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METALS AND CERAMICS DIVISION

DE83

THERMAL-CONVECTION-LOP STUDY OF THE CORROSION OF Fe-Ni-Cr ALLOYS BY MOLTEN NaN03-KN03 P. F. Tortorelli J. R. DeVan

Date

NOTICE:

Published

- December

1982

This document contains information preliminary nature. It is subject revision or correction and therefore not represent a final report.

Prepared by / :,”OAK RIDGE NATIONAL

the LABORATORY ' Oak Rtdge, Tennessee 37830 operated by '\ UNIOY CARBIDE CORPORATION for the Office of Industrial Programs

U.S. DEPARTMENT OF ENERGY Under Contract No. W-7405-eng-26

does

00422%-

CONTENTS

............................. INTRODUCTION . .. . . . . EXPERIMENTAL PROCEDURES . . . . . . . . . . . . . . . . . . . . . . RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ABSTRACT

1 1 2

19 25 26 26 29

THERMAL C O W E C T I O K LOOP STUDY OF THE CORROSION --. OF Fe-Ni-Cr

ALLOYS BY MOLTEN NaN03-KN03* P. F. T o r t o r e l l i and J. H.

*Van

ABSTRACT

We s t u d i e d the c o r r o s i o n of Fe-Ni-Cr a l l o y s by draw s a l t (60 w t % KaN03A0 w t % n o , ) v i t h thermal convection loops of a l l o y 600 and types 304L and 316 s t a i n l e s s s t e e l . %e main c o r r o s i o n processes a t 600°C and below were the growth of t h i n o x i d e s c a l e s and the d i s s o l u t i o n of chromium by t h e s a l t . S p a l l a t i o n of oxide l a y e r s occurred on type 304 s t a i n l e s s s t e e l specimens a t i n t e r m e d i a t e temperatures. R e s u l t s indicated relat i v e l y low c o r r o s i o n r a t e s (<I3 p / y e a r i n most c a s e s ) f o r temp e r a t u r e s of 600°C and less. Corrosion of type 316 s t a i n l e s s s t e e l was g r e a t l y a c c e l e r a t e d when the maximum loop temperature was r a i s e d t o 620°C. It t h e r e f o r e appears t h a t 600°C may be t h e l i m i t i n g temperature f o r use of t h e above a l l o y s in draw s a l t .

INTRODUCTION A mixture of 60 w t % NaN03-40 w t X KNO3 (draw s a l t ) h a s been proposed1 €or use i n s o l a r thermal power systems a s both a s o l a r r e c e i v e r working f l u i d and a t h r r a a l energy s t o r a g e medium.

Draw s a l t is a t t r a c -

t i v e f o r such a p p l i c a t i o n s because of i t s high s e n s i b l e h e a t c a p a c i t y , i t s low r e a c t i v i t y i n t h e event of a leak t o a i r or steam, a n d t h e low o p e r a t i n g p r e s s u r e s required for i t s u s e -

However, t h e f e a s i b i l i t y of

such a system depends p a r t l y on t h e c o m p a t i b i l i t y of t h e s a l t w i t h candidate structural alloys.

The s u b j e c t experiments were t h e r e f o r e per-

formed in closed-loop systems t o study t h e c o r r o s i o n of a l l o y 800 and t y p e s 304 and 316 s t a i n l e s s s t e e l by slowly flowing NaN03-KN03 under an impressed temperature g r a d i e n t . [49 NaN03-44 Kh’03-7 %NO2

Previous work2 w i t h a n i t r a t e - n i t r i t e

( w t %)] s a l t revealed unacceptably h i g h c o r r o s i o n

r a t e s of type 316 s t a i n l e s s s t e e l a t 550°C.

The study p r e s e n t s comparable

d a t a f o r a pure n i t r a t e s a l t system. *Research perlormed under s u b c o n t r a c t with Sandia National L a b o r a t o r y , Lirermore, C a l i f o r n i a , p e r purchase agleement 92-8568, spons o r e d by t h e U.S. Lkpartaent of Energy, Division of Energy S t o r a g e Systems.

The t e s t s r e p o r t e d here are companion experiments t o t h o s e conducted The a t Sandia N a t i o n a l L a b o r a t ~ r y . ~ ~ Sandia experiments w e r e matched t o o u r experiments i n terms of l o o p m a t e r i a l s , specireens, and temperature conditions.

Tbwever, the Sandia experiments operated under a i r , w h i l e

o u r s were s e a l e d , t h e cover gas being composed of s a l t decomposition products

EXPERIMEhTAL PROCEDURES

The as-received draw s a l t , with a nominal c o r p o s i t i o n of 60 w t % NaN03-40 w t X I;NO3* and a m e l t i n g p o i n t of 221"C, was outgassed i n a c l o s e d type 304 s t a i n l e s s s t e e l v e s s e l at 250 t o 300째C w h i l e simult a n e o u s l y being purged by argon flowing above and through t h e s a l t . s i g n i f i c a n t a m u n t of water vapor was removed i n t h i s way.

Subsequently,

t h i s pot of s a l t wa3 used t o f i l l t h r e e thermal convection loops (TCLs) of t h e type shown s c h e m a t i c a l l y i n Fig. 1.

N a t u r a l c i r c u l a t i o n of t h e s a l t

was induced i n t h e s e closed l o o p s by c o n t r o l l i n g t h e l o o p ' s temperature p r o f i l e t o produce c o n v e c t i v e tlou. s t r u c t e d of d i f f e r e n t a l l o y s : a l l o y 800 (Table 1).

Each of t h e t h r e e loops was con-

types 304L and 316 s t a i n l e s s s t e e l and

Tne loops were desigr,ed t o permit t h e i n s t r t i o n and

removal of c o r r o s i o n coupons and s a l t samples with minimal d i s r u p t i o n of t h e salt flow.

In t h i s

vay,

changes in s a l t composition, specimen

weight, and m i c r o s t r u c t u r e were monitored as f u n c t i o n s of exposure t i m e . Tubing f o r the loops ranged from 1.9 t o 2.5 c m i n o u t s i d e diameter and from 0.2 t o 0.3 cm i n w a l l t h i c k n e s s . 1.9

I n s e r t coupons were approximately

0.8 X 0.1 cm and uere i n t h e as-annealed

c o n d i t i o n (unpolished).

a n y given loop, t h e i n s e r t specimens and t u b i n g were of t h e same nominal c o m p o s i t i o n s , although, i n t h e c a s e of t h e t y p e 304L s t a i n l e s s s t e e l loop, specimens of bot':

normal- and low-carbon g r a d e s were exposed.

TCL, t h e p r e s s u r e above t h e s a l t r a p i d l y i n c r e a s e d t o above 6.9

In each X

lo4

gage (10 p s i g ) when t h e temperature was i n i t i a l l y r a i s e d above about *Partherin 430, Park Chemical Co.

Pa,

Fig. 1. Thermal convection l o o p used i n NaN03-KN03 s t u d i e p . %e "H" and "C" numbers denote specimen p o s i t i o n s i n t h e hot and cold l e g s , r e s p e c t i v e l y . Coupons Hl and C1 a r e l o c a t e d i n t h e cover gas and are not w e t by the s a l t .

520째C.

Analysis of t h e evolved gas showed t h a t i t was p r i n c i p a l l y oxygen.

Subsequently, care was t a k e n t o graintain t h i s gas composition o v e r the loop s a l t by u s i n g o n l y oxygen as a cover gas durin.? specimen manipulation and s a l t sampling.

Lkring most of our s t u d y t h e t h r e e TCLs o p e r a t e d with

maximum temperatures and temperature d i f f e r e n t i a l s A!Fs of about 603 and

Table 1.

Nominal cornpositions of t e s t e d a l l o y s Concentration (ut X )

A1loy

Alloy 800

Type 304 stainless steelb

5 p e 316 stainless steel

0.2

0.5

0.08

0.10

nMaximum c f l n c e n t r a t i o n s . 1)Type 304L s t a i n l e s s s t e e l i s of s i m i l a r composition except f o r a maximum carbon c o n c e n t r a t i o n of 0.03.

225"C, r e s p e c t i v e l y .

R e AT was e s t a b l i s h e d by f i r s t s e t t i n g t h e minimum

temperature a t t h e bottom o f t h e cold l e g and then i n c r e a s i n g t h e o u t p u t of t h e h e a t e r s along the h o t l e g t o achieve a maximum temperature of 600째C j u s t below the bottom of t h e hot-leg surge t a n k (Fig.

1). T y p i c a l

temperature p r o f i l e s f o r each of t h e t h r e e TCLs a r e shown in Fig. 2. nominal s a l t v e l o c i t y was 4 m / s .

The

After 5300 h of coupon exposure, t h e

ATof t h e type 316 s t a i n l e s s s t e e l TCL was changed t o 150"C, and i t s maximum temperature w a s maintained a t 600째C t o assess any changes i n behavior due t o mass t r a n s f e r e f f e c t s (see below).

This t e m p e r a t u r e pro-

f i l e i s also shown i n Fig. 2., B e f o r e weighing, t h e l o o p specimens were cleaned w i t h water a f t e r each exposure t o flowing s a l t .

After 1500 and 4500 h of exposure, c o r n e r s were

c l i p p e d from t h e specimens i n each loop f o r m e t a l l o g r a p h i c examination. P e r i o d i c a l l y , samples of s a l t from t h e r e s p e c t i v e l o o p s were t a k e n and analyzed f o r Fe, N i , Cr, Mo, N02, and NO3.

T o t a l chromium and chromium(V1)

c o n c e n t r a t i o n s i n t h e s a l t were measured s p e c t r o p h o t o m e t r i c a l l y , 6

and

n i c k e l and molybdenum l e v e l s were determined by t h e atomic a b s o r p t i o n s p e c t r o s c o p i c g r a p h i t e f u r n a c e technique.

The s a l t samples were analyzed

f o r iron s p e c t r o p h o t o m e t r i c a l l y with t h e o-phenanthrolfne method.8

The

n i t r a t e c o n c e n t r a t i o n s were determined by a modified DeVarda r n e t h ~ d ;t~h e

n i t r i t e l e v e l s were measured voliimetricallp by t i t r a t i o n with cerium(1V) (ref.

10).

550

9 Y

V oO 3

a 450

-.---

I-

TYPE 316 LOOP. AT= 150% LOC ALLOY 800

-- - TYPE 304

PI,

350 300 0

40 15 20 25 30 35 DISTANCE FROM BOTTOM OF Gou) LEG IN DIRECTION OF FLOW (cm)

Fig. 2. Typical temperature p r o f i l e s f o r t h e t h r e e draw s a l t thermal convection l o o p s ; AT, temperature d i f f e r e n t i a l .

RESULTS

As p r e v i o u s l y d i s c u s s e d , t h e weights

measured as a f u n c t i o n of exposure time.

of t h e loop specimens were Such d a t a are s h w n i n Fig. 3

f o r t h e specimens a t t h e two h o t t e s t coupon p o s i t i o n s (H3 and H4) i n t h e a l l o y 800 TCL.

Very l i t t l e weight change occurred.

Similar b e h a v i o r was

found f o r specimens i n t h e s e p o s i t i o n s i n t h e type 304L s t a i n l e s s s t e e l loop, which contained both types 304 and 304L s t a i n l e s s s t e e l a t t h e 600째C

(H3) p o s i t i o n (Fig. 4).

Equivalent u e i g h t change d a t a f o r t h e t y p e 316

s t a i n l e s s s t e e l loop a r e shown i n F i g . 5.

As w i t h t h e other two loops,

t h e weight changes of t h e H3 (595째C) specimen were small.

Bowever, t h e

weight of t h e H4 (565째C) specimen s t e a d i l y decreased during t h e i n i t i a l

3000 h of exposure t o t h e s a l t , a f t e r which it became approximately

4r' OQNL-DWG 81 -20927 0

-2t 0

~ m m 3 o o o 4 o O 0 5 o 0 o 6 o o o 7 o O 0 ~ EXPOSURE TIME ( h )

Fig. 3. Weight change versus exposure time for alloy 800 exposed t o flowing NaNO3-KNO;.

TYPE 304L. 600'C

--4 2

0 TYPE

304. 6o(rc

A TYPE 304L. 570'C

A -6

(OOO

2000

I 3000

4OOO

5000

6000

7WO

8032

EXPOSURE TIME (h)

P i g . 4. Weight change versus exposure time for types 304 and 304L s t a i n l e s s steel exposed to flowing NaN03-KN03.

1 ATCHANGED To f5OOc

0 0

E \

0 0 0

-2

CHANGED TO 6x)Qc,AT=I W T

Ti,$

565-59OOC a

-4

z 4

-6 a

3 -8

a a

-4 0

-12

Fig. 5 . Weight change versus exposure time f o r type 316 s t a i n l e s s s t e e l exposed t o flowing &aN03-KN03.

constant.

Changing t h e A? of the type 316 s t a i n l e s s s t e e l l o o p from 225

t o 15OoC a f t e r 5000 h of coupon exposure had l i t t l e e f f e c t on t h e weight changes, but r a i s i n g t h e maximum loop temperature t o 620째C a f t e r 6000 h r e s u l t e d i n t h e onset of s i g n i f i c a n t weight-loss

rates.

As noted, t h e weight change d a t a i n Figs. 3 through 5 a r e f o r only two of the coupon p o s i t i o n s i n each of t h e TCLs.

The data f o r a l l loop

coupons are l i s t e d i n t h e Appendix i n Tables A.1 through A.3 ( t h e l o c a t i o n of each specimen i s shown i n Fig. 1).

With t h e exception of t h e type 316

s t a i n l e s s s t e e l H4 coupon, t h e weight changes of a l l the specimens i n the a l l o y 809 and type 316 s t a i n l e s s s t e e l loops were small.

However,

s i g n i f i c a n t weight g a i n s were measured on c e r t a i n coupons i n t h e type 304L

s t a i n l e s s s t e e l TCL.

The weight g a i n s were p a r t i c u l a r l y h i g h f o r s e v e r a l

coupons i n t h e lower h a l f of t h e h o t leg.

These weight changes are

p l o t t e d as a f u n c t i o n of exposure t f r e i n Fig. 6.

?he d e c r e a s e s i n weight

e t longer times appeared t o be r e l a t e d t o s p a l l a t i o n . Metallographic s p e c i m m s were o b t a i n e d by c l i p p i n g c o r n e r s from t h e l o o p coupons a f t e r 1500 and 4500 h of exposure t o Nat;03-KN03.

Polished

cross s e c t i o n s of specimens exposed t o t h e draw s a l t a t t h e H 3 and H4 p o s i t i o n s i n each of t h e loops a r e shown i n Figs. 7 through 9.

Each a l l o y

e x h i b i t e d m u l t i l a y e r e d c o r r o s i o n p r o d u c t s a t t h e higher temperatures.

c e r t a i n c a s e s , but p a r t i c u l a r l y i n t h e case of t y p e s 304 and 304L s t a i n l e s s s t e e l , e t c h i n g of t h e s e metallographic specimens increased t h e number of a p p a r e n t l a y e r & t h a t could be d i f f e r e n t i a t e d on a given specimen. Examples of t h i s c a n be seen by comparing the mlcrographs in Pig. 10 with t h e a s s o c i a t e d ones i n Figs. 8 and 9 .

Micrographs of specimens exposed a t

o t h e r loop p o s i t i o n s are shown i n F i g s . 11 through 1 4 .

Those i n Fig. 14

a r e of t h e s u r f a c e s of the coupons t h a t c x h i b i t e d the l a r g e r weight g a i n s

Pig. 6. Weight change versus exposure t i m e â&#x201A;Źor type 304L s t a i n l e s s s t e e l exposed t o flowing b N 0 3 - K N 0 3 .

2689LL-A 6

m l - A

11 Y-174797

Y- 174794

Y-178838

Y-178836

(a) ,

(e) I

4opm

(f)

Fig. 9. Polished cross sections of types 304 and 304L s t a i n l e s s (a) Type 304 , f o r 1500 h at 600'C. s t e e l exposed t o flowing NaN03-Kt?03. (e) (b) Type 304t for 1SOO h a t 600'C. . . Type - 304 f o-r 4500 h* at .60OoC,. e . -

-e*-

e-1-

Y-177223

Y-178952

. .

Y-179010

Fig. 10. Cross sections of stainless steel exposed to flowing NaNO3-KNO3 for 4500 h at approximately 600%; etched with aqua regia. (a) T y p e 316 s t a i n l e s s steel. (b) Type 304 stainless s t e e l . (e) Type 304L s t a i n l e s s r t e e l .

Y-177244

Y-1

4Opm

Fig. 12. Polished cross sections of type 316 stainless steel exposed t o flowing #aNOg-KNOj f o r 4500 h at (a) 505OC, (b) 41OoC, and (c) 375OC. t

Y-178812

Y-178814

Y-178816

, 40ym , Fig. 14.

Polished cross sections of type 304L s t a i n l e s s s t e e l

exposed to flowing NaN03-Kl?o3 for 4500 h a t (a) 54SoC, (b) 520째C, and

(e) 49O0C-

17 r e p o r t e d i n F i g . 6.

Note t h a t t h e l a r g e weight g a i n s a r e a l s o r e f l e c t e d

i n r e l a t i v e l y t h i c k r e a c t i o n l a y e r s and t h a t s p a l l a t i o n of t h e l a y e r s

seems imminent.

Tables A.4 through A.6

in t h e Appendix l i s t t h e t o t a l

d e p t h of t h e corrosion products x f o r e a c h coupon a s w a s u r e d from t h e r e s p e c t i v e micrographs. High-resolution

e l e c t r o n microprobe a n a l y s i s allowed d e t e r m i n a t i o n of

t h e chemical composition of the r e a c t i o n zone of t y p e 316 s t a i n l e s s s t e e l a f t e r 4500 h of exposure a t 595°C.

"tie r e s u l t s i n d i c a t e d t h r e e l a y e r s of

d i f f e r e n t compositions, shown s c h e m a t i c a l l y i n Fig. 15.

The e n t i r e reac-

t i o n zone c o n t a i n e d oxygen, and t h e i n d i v i d u a l l a y e r s comprising t h i s zone d i f f e r e d i n t h e i r r e l a t i v e r a t i o of i r o n t o chromfum.

The outermost l a y e r

was d e p l e t e d in chromium, b u t t h e u n d e r l y i n g one w a s not.

Finally, a thin

n i c k e l - r i c h l a y e r vas observed a t t h e c o r r o s i o n product z o n e a s e metal interface. The s a l t in each of t h e TCLs was p e r i o d i c a l l y analyzed t o determine

t h e e x t e n t and n a t u r e o f c o r r o s i o n product s p e c i e s ; r e s u l t s are shovn i n T a b l e 2.

(The sampling times i n t h e t a h l e r e p r e s e n t the number of hours

t h e s a l t c i r c u l a t e d in a p a r t i c u l a r loop b e f o r e a n a l y s i s and, as such, do n o t c o i n c i d e w i t h t h e coupon exposure times used In r e p o r t i n g t h e weight change and m i c r o s t r u c t u r a l r e s u l t s because of t h e p e r i o d s in which t h e coupons were o u t of t h e r e s p e c t i v e loops.)

The d a t a in t h e t a b l e c l e a r l y

show a g e n e r a l i n c r e a s e of chromium in the s a l t w i t h i n c r e a s i n g o p e r a t i n g time €n a l l t h r e e loops.

The c o n c e n t r a t i o n s of t h e o t h e r p r i n c i p a l e l e -

ments of t h e s t r u c t u r a l a l l o y s did not d i f f e r a p p r e c i a b l y from t h e i r l e v e l s i n t h e as-rece€ved s a l t .

W e a r e u n s u r e of t h e reason f o r t h e occa-

s i o n a l , u n u s u a l l y high molybdenum c o n c e n t r a t i o n i n t h e s a l t .

Most prob-

ably t h e measurements are erroneous because we observed a d e c r e a s e t o a "normal" c o n c e n t r a t i o n in t h e a l l o y 800 l o o p a t a l a t e r sampling time.

a l l t h e loops, t h e s a l t ' s n i t r € t e c c n c e n t r a t i o n i n c r e a s e d r a p i d l y t o about 1.9 w t X b u t t h e n i n c r e a s e d more slowly. As d e s c r i b e d in t h e experimental procedures s e c t i o n , t h e p r e s s u r e

above t h e l o o p s a l t r a p i d l y i n c r e a s e d d e n t h e temperature was r a i s e d above about 52OOC.

Analyzed samples of e v o l v e d g a s were mainly oxygen.

These a n a l y s e s are g i v e n in Table 3.

ORNL-DWG 84-20923

REACTION ZONE

OXYGEN TRACE

MATRIX

F i g . 15. R e s u l t s fron: electron microprobe analysis of type 315 s t a i n l e s s s t e e l exposed t o NaNO+CNO3 for 4500 h at 595째C.

S a l t composition 8s a function of loop operating time

Table 2.

Concentration i n salts

LOOP LOOP

mater 1I1

operatin(l tlmt

(h) 0 Alloy 800

(vt

(vt X)

PPI

crb

(10

2,403

130

(10

5,619

137

(10

7,755

(10

10,104c

156 237 (218)

1,894 2,715 4,249 8,809 9,672 12,122

163 120 200 222 246 322 (310)

1,126 4,034 5.355 7,419 12,578C 14,638c 15,313C

63 118 128 120 163 270 283 (282)

21 1.6 c1

1*)*

H03

0.05

65.5

1.7

68.1

3.2

63.6

3.3

64.1

1.7 2.0 2.0 3.9

67.6 66.6 67.2 63.2 63.4

(10

2.3

a0 a0

1.6

(10

<2 <2 3 4

4.0

Q (10

(10 (10 (10

<I 1.1

<2 <2 29

1.9 1.8 1.9 1.9 4.3

64.3 66.5 68.5 65.3 62.7

4 k a e u r t r r n t uncertaintier are t 5 X . k m b e r . in parenthere* repreaent coneeatration. of &(VI)

% a x i u = loop telperature , ,? vaa about 621'C; measurement timer were fc- a S; of about 600.~.

a11 other

i n ralt.

19 T a b l e 3. Analyses of gas i n i t i a l l y evolved from KaiaKO3-KS33 i n closed thermal convection l o o p s Concentration ( w t Species

A rQ

Type 3C4L l o o p

Type 316 l o o p

<0.1

4.4 0.4

(0.1 0.7 99.2

1.3 3.6 90.3

eo2

H20 N2 + CO 02

L _

aArgon was the o r i g i n a l c o v e r gas.

DISCUS S ION A s u b s t a n t i a l amount of gaseous oxygen r a p i d l y evolved when the draw

s a l t in t h e s e closed l o o p s was heated t o above about 520째C.

This behavior

is c o n s i s t e n t w i t h t h e decomposition r e a c t i o n

where X c a n be sodium o r potassium.

e a r l i e r for t h i s s a 1 t . l

Such a r e a c t i o n was r e p o r t e d

A s seen i n T a b l e 2, t h e measured n i t r i t e con-

c e n t r a t i o n s i n t h e s a l t increased i n accordance w-i th t h e decomposition r e a c t i o n and are i n approximate agreement v i t h those c a l c u l a t e d with t h e e q u i l i b r i u m c o n s t a n t for Eq. (1) (ref. 11) and t h e assumption t h a t a mass b a l a n c e w a s maintained between nitrate and n i t r i t e .

Although metal oxidat i o n r e a c t i o n s were also occurring in t h e system, as evidenced by the

growth of oxide f i l a and the buildup of chromium i n t h e s a l t , i t appears t h a t t h e above decomposition r e a c t i o n c o n t r o l s t h e o x i d a t i o n p o t e n t i a l of t h e system.

This is c o n s i s t e n t with t h e f a c t t h a t the o x i d a t i o n of the

m e t a l i s a small effect: r e l a t i v e t o the amount of s a l t i n t h e loop and t h e r e f o r e should not s i z n i f i c a n t l y a f f e c t salt chemistry.

As a l r e a d y d i s c u s s e d , metal o x i d a t i o n p r o c e s s e s were m n i f e s e e d by t h e f o r m a t i o n of s u r f a c e r e a c t i o n l a y e r s ( f i g s . 7-10)

and i n increased

20 l e v e l s of chromium i n t h e draw s a l t , and microprobe examination of exposed t y p e 316 s t a i n l e s s s t e e l (Fig. 5 ) i n d i c a t e d t h a t s i g n i f i c a n t d e p l e t i o n of chromium from h o t t e r l o o p s u r f g c e s a l s o occurred.

Both t h e

o x i d a t i o n and d e p l e t i o n r e a c t i o n s are discussed i n more d e t a i l belw.

The weight cha: c e s reported i n Tables A.l t i v e of t h e t r u e c o r r o s J o n r a t e s .

through A.3 a r e n o t i n d i c a -

The measured weight change hw i s

a c t u a l l y the d i f f e r e n c e b e t w e e n t h e weight of t h e r e a c t e d oxygen AWo and t h e sum of t h e weights o f t h e d i s s o l v e d elements ( p r i n c i p a l l y chromium) and of the s p a l l e d m a t e r i a l UrG:

The weight of t h e oxygen acquired by a specimen AVO c a n be r o u g h l y c a l c u l a t e d from the t h i c k n e s s of the o x i d e l a y e r x (see T a b l e s A . 4 4 . 6 ) :

where t h e d e n s i t y of Fe30,,,

pFe 3 4

composite oxide l a y e r .

, was

used t o approximate that of t h e

The t o t a l weight of oxidized metal AWT i s d e f i n e d

as t h e weight of metal i n both t h e oxide scale and molten s a l t .

This

weight â&#x20AC;&#x153;lossâ&#x20AC;&#x2122; i s thus t h e sum of t h e weight of t h e d i s s o l v e d elements AWD and t h e weight of the r e a c t e d ( o x i d i z e d ) i r o n and chromium AWM(*):

where

When AWT i s d i v i d e d by t h e time of s a l t exposure, an average approximate

corrosion rate f o r the p a r t i c u l a r speciaen is obtained.

The v a l u e s of AW?

c a n be o b t a i n e d by s u b s t i t u t i n g i n Eq. (4) v a l u e s of AWD and AWkt(o) o b t a i n e d by use of Eqs. ( 2 ) and ( 5 ) , r e s p e c t i v e l y , and a p p r o p r i a t e measurements of LCJ and

from T a b l e s A.1 through A.6.

For our c a s e , w e

w i l l assume hV, = 0 because o b s e r v a t i o n of p o s s i b l e s p a l l a t i o n a f t e r 4500 h

of e x p o s u r e was r e s t r i c t e d to

a v e r y few specimens i n one loop (specimens

R6 and H7 i n t h e type 304L s t a i n l e s s s t e e l TCL).

With t h i s assumption,

21 approximate values of t h e t o t a l weight of o x i d i z e d metal of each loop specimen a r e l i s t e d i n Tables A.7 through A.12.

Measurements of tbe

weights of descaled specimens a r e i n p r o e r e s s t o determine t h e acclzracy of t h e above approach.

Corrosion weight changes and r a t e s o b t a i n e d in t h i s

manner f o r t h e a l l o y 800 and t y p e 316 s t a i n l e s s s t e e l loops (Table6 A.7A.lO)

a r e c o n s i s t e n t with the expectation t h a t o x i d a t i o n k i n e t i c s , i.id

t h e r e f o r e t o t a l weight l o s s , should i n c r e a s e ui t h i n c r e a s i n g temperature ( t h i s i s not t h e case f o r the uncorrected weight changes measured wfth t h e oxide f i l m s i n t a c t ) .

I n c o n t r a s t , t h e weight l o s s e s c a l c u l a t e d f o r

specimens i n t h e t y p e 3342, s t a i n l e s s s t e e l loop a f t e r 4500 h of e x p s u r e d i d not maximize a t t h e highest loop temperature.

I n t h i s case, relati-

v e l y l a r g e corrosion r a t e s r e s u l t e d a t i n t e r m e d i a t e temperatures (see Table A.12).

The a c c e l e r a t e 6 o x i d a t i o n a t t h e s e i n t e r m e d i a t e positions

was c h a r a c t e r i z e d by t h e presence of r e l a t i v e l y t h i c k and a p p a r e n t l y p o o r l y adherent oxide f i l m s (Fig. 1 4 ) .

However, t h e a c t u a l c o r r o s i o n

r a t e s a t these p o s i t i o n s a r e smewhat less than t h o s e c a l c u l a t e d because t h e c a l c u l a t i o n s assumed a 100% dense o x i d e f i l m of t h e same t h i c k n e s s as t h a t of t h e observed porous film. Regardless of p o s s i b l e i n a c c u r a c i e s i n t h e d e t e r m i n a t i o n s of WT, t h e weight change r e s u l t s shown i n Tables A.7 through A.12 i n d i c a t e t h a t t h e metal i n t h e oxide f i l m c o n s t i t u t e s most of t h e metal l o s s and t h a t t h e t o t a l corrosion r a t e s a r e r e l a t i v e l y small.

Figure 16 shows that t h e

o x i d e t h i c k n e s s e s on t h e a l l o y s of t h i s s t u d y are about an o r d e r of magnit u d e less than those expected for annealed t y p e 304 s t a i n l e s s s t e e l

steam.l*

The t o t a l weight losses a f t e r 4500 h i n draw s a l t have been con-

v e r t e d i n t o corrosion r a t e s i n t h e l a s t column of Tables A.8,

h.10, and

A.12 and i n d i c a t e t h a t , while the "worst case" corresponds t o 25 pn'year

(1 m i l / y e a r ) ,

t h e corrosion r a t e is s u b s t a n t i a l l y less i n most cmes.

The

one p o t e n t i a l concern i n these t e s t s was the n a t u r e of t h e o x i d e films on t y p e 304L s t a i n l e s s s t e e l specimens a t i n t e r m e d i a t e temperatures, &.ere s p a l l i n g appeared imminent Although t h e exact nature of t h e m t a l l o g r a p h i c a l l p d i s t i n c t s c r f a c e l a y e r s on exposed specimens is not known, it a p p e a r s t h a t , i n g e n e r a l , t h e

8 3WL ss

2t 100

402

I 1 . r .

III ?

$0'

EXPOSURE TIME (h)

Fig. 16. Comparison of oxide thickness versus exposure t i m e f o r exposures t o NeNO3-KNO3 and steam. r e a c t i o n zone a t h i g h e r temperatures c o n s i s t s of an i r o n oxide (probably Fe3O4) with an underlying spine!

l a y e r such as Fe(Fe,Cr)204.

T h i s would

be c o n s i s t e n t with t h e aicroprobe d a t a and agrees with the r e s u l t s from a s t u d y of t h e c o r r o s i o n of type 304 s t a i n l e s s s t e e l by t h e m a l l y convective draw s a l t . 3

However, d e t a i l s revoaled by o p t i c a l metallography i n d i c a t e a

more complex r e a c t i o n zone than t h a t just described.

For example, a dark-

e t c h i n g phase was sometfnes observed o p t i c a l l y a t t h e s a l t - a l l o y i n t e r f a c e

[Pig. lO(a)], although t h i s phase was not revealed by microprobe a n a l y s i s . The o t h e r t h r e e l a y e r s seen i n Fig. 10(a) were d i s t i n c t a r e a s i n the microprobe a n a l y s i s (Fig. 15).

Another complicating f a c t o r is t h e multi-

t u d e of l a y e r s observed i n t h e r e a c t i o n zones of types 304 and 304L s t a i n l e s s steel when t h e i r polished c r o s s s e c t f o n s are etched (Fig.

10).

This v a r i e t y of m e t a l l o g r a p h i c a l l y d i s t i n c t structures was not observed i n another study of type 304 s t a i n l e s s steel,3 and t h e reason why so many l a y e r s should develop on t h e s e a l l o y s and not on t h e o t h e r materials

i n i h i s study i s not obvious from compositional d i f f e r e n c e s .

Finally, the

micrographs of t h e higher temperature a l l o y 803 coupons i n d i c a t e d some tendency f o r i n t e r n a l o x i d a t i o n a t the r e a c t i o n zone-base metal i n t e r f a c e [Fig. 7 ( a ) ] , although longer term t e s t s w i l l be required t o e s t a b l i s h t h e s i g n i f i c a n c e of t h i s observation. The observed i n c r e a s e s i n t h e chromium l e v e l s of the test s a l t i n each loop imply t h e formation of one or more chromium-containing s p e c i e s t h a t a r e s o l u b l e i n Nah'O3-KNO3.

Because most of t h e chromiuin i n t h e s a l t

was i n t h e +â&#x201A;Ź o x i d a t i o n s t a t e (Table 2), Na2Cr2O1 and Na2Cr0,, species.

a r e probable

Changes i n the chromium c o n c e n t r a t i o n of t h e draw s a l t a s a

f u n c t i o n of exposure rime can a l s o yield information about t h e chromium d e p l e t i o n process.

Assuming t h a t , as discussed below, t h e r e is no deposi-

t i o n of chromium from t h e s a l t , t h e i n c r e a s e i n t h e chromium c o n c e n t r a t i o n s of t h e s a l t C C ~ ( ~should ) s u b s c r i b e t o a half-pouer

time dependence i f

d i f f u s i o n of chromium i n t h e a l l o y o r i n t h e s a l t is r a t e c o n t r o l l i n g . l i n e a r time dependence would imply s u r f a c e r e a c t i o n c c n t r o l .

Recent work3

on t h e corrosion of type 304 stainless s t e e l i n draw s a l t r e v e a l e d a time dependence of

t o e 4

f o r C C ~ ( ~ ) .In agreement w i t h o t h e r work3n5 our data

g e n e r a l l y i n d i c a t e a continuing i n c r e a s e i n chronium l e v e l s of t h e s a l t w i t h time (Table 2) f o r t h e d u r a t i o n of t h e experiments.

The chromium

c o n c e n t r a t i o n of t h e s a l t i n t h e type 316 s t a i n l e s s s t e e l TCL appeared t o approach a time-independent value a f t e r s e v e r a l thousand hours and then i n c r e a s e d again when the maximum loop temperature w a s increased.

I n no

c a s e , however, was t h e time dependence of t h e chromium c o n c e n t r a t i o n s I n t h e s a l t g r e a t e r than too5 f o r o p e r a t i o n a t a maximum temperature of

600'C;

the b e s t - f i t exponent was always between 0.1 and 0.5.

This would

t h u s i n d i c a t e that o d i f f u s i o n a l process involving chromium is t h e r a t e c o n t r o l l i n g step.

Accordingly, t h e c o n c e n t r a t i o n s are p l o t t e d v e r s u s t h e

s q u a r e root of exposure t i m e i n Fig. 17. One p o s s i b l e c o r r o s i o n e f f e c t is t h e movement of s u r f a c e a t * -

test a l l o y s from h o t t o c o l d r e g i o n s of t h e l o o p v i a convection s a l t phase.

This phenmenon of thermal g r a d i e n t mass t r a n s p o r t

. 'le

Pig.

17.

Chromium c o n c e n t r a t i o n i n NaN03-KN03 v e r s u s t h e s q u a r e root

of l o o p o p e r a t i n g t i m e . observed i n o t h e r molten s a l t systems ( p r i n c i p a l l y hydroxides or f l u o r i d e s ) s u b j e c t e d t o a thermal g r a d i e n t . 1 3

The t r a n s p o r t mechanism can i n v o l v e

o x i d a t i o n and d i s s o l u t i o n of r e a c t i v e metals as c o r r o s i o n p r o d u c t s in t h e h o t zone w i t h subsequent reduction and deposition of the m e t a l s in the

cold zone.

Our tests i n d i c a t e t h a t an o x i d a t i o n r e a c t i o n o p e r a t e s t o

d i s s o l v e chromium i n t h e s a l t ; however, the weight change d a t a gave no evidence t h a t t h e r e a c t i o n leads t o a net t r a n s p o r t of chromium from h o t t o cold regions.

There was a l s o no evidence of cold-leg d e p o s i t s i n

v i s u a l or m e t a l l o g r a p h i c examinations of cold-leg

specimens.

Furthermore ,

changes in t h e &T of t h e type 316 s t a i n l e s s s t e e l loop d i d not s i g n f f i c a n t l y a f f e c t t h e measured weight changes (Fig. 5 ) .

Thus, o u r r e s u l t s are

e n c o u r a g i n g in t h a t thermal g r a d i e n t mass t r a n s p o r t w i l t not be a s i g n i f i -

cant o p e r a t i o n a l problem i n draw s a l t systems up t o 600째C.

This s t u d y i n d i c a t e s two p r i n c i p a l c o r r o s i o n r e a c t i o n s r e s u l t i n g from e x p o s u r e of Fe-Xi-Cr

a l l o y s t o NaaN03-KN03:

t h e s o l i d - s t a t e o x i d a t i o n of

t h e exposed a l l o y s and t h e s e l e c t i v e d i s s o l u t i o n of chromium from t h e

matrix.

As such, o u r r e s u l t s agree w i t h t h o s e r e p o r t e d 3 * " f o r similar

a l l o y s exposed t o NaN03-KN03 i n Tns t h a t w e r e open t o a i r .

Spallation

was not e x t e n s i v e , and i n t e r g r a n u l a r a t t a c k v a s not observed. The r e s u l t s of t h i s study i n d i c a t e t h a t a l l o y 800 and t y p e s 304,

304L, and 316 s t a i n l e s s s t e e l have low c o r r o s i o n r a t e s i n t h e r m a l l y conv e c t i v e NaNO3-KNO3 a t a rnarimum temperature of 600째C.

Vnder such con-

d i t i o n s , t h e g r e a t e s t metal l o s s r a t e corresponded t o 25 p l y e a r , but i n most c a s e s t h e r a t e was less t h e 3 1 3 p l y e a r .

These l a t t e r r a t e s a r e

sc?mewhPt s m a l l e r than t h o s e reported f o r s i m i l a r loops operated with t h e s a l t exposed t o t h e a t m ~ s p h e r e ~but , ~ are w i t h i n a f a c t o r of 2 t o 5 . S i g n i f i c a n t metal o x i d a t i o n w a s observed o n l y a t i n t e r m e d i a t e temperatures i n t h e type 304L s t a i n l e s s steel TCL a f t e r 5000 h of s a l t exposure (Fig. 6), but t h e o v e r a l l r a t e d i d not exceed 25 p d y e a r .

R a i s i n g t h e maximum l o o p

temperature t o 620째C r e s u l t e d i n uuch g r e a t e r c o r r o s i o n r a t e s f o r type 316 s t a i n l e s s s t e e l (Fig. 5).

This r a p i d a c c e l e r a t i o n of t h e c o r r o s i o n pro-

cess w a s a l s o observed i n o t h e r s t u d i e s of types 304 and 316 s t a i n l e s s

steel and a l l o y 800 i n draw s a l t loops,5s5 where a t 630째C s i g n i f i c a n t s p a l l i n g of t h e o x i d e f i l m s on type 316 s t a i n l e s s s t e e l was observed.5 Our r e s u l t s t h e r e f o r e confirm t h a t 600째C probably r e p r e s e n t s a n upper t e m -

p e r a t u r e s e r v i c e l i m i t f o r such a l l o y s in NaNOj-mO3

( r e f . 5).

SUMMARY

Alloy 800 and t y p e s 304, 304L, and 316 s t a i n l e s s s t e e l were exposed t o t h e r m a l l y c o n v e c t i v e NaN03-KN03 draw s a l t a t 375 t o 60OoC f o r more than 4500 h.

The exposure r e s u l t e d i n t h e growth of t h i n oxide f i l m s on

a l l a l l o y s and t h e d i s s o l u t i o n of chromium by the s a l t .

The weight change

d a t a f o r t h e a l l o y s i n d i c a t e d t h a t (1) t h e metal i n t h e oxide f i l m c o n s t i t u t e d most of t h e metal loss; (2) t h e c o r r o s i o n r a t e , in g e n e r a l , i n c r e a s e d w i t h temperature; and ( 3 ) although t h e g r e a t e s t metal l o s s corresponded

t o a p e n e t r a t i o n rate of 25 w / y e a r (1 a i l / y e a r ) , t h e r a t e was less than 1 3 p / y e a r i n most c a s e s .

S p a l l a t i o n had a s i g d f i c a n t e f f e c t on metal

l o s s a t i n t e r m e d i a t e temperatures i n t h e t y p e 3t4L s t a i n l e s s s t e e l loop. M e t a l l o g r a p h i c examinations showed no evidence of i n t e r g r a n u l a r a t t a c k

o r of s i g n i f i c a n t cold-leg d e p o s i t s .

Weight change d a t a f u r t h e r confirmed

t h e absence of thermal g r a d i e n t mass t r a n s p o r t processes i n t h e s e draw

s a l t systcms. R a i s i n g t h e m a x i m u m temperature of t h e type 316 s t a i n l e s s steel loop from 595 t o 620째C d r a m a t i c a l l y increascd the c o r r o s i o n rate.

It t h u s

appeared t h a t 600째C may be t h e l i m i t i n g t e m p e r a t u r e for use of such a l l o y s

i n draw s a l t .

T h i s study was funded by a s u b c o n t r a c t from Sandia N a t i o n a l Laboratory; R. W. C a r l i n g of Sandia waE t h e t a s k manager.

Ihe a u t h o r s

acknowledge t h e following ORNL s t a f f members vho c o n t r i b u t e d t o t h i s wo-k:

E. J. Lawrence, who designed and operated the thermal convection l o o p s ; C. C. Marsh and

R. S. Crouse f o r t h e f r S t a l l o g r a p h f c and microprobe work;

D. A. Costanzo f o r h i s a i d i n t h e chemical a n a l y s e s ; I r e n e Brogden f o r e d i t i n g ; and D. L. LeComte f o r p r e p a r a t i o n o f f i n a l copy. REFERENCES

Conceptual ~ e s i g nof Admnced CentTvzZ Receiver Power system,

Final Report, DOE/ET/20314-1/2, H a r t i n Marietta Corporation, Denver, September 1978. 2.

J. R. Keiser, J. E. DeVan, and E. J. Lawrence, " C o m p a t i b i l i t y of

Molten S a l t s with Type 316 S t a i n l e s s S t e e l and Lithium," J. NucZ. Mater.

85&86(II, A), 295-98 (1979). 3.

R. W. Bradshaw, C o r r o s b n of 334 S t a i n l e s s Steel by Elblten RczN03-KNOj i n a !l%ermzZConvection L o q , S m O - 8 8 5 6 , Sandia N a t i o n a l Laboratory, Livermore, C a l i f . ,

December 1980.

R. W. C a r l i n g e t a l . , Molten E t m t e S a l t Technology Development S t a t u s Report, SAND80-8052, L i v e m o r e , Calif March 1981, pp. 33-44. 5.

R. W. Btadshaw, ThentmZ C'O'onVe&iOn Loop Corrosion Test6 of 316

S t a i n t e s e Steel and IN8OO i.t Molten Nitrate N a t i o n a l Laboratory, Livermore, C a l i f . ,

3Zt6,

SAND81-8210, Sandia

February 1982.

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E. B. S a n d e l l , CoZorimetrYic k t e m i r - l t i o n of Duces of Metals,

I n t e r s c i e n c e , Hew York, 1959, p. 392.

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409 (1966).

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&em.

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(1955).

W. W. Scott, Stanrhd Wethods of &mica2 b2a?ysis, 6 t h ed.,

Van Nostrand, New York, 1962, p. 748. 10.

I. M. Kolthoff and E. B. S a n d e l l , Textbook of Quantitative

Inorganic Anatysis, 3d ed., MacMillan, New Yo&, 1952, p. 581. 11. D. A. Nissen, Tfie Ozer6stry of the 3inary .r3z.'i03-ENOj Qstem, SAND81-8007, Sandia N a t i o n a l Laboratory, Livermore, C a l i f . , 12.

June 1981.

J. C. Griess and W. A. Maxwell, f i e Long-?erm Cbcidat&m

Selected AZZoys i n Supwheated Steam at 482 a d 538OC, OW-5771, J u l y 1981. 13. J. H. &Van and R. B. Evans 111, "Corrosion Behavior of R e a c t o r M a t e r i a l s i n F l u o r i d e Salt Mixrures," pp. 5 5 7 4 0 i n Corrosion of &actor

Materhls, I n t . Atomic Energy Agency, Vienna, 1962.

Appendix

CORROSION DATA

Table A . l .

Spec

;

1500

2002

3010

4498

4998

6146

6650

+0.3 +1.7

+0.9 +2.0

+le4

+2.6

+3.7

+3.0 +3.7

+1.1

+0.9 -6.3

+1.4

+2.0

+2.5

+O.3

M.3

6.6

-0.3 4 . 3 -0.9 0.0

4.3

6.3

+0.2

+0*7

0.0

-1.1 -0.6 -1.1 6.6

0.0

495 470

4.6 0.0 4.6 +0.3

+0.8 0.0

+3.4 +4.7 +3*0 +le3 +1.0

+3.9

+2.5

600 570 545 520

0.0 iQ.3 M.3 -0.6 -0.6 0.0

-0.6 0.0

4 . 6 0.0

-0.6 0.0

485 455 435 410

-0.6 -0.6 -0.3

4.8 6.3 4.3 4.3 -0.3 -0.6

-1.1 4 . 6

-0.6 6 . 3 -0.3

-1.4 4.9 -1.0

-0.4 -0.4 4.5

+0.1

4 . 6 4 . 6

0.0 -1.44 0.0 -0.3

6.3

-0.6 -0.9

4 . 3

0.0 4.3

4 . 5 4 . 3

-0.3 0.0

595

c4 c5 C6 c7 C8

for t o t a l exponuts in h

378

580

("a

H2 H3 H4

Weight change (R/rn2)

Temperature

H9 H6 I T7

Weight changes for alloy 800 specimens exposed to draw salt in an alloy 800 thermal convection loop

390

-0.6 -0.6

375

-0.6

-2.0

-0.3

4.8

'5.7 +3*5

+le8 +1.5

-0.9

0.0 +O.2

0.0

-0.3

w I-

Table A.2. Weight changes f o r t y p e s 304 and 304L e t a i n l e s s s t e e l specimens exposed t o draw s a l t i n a type 304L s t a i n l e s s s t e e 1 thermal convect i o n loop

Specimen

Temp e r a ture ("C)

Weight change (g/m*) for t o t a l exposure I n h 500

H1 H2 H3

580 595 600

-1.7 -1.1

If3W

600

-2.8

H4 H5

570 54 5

M.8 W.8

520

0.0

14 8

490 465

0.0 3.0.3

c3 c4

485 455

C4Aa c5

455 435 410 390 375

M.6 49.3 0.0 0.0

C6 c7 C8

M.3 M.6 M.3

1500

2121

3006

4488

4988

5489

6279

6804

+0.3 -0.3 0.0 4.8 +2.3 +le1 +0.6 +0.8 +2.0

+0.3 -2.0

+0.3 -2.0 W.9

M.6

+Os6

-1.3

+0.9

+3.5

-0.9 +3.1

+2.0 +11.4

+le7

+4.0

+2.0

+2.8

+le7 +1.1 +0.8 +1.7 +2.8

+3.1 +2.0

+0.9

4.3

4-16.8 +4.8 +2.6

+1.4 +le7

+0*6

+2.6 6.2

0.0

-1.8 +2.2 -1.3

0.0

+O,9

+O.Z

+Q.1

-107

-105

+le7 +8.8 +14.5 +4.6 +2.8

+2.0 +Om6 +15.4 +4.8 +3.4

+0.7

-0.2 H.6 +11.0 +3.5 +2.5

-1.5 -0.8 +10.5 +3.5 +1.6

-5.1

+4.6 +3.7 +3.7

+5.7 +5.7 +4.6

+4.7 3.4.2 3.3.2

+5.2

+3.4 +2.6 +2.3 +3.1

+4.0 +2.8 +3.7

+3.6 +4.0

+3.2 +3.1 +2.0 +2.8

+le1 +16.9 +4.8 +2.5

+2.4 +2.8

+6.7 +2.7

+2.8

aSpecimens H3A and C4A are t y p e 304 s t a i n l e s s steel; remainder a r e type 304L.

-0.9

-1.3 +11.0 4-3.5

+lo6

+5.7 +6*7 +3.2

+6*7 +Go2 +3*2

+3-2 +2*2

+3*2 +2*2 +lo5

+lo1

+2.4

w N

Table A.3. Weight changes f o r type 316 stainless steel specimens exposed to draw salt in a type 316 stainless steel thermal convection loop Temperaturea ('C) Specimen

AT 225'C

150'C

Weight change (g/a2) â&#x201A;Źor total exposure in h 499

1500

3010

2001 ~

H1 H2

5RO

590 595

~~~

4501

5102

6108

7133

4.7 -2.5 -1 e4

-2 .o

+o. 5 4 . 6

580 580

0.0

+l.l

M.6

+0.7

+0.6

M.3

-2.8

-2.3

595

-4.0 -0.6

-1.4 -6.6

-1.1

-2.0 4.9 -8.6 +0.6

565

575

-1.4 M.3

H5 H6 87 H8

535

555

M.8

+0.8

505 47 5 445

535 520 500

44.6 0.0 0.0

+0.6 +Om3

+0.3 +0.3

-8.6 +0.3 M.6 M.6

0.0

-0.3

0.0

-0.3

c3 c4 CII

485

530

M.8

+0.6

+le1 -1.7 +la4 +3.1 M.8 -1.1

+le4

83 H4

C6 c7 C8

455 491)

410 390 375

-3.1

505

M.3

+0.8

495 It80 465

M.6

M.8

44.3 W.3

450

-1.1

+2.8 W.3 -1.1

0.0

-1.1 M.8 +2.8 H.6 -1.1

7658

8660

~~~~~

W.6 +0.3

-1.1 +1.7 +3.4 M.6 -1.1

-8.1 fl.1 +1.1 +0.8 +le3

+2.8 -1.6 +le7 +3.4 #e1 -1.1

+c).

M.5

+0.5

-0.1 -6.6 -8.0

4.1 4.0

-7.6 +1.6 +1.1 +1.3

-1.8 -3.7 +5.7 +3.5 +1.8

+le8

+3.4

+2.3

+7.0

+So1

-255

M.8 +3.2 +6.2 +2.9 +le3

-1.1

-0.9

+2.7 +3.4 M.1 -0.6

+3.3 +2.5 +1.8

+3.4

+3*2 +5.7 +2*4 Me8

aAT (temperature differential) changed from 250 to l50'C after 5102 h; Tmax (maximum temperature) changed from 595 to 620'C after 6108 h with corresponding changes in all epecimen tcmperaturee.

-7.5 +5.2

+4.9 +4.2 +5*0 +8*8 M.3 +To1

+7.6 +4*8 +3*2

Table A.4. Average corrosion product thickness for alloy 800 specimens exposed t o flowing drav s a l t (Average of ten measurements)

Specimen

Tempera t u re

Thickness x

(m)

,O".

1500 h

4500 h

580

3.7

5.1

595

3.7

6.2

600

5.4

6.8

570

2.8

5.3

545

2.3

3.6

520

1.0

1.2

495

0.4

1.1

470

0.3

1.3

485

1.8

2.5

455

1.6

2.7

435

0.9

1.7

410

0.4

2.7

390

0.4

1.2

37 5

0.5

2.3

Table A. 5. Average corrosion product thickness for types 304 and 304L s t a i n l e s s s t e e l exposed t o flowing drav s a l t (Average of ten measurements) Temperature Spec iaen

Thickness z (p)

f OF\

1500 h

4500 h

580

4.1

595

2.6

6.8

600

3.6

5.6

H 3Aa

600

3.6

5.0

570

1.9

6.0

545

1.2

11.9

520

0.4

22.8

490

1.1

3.5

465

1.4

1.3

485

2.9

5.1

45s

2.0

3.1

C4Aa

455

1.7

2.4

435

1.4

3.1

410

0.7

0.6

390

<0.4

1.4

375

1.1

0.7

?Qpe 304 s t a i n l e s s s t e e l ; others are t y p e 304L.

Table A.6. Average corrosion product thickness for type 316 s t a i n l e s s steel exposed to flowing draw s a l t (Average of ten measurements) Temperature Specimen

Thickness x (pm)

IO-\

1500 h

4500 h

58 0

0.5

595

1.9

4.4

59 5

4.0

7.7

565

0.2

8.2

535

1.7

5.7

505

0.6

2.8

475

0.2

0.0

445

0,o

0.0

48 5

2.2

7.4

455

1.7

4.4

43 5

0.9

1.3

410

6.1

4.0

390

0.3

0.2

375

0.0

Table A.7.

Weight change r e s u l t s for a l l o y 800 exposed t o flowing b N 0 3 - K N 0 3 f o r 1500 h Weight change

(g /m2

M.3 +1.7 +1.1

3.7 5.4 2.8

5.4 5.4 7.9 4.1

-0.3

2.3

3.3

4.6

1.0 0.4 0.3

1.5 0.6 0.4

0.0

H3 H4

580 595 600 5 70

H5 H6 H7 H8

545 520 495 470

c1 c2 c3 c4

470 475 485 455

1.8 1.6

2.6 2.3

c5 C6 c7 C8

435 410 390 375

0.9 0.4 0.4 0.5

1.3 0.6 0.6 0.7

H1 H2

3.7

-0.6 M.3 -2.8 +l.1 -0.8

4.3 -0.3 -0.3 -0.3

-0.6

3 6.8 4.4

13.8 13.8 20.2 10.5

18.9 17.5 27.0 14.9

4.0 1.5 1.2 0.2

8.6 3.7 1.5 1.1

12.6 5.2 2.7 1.3

3.4 2.6

6.8 6.0

10.2 8.6

1.6 0.9 0.9 1.3

3.4 1.5 1.5 1.9

5.0 2.4 2.4

3.2

QT, temperature. hx, t h i c k n e s s . CAWo, weight of reacted oxygen; Abâ&#x20AC;&#x2122;, measured ,,.ght change; ~ W D ,t o t a l weight of dissolved e l e m e n t s ; A k k ( o ) , weight of reacted I r o n and chromium; and hWT, t o t a l weight of o x i d i z e d metal.

Table A.8.

Weight change r e s u l t s f o r a l l o y 800 exposed t o flowing NaNO3-KNO3 f o r 4500 h ~-

Specimen

la ("C)

(m)

Weight change= (g/m* ) A WO

Avo

+2.6 +3.7 +2.0 4.3

580

5.1

7.4

595

6.2

9.0

600 570

6.8

9.9 7.7

545

3.6

520

1.2

495 470

1.1 1.3

5.2 1.7 1.6 1.9

470 475 485 455

3.2

4.7

2.2

3.2

2.5 2.7

3.6 3.9

435 4 10 390 375

1.7 2.1 1.2

2.5 3.9 1.7

2.3

3.3

H5 H6 H7 H8 c1 c2

c3 c4 c5

C6 c7 C8

5.3

%(o)

AWT

4.8

19.1

23.9

0.2

5.3

23.2

28.5

0.3

7.9 7.4

25.5 19.8

33.4 27.3

8 8

0.3 0.3

0.0

5.2

-0.3

2.0 2.2

13.5 4.5 4.1 4e9

18.7 6.5

5 3

6.3

6e8

0.2 0.1 0.1 0.1

12.0 8.2

18.6

1.8

0.2 0.1

3.6 5.3

10.1

5 3 3 3 3 3

0.1 0.1 0.1

0.1

-0.6 0.0

1.9

-2.0 +1.4 0.0 -1.4

6.6

0.0 -0.3 0.0 -0.3

2.5 4.2 1.7 3.7

9.4 6.3 10.1 4.5

8.6

10.0 13.0 15.4 8.8 14.3 6.2 12.3

(rzm/year)

(mil/year)

0.1 0.1

aT, temperature.

bz, t h i c k n e s a . CAWo, weight of r e a c t e d oxygen; AW, measured weight change; WD, t o t a l weight of d i s s o l v e d elements; A W M ( ~ ) , weight of r e a c t e d i r o n and chromium; and AWT, t o t a l weight of o x i d i z e d metal.

&R,

carroeion r a t e .

CRd

Table A.9. Weight change r e s u l t s for type 316 s t a i n l e s s steel exposed t o flowing NaNO3-KNO3 for 1500 h

Speci men H1 H2

H3 H4 85 H6

H7 Ha

("C)

(Fnn)

580 595 600 565

1.9 4.0

0.2

2.e 5.8 0.3

535 505 47 5 445

1.7 0.6 0.2 0.0

2.5 0.9 0.3 0.0

e2 c3 c4

470 475 485 455

c5 C6

435 410

390 375

Weight changeC (g/m2)

+1.1 -4.0 4.6

-3.1 H.8 H.6

4-0.3 0.0

6.8 6.4 3.4

7.1 15.0 0.8

21.4 4.1

1.7 0.3 0.0 0.0

6.4 2.2 0.7 0.0

8.C 2.5 0.7 0.0

2.6 -1.7

8.2 6.3

10.8

3.4 22.8 1.2 0.0

3.9 28.9 1.3 1.1

13.9

0.0

2.2 1.7

3.2 2.5

0.9 6.1 0.3 0.0

1.3 8.9

0.4 0.0

-0.6 M.6

+o.a M.8 +2.8 M.3 4.1

0.5 6.1 0.1 1.1

8.0

QT, temperature.

bz, thickness. =AVO, v e i g h t of r e a c t e d oxygen; AV, measured weight change; AVg, tot31 v e i g h t of dissolved elements; AKMfO), weight of reacted iron and chromium; and AWT, total weigh' of ox!dized metal. aNegative number i n d i c a t e s p h y s i c a l i m p o s s i b i l i t y .

Table A.10.

Specimen

Weight change results f a t type 316 stainless steel exposed to flowing NaNO3-KNO3 for 4500 h

H1 H2

580

CRe

Weight changeC (g/m2)

A'd

0.5 4.4

0.7 6.4

+0.6 -2.0

0.1 8.4

7.7

11.2

4.9

12.1

AwMfo)

(m/year)

1.9 16.5 28.8 30.7

2.0

(0.1

24.9

0.2

40.9

0.4

51.2

0.5

21.3 10.5

29.0 14.0

0.3

0.1

0.0

-0.3

0.0

0.3

(0.1

0.3

<3 3

(0.1

0.4 0.2

595 600

565

8.2

11.9

-8.6

20.5

H5 H6 H7 H8

535

5.7 2.8 0.0 0.0

8.3 4.1

+0.6 +0.6

7.7

505 475 445

0.0

+Om3

0.0

-0.3

c1 c2

470 475

0.0

1.9

0.0 7.1

485

7.4

455

4.4

9.4 7.5

27.7

4.3 -0.3 +1.4 -1.1

0.3 3.1

2.8 10.8 6.4

c5 C6

435

1.3

0.2

4.9

5.1

4 .O

1.9 5.8

+1.7

410 390

+3.4

2.4

15.0

17.4

+0.6

375

0.0

0.3 0.0

-0.3 1.1

0.7 0.0

0.4 1.1

(3 (3

c7 C8 OT,

0.2

-1.1

3.5 4.3 0. 3

(mil/year)

16.5

10.2 37.1 24.0

0.1

<o. 1 0.2 <o. 1 (0.1

temperature.

bz, thickness. CAWo, wnfght a f reacted oxygen: AW, measured weight cliango; b w ~ ,total weight of dissolved elements; A W M ( , ) , weight of reacted iron and chromium: and AVT, total weight of oxidized metal.

dNegative number indicates physical impossibility. OCR, corrosion rate.

Weight change r e s u l t s for types 304 and 304L s t a i n l e s s s t e e l exposed t o flowing NaNO3-KSO3 for 1500 h

Table A.11.

H2 H3 H 3A R4

580 595

600 600 570

545

520

87 H8

490 465

470

c2 c3 c4 C4A

475

2.6 3.6

5.2 5.2

3.6 1.9

2.8

1.2 0.4 1.1 1.4

1.7 0.6 1.6 2.0

-0.3 0.0

4.1

9.7

13.8

5.2

-0.8 +2.3

6.0

18.7 19.5

0.5

13.5 13.5 7.1

+1.1

0.6

4.5

+0.6 +0.8 +2.0

0.0

1.5

7.6

0.8

4.1

0.0

5.2

5.1 1.5 4.9 5.2

10.9 7.5

13.4 8.4

6.4

7.1

5.2

6.2 2.8 0.4 2.9

H.8

+2.3

485

2.9

455

2.0

455

1.7

c5 C6

435 410

1.4 0.7

390 375

(0.4

3.8

1.1

4.2 2.9 2.5

+1.7 +2.0 +1.7

2.5 0.9 0.8

2 .o 1.0 0.6 1.6

+1.1 +0.8 +1.7 +2.8

0.9 0.2

-1.1 -1.2

2.6 1.5 4.1

aT, t e m p e r a t u r e . bx, t h i c k n e s s . CAW,, weight of r e a c t e d oxygen; AW, measured W e i g h t change; t o t a l weight of dissolved elements; A$!(*), weight of r e a c t e d iron and chromium; and AVT, t o t a l weight of o x i d i z e d metal.

AWD,

dNegative number i n d i c a t e s p h y s i c a l i m p o s s i b i l i t y .

Table A.12.

Weight change r e s u l t 8 for types 304 and 304L s t a i n l e s s steel exposed to flowing NaN03-KN03 for 4500 h

H1 H2 H3 H 3A H4

580 595 600 600 570

4.1

H5 H6 H7 H8

545 520 490 46 5

c1 c2 23 c4 C4A

470 475 485 455 455

5.1 3.1

7.4 4.5

2.4

435 410 390 375

3.1

0.6

C6 c7 C8 a!l",

5.6 5.0 6.0

6.0 9.9 8.2 7.3 8.7

+6.0 +O. 9 +0.6 +O. 9 +2.0

11.9 22.8 3.5 1.3

17.3 33.2 5.1 1.9

+0.6 +15.4

6.8

1.4 0.7

5.4 9.0

15.4

20.7

34.5

6.4 6.7

25.5 21.0 18.7 22.5

8 8 5 8

44.6 85.4 13.1 4.9

61.3 103.2

4.3.4

6.7 17.8 0.3 -1.5

3.4

15 25 3 (3

1.7 -1.2 -1.1

19.1 11.6 9.0

20.8 10.4 7.9

0.2

3 3

00 1

3.5

+2.3 +2.0 +5.7 +5.7 +4.6

4.5 0.9 2.0 1.0

+4.0 +4.0 +2.8 +3.7

0.5 -3.1 7 -2.7

11.6 2.2 5.2 2.6

12.1

0.1

4.9 4.5

<0.1

+4*8

7.6

- 0 0

28.5 25.1 29.2

13.4

0.2 0.3 0.3 0.2 0.3 0.6 1.0

0.1 <o. 1

0.1

-1.99

temperature.

bz, t h i c k n e s s .

weight of reacted oxygen; AW, measured weight change; W D , total weight of d i s s o l v e d elements; A W M ( ~ ) , weight of reacted iron and chromium; and AWT, t o t a l weight of o x i d i z e d metal.

dNegative number represents physical impossibility. eCR, c o r r o s i o n r a t e .