Q/

ORNL / zms- 5 7 8 2

Contract N o . W-7405-eng-26

METALS G E W I C S D-fVLSPQN

The Corrosion Resis of 316 Stainless Steel t a L i z

J. R. Keiser, J. H. D n, and D. L. Manning

Date Published - A p r i l 1 9 7 7

QPlCE This document contains information of a preliminary nature. It is subject to revision OF correction and therefore does not represent a final report.

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THE CORROSION RESISTANCE OF TYPE 396 STAINLESS STEEL TO Li2BeFb

J. R. K e i s nd D. LI. M a

orrela ted WI

A cor ros ion was observ e a s - r ece i

a l t f u r t h e r

blanket mat t-

eactors w i l

a t i o n of neut

h f ab r i ca t ion ,

roduction of

have r e s u l t nl.ess steel

ion f o r the t-wall. mate

ke t capable CI are limite

bear ing eompa kes material e Because

may impose 8 re

t, s t r o n g c o n s i d e r a t i

1.

2

the sal ts TdiF and BeF2 f o r a b l anke t of a f u s i o n reactor w i t h a s t a i n l e s s ,

s tee l f i r s t wall. The p u r p o s e of t h i s paper i s t o r e p o r t on t h e c o r r o s i o n

r e s i s t a n c e of type 316 s t a i n l e s s s tee l t o %i2BeFt+.

There are p o t e n t i a l problems a s s o c i a t e d wi th t h e use of Li.F-BeF2

as a combined coolant-breeding material. These inc lude t h e e f f e c t i v e

breeding and t h e e f f i c i e n t recovery of tr i t ium, t h e e f f e c t on t h e salt:

of induced e l e c t r i c f i e l d s and o f t ransmuta t ion products , and t h e compati-

b i l i t y of t h e s a l t w i th t h e component i t contac ts . The % r i t i . u m breeding

r a t i o of t h i s s a l t i s n o t as h igh as i s t h a t of l i t h i u m , and some des igns

would probably r e q u i r e t h e a d d i t i o n of a neutron mml..ti-plier such as

bery l l ium.

AnotI ie~ problem coul-d be the e l s c t ~ i ~ . f i e l d induced i n t h e c~n iduc t ing

s a l t when i t f lows through t h e i n t e n s e magnetic f i e l d s r equ i r ed f o r plasma

containment This p o t e n t i a l d i f f e renee occur r ing hetwcen the salt and

the p ipe wall could be g r e a t enough to e l e c t r o l y z e t h e salt and thus

cause seve re c o r r o s i o n problems f o r the metallic c o n t a i n e r e Means OP

reducing the e f f e c t of t h e induced f i e l d to a t o l e r a b l e level. have been

suggested by Horneyer. According t o him, e l e c t r o l y t i c c o r r o s i o n could

b e minimized by avoid ing , as much as p o s s i b l e , h igh f l u i d v e l o c i t i e s

perpendicular t o t h e magnetic f i e l d . Solution of t h i s problem w i l l

most probably have t o be a r e su l t of engineer ing des ign changes r a t h e r

than through a d d i t i o n s or modi f i ca t ions t o t h e s a l t ,

An a d d i t i o n a l concern w i l l be t h e c o r r o s i v e e f f e c t of t h e salt on

the. s t a i n l e s s s tee l . A s seen from t h e f ree ene rg ie s of formati-on in

T a b l e 1, L i F and BeF2 arc much more s table than t h e f l u o r i d e s of the

major c o n s t i t u e n t s of s ta inless s tee l , E e , Ni, and Cr. Consequently,

no s i g n i f i c a n t r e a c t i o n of t h e s a l t components w i t h the metall ic con ta ine r

i s expected; however, i m p u r i t i e s i n the s a l t may cause c o r r o s i o n , Impuri-

t i e s expected t o be p x c s c n t are HP and N i F 2 , and t hese w i l l cause

r e a c t i o n s of the? type :

2W. R. G r i m e s and S tan ley Carter, “MoPten Salt Blanket F l u i d s i n Cont ro l led Ftasion Reac tors pp 161--9(4 i n The Chemistry of Fusiovi TechnoZogy D. M. Gruen, e d . Ylenan Press, New York, 1972.

3W. G. Horneyer flTherrnal and Chemical A s p e c t s of t h e Thernnoniiclear Blanket Problem,” Technical Report 4 3 5 , M. 1, T. Research Laboratory of E l e c t r o n i c s (1965).

3

T a b l e 1. Foma of F l u o r i d e

L i F

-448 -107

---e, 10 ---9 8 @rFz

FeF2

HF

NiF2

MOF6

-7 5 --67 -66

-5 5 -5 0

a C. F , B dynamics of M 15, 6 2 4 , P , C 690801 (196’3) e

MF2 + Cr ould be Ni o

ions of impuri n t a i n e r w i l l I l y b u t w i l l s1 i m p u r i t i e s are

d t o the system, a o additional.

hours t h e s e are not exp

of a r edue t

e t he corrosive p u r i t i e s by

s . The t r a n

neu t ron react r ep resen t axlo

e elemental C 8

are expected

e of oxidants in t

T h e reactions o f l i t h i u m ,

3 7LiF 4- n -f H e -E. ,WF f nc

are, of course, d e s i r e d became t h i s is thc means of producing the

tritium f u e l . Hocdcver, the TF p ~ o d ~ e d can react w i t h t h e s t a i n l e s s

steel. con ta ine r t o form metals f l u o r i d e s as shown i n ~ q . (1).

Transmutation of be ry l l i um can occur i n either of ~ W Q ways:

( 3 )

6 The ,Ke undergoes beta decay w i t 1 1 an O.$-sec h a l f - l i f e yir?l.di.ng f a r t h e

r i g h t s ide of Kquation 6,

‘The f l u o r i n e produced i n these beryll . ium t ransmuta t ion reac t i a n s

can react w i t h t h e s ta in less s tee l conta iner , c o n s t i t u t i n g another

source of co r ros ion .

The o t h e r t ransmuta t ion of t h e s a l t cornponents i s that o€ f l u o r i n e :

16 -~ 1 6 The , N undergoes beta decay wi th about a 7-sec ha8f-l . ife y i e l d i n g 0,

which can be expected to react wi th the containment metal causing addi -

t ional. co r ros ion . The net r e s u l t of all.. three tuansrnritation r e a c t i o n s

i s the product.i.on o f subs tances that w i l l cause ear-rosion of the stainless

5

steel containment a l l o y this corrosion

nt and poss ib One means of

p s o f clliminat-ing t

. This buffer o be capable of (

T2 and a so lub le f c t i n g wi th the f

01pre.s i n s a l t and can I t of the s a l t .

One o the r problem a the use of LizBe 8 the recovery

tritium from the s a l t , er r e q u i r e d t o

in the form of Tz bec ity of TF with s ta inless steel

b l y make i t e: the tritium f r

and tritium

r i f a CizBeFt, b n-

me sel, A great de with molten s a

redox b u f f e r has been a as a r e s u l t of work f a r the

en S a l t Breeder R e The f u e l salt 6 reactor

sed of L i F and E3 f e r t i l e mat

Cause of r ex .

h p repa ra t ion e containment

i s a c t u a l l y fou i n both t h e

her t hese compo

of t h e f u e l t sf material

t addition of

a w e serious containment

f an o x i d i z i s i n some U ( I I

nd, converse1

of U ( I V ) to u 11) r a t i o migh se of t h i s b

t i c swings in th e n t i a 1 w i l l . take

6

t h a t could make t h e s a l t s u f f i c i e n t l y "reduciiig'l t o p r a c t i c a l l y

e l i m i n a t e t h e c o r r o s i o n caused by i m p u r i t i e s and t ransmuta t ion p r o d u c t s

i s bery l l ium, Typical. r e a c t i o n s could be:

To determine t h e s u i t a b i l i t y o f s t a i n l e s s s t e e l f o r con ta in ing Li2BeF4,

cor ros ion measurements have been made wi th sa1.t i n t h e as-received

candi t ion and i n t h e reducing c o n d i t i o n reached by adding bery l l ium t o

i t

EXPERIMENTAL METHODS

The experimental assembly used f o r t h i s c o ~ r o s i o n work, cal.l.ed

a thermal-convection loop , i s shown in Fig . 1. The l o o p p o r t i o n of

t h e assembly is cons t ruc t ed of t y p c 316 s ta in less steel tubing and

is hea ted on t h e bottom and one ve r t i c l e s i d e . Cooling t h e o the r t w o

s i d e s causes the molten s a l t i n t h e tubing to f low because o f t h e

v a r i a t i o n i n d e n s i t y of t h e salt wi th temperature, The v e l . o c i t y of

the sa l t i s about 1 m l m i n , Other important features of t he I S O ~ are

t h e removable c o r r o s i o n specimens and t h e accesses f o r i n s e r t i o n o f

e l e c t r o d e s f o r voltammetry measurements and f o r a d d i t i o n of material

to t h e s a l t i n t h e I-oop o r removal of salt samples from t h e 100p.

S ix t een c o r r o s i o n specimens are I I S F Z . ~ , wi th e i g h t i n se r t ed i n the hot

l e g (heated v e r t i c a l s e c t i o n ) and e i g h t i n t h e c o l d keg (cooled

v e r t i c l e s e c t i o n ) . These specimens are removed from. the loop every

500 h r f o r weighing and examination.. When t h e irivestigationas t o be

made w i t h a p a r t i c u l a r set s f spcci.rnens are completeds s e v e r a l of the

speci.mens a re examined me ta l log raph ica l ly and, i f b7arwanted w i t h t h e

electron microprobe. These resu1 . t~ reveal at which tempera ture weight

7

ORWL-DWG 68-598TR3

SAMPLER

FLUSH 1 TANK 1 TANK

I

ion t

8

ga ins o r l o s s e s have occi.mred, and t h e microprobe examination i d e n t i f i e s

which components of t he specimen material. have been t r a n s p o r t e d ,

One of the most s i g n i f i c a n t improvements i n thermal-convec t i o n

l o o p operation has been the a p p l i c a t i o n of voltanmetry e Control led-

p o t e n t i a l v o l t a e t r y is a modern e lec t rochemica l technique, w1-nicl-n

may be used t o examine t h e n a t u r e of an electrochcnaical react ion i n

d i f f e r e n t media, such as aqueous and nonaqueous s o l u t i o n s and high-

temperature salt systems * For measurements i n mol.ten L ~ ~ B E F I . ~ , the

electrodes cons i s t ed of the 1 . 0 0 ~ i t s e l f , which s e r v e d as the counter

e l e c t r o d e , and two i r i d i u m e l e c t r o d e s , which were the quasi-refermce

and working e l e c t r o d e s . These e l e c t r o d e s were t y p i c a l l y 25-nm (1-in.)

l eng ths of 1.8-gage i r i d i u m wires welded t o 3-mm-OD (118-in.) ni-ckel

risers.

voltammograms were ~ e c ~ r d e d versiis the i r i d i u m qziasli-reference e l e c t r o d e

(IK QRE), which i s poised a t t h e equi. l . ibrium p o t e n t i a l (E ) of the

m e l t . I n this mamner rel.atiwe chan.ges i n t h e ox ida t ion po ten t i a l . o f

t h e sa l t anid of t h e c o n c e n t r a t i o n of c e r t a i n i m p u r i t i e s and COKI-OS~CIII

p roducts can be measured on-l ine. The e q u i l i b r i u m p o t e n t i a l o f t h e

s a l t is a measure of the tendency of t h e m e l t t o r e a c t w i t h ox id i zab le

materials t h a t i t contac ts . For m e l t s that are n o t po i sed by a redox

bu f fe r relati-ve changes i n t h e equiSi.briurr1 p s t e , n t i a l a re convenient ly

observed by determining vol. tametrical-I*y the potential d i f f e . r ence (AE)

between t h e m e l t e q u i l i b r i u m potential . . and the ca thodic l i m i t of the

melt, which f o r E ~ ~ B ~ F L , i s the r e d w t i . o ~ n of beryPl ium(I1) . We d e f i n e

t h e ca thodic l i m i t f o r these measurements as t h e potential a t 80 n ~ 4

c e l l cur ren t .

same c o n d i t i o n s are maintained, A d e c r e a s e i n A&’, r e s u l t i n g i n a

c a t h o d i c s h t f t i n Eeq f o r example, indicates that t h e melt has beconlle

l ess o x i d i z i n g ; and an i n c r e a s e i n AI7 i n d i c a t e s t h a ~ t h e o p p o s i t e has occurred A voltammetric scan t h a t shows t h e ca thodic l i m i t r e l . a t i v e

t o Eeq i s i1l.usltrated i.n F i g . 2,

TIE e1.ectrode area exposed t o the m e l t was 10 t o 20 mm2- T R ~

eq

Other r e fe rence markers may a l s o be used as long as the

9

80

60

40

20

L i F i g , 2 . n F ina l L i m i t

2BeF4 i n I,

r e d u c t i o n of

mmogram shown i n h e i g h t s of t h e t w o waves

t o about 1 3

r e s u l t s of a vol tammetr i

due t o i r o n has eral weeks a f t e r that

le the chro

the same h e i g h t . It is a t o note t h a t ca thodic s h i f

k potential of

of t h e analyse

tential voJta

1.0

F i g . 4 * Reduction. of Chromium in Loop NCL 31,

11

16 stainless were f i r s t exp

t a i n l e s s stee "on loop con t

190 ppm Fe, 60

e of 650°C and

500 and 1000

ans of v o l t a

Following thi , a be ry l l i um

It and a n e t a i n l e s s steel

. These s p e c i

ng af the cond

n a t i o n of t h e he f i r s t 500

t s i g n i f i c a n t d occurred , m

because of r e a c t i o n s with . Figure 5 shows t

st and lowest temp

1 2

weight l o s t by t h e h o t t e s t specimen dur ing the f irst 508 h r , i f w e

assume t h a t t h e material w a s l o s t uniformly f rom the su r€ace ,

corresponds t o a co r ros ion r a t e of about 10 ym ( 8 . 4 mil ) pe r year.

ow ever, i n prev ious W O ~ P w i t h s t a i n l e s s s tee l i n f luoricie sa l ts

con ta in ing UF4 as an ox id iz ing s p e c i e s , c o r r o s i o n d i d not r e s u l t i n

uniform removal of material; r a t h e r , a porous s t r u c t u r e w a s found

(Fig. 6 ) . The 1000-hr exposure of OUT specfinens w a s no t long e n ~ i ~ g l i

f o r ex tens ive c o r r o s i o n t o occur, bu t F ig . 'I i n d i c a t e s that material

w a s n o t removed uniformly from t h e h o t t e r specimen. Consequently, t h e

dep th of a t t a c k can be expected t o exceed that c a l c u l a t e d f o r uniform

material removal. Futhermore, i n t he sa l t of a f u s i o n r e a c t o r , impuri-

t ies would be p ~ o d u c e d cont inuous ly and i n s i g n i f i c a n t q u a n t i t i e s , and

these i m p u r i t i c s would be mure ox id i z ing than t h e i m p u r i t i e s found

i n t h i s experimental s a l t . Thus, c o r r o s i o n of s t a i n l e s s s teel i n a

L-usion reactor coiz'hd be a s e r i o u s prohl-cm i f no e f f o r t is made t o

b u f f e r t h e s a l t .

Y-'3 277

13

Y- 1 34722

. 7 . Type 316 ecimens Expos

TO e v a l u a t e t h e e f f e c t r on t h e coxros

i2BeF4, a p i e c e o s i n s e r t e d i n t

wed t o react. A s s e equ i l ib r ium

c a l l y , i n d i c a t i n g made less o x i d i

ts i n d i c a t e d t t i o n t o be les

A new set of ed t o t h i s re

Examination ter 500 and

weight chan of d e t e c t i o n .

e weight changes f a en temperatu

sma l l e r f o r t h e s he reducing s

h r than i n t

F igure 5 shows t h a t o ium w a s removed from

t h e f i r s t 500- e t o t h e r ed

n t h e h o t t e s t i n c r e a s i n g w e

f time. T h i c u r r e d becaus

urn w a s remov t h e s a l t w e r

urn and t h e sa i v e l y more ox

n a l o w c o r r ium would have

cont inuous ly o r a t least If t h a t were done

e of t ype 316 s t a i n l . eF4 s a l t could be

a range a c c e p t a b l e f o r f

14

ORNL-DWG 77-3822 -4.1 5

- 1.10

0

> d

t- z w )-.

v

-4.05 5

-1.00 B t 5 -.J

-0.95

-0.90

-----e- < t

Be ADDITION

0 20 4Q 60 400 TIME (days)

F i g . 8. V a r i a t i o n of the L i m i t f o r TAZBeFb S a l t i n NCL 3 1 .

CONCTAJS IONS

The conc lus ions t h a t can be drawn from t h i s r e s e a r c h are:

1. The o x i d a t i o n p o t e n t i a l o f t he sa l t can be c o n t r o l l e d by

a d d i t i o n s o f a reducing metal,

2 . V o l t a m e t r i c techniques p rov ide on- l ine measurement o f the

o x i d a t i o n p o t e n t i a l of t h e s a l t and t h e c o n c e n t r a t i ~ ~ ~ of i m p u r i t i e s

i n t h e s a l t .

3 . High c o r r o s i o n rates are encountered ini t ia l .1-y i n s a l t w i t h

a normal impur i ty c o n c e n t r a t i o n .

4 . The c o r r o s i o n r a t e of type 316 s t a i n l e s s s tee l i n Li2ReF1,

s a l t w a s reduced s i g n i f i c a n t l y by t h e d i r e c t a d d i t i o n of be ry l l i um

metal t o t h e s a l t ,

9 5

T h e au tho r s would e f f o r t s of E . S. Lawrence Soar o p e r a t i n g t h e thermal-c e DJStefano and J. L. Scott

* ing, S. Peter 1 G01l . ihe~ f o r f i n a l n o f t h e manu

17

1-2 . 3 . 4 . 5. 6.

8. 7.

11 . 12. 1 3 .

16. 17 *

20 21. 22. 23.

23.

I B U T I O N

C e n t r a l Research L i Document Referen Laboratory Rec

o r a t o r y Rec L Patent O f

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J. F. Clarke R, E . Clausing S I D. Clinton

E. Cunningha a A* Dandl

J. H. DeVan P. Fraas sion Energy D sion Energy D O f f ice . A. Gabriel

M. L , Grossbeck P. M. Haubenre M, R. Hill J. A. Horalc

6 4 , W.C.T. 65. J , S, W 66. J. R, W 67. W. M, Wells

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