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UNION CARBIDE CORPORATION

NUCLEAR DIVISION for the

U.S. ATOMIC ENERGY COMMISSION

ORNL- TM- 2724

COMPATIBILITY OF MOLYBDENUM-BASE ALLOY TZM,WITH c

LiF-BeF

ThF UF (68-20-1 1.7-0.3 mole %) at 1100% 4- 4 J. W. Koger and A. P. Litman

NOTICE This document contains information of a preliminary nature and was prepared primarily for internal use a t the Oak Ridge National Laboratory. It i s subiect to revision or correction and therefore does not represent a final report.

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ORNLTM-2724

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 O F MOLYBDENUM-BASE ALLOY TZM WITH LiF-BeF*-ThF&-UF4 (68-20-11.7-0.3 m o l e at 1100째C

J. W. Koger and A. P. Litman

DECEMBER 1969

OAK RIDGE NATIONAL LABORATORY

O a k R i d g e , Tennessee operated by UNION CARBIDE CORPORATION f o r the U.S. ATOMIC ENERGY COMMISSION

iii

CONTENTS Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental Procedure . . . . . . . . . . . . . . . . . . . . . Results and Discussion . . . . . . . . . . . . . . . . . . . . . S a l t Analysis . . . . . . . . . . . . . . . . . . . . . . . Weight Changes . . . . . . . . . . . . . . . . . . . . . . X-Ray Fluorescence and Microprobe Analysis . . . . . . . . Microstructural Changes . . . . . . . . . . . . . . . . . . Re c r ys t a l l i z a t i o n . . . . . . . . . . . . . . . . . . . . . Strength . . . . . . . . . . . . . . . . . . . . . . . . .

............. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . Corrosion Reactions and Kinetics

Page 1 1

2 3 3

4 4

5 8 9 11

COMPATIBILITY OF MOLYBDENUM-BASE ALLOY TZM WITH LiF-BeF2-ThF4-UF4 (68-20-11.7-0.3 mole %) a t 1100"C

J. W. Koger and A. P. Litman

ABSTRACT

The TZM a l l o y (Mo-O.5% Ti-0.08% Zr-O.02$ C ) showed a very small amount of a t t a c k by t h e fused f l u o r i d e s a l t (LiF-BeF2-ThF4-UF4, 68-20-11.7-0.3 mole %) a t 1100"C for 1011 h r . Corrosion manifested i t s e l f as leaching of t i t a n i u m and possibly zirconium from t h e a l l o y . The TZM a l l o y exposed t o t h e s a l t p a r t i a l l y r e c r y s t a l l i z e d , while t h a t exposed t o t h e vapor did not. This r e c r y s t a l l i z a t i o n w a s a t t r i b u t e d t o t h e removal of t i t a n i u m and zirconium. On t h e basis of t h i s s i n g l e t e s t t h e magnitude and mechanism of corrosion i n d i c a t e no s e r i o u s problems f o r longterm use of TZM i n t h e vacuum d i s t i l l a t i o n processing scheme f o r t h e Molten S a l t Breeder Reactor. However, t h e s t r e n g t h p r o p e r t i e s of t h e TZM a l l o y would approach those of unalloyed molybdenum as s a l t exposure time increased; t h i s i s not considered a problem now.

INTRODUCTION

The current success of t h e Molten S a l t Reactor Experiment a t ORNL has stimulated work on a thermal Molten S a l t Breeder Reactor (MSBR).' One of t h e r e q u i r e m e n t s for a s u c c e s s f u l MSBR s y s t e m w i l l be t h e con-

tinuous reconditioning of t h e fuel s a l t t o remove unwanted f i s s i o n products.

A p o s s i b i l i t y under study f o r one s t e p of t h e s a l t reprocessing

i s vacuum d i s t i l l a t i o n .

Uranium would be s t r i p p e d from t h e f u e l s a l t ,

and t h e remaining s a l t would be d i s t i l l e d a t 1000째C and 2 t o r r .

The

d i l u e n t s o f t h e f u e l s a l t , l i t h i u m and beryllium f l u o r i d e s , would d i s t i l r e a d i l y and leave behind t h e r a r e - e a r t h and a l k a l i n e - e a r t h f i s s i o n products.

This process has been demonstrated i n laboratory experiments

and w i t h some r a d i o a c t i v e s a l t from t h e E R E .

1968,

'M. W. Rosenthal et al., MSR Program Semiann. Progr. Rept. Aug. 31, ORNL-4344, pp. 53-108.

2J. R. Hightower and L. E. McNeese, MSR Program Semiann. Frogr. Rept. Aug. 31, 1968, ORNL-4344, pp. 306-308.

The s t r e n g t h and corrosion r e s i s t a n c e required o f a container m a t e r i a l f o r t h e high-temperature vacuum d i s t i l l a t i o n s t e p eliminate most conventional a l l o y s from consideration.

Our preliminary survey

d i s c l o s e d t h a t c e r t a i n r e f r a c t o r y a l l o y s , p a r t i c u l a r l y molybdenum-base m a t e r i a l s , may be s u i t a b l e f o r t h i s s p e c i a l s e r v i c e .

(Mo-O.5% Ti4.08’$ Zl-o.O2$

The a l l o y TZM

C ) w a s s e l e c t e d f o r an i n i t i a l experiment

because it i s s t r o n g e r and u s u a l l y m r e f a b r i c a b l e t h a n pure molybdenum. Accordingly, t h e experiment reported here provides a preliminary t e s t of t h e c o m p a t i b i l i t y of TZM a l l o y with a t y p i c a l f e r t i l e - f i s s i l e s a l t (LiF-BeF2-ThF4-UF4, 68-20-11.7-0.3

mole $) a t 1100°C.

This s a l t i s a

s t r o n g candidate f o r t h e s i n g l e - f l u i d MSBR now being designed.

No t e s t s

were s p e c i f i c a l l y conducted t o determine t h e s t r e n g t h p r o p e r t i e s of TZM a l l o y , but conditions caused by t h e exposure t o t h e s a l t t h a t could a f f e c t t h e s t r e n g t h were noted. EXPERIMENTAL PROCEDURE

The experimental system used f o r t h i s study c o n s i s t e d of a simple capsule f a b r i c a t e d of cold-worked TZM a l l o y , containing specimens of t h e same a l l o y , and shown i n Fig. 1.

Note t h a t t h e specimens were

l o c a t e d i n t h e s a l t , at t h e salt-vapor i n t e r f a c e , and i n t h e vapor.

The

p u r i f i e d s a l t (60 g ) w a s supplied by t h e Fluoride Processing Group of t h e Reactor Chemistry Division.

P u r i f i c a t i o n involved sparging w i t h an

HF-Hz mixture a t 600°C t o remove oxides and s u l f i d e s and s t r i p p i n g w i t h H2

at 790°C t o remove m e t a l l i c impurities.

The loading operation, which

c o n s i s t s of introducing t h e f l u o r i d e s a l t i n t o t h e capsule, welding t h e t e s t capsule, and s e a l i n g t h e outer Inconel p r o t e c t i v e container, was c a r r i e d out i n an inert-gas atmosphere charriber containing argon purer than 99.995%. After being t e s t e d i n t h e p o s i t i o n shown i n Fig. 1 f o r 1011 h r a t

llOO°C, t h e capsule w a s removed f r o m t h e furnace, i n v e r t e d t o keep t h e specimens out of t h e s a l t , and quenched i n l i q u i d nitrogen t o r e t a i n high-temperature corrosion products.

Af’ter t e s t , weight changes of t h e

specimens were determined, t h e salt was analyzed f o r i m p u r i t i e s , and t h e specimens and capsules were analyzed by x-ray fluorescence and examined metallographically.

3 ORNL-DWG

69-10034

1.625 in

0.88 in.

/0.125-in. WALL INCONEL PROTECTIVE CONTAINER

_ 0.040 , -in. WALL T Z M CAPSULE

--00.004-in.

TANTALUM FOIL L I N E R

,-ARGON /â&#x20AC;&#x2122;

L i F- Be F2 -Th

5 - U F4 SALT

(68- 2 0 - 11.7 - 0.3 MOLE % ) -TZM

SPECIMEN (0.30 in.x 1.0 in. x 0.02 in.)

Fig. 1. Schematic Drawing of Corrosion Test Capsule Used t o Study Compatibility of TZM Alloy w i t h a Fused Fluoride S a l t . RESULTS AND DISCUSSION S a l t Analys i s

Concentrations of t h e c o n s t i t u e n t s of t h e s a l t and i t s i m p u r i t i e s b e f o r e and a f t e r t e s t a r e given i n Table 1.

During t h e experiment

t i t a n i u m , zirconium, and chromium concentrations i n t h e s a l t increased and t h a t of i r o n decreased.

The t i t a n i u m and zirconium a r e i n t e n t i o n a l

a l l o y i n g a d d i t i o n s , b u t chromium i s an unwanted impurity. Weight Changes The specimens exposed t o t h e s a l t showed small (0.5 mg/cm2) weight gains, and t h e one exposed t o t h e vapor did not change weight measurably.

Table 1. Chemical Analysis of F e r t i l e - F i s s i l e S a l t Exposed t o TZM Alloy Capsule f o r 1011 h r at 1100°C (2010°F)

Constituent Mo

Zr Ti Fe Cr 0 H20

Content, ppm Before

After

< 5 37

< 10

74 80

151

58 40

Constituent

134

38 97 < 50 70

Li Be Th U F

Content, w t $ Before

After

6.71 2.65 43.1 1.75 45.5

7.01 2.55 42.6 1.93

45.7

X-Ray Fluorescence and Microprobe Analysis Table 2 gives t h e concentrations of t h e major elements i n t h e TZM a l l o y as determined by x-ray fluorescence before and a f t e r t e s t .

Iron

w a s found on t h e s u r f a c e and probably caused a major p o r t i o n of t h e

weight gains, but no q u a n t i t a t i v e value w a s obtained.

Significantly,

t h e q u a n t i t a t i v e a n a l y s i s shows a decrease i n t i t a n i u m concentration, no s i g n i f i c a n t change i n zirconium concentration, and a corresponding increase i n t h e concentration of molybdenum a f t e r exposure t o t h e s a l t . Care must be taken i n i n t e r p r e t i n g t h e s e r e s u l t s , s i n c e t h e s e n s i t i v i t y of t h e fluorescence a n a l y s i s i s questionable a t t h e s e low concentrations and i r o n w a s deposited over t h e surface.

The e l e c t r o n microprobe

a n a l y s i s showed 0.3% T i on t h e surface and 0 . 5 $ T i i n t h e matrix.

The

zirconium content w a s about ‘2.1%i n a l l portions of t h e specimen.

Any

changes at t h e l e v e l of O.l$ a r e beyond t h e l i m i t of d e t e c t i o n of t h e instrument.

Hawever, t h e s e r e s u l t s agree reasonably with t h e increase i n

concentration of c e r t a i n a l l o y i n g elements i n t h e s a l t and a r e i n accord with t h e proposed corrosion mechanism(s).

(See Corrosion Reactions and

Kinetics. ) Microstructural Changes Figure 2(a) shows t h e t y p i c a l cold-worked s t r u c t u r e of t h e specimens and capsule before t e s t .

This f i g u r e i s a l s o t y p i c a l of t h e specimen

5 Table 2. Concentration of Alloying Elements i n TZM Alloy Specimen Before and After Exposure t o a F e r t i l e - F i s s i l e S a l t a t 1100째C f o r 1011 h r , as Determined by X-Ray Fluorescence Analysis" Content. w t %

Sample Analyzed Mo

Untested a l l o y

99.4

0.08

0.5

Exposed specimens i n vapor at interface i n salt

99.8 99.87 99.90

0.08 0.09 0.0%

0.1 0.04

0.013

The a n a l y s i s disclosed s u b s t a n t i a l i r o n on t h e a l l o y surface a f t e r t e s t , but i r o n w a s not considered i n determining t h e quantit i e s above. exposed t o t h e vapor, where no m i c r o s t r u c t u r a l change occurred.

unetched specimen, Fig. 2 ( b ) , exposed t o t h e s a l t shows no a t t a c k a t t h e surface.

The same specimen etched, Fig. 2 ( c ) , shows r e c r y s t a l l i z a t i o n

for a maximum depth of about 0.004 i n .

Examination of t h i s specimen at

a lower magnification, Fig. 2 ( d ) , shows t h a t both surfaces r e c r y s t a l l i z e d as t h e r e s u l t of t e s t .

The i n s i d e capsule w a l l a l s o r e c r y s t a l l i z e d i n

t h e same manner. Recrsst a l l i z a t ion I n view of t h e m i c r o s t r u c t u r a l and chemical changes induced i n t h e TZM a l l o y by t h i s t e s t , we compared reported r e c r y s t a l l i z a t i o n tempera-

t u r e s f o r molybdenum and TZM a l l o y (Table 3 ) .

It i s c l e a r from t h e

above and from general m e t a l l u r g i c a l considerations t h a t an increase i n annealing tlme from 1 h r t o s e v e r a l thousand hours should lower t h e r e c r y s t a l l i z a t i o n temperature of TZM a l l o y only 100 t o 200째C.

Moreover

t h e presence of as l i t t l e as 0.01% of a f o r e i g n element i n s o l i d s o l u t i o n can r a i s e t h e r e c r y s t a l l i z a t i o n temperature as much as s e v e r a l hundred degrees.

Conversely, t h e removal of a l l o y i n g c o n s t i t u e n t s would f r e e

3R. E. Reed-Hill, Physical Metallurgy P r i n c i p l e s , V a n Nostrand, Princeton, N. J . , 1964, p. 198.

Fig. 2. TZM Alloy Exposed t o F e r t i l e - F i s s i l e S a l t , LiF-BeF2-ThF4-UF4 (68-20-11.7-0.3 mole $) for 1011 hr at llOO째C. (a) Typical cold-worked s t r u c t u r e of capsule and specimens before t e s t ; a l s o the s t r u c t u r e of t h e specimen exposed t o the vapor during t e s t . 50Ox. Etchant : H20, H202, H2S04* (b) &-polished capsule and specimen exposed t o salt. 500x. and specimen exposed t o salt. 50Ox. Etchant: H20, H202, Specimen exposed t o salt. 1OOx. Etchant : H20, H202, H2SO4.

7 Table 3. R e c r y s t a l l i z a t i o n Behavior of Wrought, S t r e s s Relieved, Unalloyed Molybdenum and TZM Alloy Temperature

("C)

Time ( h r)

Percent Recrys t a l l i z a t ion

Reference

1130

100

TZM

560

4400

TZM

1100

TZM

1160

4400

TZM

1250

4400

100

TZM

1390

100

Unalloyed Mo

%. A. Wilcox, p. 26 i n Refractory Metal Alloys, Metallurgy and Technology, ed. by I. Machlin, R. T. Begley, and E. D. Weisert, Plenum P r e s s ; New York, 1968. bD. H. Jans en, Fue Is and Materials Development Program Quart. Progr. Rept. Sept. 30, 1968, ORNL-4350, pp. 107-111, and p r i v a t e communi c a t ion.

t h e g r a i n boundaries and allow them t o move t o form new g r a i n s .

Thus,

t h e enhanced r e c r y s t a l l i z a t i o n (lower r e c r y s t a l l i z a t i o n temperature) i n t h e samples and capsule of t h i s experiment i s due p r i m a r i l y t o t h e removal of t h e t i t a n i u m and p o s s i b l y zirconium f r o m t h e molybdenum matrix.

This i s f u r t h e r s u b s t a n t i a t e d by t h e l a c k of r e c r y s t a l l i z a t i o n

i n t h e samples exposed t o t h e vapor, where t h e composition changed much less. The a d d i t i o n of carbon and one o r more group IV-A elements t o molybdenum g r e a t l y i n c r e a s e s t h e r e c r y s t a l l i z a t i o n temperature.

Thus,

carbon removal from t h e a l l o y should likewise change r e c r y s t a l l i z a t i o n behavior.

However, carbon analyses show no d i f f e r e n c e (about 0.035% C

i n each) between exposed and unexposed TZM samples, s o t h i s e f f e c t i s very small or absent.

Although carbon mass t r a n s p o r t i s comon i n

l i q u i d metal systems, e s p e c i a l l y a l k a l i metals, it i s not considered a problem i n fused f l u o r i d e systems.

'W. H. Chang, A Study of t h e Influence o f Heat Treatment on Micros t r u c t u r e and P r o p e r t i e s of Refractory Alloys, ASD-TDR-62-211 ( A p r i l 1962).

8 Strength The molybdenum-base TZM a l l o y i s about t h e b e s t documented r e f r a c t o r y a l l o y i n which base metal s t r e n g t h i s improved by p r e c i p i t a t i o n hardening.

This a l l o y i s strengthened by t h e formation of f i n e carbides

of t i t a n i u m and zirconium as w e l l as by cold working, and i t s u l t i m a t e t e n s i l e s t r e n g t h i s double or t r i p l e t h a t of unalloyed molybdenum.

The

100-hr rupture s t r e n g t h of TZM at 1100째C i s a l s o much g r e a t e r than t h a t of molybdenum.5

Although TZM i s much m r e d i f f i c u l t t o f a b r i c a t e t h a n

commercial a l l o y s and many r e f r a c t o r y a l l o y s , it i s u s u a l l y much e a s i e r t o work than unalloyed molybdenum.

Thus, as an engineering m a t e r i a l TZM

has many advantages over molybdenum. Comparing t h e s t r e n g t h and d u c t i l i t y o f wrought, s t r e s s - r e l i e v e d , and r e c r y s t a l l i z e d TZM, Wilcox et a L 6 noted a s i g n i f i c a n t i n c r e a s e i n y i e l d and u l t i m a t e s t r e n g t h s due t o working a t t e s t temperatures of 1200 t o 1300째C.

A t 1550째C a f t e r r e c r y s t a l l i z a t i o n of t h e wrought sample

there was r e l a t i v e l y l i t t l e difference i n t h e materials.

However, at

l l O O 째 C , t h e temperature of our capsule t e s t , Wilcox's r e c r y s t a l l i z e d

a l l o y had much lower s t r e n g t h than t h e wrought a l l o y .

Thus, t h e use of

a s t r e s s - r e l i e v e d TZM a l l o y f o r conditions given i n t h i s experiment should a l s o be considered. Although TZM would g e n e r a l l y be favored over molybdenum f o r t h e previous reasons, through t h e l o s s of i t s a l l o y i n g elements ( t i t a n i u m and zirconium) during exposure t o t h e fused f l u o r i d e s a l t t h e composit i o n and t h e s t r e n g t h p r o p e r t i e s of t h e cold-worked TZM approach those of unalloyed r e c r y s t a l l i z e d molybdenum.

Although unalloyed molybdenum

or r e c r y s t a l l i z e d TZM i s weaker than t h e i n i t i a l cold-worked m a t e r i a l , t h e s t r e n g t h of t h e exposed m a t e r i a l would probably be ample f o r t h e loads proposed i n t h e lvlSBR vacuum d i s t i l l a t i o n system.

However, before

t h e depleted TZM i s used, it should be t e s t e d t o more c a r e f u l l y define 5T. E. Tietz and J. W. Wilson, Behavior and P r o p e r t i e s of Refractory Metals, Stanford University Press, C a l i f o r n i a , 1965, pp. 156-205.

6B. A. Wilcox, A. G i l b e r t , and B. C. Allen, Intermediate Temperature D u c t i l i t y and Strength o f Tungsten and Molybdenum TZM, AFMETR-66-89 ( A p r i l 1966).

9 t h e s t r e n g t h p r o p e r t i e s of t h e r e c r y s t a l l i z e d material.

An advantageous

t r a d e - o f f with t h e s e mechanical property changes i s , of course, t h a t pure molybdenum i s more r e s i s t a n t than T Z M t o t h e f l u o r i d e s a l t s of i n t e r e s t t o t h e MSRP.

Thus, s e v e r a l b e n e f i t s come from f a b r i c a t i n g t h e

system with TZM while o t h e r s accrue from t h e "conversion" of TZM t o molybdenum during t h e f l u o r i d e s a l t exposure. Corrosion Reactions and Kinetics In f l u o r i d e salt systems one of t h e major corrosion r e a c t i o n s i s t h e oxidation of one of t h e c o n s t i t u e n t s o f t h e container a l l o y by t h e reduction of a l e s s s t a b l e impurity metal f l u o r i d e i n i t i a l l y i n t h e s a l t , ? f o r example

The reduced metal s u b s t i t u t e s f o r t h e oxidized metal on t h e container material.

This type of r e a c t i o n apparently occurred i n our experiment

involving t h e strong reducing agents t i t a n i u m and zirconium:

+ 2FeF2

--3

ZrFL + 2Fe

(3)

The reported* free energy changes for t h e r e a c t i o n s shown by

Eqs.

(1) (2), and (3) a r e s t r o n g l y negative, and E q s .

(3) seem t o be i n d i c a t e d by t h e r e s u l t s r e p o r t e d above.

( 2 ) and p o s s i b l y

A s noted e a r l i e r ,

t h e i r o n metal t h a t formed i n t h e s e r e a c t i o n s deposited i n t h i n l a y e r s on t h e container and specimens.

7W. R. Grimes, G. M. Watson, J. H. DeVan, and R. B. Evans, "Radio-Tracer 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.

'A. Glassner, The Thermochemical Properties of t h e Oxides, Fluorides, and Chlorides t o 2500"K, ARE5750 (1957).

10 A l t e r n a t i v e l y t h e f u e l s a l t corrosion r e a c t i o n s i n which UF4 i s

reduced t o UF3 a l s o may have occurred t o remove t i t a n i u m and zirconium from t h e a l l o y :

However, no data w t h which t o determine t h e extent of L e s e r e a c t i o n s are available. Assuming t h a t t h e removal of t h e elements from t h e TZM was 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 , one can c a l c u l a t e f r o m t h e increase of t h e

t i t a n i u m and zirconium i n t h e s a l t t h e apparent d i f f u s i o n c o e f f i c i e n t s of t i t a n i u m and zirconium i n t h e TZM a l l o y .

Fromthese one can estimate t h e

amount of those m a t e r i a l s t h a t would be removed a t d i f f e r e n t times and temperatures.

I n regard t o t h e zirconium removal, we f e e l t h a t t h e salt

a n a l y s i s i s c o r r e c t and t h a t t h e instruments involved i n t h e fluorescence and microprobe analyses a r e not s u f f i c i e n t l y s e n s i t i v e t o measure t h e mvement of t h e zirconium. The t o t a l amount of m a t e r i a l , Mt,

t h a t d i f f u s e s from t h e a l l o y h e l d

under isothermal conditions w i t h a zero s u r f a c e concentration i s given by

where C O = t h e concentration of t h e d i f f u s i n g element, D = t h e d i f f u s i o n c o e f f i c i e n t , and

t h e time.

We c a l c u l a t e d D = 1 . 2 x 2.9 x

cm2/sec f o r t i t a n i u m i n TZM and

cm2/sec f o r zirconium i n TZM a t 1100째C. We d i d not c a l c u l a t e

f o r chromium removal, as i t s concentration f l u c t u a t e d from sample t o sample and we could not assume t h a t it w a s d i s t r i b u t e d homogeneously through t h e a l l o y .

9J. Crank, The Mathematics of D i f f i s i o n , Clarendon Press, Oxford, England, 1956, p. 11.

11 W

The expression

- X2/D ,

(7)

where X i s t h e d i s t a n c e of composition change, i s very u s e f u l i n calcul a t i n g approximately whether t h e composition has changed appreciably by d i f f u s i o n under a given s e t of circumstances.

For example, we can calcu-

l a t e t h e time required f o r appreciable removal

t h e i n i t i a l and u l t i m a t e concentrations

concentration between

o f t h e d i f f u s i n g element a t a

c e r t a i n d i s t a n c e from t h e surface. From t h e c a l c u l a t e d d i f f u s i o n c o e f f i c i e n t s and t h e experimental time of 1011 h r , we f i n d t h e depths of removal of t i t a n i u m and zirconium, r e s p e c t i v e l y , a r e 0.0008 and 0.0040 i n .

Since t h e microstructures shuw

r e c r y s t a l l i z a t i o n f o r a d i s t a n c e of about 0.004 i n . , we may assume t h a t t h e c a l c u l a t e d d i f f u s i o n c o e f f i c i e n t f o r zirconium may be more accurate than t h a t f o r t i t a n i u m

- that is,

be somewhat i n e r r o r .

Extrapolation of t h e c a l c u l a t e d values shows t h a t

t h e s a l t a n a l y s i s for t h e t i t a n i u m may

it would r e q u i r e 4000 hr t o r e c r y s t a l l i z e an a d d i t i o n a l 0.004 in. of material.

This i l l u s t r a t e s t h e decrease of t h e corrosion r a t e with time

and t h e general usefulness of TZM a l l o y f o r MSR reprocessing s e r v i c e .

CONCLUS IONS 1.

This t e s t shawed n e g l i g i b l e corrosion of t h e TZM a l l o y by t h e

fused fluoride s a l t (LiF-BeFz-ThF4-UF4, 68-20-11.7-0.3 mole ’$) at 1100°C

f o r over 1000 h r .

Corrosion manifested i t s e l f as leaching of t i t a n i u m and p o s s i b l y

zirconium from t h e a l l o y .

FeF2 i n i t i a l l y present i n t h e s a l t oxidized

t h e a l l o y i n g elements t o f l u o r i d e s dissolved i n t h e s a l t bath.

The i r o n

metal r e s u l t i n g from t h e r e a c t i o n deposited i n t h i n l a y e r s on t h e specimens and container. 2.9 x

We found t h a t DTi and DZr were 1 . 2 x

and

cm2/sec, r e s p e c t i v e l y , a t 1100°C i n t h e a l l o y . 3.

The TZM a l l o y exposed t o t h e s a l t p a r t i a l l y r e c r y s t a l l i z e d ,

while t h e TZM a l l o y simultaneously exposed t o t h e vapor d i d not.

This

r e c r y s t a l l i z a t i o n w a s a t t r i b u t e d t o t h e removal o f t i t a n i u m and zirconium.

12 4.

On t h e b a s i s of t h i s s i n g l e t e s t , t h e magnitude and mechanism

of corrosion i n d i c a t e no s e r i o u s problems for long-term use of TZM a l l o y

i n t h e IGBR vacuum d i s t i l l a t i o n processing scheme.

However, t h e s t r e n g t h

p r o p e r t i e s of t h e TZM a l l o y would approach those of unalloyed molybdenum

as s a l t exposure time increased. ACKNOWLFEMENTS

It i s our pleasure t o acknowledge t h e a s s i s t a n c e of F. D. Harvey, E. J. Lawrence, and J. B. P h i l l i p s with t h e s e experiments.

We a r e a l s o

indebted t o J. R. DiStefano, W. 0. H a r m s , and H. E. McCoy, Jr., f o r t h e i r constructive review of t h e manuscript. Thanks a l s o go t o H. R. Gaddis of t h e Metallography Group, H a r r i s Dunn and o t h e r members of t h e Analytical Chemistry Division, t h e Graphic Arts Department, and t h e Metals and Ceramics Division Reports Office f o r invaluable a s s i s t a n c e .

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