microprobe traces7 of this hottest

section for all the alloying elements showed no compositional gradients. The chromium trace f o r t h i s section i s given i n Fig. 8 .

These results

indicate a general dissolutive attack in the hot-leg section.

An area a t the entry of the cold l e g (520OC) showed a duplex surface structure, and the areas numbered i n Fig. 9 were analyzed w i t h the microprobe f o r the various elements (Fig. 10). is quite high in nickel

- trp t o 87 w t $.

The outermost layer, area 1,

We believe t h i s i s most l i k e l y

a region where nickel deposited during the t e s t . taining essentially only molybdenum and nickel.

Area 2 is a region conClose t o the original

metal surface there is an increase in nickel, a smaller increase i n molybdenum, and l i t t l e change in chromium and iron.

Note that w i t h an

increase i n nickel and molybdenum metal the concentration of chromium and iron would show a decrease even though there were no change i n the actual amounts. Figure ll, a photomicrograph of the coldest section NCL-10 (46OOC)

shows a spongy deposit on the surface and a loosely adherent corrosion product beyond t h i s region.

A t higher magnifications,

it was seen t h a t

the t i g h t l y attached spongy deposit varied in thickness from 0 t o 6 pm

7Data not corrected f o r absorption, secondary fluorescence o r atomic number effects.

ORNL-DWG 67-9342R

,’ ,’

I I

0.007 inches

32.5 in.

0.007 inches

LOCATION OF Na, Cr F6 PLUG,

NCL 10 SALT: NaBF$NaF-KBF4 (90.4-6 mole %) . TEMPERATURE:605’C,AT= 145”C,TIME = 8760 hr

Fig. 6.

&face

Layers in NCL-10 (Hastelloy N) After 8760 hr Operation.

P i g . 7. Hastelloy N Hot-Leg Section E L - 1 0 (605째C) s h m a t Different Magnifications After 8760 h r i n Fluoroborate S a l t . Etchant: Glyceria Regia.

ORNL-DWG

68- 9658

I TRACES OF Ni,Fe, AND Mo SHOWED NO GRADIENT

I I

20 DISTANCE ( p )

Chromium i n Hot Leg (605째C) of NCL-10 Fluoroborate Salt.

ast e u o y N)

After 8760 h r i n

U P

Fig. 9. Entry t o Cold Leg (520째C) of NCL-10 (Hastelloy N) Operated for 8760 h r i n Fluoroborate Salt. Etchant: Glyceria Regia. (a) 500x. (b) Numbered areas were anaLyzed by microprobe. 2000X.

ORNL-DWG 68-9659

-.e --.-.\

-0-0-

-*-e-.--.-.-

-.-e--t-.-.e-

0 0

DISTANCE ( p )

Fig. 10. Penetration Curve of Constituents of Hastelloy N a t Cold-Leg Entry (52OOC) of NCL-10 Operated for 8760 h r i n 3luoroborate Salt.

Fig. ll. Coldest Section ( 4 6 0 째 C ) of NCL-10 (Hastelloy N) Shown a t Different Magnifications After 8760 hr i n Fluoroborate Salt. Etchant: Glyceria Regia.

19 and that it resemble much thinner.

area seen i n the

r y t o the cold leg, although

The spongy layer i s high i n nickel, w i t h some molybdenum,

and appears, l i k e the area 1 of Fig. 9, t o have resulted f’rom nickel deposition (Fig. 1 2 ) .

There was only 63f metal i n t h i s spongy layer;

the b a l a x e was fluoride compounds.

The original surface i s high i n

nickel and molybdenum and low i n chromium and iron and is a n area where nickel and molybdenum have diff’used into the matrix.

No analysis of the

loosely adherent material was possible. The last section of the loop examined was an area a t the entrance I

Although the salt was being heated here, the temperature

t o the hot leg.

was s t i l l low enough (< 530°C) t o make t h i s an area where material was Again, a duplex layer was seen (Fig. 13) and found t o

being deposited.

be predominantly nickel, 65 t o 90$ metal and the balance fluoride com-

pounds (Fig. 14).

X-ray diff’raction analysis of the fluoride compounds

disclosed an unidentifiable crystal structure, probably a mixture of several fluoride compounds.

The surface layer had somewhat more nickel

and molybdenum than originally, and, thus, smaller concentrations of chromium and iron. E

NCL-12 (Croloy 9M)

Visual.

When t h i s loop was sectioned for examination, a dark

gray plug that completely f i l l e d the cross-sectional area for a v e r t i c a l distance of 1 in. was found i n the cold l e g (Fig. 15). Also, small, green crystals were see i n line, and a metallic layer w a s found against the i n s i

ing i n the cold section and i n the

salt. The thin metallic 1 inside of the tubing i loop).

2.5 mils thick was deposited on the cold-leg section (over one-half the

In some places, the material had become detached f’rom the tubfng

and was found i n the f’rozen salt (Fig. 16). 1

The metallic layer repre-

ss of the m t e r i a l (mostly salt) removed op. Chemical analysis revealed the layer (by mechanical means) fko t o be 90 wt 4 Fe and 10 wt $ C r metal.

sented about 4$ of the t o t

ORNL- DWG 68-9660

- .._.

Ni u

60 h

6? c

3: U

0 40

tu 0

I-

2 0

SPONGY LAYER

I I

0 DISTANCE ( p )

Fig. 12. Penetration Curve of Constituents of Hastelloy N i n Coldest Section (460째C) of NCL-10 Operated for 8760 h r i n Fluoroborate S a l t .

Fig. 13. Entry t o Hot Leg (530'C) of NCL-10 (Hastelloy N) Shown a t Different Magnifications After 8760 h r i n Fluoroborate S a l t . Etchant: Glyceria Regia.

22 DRNL-DWG 68-966f

o o-*-.-4*1-.-.

* ?I

0-0-0-

I 60

-f

65-9070 METAL I BALANCE i FLUORIDE SALT I

-I ~

LAYER

ESTIMATED ORIGINAL SURFACE

I 40

I 1 I I

i5 V

I 20

Fig. 14. Penetration Curve of Constituents of Hastelloy N i n HotLeg Entry (530째C) of NCL-10 Operated f o r 8760 h r i n Fluoroborate S a l t .

Fig. 15. Iron. Dendrite Plug i n Coldest Section (460째C) of NCL-12 (Croloy 9M) Operated i n Fluoroborate Salt f o r 14.40 h r a t a Maximum Tem, perature of 605째C and a Temperature Difference of 145째C.

Fig. 16. Cross Section of Tubing of NCL-12 (Croloy 9M) Shuwing Deposited Metal Layer. Loop operated for 14-40h r i n fluoroborate salt a t a maximum erature of 605째C and a temperature difference of 145째C. Reduced 22%. Cross section of NCL-12 tubing. (b) Enlarged view of metal and salt interface. 30X.

24 The dark-gray plug located i n the coldest p a r t of the loop was come found similar crystals adhering t o speciposed of dendritic crystals. W

mens i n the hot l e g (Fig. 171, but we assume that these crystals, grming and circulating i n the salt stream, attached themselves t o the specimens

while the loop was cooling. Chemical. -Chemical analysis showed t h a t the dark-gray plug and the material on the specimens were essentially pure iron with l e s s than 1% of other elements (sham below). Elements

Content ( w t 99.00

0.03

< 0.05 < 0.01

0.05

Pb Si

0.02 0.02

0.05

0.02

The results of chemical analysis of the green crystals i n the drain leg are given belm.

Elements

wt%

10-15

2 4 45.9 0.058 18

F K

1.5

The stoichiometry of t h i s green deposit, based on t h i s analysis, color, and other factors, is roughly 2NaF-FeFa-CrF3-BF3, which corresponds t o chromium and iron fluorides mixed with the salt.

Fig. 17. h e Iron C tals f k o m NCL-12, Which Ope d for 1440 hr i n Fluoroborate Salt at a Maximum Temperature of 605째C and a Temperature Difference of 145째C. 1OX. Reduced 12%~. (a> Crystals adhering to specimen, and (b) c r y s t a s removed from specimen for photographing. \

Table 5 gives t h e composite of the salt before and a f t e r operation.

The significant changes i n the salt chemistry due t o t e s t axe the increases

i n the chromium and iron content from 54 t o 255 and 28 t o 700 ppm, respectively. Composition of Salt Before and After Operation i n NCL-12

Table 5.

Composition ( w t K

Na ~

$1 F

Composition (ppm) Cr

Before Test Theoretical (calculat ed) Before F i l l

2.10

20.00

9.60

68.3

2.20

25.80

9.65

60.4

During F i l l

1.98

18.83

9.38

66.2

3000

83 146

1400

After Test Hot l e g

1.55

19.72

9.29

67.1

265

700

Cold l e g

1.89

20.71

9.27

67.3

255

700

< <

7 3000

< 2 3200

Metallurgical. -Metallographic examination of the hot-leg loop tubing (Fig. 18) disclosed a f a i r l y smooth surface, and micrometer measurements shared an average 2.5 mil loss fiom a nominal pipe diameter. Electron microprobe analyses were made on tubing from the hot- and cold-leg sections of NCL-12:

the hot-leg analysis (Fig. 191, consistent

with the metallographic results, shared no iron or chromium concentration gradients; the cold l e g analysis (Fig. 2 0 ) showed an increase of about

4 w t $ i n iron concentration and a decrease of about 4 w t $ i n chromium concentration ( i . e.

, an iron-rich

surface layer 1.

. DISCUSSION

The tests described are the first study of the compatibility of a

fluoroborate salt with container materials of interest t o molten-salt reactors.

Unfortunately, l i t t l e was known at the time of the t e s t about

the purification of the salt o r the characteristics of mass transfer i n

FA&. 18. Hot Leg (605'C) of NCL-12 (Croloy 9M), Which Operate for 1440 h r i n Fluoroborate S a l t . Etchant: Picric acid, hydrochloric acid, and ethyl alcohol.

ORNL-DWG 60-9663 I

-6

- 86 e

-OO o

0 0 0

0 0

Fig. 19. Penetration Curve f o r Iron and Chromium i n the Hot Leg Which Operated for 1440 h r i n Fluoroborate (605째C) of NCL-12 (Crolo

salt.

94 0

-t9 90

v -

0 42

k t (L 8

a .

--..-..a-.

o u u0

0 0

o w

0 0

0 ' 0 "

TO-.'.

.a.

. .

.-,.-n Cr

Fig. 20. Penetration Curve for Ikon and Chromium i n the Cold Leg (46OOC) of NCL-12 (Croloy 9M), Which Operated f o r 14.40 h r i n Fluoroborate

salt.

systems containing such salts.

The use of loops of an old design pre-

vented our having any removable specimens, but permanent hot-leg specimens

were used i n NCL-12. Our expectation, based on previous experience with fluoride salts,

was t h a t temperature gradient mass transfer of the l e a s t noble constituent (i.e., chromium and iron) would be the limiting factor. But salt analyses after the t e s t indicated t h a t all the major alloying elements of the container materials had mass transferred. The nonselective attack was a l s o confirmed by metallographic examination and microprobe analysis of the loop specimens and piping. I n view of the movement of nickel and molybdenum, it is obvious that highly oxidizing conditions, due t o water and oxygen i n the salt, were present during the operation of the loops.

In analyzing NCL-10 and -12, we reviewed e a r l i e r studies8 ( l a t e 1950's) t o single out the cases of mass transfer of nickel and molybdenum. 8J. H. DeVan, unpublished data, 1957-1959.

We found several instances where t h i s had occurred i n salts containing KF.

One such loop, constructed of HastelIoy N, operated for a year with

a NaF-LiF-KF salt (11.5-46.5-42

mole

with a temperature difference of 9OOC. i

a t a max-

temperature of 690Â°C

No analysis for oxygen or water

was made before t e s t . Examination a f t e r t e s t disclosed green crystals embedded i n the salt. S a l t analyses showed significant increases i n nickel, iron, molybdenum, and chromium content. X-ray analysis showed that the green crystals were mixtures of sodium and potassium chromium fluoride complex compounds and that most of the XI? present i n the salt was actually KF-2H20. Another loop, constructed of Inconel, (~i-18%Cr-lO$

Fe) operated

about one-half year with the same salt and a t the same temperatures as above.

After t e s t , metallographic examination showed heavy attack i n

the hot-leg portion of the loop.

After t e s t only the chromium and iron

contents of the salt were determined; the chromium content had increased significantly from 60 t o 900 ppm. X-ray analysis of the salt disclosed that the salt was about 15%KF-2H20.

The relatively well-defined x-ray

pattern of t h i s hydrate suggested that it was not formed when the cold melt was exposed t o a i r but was carried as a part of the salt mixture during operation.'

The compound KFe2H20 is knuwn t o be thermally stable.

The l i t e r a t u r e s t a t e s that KF easily forms a series of crystal hydrates, whereas LiF and NaF crystallize ar~hydrously.~It is a l s o noted t h a t it

is easy t o form KBF3OH and NaBF3OH and that they are quite stable. Thus we have seen that during the molten-salt reactor corrosion program, salts containi metals.

We believe

ciated with t h a t a l k a l i found no leakage of .

have on occaSion been very aggressive toward

is is the coxtibinedwater assofluoride salt. We again s t r e s s that we of the loops.

This suggests t h a t

hydrated KF is not r

purification process.

it appears t h a t con21 released and w i l l reac

the fluoroborate salt mixture can be

Paradoxically, c

*â&#x201A;ŹE' + BF3OH

(2)

'1. G. Ryss, The Chemistry of Fluorine and Its Inorganic Compounds, pp. 521-28 and 815-16, AEC-tr-3927, Pt. 2, (February 1960).

30 The generation of HF i n the system leads t o the following reactions with

the elements of the container material: 2HF

, + 3H2 , + H2 , + H2 .

+ N i eNiF2 +

6HF + b * k F 6 2HF + Fe t F e F 2 2HF + C r t C r F 2

(3 1

(41 ( 51

Note that, i n the temperature range of interest, changes i n the standard free energy of formation are not favorable f o r all the reactions

as written. scheme"

However, studies on the Fluoride Volatility Processing

a t ORNL shawed that i n a Hastelloy N hydrofluorinator containing

fused fluoride salts and HF, chromium and iron were rapidly leached f r o m the alloy at elevated textperatures (500 t o 650째C) even when the HI? activi t y w a s quite law.

The main reaction i s the oxidation of the metal by

the HF t o form fluorides soluble i n the melt.

It was found t h a t NiF2 i s

produced by driving the reaction through the continuous removal of hydrogen and reaction products, since the free energy of formation (above 49OOC) of the nickel fluoride reaction by Eq. (3) is not favorable.

For

the same reason, molybdenum can also be forced t o react with HF even though a positive change i n free energy is involved.

Evidence of s m a l l

but f i n i t e dissolution rates of molybdenum metal during hydrofluorination conditions have been reported.

In l i g h t of the discussion above, we can now consider why two different types of products (i.e.,

metal and fluoride compounds) were mass

transferred i n these (NCL-10 and 1 2 ) thermal convection loops. 11

''A. P. Litman and R. P. Milford, Corrosion Associated with the Oak R i d g e National Laboratory Fused S a l t Fluoride Volatility Process, I t paper presented a t the Symposium on &sed Salt Corrosion a t the Fall

Meeting of the Electrochemical Society, Detroit, Michigan, Oct. 1-5, 1961. "A. E. Goldman and A. P. Litman, Corrosion Associated With Hydrofluorination i n the.Oak Ridge National Laboratory Fluoride Volatility Process, ORNL-2833 (November 1961).

12A. P. Litman, Corrosion of Volatility Pilot mant MARK I INOR-8 Hydrofluorinator and MARK I11 L Nickel Fluorinator After Fourteen Dissolution Runs, ORNL-3253 (Feb. 9, 1962).

31 Thermodynamics of System Corrosion The cycle of mass transfer i n i t i a t e d by the corrosion of metals by

fluoride salts i s k n m t o begin i n the hotter regions of a loop system w i t h the formation of structural metal fluorides t h a t are soluble i n the

salt : M+F'-==M!?

(7)

where M is the attacked metal of the container. The equilibrium constant for Eq. (7) increases with increasing temperature; thus, i n a Multicomponent alloy the concentration of the attacked c o q t i t u e n t , M, w i l l decrease at loop surfaces a t high temperature (weight loss) and increases a t surfaces a t l m e r temperatures (weight gains).

A t some intermediate temperature, the i n i t i a l surface composi-

t i o n of the structural alloy w i l l be i n equilibrium with the fised salt (no weight change).

In the cooler regions of a closed system, the r a t e a t which the metal is deposited (steady s t a t e condition before plugging) i s generally equal t o the r a t e at which it diffuses into the metal maintain i t s equilibrium a c t i v i t y on the surface.

- a necessity t o

Thus the r a t e of

diffusion of the metal i n the alloy i n the cooler regions usually cont r o l s the r a t e of corrosion i n the hot zone.

A t t h i s stage i n the tem-

perature-gradient mass-transfer process, weight gains would be found on specimens i n the cold section but l i t t l e i f any surface change (deposits) would be seen. This generalization is valid only when the equilibrium concentration of a metal Eq.

(711

does not exceed

e compound i n the salt [produced by ation concentration of a metal fluoride

compound i n the salt at the l a w temperature. Early studies reported by Grimes13 clearly i l l u s t r a t e these points. Table 6 shms the chromium concentrations for two salts as functions of the equilibrium between the salts and Inconel or pure chromium. t h a t when the alkali-met

Note

fluoride salt (NaF-fCF-LiF-UF4) i s i n contact

with Inconel a t 800째C it w i l l support a higher concentration of chromium 13W. R. Grimes,

pp- 96-99.

ANP &uart. Progr. Rept. June 10, 1956, Om-2106

32 Table 6.

crd

Equilibrium Concentrations of Chromium Fluorides W i t h !bra S a l t s

Chromium Concentration (ppm) NaF-KF-LiF-UF4

NaF-ZrF4 -UT4

Calculated salt equilibrium with Inconel

Ncr =

0.16 a t 800째C (ref. a )

1660

1400

1100

2400

Experimental salt equilibrium with pure chromium

Ncr =

1.0 a t 600째C (ref. a > ~~

'concentration of chromium i n mole fraction. fluorides

(1660 ppm Cr) than it w i l l when it is i n contact with pure

chromium a t 600째C (U00 ppm C r ) salt in

Accordingly, circulation of such a

an Inconel loop would result i n the deposition of essentially

pure chromium metal i n the cold zone.

In that case, the t o t a l r a t e of

attack would be controlled simply by diffusion of chromium t o the interface between metal and salt in the hot zone.

W e believe that the deposi-

tion of pure iron i n NCL-12 (Croloy 9M) occurred t h i s way.

This i s not

a new phenomenon.

Rapid plugging by deposition of metal dendrites occurred frequently in the l a t e 1950's especially i n iron-base alloy loops. 14, l 5 Examination of the data f o r the other s a l t sham i n Table 6 (NaF-ZrFc-UF4) leads t o the conclusion that chromium metal would not p l a t e out i n t h a t system and t h a t any product of mass transfer would be

a fluoride compound. A t 600째C t h i s salt i n equilibriumwith pure chromium w i l l support a higher concentration of chromium fluorides than it w i l l I 4 G . M. Adamson, R. S. Crouse, and W. D. W y , Interim Report on Corrosion by Alkali-Metal Fluorides: Work t o k y 1, 1953, ORNL-2337 (March 20, 1959). 15G. M. Adamson, R. S. Crouse, and W. D. Manly, Interim Report on Corrosion by Zirconium-Base Fluorides, ORNL-2338 (Jan. 3, 1961).

i n contact with Inconel at 800째C.

For gross deposition of compounds t o occur, the concentration of the compound a t the temperature of interest (cold zone) must exceed the saturation concentration of the metal fluoride corrosion product present in the system. We believe t h a t the mechanism i n NCL-10 (Hastelloy N) i s similar t o the one discussed above for the NaF-ZrF4 system and t h a t i n time the con-

- or more accurately the concentration of mixed metal chromium fluoride - i n the fluoroborate salt at 605째C exceeded the centration of chroplium

saturation concentration a t 460째C and allowed deposition of large quant i t i e s of complex compounds. Both NCL-10 and NCL-12 operated with the same salt a t the same temperature, yet they were plugged by different mechanisms at different

rates. A chromium-rich plug was found i n the Hastelloy N loop (containing 7$ C r 3 $ Fe) and an iron plug was found i n the Croloy 9M loop (containing 9%C r - b a l Fe). Thus, it appears t h a t when these fluoroborate salts are contained i n a l l o y s w i t h between 7 and 9 w t 4 Cr, the iron content of the alloy controls the composition of the temperature-gradient mass-transfer deposit. i

Kinetics of System Corrosion With the foregoing mine when plugging start element a t various times. We calculated the weight of the iron plug

an assumed reaction-rate constant and mode

i n the cold section, 70

&Ir

oncentrations for chromium and iron i n amount of these elements a f t e r t e s t , the

culations, presented i n T

n NCL-10 and -12 and the disposition of each

NCL-12 from knowledge o of chemical attack (from 1

and evaluation, it is now possible t o deter-

xperiments). l6

The results of the cal-

, show that

the saturation value of iron 1120 mg, was exceeded shortly after

130 hr; at t h a t time pure started depositing and eventually caused a complete plug. Table 8 shows the same kind of calculations f o r the operation of NCL-10. In t h i s case, it appears t h a t the saturation W u e of chromium 16J. W. Kbger and A. P. Litman, M SR Program Semi-. Feb. 29, 1968, ORNL 4254 pp. 218-225.

Progr. Rept.,

Table 7.

Calculated Concentration of Alloying Elements from NCL-12 Present i n the S a l t a t Various Times Concentration i n Salt

Maximy Attack

Time (hr)

(%/an21

14.4 57.6

Average Attack (mg/cm2)

Area Attackedb (an21

Total

Material

Deposited i n Ribbon Form

Lost (mg)

(w)

Deposited As P l w (mg)

(mg)

(ppm)

(mg)

(ppm)

2 4

1 2

650 650

650 1300

250 500

360 720

225 450

40 80

25 50

130

650

1950

750

1080

675

120

1440

650

6500

2450

1120

700'

400

250'

'Calculated

from LW = 2/&

where K = rate constant,

& an2/sec a t 6070C,

LW = weight loss, c = concentration of chromium and iron, P = density

of Croloy 9M, and

t = time. bIncludes surge tank and l i n e s and one-half of t o t a l area. C

By chemical analysis.

2530

Table 8.

Calculated Conceritration of Alloying Elements from NCL-10 Present in the Salt at Various Times Concentration

Th? 4000 8760

Mt-l&m Attack& brig/cm* ) 38: 56

Average Area Attack Attackedb blg/u!l* > ,h?m2) 19 28

550 556

Deposited on TotE!l Material Cold Surface "l.ost bug> 10,450 15,400

2100 3300

in Salt Deposited

MJ Arlmgp;ukl

h3)

(ppm)

brig)

(ppm)

bg)

Cppm)

bug)

350 509

383 555c

420 427

459 470'

6350 9300

6950 ll50 10,OOOc 1560

(ppm) 1260 1700'

220 %

aCsbul.ated f&m &I = 2/J p & where K= rate constant, 10-l* cm*/sec at 607Y, LS?= weight loss, c = concentration of chromium, iron, nickel, and molybdenum, P = density of Hastelloy N, and ,. t .= time. b Includes surge tank and lines and one-half' total area.

i n the cold leg, 470 ppm, was reached a f t e r about 4000 h r of operation;

a t that time, the chromium, as Na3CrF6, started depositing i n large amounts. 17

It i s interesting t o note t h a t i n both cases the amounts of the alloying elements i n the salt and i n the plug were i n about the same r a t i o as they are i n the base metal.

Capsule t e s t s and other loop t e s t s

have shuwn t h a t when constituents of an alloy react w i t h the l i q u i d medium they w i l l be found i n the liquid and/or deposited i n the r a t i o at which they existed i n the alloy. l8 This behavior was found i n both loops of t h i s experiment. Nickel and molybdenum mass transfer products, not normally found i n relatively pure fluoride salt systems, also follow t h i s pattern.

As has

been stressed here, the presence of gross quantities of nickel and molybdenum indicates strongly oxidizing conditions. Salt Purification

The salt used i n these experiments contained many impurities that the old techniques, successful with other fluoride salts, did not remove. Since these experiments, improvements have been made i n the fluoroborate

salt preparation. Only very pure salt (99.9%) is now used as s t a r t i n g material. I n fact, the salt has so few impurities t h a t no steps are "Several suggestions, besides the removal of the water or oxygen from the system, are offered t o improve the service of the container materials using t h i s fluoroborate salt. The most obvious change is t o lower the hot-leg temperature t o 540째C thus lowering the reaction-rate constant an order of magnitude. It would then take three times as long t o duplicate the previous plugging. Another possibility is t o r a i s e the temperature i n the cold-leg section. This would r a i s e the saturation value of the elements i n the salt, and depositing would not occur u n t i l later. But t h i s probablywould not improve the l i f e by the same factor as above. Probably the most important improvement i s removing corrosion broducts from the system on a continuous or batch-wise basis since deterioration (thinning) of the container w a l l by dissolutive corrosion i s not a probled i n these systems. 18J. R. Weeks and D. H. G u r i n s Q , "Solid MeW-Liquid Metal Reactions i n Bismuth and Sodium," pp. 106-161 i n L i q u i d Metals and Solidification, American Society for Metals, Metals Park, Novelty, Ohio, 1958.

37 taken during processing t o remove the structural metals.

Purification procedures f o r removing water and oxygen are now under study. A new procedure is also used i n the melting and preparation of the fluoroborate

salt t o prevent loss of BF3 vapor and the ensuing change of composition. f

In brief, the loaded salt i s evacuated t o about 380 t o r r and heated t o 150째C i n a vessel lined w i t h nickel .and i s held for about 15 h r under these conditions.

If the r i s e i n vapor pressure i s not excessive, (no

v o l a t i l e impurities) the salt i s heated t o 5OO0C, while s t i l l under vacuum, and agitated w i t h helium for a few hours.

The salt i s then

ready for transfer t o the f i l l vessel. CONCLUSIONS

1. Natural circulation loops, fabricated from Hastelloy N and

Croloy 9% t h a t circulated impure (> 3000 ppm impurities) NaBF4-NaF-KBF4 (90-4-6 mole

a t a maximum temperature of 605째C with a temperature

difference of 145째C evidenced serious temperature-gradient mass transfer. The mass transfer involved migration of all major constituents of the constituents of the container materials and resulted i n r e s t r i c t i n g f l o w i n i

the Hastelloy Nloop by deposition of Na3CrF6 crystals and complete plugging of the Croloy 9M loop by iron dendrites. 2.

The nonselective corrosion observedwas due t o the presence

of water, chemically bound t o the fluoroborate salts, that reacted during heating t o form HI?. 3.

The driving force

mass transfer was the temperature depen-

dence of the equilibrium constant between the container material cons t i t u e n t s and the most stable fluoride compounds that can form i n the

system. 4.

The saturation concentrations for iron and chromium i n the t e s t

salt a t 460째C were found t o be 700 and 470 ppm, respectively. C

1. Other, more highly purified fluoroborates should be extensively

tested f o r corrosion before these coolants can be qualified for moltensalt reactor service.

38 2.

Based on knowledge t o data, iron-base and iron-containing alloys

should be avoided i n molten-salt reactor coolant circuits t h a t use fluoroborate salts.

ACKNCWLEDGMEXTS

It i s a pleasure t o acknowledge t h a t E. J. Lawrence supervised con-

struction, operation, and disassenibly of the t e s t loops during the course

of t h i s program. We are also indebted t o H. E. McCoy, Jr. and J. H. DeVan for constructive review of the manuscript. Special thanks are extended t o the Metallography Grow, especially H. R. Gaddis, H. V. Mateer, and R. S. Crouse, and t o the Analytical

Chemistry Division, Graphic Arts Department, and the Metals and Ceramics Division Reports Office f o r invaluable assistance.

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256. 257. 258. 259. 260. 261. 262. 263. 264. , 265. 266-267. 268. 269. 270. 271. 272. 273. 274. 275. 276. 277. 278. 279. 280. 281. 282. 283. 284+98.

G. J. D. C. H.

G. Allaria, Atomics International G. Asquith, Atomics International F. Cope, RDT, SSR, AEC, Oak Ridge National Laboratory B. Deering, AEC, OSR, Oak Ridge National Laboratory M. Dieckamp, Atomics International A. Giauibusso, AEC, Washington F. D. Haines, AEC, Washington C. E. Johnson, AEC, Washington W. L. Kitterman, AEC, Washington W. J. Larkin, AEC, Oak Ridge Operations T. W. McIntosh, AEC, Washington A. B. Martin, Atomics International D. G. Mason, Atomics International C. L. Matthews, RDT, OSR, AEC, Oak Ridge National Laboratory G. W. eyers, Atomics International D. E. Reardon, AEC, Canoga Park Area Office H. M. Roth, AEC, Oak Ridge Operations M. Shm, AEC, Washington J. M. Simmons, AEC, Washington W. L. SmaUey, AEC, Washington S. R. Stamp, AEC, Canoga Park Area Office E. E. Stansburg, the University of Tennessee D. K. Stevens, AEC, Washington R. F. Sweek, AEC, Washington A. Taboada,- AEC, Washington R. F. Wilson, Atomics International Laboratory and University Division, AEC, Oak Ridge Operations Division of T nformation Extension