ORNL-TM-2978 by Jon Morrow

HOME HELP OmL-TM-2978

C o n t r a c t No. W-'7405-eng-26 METALS AND CERAMICS D I V I S I O N

COMPATIBILITY OF FUSED SODIUM FLUOROBORATES AND B F 3 GAS WITH HASTELLOY N ALLOYS J. W. K o g e r and A. P. Litman

JUNE 1970

OAK RIDGE NATIONAL LABORATORY O a k R i d g e , Tennessee operated by

UNION CARBIDE CORPORATION U.S.

f o r the ATOMIC ENERGY COMMISSION

iii

CONTENTS

. .................... Introduction . . . . . . . . . . . . . . . . . . . Experimental Methods and Materials . . . . . . . . . Results . . . . . . . . . . . . . . . . . . . .

... ... ... ... E f f e c t of S a l t Composition and BF3 Pressure (Series I ) . . . Weight Changes . . . . . . . . . . . . . . . . . . . . . S a l t Chemistry . . . . . . . . . . . . . . . . . . . . . Specimen Chemistry . . . . . . . . . . . . . . . . . . . Metallography . . . . . . . . . . . . . . . . . . . . . E f f e c t of Temperature (Series 11) . . . . . . . . . . . . . . I n t e r p r e t a t ion of C o r r os ion R a t e s . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . .

Abstract

Acknowledgment

. . . .

..........................

Page .

1 1 3

7 7 8 10

11 13

14 16

COMF'ATIBILITY W I T H FUSED SODIUM FLUOROBORATES AND BF3 GAS OF HASTELLOY N ALLOYS

J. W.

Koger and A. P. Litman'

ABSTRACT

Corrosion by NaBFL-NaF mixtures (4-8 mole % NaF) low i n oxygen and water w a s s t u d i e d w i t h 6800-hr s t a t i c t e s t s a t 605째C i n standard Hastelloy N capsules w i t h titanium-modified Hastelloy N specimens. Boron t r i f l u o r i d e gas was added t o overpressures up t o 400 psig. Specimens i n t h e molten s a l t s and a t t h e i n t e r f a c e showed both weight gains and l o s s e s . The g a i n s decreased with i n c r e a s i n g NaBF4 content, and l o s s e s up t o 0.03 mg/cm2 (< 0.01 mil/year) were observed f o r specimens i n 4 mole % NaF. Weight changes of specimens i n vapor were very small and independent of overpressure. The main r e a c t i o n w a s t h e oxidation of chromium i n t h e a l l o y by i r o n f l u o r i d e i n t h e s a l t t o form Na3CrF6 and deposit i r o n on both specimens and capsule. Metallographic examination showed only minor a t t a c k and no d i f f e r e n c e i n t h e two Hastelloy N a l l o y s . The r a t e of chromium d e p l e t i o n a t 605째C w a s c o n s i s t e n t with t h e r a t e of s o l i d - s t a t e d i f f u s i o n of chromium t o t h e a l l o y surface. Additional t e s t s from 538 t o 760째C showed Arrhenius behavior t h a t confirmed t h i s mechanism. Fluoroborate salt mixtures were found compatible with Hastelloy N a l l o y s under t h e conditions of t h e s e experiments, and comparison with other experiments showed t h a t i n c r e a s e s i n water and oxygen content increased t h e chromium uptake and corrosion.

INTRODUCTION

The s u c c e s s f u l use of molten f l u o r i d e s a l t s i n a r e a c t o r system, as demonstrated by t h e Molten S a l t Reactor Experiment (MSRE) a t t h e Oak Ridge National Laboratory, has l e d t o development s t u d i e s f o r a Molten S a l t Breeder Reactor (MSBR).

One of t h e most promising f e a t u r e s of

molten s a l t s has been t h e low corrosion r a t e s experienced with m a t e r i a l s of construction, p r i n c i p a l l y nickel-base a l l o y s .

N ' ow with AEC Division of Space Nuclear Systems, Washington, D.C.

Theory and experiment

have shown t h a t t h e corrosion r e s i s t a n c e of

metals t o molten f l u o r i d e s a l t s v a r i e s d i r e c t l y with t h e " n o b i l i t y " of t h e metal; t h a t i s , t h e r e s i s t a n c e t o a t t a c k v a r i e s i n v e r s e l y with t h e magnitude of t h e f r e e energy of formation of t h e f l u o r i d e s of metals i n t h e container m a t e r i a l ,

Therefore, t h e container a l l o y f o r t h e f l u o r i d e

s a l t must have only a small percentage of c o n s t i t u e n t s t h a t a r e e a s i l y oxidized by t h e components of t h e s a l t .

Considering t h e s e f a c t s and

u t i l i z i n g information gained i n corrosion t e s t i n g of commercial a l l o y s , ORNL developed3 a high-strength nickel-base a l l o y , Hastelloy N, containing

1'7%Mo, '7% C r y and 5% Fe, f o r use i n t h e MSRE. as well as many recent corrosion tests4-'

These e a r l y experiments,

i n LiF-BeF,-ThF&, LiF-BeF2-UF4,

and LiF-BeF2-UF4-ThF4, have Shawn t h a t Hastelloy N i s very r e s i s t a n t t o a t t a c k and corrosion r a t e s a r e below 0 . 1 mil/year. Recently t h e s e l e c t i o n of a f l u o r i d e s a l t t h a t can be used as a coolant t o t r a n s f e r heat from t h e f u e l s a l t t o a steam power conversion system has been s e r i o u s l y considered.

Cost and melting p o i n t considera-

t i o n s favor t h e sodium f l u o r o b o r a t e f i x t u r e , NaBFI+-8mole

l i t t l e information e x i s t s on corrosion by fluoroborates.

I n i t i a l corro-

NaF.

However,

s i o n r e s u l t s were obtained from an impure f l u o r o b o r a t e s a l t (high i n oxygen and water) i n thermal convection loop t e s t s , ? but t h e s e r e s u l t s were not considered i n d i c a t i v e of behavior with purer s a l t .

Since t h a t

time, new methods of p r e p a r a t i o n have g r e a t l y increased t h e p u r i t y of a v a i l a b l e s a l t with respect t o oxygen and water.

2J. H. DeVan and R. B. Evans 111, Corrosion Behavior of Reactor Materials i n Fluoride S a l t Mixtures, ORNL-TM-328 (Sept. 19, 1962). 3W. D. Manly, J. H. Coobs, J. H. DeVan, D. A. Douglas, H . Inouye, P. P a t r i a r c a , T. K. Roche, and J. L. S c o t t , "Metallurgical Problems i n Molten Fluoride Systems," Progr. Nucl. Energy Ser. I V 2, 164-1'79 (1960).

'J. W. Koger and A. P. Litman, MSR Program Semiann. Progr. Rept. Feb. 29, 1968, ORNL-4254, pp. 218-221.

5J. W. Koger and A. P. Litman, MSR Frogram Semiann. Progr. Rept. AUg. 31, 1968, ORNL-4344, pp. 257-264. 'J. W. Koger and A. F. Litman, MSR Program Semiann. Progr. Rept. Feb. 28, 1969, ORNL-4396, pp. 243-246.

3 W

The o v e r a l l o b j e c t i v e of t h i s study w a s t o determine t h e corrosion c h a r a c t e r i s t i c s of pure (low oxygen and water) NaBF4-NaF mixtures under O f p a r t i c u l a r concern w a s t h e temperature-

isothermal conditions.

dependent d i s s o c i a t i o n of t h e salt i n t o l i q u i d NaF and BF, gas.

Thus,

we f i r s t determined t h e e f f e c t s of varying t h e concentration of our s a l t mixture and t h e BF3 pressure over it i n 6800-hr t e s t s a t 605째C.

In a

l a t e r s e r i e s of experiments we determined t h e r a t e of chromium uptake by NaBF4-8 mole $, NaF between 538 and 760째C i n 1200-hr t e s t s . EXPERIMENTAL METHOD

AND MATERIALS

The capsule design used i n studying t h e e f f e c t of s a l t composition and BF3 overpressure ( S e r i e s I ) i s p i c t u r e d i n Fig. 1. Each capsule cont a i n e d t h r e e specimens of titanium-modified Hastelloy N, one l o c a t e d i n t h e vapor space, one i n t h e s a l t , and one a t t h e liquid-vapor i n t e r f a c e . The capsule was constructed of commercial Hastelloy N and w a s 2 i n . i n diameter x 6 i n . high.

The titanium-modified a l l o y i s considered

because o f i t s s u p e r i o r r a d i a t i o n - r e s i s t a n t p r o p e r t i e s . PHOTO 762074

GAS AND F I L L LINES

DRAIN TANK

SPEC1M ENS

Fig. 1. Hastelloy N Capsule Assembly f o r Studying E f f e c t s o f S a l t Composition and BF, Overpressure. The capsule used i n studying t h e e f f e c t of temperature ( S e r i e s 11)

w a s b a s i c a l l y t h e same.

However, t h e t e s t specimens were commercial

Hastelloy N i n t h e form of 0.030-in.-thick area.

s t r i p s t o provide more surface

Each capsule w a s 2 i n . i n diameter x 8.5 i n . high and contained

16 specimens.

The capsule i s p i c t u r e d i n Fig. 2, and t h e t e s t specimens

are shown i n Fig. 3.

Table 1 l i s t s t h e compositions of t h e a l l o y s used

f o r t h e capsules and specimens.

DUMP TANK HASTELLOY N

F i g . 2.

Hastellay N Capsule Assenibly f o r Temperature Effect Study.

F i g . 3 . Standard Hastellay N Specimens Made t o Provide Maximum Surface Area f o r Study of Temperature Effects. (a) End vi&. (b) Side view. \

5 Table 1. Composition of Hastelloy N

est Series I

Composition (wt

c omponent

$1"

Cap s u l e s

15.8

7.4

2.4

0.2

0.3

0.02

Spech e n s

12.0 7.3 < 0.1 < 0.01 0.14 0.5

Capsules and Specimens

17.0 7.2

4.3

0.45 0.5

0.02

a Balance nickel. The salt f o r t h e s e t e s t s was processed by t h e Fluoride Frocessing Group of t h e Reactor Chemistry Division.

The very pure (> 99.9%) s t a r t i n g

m a t e r i a l s were evacuated t o about 380 t o r r , heated t o 150째C i n a v e s s e l l i n e d with n i c k e l , and t h e n held f o r about 15 h r under t h e s e conditions. After t h e r i s e i n vapor pressure was observed t o be not excessive ( i n d i c a t i n g no v o l a t i l e i m p u r i t i e s ) , t h e s a l t was heated t o 500째C while s t i l l under vacuum and a g i t a t e d by bubbling helium through t h e capsule f o r a few hours.

The s a l t w a s t h e n t r a n s f e r r e d t o t h e f i l l v e s s e l and forced

i n t o t h e capsules with helium pressure.

S a l t chemistry i s discussed

under "Results" i n t h i s r e p o r t . A l l capsules were heated i n v e r t i c a l muffle furnaces (Fig. 4) and

monitored with Chromel-F v s A l u m e l thermocouples, which had been spot welded t o t h e capsule and covered with shim stock.

I n our f i r s t s e r i e s

of t e s t s a l l capsules were operated a t 605"C, c l o s e t o t h e maximum t e m -

p e r a t u r e proposed f o r t h e coolant s a l t i n t h e MSBR.

Capsule 1 of t h i s

s e r i e s contained a helium overpressure of 4 p s i g and no added BF3. Capsules 2, 3, and 4 contained BF3 corresponding t o overpressures of 50,

100, and 400 p s i g , r e s p e c t i v e l y .

I n i t i a l l y t h e added BF3 dissolved i n

t h e molten s a l t o r combined with t h e NaF, b u t a f t e r a few minutes t h e p r e s s u r e i n t h e capsule gradually increased.

However, when t h e capsules

had been pressurized t o t h e operating pressures, no changes i n BF, cont e n t were necessary during t h e t e s t .

The compositions of t h e s a l t s

c a l c u l a t e d from amounts of BF3 added a r e shown i n Table 2.

7 Table 2.

Capsule

Effect of S a l t Composition and BF3 Overpressure on Weight Changes of Hastelloy N

BF3 Overpressure ( P d

Salt Compos it i o n

Weight Change (mg/cm2)

(mole %) NaBF4 NaF

Vapor

Interface

Liquid

-0.03

+O. 5

+1.3

-0.3

+0.4

+1.2

100

4.4

+o. 2

+O. 06

400

0.0

-0.03

A f t e r 6800 h r t h e s a l t i n each capsule was removed and sampled.

The

capsules were opened and t h e specimens were removed, washed i n warm d i s t i l l e d water, d r i e d with e t h y l alcohol, and weighed.

The specimens and

capsules were examined metallographically and by x-ray fluorescence and microprobe a n a l y s i s .

The s a l t s were analyzed before and a f t e r t h e t e s t s .

I n our second s e r i e s of experiments, t h e capsules were operated a t 427°C (8OO0F), 538°C (lOOO°F), 649°C (1200"F), and 760°C (1400°F) with 5 p s i g He overpressure and c a l c u l a t e d equilibrium BF3 pressures of 0.005,

0.07,

0.5, and 2.5 p s i a , r e s p e c t i v e l y .

After 1200 h r t h e s a l t i n each

capsule was removed and sampled f o r a n a l y s i s .

The s a l t had a l s o been

analyzed before test. RESULTS

Effect of S a l t Composition and BF3 Pressure ( S e r i e s I ) Weight Changes The compositional changes i n t h e s a l t e f f e c t e d by BF3 gas a d d i t i o n s i n our first s e r i e s of t e s t s caused a measurable d i f f e r e n c e i n t h e weight changes of our t e s t specimens.

A s shown i n Table 2, t h e weight changes

were r e l a t i v e l y small, and some specimens showed weight gains i n s t e a d of t h e expected l o s s e s . W

There w a s no c o r r e l a t i o n between t h e pressure of

BF3 and t h e weight change of t h e specimens exposed t o t h e vapor.

The specimens immersed in salt i n capsules 1, 2, and 3 showed progressively decreas,ing weight gains. The salt specimen-in capsule 4 showed

a weight loss equivalent t o a corrosion r a t e l e s s than 0.01 mil/year. The same weight change pattern w a s observed f o r t h e specimens a t the interface. Thus, the weight gain of specimens i n the liquid and at the interface increased as t h e amount of NaBF4 i n the s a l t decreased. Figure 5 shows t h e specimens after t e s t . Very l i t t l e corrosion was noted at any position on the specimens o r capsules.

VAPOR

-I NTERi:ACE

SALT

Fig. 5. Titanium-Modified Hastellay N Specimens Af NaEiF44 mole $ NaF a t 605째C f o r 6800 hr. m e r s identi

Exposure t o capsules.

Salt Chemistry Chemical analyses of t h e salt before and a f t e r t e s t f o r each capsule are given i n Table 3.

!be most significant changes a r e an

increase in chromium concentration and a decrease i n iron concentration.

No titanium concentrations a r e included i n Table 3 because, as expected,

9 W

Table 3.

S a l t Analyses Before and After Test (Series I )

Concentrat ion Element

As Received

Capsule 1

Capsule 2

Capsule 3

Capsule 4

21.8

21.0

Weight Percent 21.9

9.57

68.2

21.4 9.54 69.2

21.0 9.48

9.48

68.5

9.49

68.7

68.8

P a r t s Per Million Cr

< 10

223

< 10

< 5

45 9

H20

400

< 5

194

576

372

1420a

400

460

350

460

Questionable r e s u l t .

no change w a s noted; only t h e t e s t specimens contained titanium, and t h e y c o n s t i t u t e d only a small p a r t of t h e t o t a l system.

The only change

i n t h e water and oxygen concentrations occurred i n capsule 4.

The high

oxygen concentration r e p o r t e d f o r t h i s capsule would normally have produced h i g h l y oxidizing conditions

and high corrosion r a t e s .

The

r e p o r t e d value i s believed t o be i n e r r o r because close examination of t h e system d i s c l o s e d no leaks o r signs of oxidation.

Apparently some

BF3 escaped from t h e capsules a f t e r cooling and during opening, s i n c e

t h e concentrations of sodium, boron, and f l u o r i n e i n t h e s a l t were about t h e same i n t h e f i n a l a n a l y s i s of each capsule. The increase i n chromium and t h e decrease i n i r o n i n t h e s a l t sugg e s t a r e a c t i o n of t h e type

8H. E. McCoy, J. R. Weir, Jr., R. L. Beatty, W. H. Cook, C. R. Kennedy, A. P. Litman, R. E. Gehlbach, C. E. Sessions, and J. W. Koger, Materials f o r Molten-Salt Reactors, ORNETM-2511 (May 1 9 6 9 ) .

10 Cr(s) + FeFZ(d) f C r F z ( d ) + Fe(deposited)

(1)

where ( s ) i n d i c a t e s s o l i d s o l u t i o n i n Hastelloy N and (d) i n d i c a t e s dissolved i n s a l t .

Any water or oxygen-type impurity i n t h e s a l t would

a l s o have caused some oxidation of t h e container m a t e r i a l s , but t h e s e impurities appear t o have been n e g l i g i b l e i n t h i s s e r i e s of t e s t s . Because t h e s a l t i n t h e s e experiments contained NaF, t h e chromium f l u o r i d e i n t h e corrosion product w a s found as NasCrF6.

This m a t e r i a l has

a l s o been i d e n t i f i e d i n other corrosion experiments'

and i t s c r y s t a l

s t r u c t u r e has been determined by x-ray d i f f r a c t i o n a n a l y s i s .

Also,

t h e i r o n f l u o r i d e i n t h e s e experiments may have e x i s t e d as NagFeFg, but t h i s compound w a s not i d e n t i f i e d . SDecimen Chemistry The proposed corrosion r e a c t i o n w a s v e r i f i e d f u r t h e r by x-ray fluorescence a n a l y s i s of t h e specimens.

Table 4 gives t h e composition

of t h e near-surface region of a specimen from capsule 4 .

The composi-

t i o n w a s determined by comparing x-ray i n t e n s i t i e s with t h o s e of a specimen not exposed t o s a l t o r vapor.

For any given region i n t h e

capsule, t h e q u a l i t a t i v e r e s u l t s from a l l t h e specimens were q u i t e

similar.

These r e s u l t s show a depletion of chromium and enrichment of

i r o n i n t h e near-surface region. titanium. changes.

A l l specimens a l s o showed a loss of

Microprobe a n a l y s i s showed e s s e n t i a l l y t h e same s u r f a c e A chromium gradient w a s seen b u t w a s s o s m l l t h a t we could

not d i s t i n g u i s h between edge e f f e c t s and an a c t u a l g r a d i e n t . According t o Cantor,

'' NaBF1-8

mole $I NaF may a l s o oxidize chro-

mium by t h e r e a c t i o n s

'J. W. Koger and A. P. Litman, Compatibility of Hastelloy N and Croloy 9M w i t h NaBF4-NaF-KBF4 (90-4-6 mole $) Fluoroborate S a l t , ORNLTM-2490 (April 1 9 6 9 ) . " G . Brunton, "The Crystal S t r u c t u r e of Na3CrF6," Mater. Res. Bull. 4, 621-626 ( 1 9 6 9 ) .

"S. Cantor, MSR Program Semiann. Progr. Rept. Aug. 3 1 , 1968, ORNL4344, p. 160.

11 Table 4.

Composition of Near-Surface Regions of Test Specimens as Determined by X-Ray Fluorescent Analysis Concentration ( w t %)

Exposure Conditions

Unexposed

12.0

79.9

0.1

7.5

0.5

Vapor

12.3

79.2

0.13

7.1

0.19

Interface

13.7

69.6

13.4

3.0

0.2

Liquid

18.0

22.7

56.4

2.7

0.16

(1+ x ) Cr(s) + NaBFr,(d) '$NaF(d)

3NaF(d)

+ CrF3(d)

CrF3(d) f N a $ r F 6 ( s )

CrxB(s)

(2) (3)

However, no borides were i d e n t i f i e d e i t h e r on our specimens or i n t h e

s a l t , and t h e experimental evidence t h a t i r o n replaced t h e chromium i n d i c a t e s t h a t r e a c t i o n (1)r a t h e r than ( 2 ) w a s t h e predominant mode of chromium oxidation.

If e s s e n t i a l l y no i r o n f l u o r i d e compound were

present i n t h e melt, r e a c t i o n (2) could w e l l c o n t r o l t h e corrosion process. Metallography Figure 6 shows t h e microstructure of t h e specimen f r o m t h e vapor region of capsule 1.

There was some i n d i c a t i o n of a v e r y t h i n , discon-

tinuous deposit at t h e s u r f a c e i n t h e as-polished condition. etching, t h i s deposit w a s no longer v i s i b l e .

After

Figure 7 shows t h e speci-

men t h a t was exposed t o salt i n capsule 1. A deposit i s v i s i b l e on t h e as-polished specimen.

Etching again removed t h e deposit b u t i n t h i s

case produced considerable a t t a c k a t t h e exposed s u r f a c e . observation of t h e capsule w a l l showed similar behavior.

Metallographic This deposit i s

apparently i r o n r i c h , and t h e a r e a revealed by t h e etchant i s chromiumdepleted Hastelloy N.

Thus, no d i f f e r e n c e i n t h e behavior of titanium-

modified and standard Hastelloy N was noted i n t h i s t e s t .

The i n t e r f a c e

between t h e s a l t and gas could be seen on t h e capsule, b u t no a t t a c k w a s noted t h e r e .

Fig. 6. Titanium-Modified Hastelloy N from Capsule 1 Exposed to BF3 Vapor for 6800 hr at 605째C. Weight change, +0.03 m g / c m 2 . 500x. (a) As polished. (b) Etched with glyceria regia.

. I

-. .

... .. .

Fig. 7. Titanium-Modified Hastelloy N from Capsule 1 Exposed to NaBF4-8 mole $ NaF for 6800 hr at 605째C. Weight change, +l.3 mg/cm2. 5OOx. (a) As polished. (b) Etched with glyceria regia.

Our second s e

mine the effects of

temperature on chromium

transfer from the Hastelloy N t o the salt.

The salt was analyzed be

and a f t e r t e s t t o determine impurity pickup.

The NaBF4-8 mole $ NaF contained approximately 400 ppm each of oxygen and water before and after t e s t . .

The chromium concentration i n the salt

No changes i n

increased while the iron decreased, as seen i n Table 5. t h e concentration of n i

and molybdenum were noted.

Again, t h e

r e s u l t s suggest chromium oxidation by reduction of an iron fluoride compound.

Metallography of Hastelloy N f r o m t h i s t e s t shmed, after

etching, no attack of the specimen (Fig. 8). Table 5.

Analysis of S a l t f o r Impurities Before and After Test (Series 11)

Impurity

As Received

Cr Fe

15 200 460 320

’

0 €I20

Average Concentrationa (ppm> Capsule Capsule Capsule 1 2 3 45 190 430 400

87 180

450 530

225 150 412

480

Capsule 4 575

40 550 490

&Nickel and llbolybdenum were not detected (< 10 ppm) i n any samples.

Fig. 8. Hastelloy N Exposed t o NaBF4-8 mole 760°C. Etched with glyceria

N a F f o r 1200 h r at

14 INTERPRETATION OF CORROSION RATES If t h e chromium s u r f a c e concentration remains constant (only t r u e i f t h e s a l t contains an excess of i r o n ) a t any given point i n t h e cap-

s u l e , t h e amount of chromium leaving a capsule i n S e r i e s I i s given by t h e equationI2

where N

i n t e g r a l f l u x of C r leaving metal, g/cm2

p = d e n s i t y of metal, g/cm3

i n i t i a l weight f r a c t i o n of C r

s u r f a c e weight f r a c t i o n of C r

D = d i f f u s i o n c o e f f i c i e n t of C r i n a l l o y , cm2/sec

exposure time, sec.

Using t h e following information S a l t i n capsule = 282 g C r increase i n s a l t = 55 ppm (by weight)

Capsule s u r f a c e a r e a = 142 cm2 t o obtain N and then using

7.5%

0 ( c o n s i s t e n t with excess of Fe)

p =

8.8 g/cm3

6800 h r ,

we obtain Dcalcd

= 3.3 x

cm2/sec a t 605째C.

This compares favorably with 3 x

cm2/sec a t 605째C e x t r a p o l a t e d

from t h e range 700 t o 850째C from datal3 of DeVan and Evans f o r chromium 12L. S. Darken and R. W. Gurry, Physical Chemistry of Metals, McGraw-Hill, New York, 1953.

'%. R. Grimes, G. M. Watson, J. H. DeVan, and R. B. Evans, "RadioTracer Techniques i n t h e Study of Corrosion by Molten Fluorides," pp. 559-574 i n Conference on t h e Use of Radioisotopes i n t h e Physical Sciences and Industry, September 6-17, 1960, Proceedings, Vol. 111, I n t e r n a t i o n a l Atomic Energy Agency, Vienna, 1962.

15 d i f f u s i o n i n Hastelloy N.

From t h i s comparison, we conclude t h a t t h e

c o n t r o l l i n g r a t e mechanism i n t h i s experiment was t h e s o l i d - s t a t e d i f f u s i o n of chromium i n t h e a l l o y and t h a t t h e oxidation p o t e n t i a l of t h e

s a l t w a s e s t a b l i s h e d by t h e presence of i r o n f l u o r i d e . Figure 9 shows t h e v a r i a t i o n of chromium content with t e s t tempera t u r e f o r t h e experiments of Series 11.

Included f o r comparison a r e

chromium concentrations measured i n loop t e s t s a f t e r 1200 hr a t various H20 impurity l e v e l s . l4-I7

An Arrhenius-type r e l a t i o n s h i p appears t o

1 4 J . W. Koger and A. P. Litman, MSR Program Semiann. Progr. Rept. Feb. 29, 1968, ORNL4254, pp. 218-221.

"J. W. Koger and A. P. Litman, PER Program Semiann. Progr. Rept. AUg. 31, 1968, ORNL-4344, pp. 257-264. 16J. W. Koger and A. P. L i t m n , MSR Program Semiann. Progr. Rept. Feb. 28, 1969, ORNL-4396, pp. 243-246. I7J. W. Koger and A. P. Litman, Compatibility of Hastelloy N and Croloy 9M with NaBF4-NaF-KBF4 (90-4-6 mole 7) Fluoroborate S a l t , ORNLTM-2490 (April 1969).

69- 12238R

ORNL-DWG

800

TEMPERATURE ("C) 700 650 600 550 500

450

400

h (L

2 0 3 1 LI

402

STATIC CAPSULE TESTS (1200 h r )

THERMAL CONVECTION LOOPS CALCULATION BASED ON Cr DIFFUSION IN HASTELLOY N

0.90

1-0

1.1

1.3

1,2 'ooo/T

I .4

1.5

(OK)

Fig. 9. Temperature Dependence of Chromium Concentration i n Sodium Fluoroborate S a l t .

16 hold between 538 and 760°C.

This would be expected i f t h e r a t e of

chromium buildup i n t h e salt were c o n t r o l l e d by s o l i d - s t a t e d i f f u s i o n

of chromium t o t h e capsule w a l l .

Using d i f f u s i o n d a t a f o r chromium i n

Hastelloy N obtained by DeVan and Evans13 and assuming t h e chromium s u r f a c e concentration t o have been reduced t o zero by t h e s a l t , we p r e d i c t a chromium buildup shown by t h e dashed l i n e i n Fig. 9.

The

agreement between t h e slopes ( a c t i v a t i o n energy) of t h e p r e d i c t e d and experimental curves, although not exact, gives credence t o t h e assumpt i o n t h a t chromium mass t r a n s f e r t o t h e s a l t i s l i m i t e d by s o l i d - s t a t e d i f f u s i o n of chromium t o t h e Hastelloy N s u r f a c e .

CONCLUSIONS

Titanium-modified Hastelloy N specimens were exposed f o r

6800 h r t o isothermal NaBFL-NaF s a l t mixtures containing only a few

hundred p a r t s p e r million oxygen and w a t e r and varying i n composition from 4 t o 8 mole $ NaF.

a l l but t h e 96 mole

Specimens i n t h e vapor region l o s t weight i n

?’ NaBF4 mixture.

Specimens f u l l y immersed i n t h e

s a l t gained weight i n a l l but t h e 96 mole $ NaBF4 mixture.

The weight

gains decreased as t h e NaBF4 content of t h e s a l t increased.

No differences i n t h e corrosion of specimens i n t h e vapor

region were noted as a function of BF3 overpressure t o 400 psig.

I n t h e main corrosion r e a c t i o n , chromium f r o m t h e a l l o y was

oxidized by i r o n f l u o r i d e impurities i n i t i a l l y i n t h e s a l t .

The i r o n

t h a t w a s reduced deposited on t h e specimens and capsule.

Metallographic observation o f both standard Hastelloy N capsules

and titanium-modified Hastelloy N specimens showed very l i t t l e a t t a c k , although a t h i n discontinuous deposit r i c h i n i r o n w a s observed a t t h e s u r f a c e of specimens exposed t o vapor. 5.

A d i f f u s i o n c o e f f i c i e n t f o r chromium i n Hastelloy N c a l c u l a t e d

from t h e r a t e of chromium removal from standard Hastelloy N by NaBFr;-8 mole $ NaF h e l d between 538 and 760°C agreed w e l l w i t h an e x t r a p o l a t i o n of published r e s u l t s .

Thus, s o l i d - s t a t e d i f f u s i o n of

chromium appeared t o be t h e r a t e - c o n t r o l l i n g mechanism f o r corrosion i n these t e s t s .

Under t h e condition of t h e s e experiments, t h e fluoroborate

s a l t mixtures were compatible with t h e Hastelloy N a l l o y s .

By v i r t u e

of o t h e r experiments we conclude t h a t increases i n water as impurity increase t h e chromium uptake and t h e corrosion.

We wish t o acknowledge E. J. Lawrence for h i s expert supervision of t h e f a b r i c a t i o n , operation, and disassembly of t h e t e s t capsules and

F. D. Harvey f o r h i s handling of t h e t e s t specimens.

We a r e a l s o

indebted t o H. E. McCoy, Jr., J. H. DeVan, and C . E. Sessions for cons t r u c t i v e review of t h e manuscript. Special thanks a r e extended t o H. R. Gaddis and Helen Mateer of t h e General Metallograghy Group, Harris Dunn and others i n t h e Analytical Chemistry Division, Graphic A r t s , and t h e Metals and Ceramics Division Reports Office for invaluable a s s i s t a n c e .

19 W

ORNL- TM- 2978 INTERNAL DISTRIBUTION

1-3.

4-5. 6-15. 16.

17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43.

44. 45. 46.

Central Research Library ORNL - Y-12 Technical Library Document Reference Sec t ion Laboratory Records Department Laboratory Records, ORNL RC ORNL Patent Office MSRP D i r e c t o r ' s Office (Y-12) G. M. Adamson, Jr. J. L. Anderson R. F. Apple C. F. Baes S. E. Beall E. S. B e t t i s F. F. Blankenship E. G. Bohlmann R. B. Briggs S. Cantor 0. B. Cavin Nancy C. Cole W. H. Cook J. L. Crowley F. L. Culler J. H. DeVan J. R. DiStefano S. J. Ditto W. P. Eatherly J. I. Federer D. E. Ferguson L. M. F e r r i s J. H Frye, Jr. R. E. Gehlbach L. 0. G i l p a t r i c k W. R. Grimes A. G. Grindell F. D. Harvey

47. W. 0. Harms 48. P. N. Haubenreich 49. R. E. Helms 50-52.

53. 54. 55. 56-65. 66. 67. 68. 69.

70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82.

83. 84. 85. 86. 87. %% .

89. 90. 91. 92.

M. R. H i l l W. R. Huntley H. Inouye P. R. Kasten J. W. Koger E. J. Lawrence R. B. Lindauer M. I. Lundin R. E. MacPherson W. R. Martin H. E. McCoy, Jr. C . J. McHargue A. S. Meyer D. M. Moulton P. P a t r i a r c a A. M. Perry H. C. Savage Dunlap Scott J. L. Scott C. E. Sessions J. H. Shaffer G. M. Slaughter A. N. Smith D. A. Sundberg J. R. Tallackson R. E. Thorn D. B. Trauger G. M. Watson J. R. Weir, Jr. J. C. White Gale Young E. L. Youngblood

EXTERNAL DISTRIBUTION

93-94. D. F. Cope, RDT, SSR, AEC, Oak Ridge National Laboratory 95. A. R. DeGrazia, Division of Reactor Development and Technology, 96. 97. W

AEC, Washington David Elias, Division of Reactor Development and Technology, AEC Washington J. E. Fox, Division of Reactor Development and Technology, AEC, Washington

20 98. Norton Haberman, Division of Reactor Development and Technology, AEC, Washington 99. Kermit Laughon, RDT, OSR, AEC, Oak Ridge Operations 1OC-109. A. P. L i t m a n , Division of Space Nuclear Systems, AEC, Washington 11c-111. T. W. McIntosh, Division of Reactor Development and Technology, AEC, Washington 112. D. R. Riley, Division of Reactor Development and Technology, AEC, Washington 113. M. Shaw, Division of Reactor Development and Technology, AEC, Washington 114. J. M. Simmons, Division of Reactor Development and Technology, AEC, Washington 115, Laboratory and University Division, AEC, Oak Ridge Operations 116-130. Division of Technical Information Extension