the influence of ph and chloride concentration on the corrosion behavior of aisi 316l steel in...

7/23/2019 The Influence of PH and Chloride Concentration on the Corrosion Behavior of AISI 316L Steel in Aqueous Solutions

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Corrosion Science Vol. 33, No . 11, pp . 1809-1827, 1992 0010--938X/92 $5.00 + 0.00

Printed in Gre at Britain. ~ 1992 Pergamon Press Ltd

T H E I N F L U E N C E O F p H A N D C H L O R I D E

C O N C E N T R A T I O N O N T H E C O R R O S I O N B E H A V I O U R

O F A IS I 3 16 L S T E E L IN A Q U E O U S S O L U T I O N S

A . U . M A L I K , P . C . M A Y A N K U TT Y , N A D EE M A . S I D D IQ I , IS M A EE L N . A N D I J A N I a n d

SHAHREER AH ME D

Research D evelopm ent and Training Cen ter , Saline Water Conversion Corp orat ion (SWCC ),

P.O . B ox 8034, A1-Jubail-31951, Kingdom o f Saudi A rab ia

Abst ract - -The ef fect of chlor ide concentrat ion, pH, dissolved oxygen and temperature on the pi t t ing

behaviour of 316L SS in aqueous solutions has been investigated under dynamic and static condit ions.

W eight loss , metal lograph y and elect rochemical polar izat ion techniques have been emp loyed dur ing the

investigation. I t has been foun d that th e corro sion rate ge nerally increases l inearly with increasing CI

concentrat ion in the range 100-5000 ppm . W ith increasing pH , the cor rosion rate de creases, being highest

at pH 4 and lowest a t pH 9. The num ber and d epth of p i ts increase wi th increasing CI concentrat ion.

From the res ults of cyclic polariz ation studies i t is inferred that the p it t ing po tentia l , Epi is shifted to a

more negat ive potent ia l wi th increasing C I- concentrat ion and tem perature an d decreasing pH. I t has

been e stablish ed that low pH, high C I- co nten t and stagnancy are the conditions m ost suitable for

init iat ion and pro pag ation o f pit t ing in A1SI 316L stainless ste el .

I N T R O D U C T I O N

PITTIN G i s a l o c a l i z e d c o r r o s i o n a t t a c k o n m e t a l s a n d a l l o y s i n a q u e o u s e n v i r o n m e n t s .

I t i s a m a j o r c a u s e o f fa i l u re o f c h e m i c a l p r o c e s s i n g a n d d e s a l i n a t i o n p l a n t s , w a t e r

s t o r a g e t a n k s a n d p i p e l i n e s , p u m p s a n d v a l v e s , p e t r o l e u m r e f in e r i e s e t c . D u e t o t h e

l o c a l iz e d n a t u r e o f p i t t i n g c o r r o s i o n , t h e f o r m a t i o n o f p i t s is c o n f i n e d t o m u c h s m a l l e r

a r e a s c o m p a r e d t o th e o v e r a l l e x p o s e d s u r fa c e . B r o a d l y s p e a k i n g , t h e i n i t i a t i o n o f

p i t t i n g is t h e r e s u l t o f t h e b r e a k d o w n o f t h e p a s s i v e f il m o n t h e m e t a l d u e t o t h e

p r e s e n c e o f c e r t a i n a n i o n s s u c h a s C 1 - a n d t h e s u b s e q u e n t e s t a b l i s h m e n t o f an

e l e c t r o c h e m i c a l c e l l i n w h i c h t h e d a m a g e d s i te a c t s a s a n a n o d e a n d t h e p a s s i v e s i t e

a c ts a s a c a t h o d e . T h e b u i l d i n g u p o f c o r r o s i o n p r o d u c t s o n t h e m o u t h o f t h e p i t m a y

r e s u l t in t h e f o r m a t i o n o f a c r e v i c e , t h u s p r o d u c i n g m o r e a g g r e s s iv e c o r r o s i o n a t t a c k .

I r o n - b a s e a l l o y s , p a r t i c u l a r l y a u s t e n i t i c s ta i n l e s s s t e e l s , a r e m o s t p r o n e t o p i t t i n g in

d i s s o l v e d C O 2 - a n d C l - - c o n t a i n i n g e n v i r o n m e n t s s u c h as t h o s e o b s e r v e d in b r i n e

r e c y c le a n d b l o w d o w n p u m p s , f la sh c h a m b e r s a n d d e m i s t e r s o f d e s a l i n a t i o n p l an t s ,

a n d i n s t e a m b o i l e r s , f e e d w a t e r h e a d e r t u b e s a n d s t e a m t u r b i n e b l a d e s o f p o w e r

p l a n t s . 1 -5

T h e e f f e c t o f c h l o r i d e o n t h e p i t t in g s u s c e p t i b i l i t y o f v a r i o u s m e t a l s a n d a l l o y s a n d

e s p e c i a l ly s t a in l e s s s t e e l s h a s b e e n e x t e n s i v e l y i n v e s t i g a t e d b y n u m e r o u s r e s e a r c h e r s

a n d is r e v i e w e d i n a n u m b e r o f b o o k s , r e v i e w s a n d a r t ic l e s . 6-~3 P a r t i c u l a r i n t e r e s t h a s

b e e n s h o w n f o r t h e C I - d u e t o i t s p r e s e n c e in s ea w a t e r a s t h e m a j o r c o n s t i t u e n t a n d

i ts r o l e a s a n a c t i v e p i t t in g a g e n t . B e s i d e s t h e i n f lu e n c e o f t e m p e r a t u r e , f lo w v e l o c i t y ,

p H a n d C I - c o n c e n t r a t i o n [ C I - ] o n p i t t i n g , t h e s y n e r g ic e f f e c t o f a n i o n s s u ch a s

S O ] - , $ 2 0 32 - , C 1 0 4 e t c . h a v e a l s o b e e n s t u d i e d . S m i a l o w s k a 14 f o u n d t h a t a m a j o r i t y

Manuscript rec eived 25 Septem ber 1991.

1809


2/19

1 8 1 0 A . U . M A LI K e t a l .

o f c o n s t r u c t i o n m a t e r i a ls s u f f e r p i tt i n g o n l y in s o l u t i o n s c o n t a i n i n g C i - o r o t h e r

h a l i d e i o n s . T h r e e m a i n r e a s o n s a r e g iv e n f o r t h e s p e c if i c e f f e c t s o f c h l o r i d e a n d i ts

a b i l i t y t o p r o d u c e p i t t i n g . F i r s t l y , t h e f o r m a t i o n o f a c h l o r i d e c o m p l e x w i t h c a t i o n

a n d h y d r o x i d e ; s e c o n d l y , a n i n c r e a s e o f h y d r o g e n i o n a c t i v it y in t h e p it e l e c t r o l y t e

a n d t h i r d l y , t h e f o r m a t i o n o f a sa l t l a y e r a t t h e b o t t o m o f p it s . T h e t h i r d fa c t o r

a p p e a r s t o e x p l a i n m o r e s p e c i f i c a l l y t h e r o l e o f h a l i d e s i n p it t i n g a t t a c k . I t i s

s u g g e s t e d t h a t t r a n s i t i o n f r o m p a s s i v i t y t o p i t t i n g c o n d i t i o n s c a n b e e x p l a i n e d b y a

c o m p e t i ti v e a d s o r p t io n m e c h a n i s m i n w h i c h c h l o r i d e io n s m o v e i n t o th e d o u b l e l a y er

( o x i d e / l i q u i d i n t e r f a c e ) , e v e n t u a l l y r e a c h i n g , a t a c r it ic a l p o t e n t i a l , E m it, c o r r e s p o n d -

i n g t o t h e [ C I -] r e q u i r e d t o d i s p l a c e t h e a d s o r b e d o x y g e n s p e c i e s . ~5-f? , N a s h i m u r a e t

a l . 1 7 1 8 f o u n d t h a t p i t i n i t i a t i o n i s s t r o n g l y r e l a t e d t o t w o d i f f e r e n t t y p e s o f b o u n d

w a t e r in a f il m . I t w a s f o u n d t h a t t h e p i tt i n g b e h a v i o u r o f a l l o y s c o v e r e d w i t h t h e

p a s s i v e f i lm i s l a r g e ly d e p e n d e n t o n t h e f i lm t h i c k n e s s a n d i o n s e l e c ti v i t y . D u r i n g

s t u d i e s 19 o n t h e e f f e c t s o f [ C I - ] o n p i tt i n g b e h a v i o u r o f s t e e l s l i n e a r r e la t i o n s h i p s

h a v e b e e n f o u n d t o e x i s t b e t w e e n p i t n u c l e a t i o n p o t e n t i a l v s l o g c h l o r i d e i o n

c o n c e n t r a t i o n , a n d l o g i n d u c t i o n t i m e v s l o g c h l o r i d e i o n c o n c e n t r a t i o n . T h e

t e m p e r a t u r e a p p e a r s t o b e a n i m p o r t a n t p a r a m e t e r i n i n f l u e n c i n g t h e p r o te c t i v i ty o f

t h e o x i d e s c a l e s .

A I S I 3 1 6 L is c o n s i d e r e d t o b e o n e o f t h e m o s t i m p o r t a n t o f s ta i n l e s s s te e l s f o r

m a r i n e e n v i r o n m e n t s a n d t h e r e f o r e i s w i d e l y u s e d a s a s t ru c t u ra l m a t e r i a l fo r

d e s a l i n a t i o n p l a n t s. E v e n t h i s a l l o y m a y f a i l u n d e r c o n d i t i o n s o f l o w p H , h i g h

c h l o r i d e c o n t e n t o r s t a g n a t i o n . 8 2 -21 R e l a t i v e l y f e w s t u d i e s h a v e b e e n c a r r ie d o u t t o

i n v e s t i g a t e t h e r o l e o f p H a n d s t a g n a n c y o n t h e p i tt i n g b e h a v i o u r o f a u s t e n i t ic s t e e l s .

T h e p r e s e n t c o m m u n i c a t i o n c o n t a i n s t h e r es u lt s o f a s tu d y e m p h a s i z in g t h e r o l e o f

p H , c h l o r i d e i o n c o n c e n t r a t i o n , s t a g n a n c y a n d t e m p e r a t u r e o n t h e p i tt i n g b e h a v i o u r

o f A I S I 3 1 6 L a u s t e n i t ic s t a i n l e ss s t e e l i n a q u e o u s c h l o r i d e s o l u t i o n s .

E X P E R I M E N T A L M E T H O D

C o m m e r c i a l g r a d e

A I S I 3 1 6 L

s t a i n l e s s s t e e l

( 1 7 . 1 C r , 1 1 . 3 N i , 2 . 1 M o , 0 . 0 2 C

a n d b a l a n c e F e , a l l i n

w t % ) , i n s h e e t a n d r o d fo r m s , w a s u s e d f o r th e s t u d i e s .

F o r i m m e r s i o n t e s t s , c o u p o n s o f a b o u t

5 c m 2

a r e a w e r e c u t f r o m t h e s h e e t a n d a b r a d e d s e q u e n t i a l l y

w i t h 1 8 0 , 3 2 0 , 4 0 0 a n d 6 0 0 g r it Si C p a p e r s . T h e a b r a d e d s p e c i m e n s w e r e c l e a n e d i n a n u l t r a s o n i c c l e a n e r

f o l l o w e d b y d ry i n g. T h e d r i e d s p e c i m e n s w e r e w e i g h e d p r io r to i m m e r s i o n .

F o r e l e c t r o c h e m i c a l m e a s u r e m e n t s , c i r cu l a r f la t t e s t s p e c i m e n s o f

1 . 5 - 1 . 6 c m

d i a m e t e r w e r e u s e d . T h e

e x p o s e d a r e a o f t h e t es t s p e c im e n s w h i c h w a s s c r e w e d i n t h e s a m p l e h o l d e r w a s

1 c m2 .

E l e c t r o c h e m i c a l

p o l a r i z a t i o n s t u d i e s w e r e c a r r i e d o u t o n a n

E G & G m o d e l 3 4 2 - 2 so f t C o r r

m e a s u r e m e n t s y s t e m . T h e

s y s t e m w a s c o n s i s t e d o f m o d e l 2 7 3 p o t e n t i o s t a t / g a l v a n o s t a t , m o d e l 3 4 2 C o r r s o f t w a r e a n d m o d e l 3 0 I B M

P S - 2 .

A l l t h e e x p e r i m e n t s w e r e c a r r i e d o u t u s i n g a c o r r o s i o n c e l l

( E G & G

m o d e l K 0 0 4 7 ) w i t h s a t u r a t e d

c a l o m e l a n d g r a p h i t e a s r e f e r e n c e a n d c o u n t e r e l e c t r o d e s , r e s p e c t i v e l y . S e v e r a l s e r i e s o f e x p e r i m e n t s w e r e

c a r r i e d o u t i n o r d e r t o s t u d y t h e e f f e c t o f

[ C I - ] , p H

a n d s t a g n a n c y o n t h e p i t t i n g b e h a v i o u r o f

A I S I : 3 1 6 L

s t e e l u s i n g t e s t s o l u t i o n s u n d e r f o l l o w i n g c o n d i t i o n s :

p H

[ CI I ( p p m )

A r t i f i c i a l s e a w a t e r

I m m e r s i o n t i m e ( w e e k s )

C o n d i t i o n

T e m p e r a t u r e

D i s s o l v e d o x y g e n ( p p m )

4 , 7 a n d 9

0 , 1 0 , 1 0 0 , 3 0 0 , 5 0 0 , 1 0 00 a n d 50 0 0

C I 2 4 1 5 0

p p m

4 , 8 , 1 6 a n d 2 4

S t a ti c o r d y n a m i c

2 0 , 25 , 3 0 , 5 0 a n d 8 0 ( 2 C )

6 0 . 5


3/19

p H an d [C I - ] an d t h e co r ro s i o n o f A IS I 3 1 6L s t ee l 1811

Prev i o u s ly w e i g h ed co u p o n s w ere i m mers ed i n te s t s o l u ti o n s fo r v a r io u s t i me i n t e rv a l s o f 1 , 2 , 4 an d 6

mo n t h s . Imm ers i o n t e st s u n d e r s t a ti c co n d i t i o n s w ere p e r fo rm ed fo l l o w i n g t h e A ST M G 3 1 -7 2 p ro ced u re .

S i mi l a r t e s ts w e re ca r r i ed o u t u n d e r d y n am i c co n d i t io n s u s in g a t h e rmo s t a t i c s h ak e r m o v i n g a t a s p eed o f

60 osc i l l a t ions min - 1. A t the end o f the t es t per iod s the coup ons w ere t aken ou t , washed in d i s t il l ed water ,

d r i ed an d t h e i r w e i g h ts w e re d c t e rm i n cd .

Fo l l o w i n g th e i mm ers i o n t e s t , m i c ro s tru c t u ra l ex am i n a t i o n o f a ll t h e t e s t s p ec i men s w as ca r r i ed o u t t o

assess the na tu re and ex ten t o f the local i zed a t t ack . Th e shap e , s i ze and dens i ty (d i s t r ibu t ion) o f the p it s

w ere d e t e rm i n ed me t a l l o g rap h i ca l l y u s in g an o p t io n a l m i c ro s co p e an d fo l lo w i n g A S T M G 4 6 -7 6 p ro -

ced u re .

O p en c i r cu i t p o t en t i a l (O CP) w ere meas u red u s i n g 3 1 6 L co u p o n a s a w o rk i n g e l ec t ro d e (W E ) an d

s a t u ra t ed ca l o me l (b e l o w 8 0 C) o r s i l v e r - s i lv e r ch l o r i d e a s a r e fe r en ce e l ec t ro d e . T h e O C P s t u d ie s w crc

carr i ed o u t w i th 100 , 300 , 500 , 1000 and 5(100 ppm ch lor ide so lu t ions o f pH 4 , 7 o r 9 and a t d i f feren t

t emp e ra t u re s . I t to o k 2 4 -4 8 h t o ach i ev e a co n s t an t p o t en t ia l co r r e s p o n d i n g t o o p en c i r cu i t co r ro s i o n

poten t i a l .

Po ten t iodyn am ic po lar i za t ion ex per im ents we re car r i ed ou t us ing a scan ra te o f 0 .1 m V s i

co mmen c i n g a t a p o t en t i a l ab o u t 2 5 0 mV mo re ac t i v e t h an t h e s t ab l e O CP. Be fo re s t a r t i n g t h e

polar i za t ion sca n , the spec ime n in the sam ple ho lder (W E) was l ef t in the ce l l fo r about 1 h to a t ta in a

s teady s t a te which i s shown by a cons tan t po ten t i a l and curren t a t the beg inn ing of the expe r ime nt . Thc

p o t cn t i o d y n am i c ru n s w ere p ro g ram me d s u ch th a t u p o n a t t a in i n g a cu r r en t d en s i ty o f 2 5 / t A cm

2,

the

scan d i rec t ion was reversed and the po ten t i a l s were scanned back to the s t ar t ing po ten t i a l .

Po lar i za t ion res i s t ance m easu rem ents w ere con ducted a t a scan ra te o f 0.1 mV s ~ wi th s tar t ing and

f in a l p o t en t i a ls co r r e s p o n d i n g t o - 2 0 an d + 2 1) mV (O C P) , r e s p ec t iv e l y . T h e max i m u m cu r r en t r an g e w as

0 . 1 / A c m

2

E X P E R I M E N T A L R E S U L T S

W eigh t lo ss s tud ie s

W e i g h t l o s s s t u d i e s s h o w e d t h a t n o p e r c e p t i b l e w e i g h t lo s s o c c u r s d u r i n g

i m m e r s i o n p e r i o d s o f u p t o 6 w e e k s i r r e s p e c t i v e o f [ C 1 - ] , p H a n d d y n a m i c o r s t a t i c

c o n d i t i o n s . E x t r e m e l y l o w w e i g h t l o s se s ( 5 0 - 7 0 k tg c m 2) w e r e n o t e d d u r i n g

i m m e r s i o n t i m e v a r y i n g f r o m 1 6 t o 2 4 w e e k s . T h e w e i g h t l o s se s r e c o r d e d w e r e

h i g h e s t a t p H 4 u n d e r s t a t i c c o n d i t i o n s a n d w e r e l o w e s t a t p H 7 u n d e r d y n a m i c

c o n d i t i o n s .

M e t a l l o g r a p h ic e x a m i n a t i o n s

U n d e r s i m i la r c o n d i t i o n s , a m a x i m u m n u m b e r o f p i t s w e r e f o u n d o n t h e

s p e c i m e n s i m m e r s e d i n s o l u t i o n s o f p H 4 a n d a m i n i m u m o n s p e c i m e n s i m m e r s e d i n

s o l u t i o n s o f p H 7 . U n d e r d y n a m i c c o n d i t i o n s ( w h e n t h e s o l u t i o n s w e r e a g i t a t e d

c o n t i n u o u s l y d u r i n g t h e e n t i r e p e r i o d s o f i m m e r s i o n t e s t ) t h e n u m b e r o f p i t s

o b s e r v e d o n t h e s u r f a c e o f t h e s p e c i m e n w a s s m a l l e r t h a n u n d e r s t a t i c o r s t a g n a n t

c o n d i t i o n s . I n g e n e r a l , t h e n u m b e r a n d d e p t h o f t h e p i ts i n c r e a s e d w i t h in c r e a s i n g

[ C I ] a n d i m m e r s i o n t i m e .

P i t d e p t h m e a s u r e m e n t s

T h e d e p t h o f t h e p i ts o n 3 1 6 L s p e c i m e n s w a s m e a s u r e d m i c r o s c o p i c a l l y . M i n i -

m u m a n d m a x i m u m p i t d e p t h s w e r e m e a s u r e d f o r a p a r t i c u l a r s p e c i m e n . A v e r a g e

d e p t h s w e r e d e t e r m i n e d b y c o n s i d e r i n g t h e d e p t h o f a ll t h e p i ts p r e s e n t . F i g u r e 1

s h o w s p l o t s o f m a x i m u m p i t d e p t h v s [C1 ] a n d d i f f e r i n g p H f o r t h e i m m e r s i o n p e r i o d

o f 4 m o n t h s . T h e p i t d e p t h i n c r e a s e s w i t h in c r e a s i n g [ C I ] . A p a r a b o l i c r e l a t i o n s h i p

a p p e a r s t o e x i s t b e t w e e n p i t d e p t h a n d c h l o r i d e c o n c e n t r a t i o n a s i n d i c a t e d b y t h e

l i n e a r n a t u r e o f p i t d e p t h v s [C 1 ]1/2 p l o t s ( F i g . 2 ) .


4/19

1 8 1 2 A . U . M A L I K e t a l .

5 o o

3 0 0

cL 200

o

o pH 4 S

0 pH 7S

t - b

p H 9 S _ ~

I O 0

0 ~ I I I I I I

4 0 8 0 1 20 1 6 0 2 0 0 Z 4 0 2 8 0

C I - ( p p m )

M a x i m i m p i t d e p t h v s [ C 1 - ] f o r A I S I 3 1 6 L in C l - - c o n t a i n i n g a q u e o u s s o l u t i o n o f

[ c . 1 .

v a r i o u s p H v a l u e s u n d e r s t a t ic ( S ) a n d d y n a m i c ( D ) c o n d i t i o n s . I m m e r s i o n t i m e : 4 m o n t h s .

F i g u r e s 3 a n d 4 s h o w t y p i c a l o p t i c a l m i c r o g r a ph s o f t h e p i t s pr o d u c e d b y

p o t e n t i o d y n a m i c a n o d i c p o l a r i z a t i o n a t t w o d i f f e r e n t [ C I - ] . M o s t o f t h e p i t s a r e

g e n e r a ll y o f s m a ll d i a m e t e r ( < 5 0 ~ m ) .

E lec tr o chem i ca l measu r em en ts

Open c i r c u i t c o r r o s i o n po t en t i a l

F i g u r e s 5 - 7 s h o w s o m e t y p i c a l t i m e v s o p e n

c i r c u i t po t e n t i a l p l o t s f o r 3 1 6 L s t e e l i mme r s e d i n a q u e o u s s o l u t i o n s c o n t a i n i n g

vary ing [C1- ] a t pH 4 , 7 and 9 , a nd a t 30 , 50 and 80C. Th e indu c t ion t im e , t i, f or p i t

F r o . 2 .

5 0 0

[ ] S to t i c J

0 I I I I I I I

3 5 7' 9 11 13 15 17

1

[cl ]~

P l o t o f [ E l - ] 1/2 v s m a x i m u m p i t d e p t h f o r A I S I 3 1 6 L i m m e r s e d i n C l - - c o n t a i n in g

s o l u t i o n s o f

p H 4

u n d e r s t a t ic ( S ) a n d d y n a m i c ( D ) c o n d i t io n s .

3 0 0

"o

2OO

o


5/19

FIG. 3.

FIG. 4.

P h o t o m i c r o g r a p h o f a c r os s s e c ti o n o f A I S I 3 1 6 L s p e ci m e n p i t te d p o t e n t i o -

d y n a m i c a l l y ( [C I ] : 5 0 0 0 p p m , p H 4 ) . x 4 0 0 .

P h o t o m i c r o g r a p h o f a c r o s s s e c t i o n o f A I S I 3 1 6 L s p e c i m e n p i t t e d p o t c n t i o -

dyn amic a l ly ([C1 ] : 1000 ppm , pH 7 , un de r de -a e ra t ed co nd i t i o n) . )


6/19


7/19

E

o

5

(3-

FIG.

5.

p H a n d [ C I - ] a n d t h e c o r r o s io n o f A 1 S I 3 1 6 L s t e e l

2 0 0

100

-100

3 0 0

o 5 0 0

. 1 0 0 0

-2oo ~ 1 L _ _ _ i _ _ J . _ _ ~

0 4 8 12 16 20 24 23 32 36 40 44 48

T i m e ( h )

V a r i a t i o n o f O C P w i t h t i m e f o r A I S I 3 1 6 L i m m e r s e d i n a q u e o u s s o l u t i o n s o f

v a r y i n g

[ C I - ]

( in p p m ) a t

2 0 C ( p H 4 ) .

1815

i n i t ia t i o n h a s b e e n d e t e r m i n e d f r o m t h e p l o t s. A t a n y g i v e n t e m p e r a t u r e , ti i s a l i n e a r

f u n c t i o n o f [ C I- ] a n d c a n b e r e p r e s e n t e d b y t h e r e la t i o n s h i p:

l o g t = C + D l o g [ C I - ] .

B o t h C a n d D c o e f f i c i e n t s a r e t e m p e r a t u r e d e p e n d e n t . F i g u r e 8 s h o w s p l o t s f o r

l o g i n d u c t i o n t i m e f o r pi t i n i t ia t i o n v s l o g [ C I- ] a t p H 4 a n d a t d i f fe r e n t t e m p e r a t u r e s .

F ig u r e 9 s h o w s p l o ts o f l o g in d u c t i o n t i m e v s t e m p e r a t u r e a t t w o [ C l - ] c o n c e n -

t r a ti o n s . T h e p l o t s i n d i c a t e t h a t i n d u c t i o n t i m e f o r pi t i n i t ia t i o n , t d e c r e a s e s w i t h

i n c r e a s i n g [ C I - l ] a n d in c r e a s i n g t e m p e r a t u r e .

E

"6

F1o. 6.

40

O'

- 40

[J 30 C

-8 0 ,,, 50oc

o

80C

-120

- 1 6 0

- 2 0 0 . ~

0 4 8 12 16 20 24 28 32 36 40 44 48

T i m e

( h )

V a r i a ti o n o f O C P w i t h ti m e f o r

A I S I 3 1 6 L

i m m e r s e d i n 10 0 0 p pm

[C1- ]

a q u e o u s

s o l u t i o n s a t v a r i o u s t e m p e r a t u r e s

( p H 4 ) .


8/19

1 8 1 6 A . U . M A L IK etal

8O

- _

/ o 4

~T o pH 9

- 2 0 0

0 4 8 12 16 20 24 28 32 36 40 44 48

T i m e (h)

FIG. 7. Var iat ion of OCP w i t h t i m e f o r A[S[ 316L i m m e r s e d i n 1 00 0 p p m C I - a q u e o u s

s o l u t io n s o f d i f f e r e n t pH v a l u e s a t 20C.

Polar izati on resistance P o l a r i za t i o n m e a s u r e m e n t s w e r e c a r r ie d o u t u s i n g 3 16 L

c o u p o n s i m m e r s e d i n c h l o r i d e - c o n t a i n i n g a q u e o u s s o l u t i o n s u n d e r c o n d i t i o n s o f

v a r y i n g t e m p e r a t u r e , p H a n d [ C I - ] . Ty p i c a l p o l a r iz a t i o n r e s i s t a n c e p lo t s a r e s h o w n

i n F i g s 1 0 - 1 2 . Ta b l e 1 l i s t s t h e c o r r o s i o n r a t e v a l u e s c o m p u t e d f r o m t h e p l o t s . Th e

c o r r o s i o n r a t e v a l u e s a s d e t e r m i n e d f r o m t h e p o l a r i z a t i o n r e s i s t a n c e e x p e r i m e n t s

w e r e b a s e d o n a n o d i c a n d c a t h o d i c T a fe l v a l u e s w h i c h w e r e o b t a i n e d f r o m p r e v i o u s l y

c a r r ie d o u t Ta f e l p l o t r u n s . Th e p o l a r i z a t i o n d a t a p r o v i d e t h e f o l l o w i n g i n f o r m a t i o n

r e g a r d i n g t h e b e h a v i o r o f 3 1 6 L i n c h l o r i d e - c o n t a i n i n g s o l u t i o n s : ( i ) a t p H 4 , t h e

2.0

FIG. 8.

i.8

1.6

1.4

12

~ - 1.0

...I Q8

0.6

0.4

Q2

[] 30 C

z~ 5 0 C

o 80 c

0 I I I

1 2 3

L o g CI -

L o g i n d u c t i o n t i m e f o r p i t i n i t i a t i o n v s l o g [ C I - ] a t v a r i o u s t e m p e r a t u r e s ( p H 4 ) .


9/19

pH and [C1-] and the corrosio n of AISI 316L steel 1817

FIG. 9.

2.0

.J

1.8

1 . 6 -

1 . 4 -

1.2 -

1.0

-

0 . 8 -

0.6- -

0.4 --

0 .2 - -

01

0

1000 ppm

I I I I

20 4O 60 80 100

Tempereture (C)

Log induction ti me r or pit initiatio n vs temperaturc at 100 and 1000 pp m[ Ct ]

(pH4) .

TABLE 1. RE SULT S FROM POLARIZATION RESISTANCE MEASURE-

MENTS

Tem p [Cl ] pH E ..... Corro sion rate

C ppm mV MPY

20 100 4 -1 56 0.104

20 300 4 -1 20 0.116

20 500 4 -14 4 0.119

20 100(I 4 -168 0.135

20 5000 4 -1 85 0.148

30 1000 4 -203 0.143

5(1 1000 4 -127 0.154

80 1000 4 -93 0.295

20 1000 7 -50 0.071

30 1000 7 -15 0 0.051

50 1000 7 -12 2 0.03

80 1000 7 -200 0.169

20 1000 9 -2 31 0.011

30 1000 9 -2 19 0.041

50 1000 9 -238 0.(155

8(1 1000 9 - 287 0.163

25 24153* 7.3 -2 49 0.064

20 1000t 4 -3 43 0.049

20 1000t 7 -2 35 0.041

20 1000+ 9 -238 0.010

* Artificial sea water.

t De-aerated.


10/19

1 8 1 8 A . U . M A L IK

e t a l .

FIG. 10.

- 1 4 5

- 1 5 6

- 1 6 7

- 1 7 8

- 1 8 9

I

- 0 . 0 4 4

100 p p m

1 0 0 0 p p m

. r ,

5 0 0 0 p p m

- 20 0 ' ' ,

- 0 .060 - 0 .028 - 0 .012 0 .00 4

I ( / c A m - 2 )

P o l a r i z a t i o n r e s is t an c e c u r v e s f o r A I S i 3 1 6 L i n a q u e o u s s o l u t i o n s o f v a r i o u s

c h l o r i d e c o n c e n t r a t i o n s a t p H 4 a n d 2 0 C .

FIG. II.

E

uJ

- 8 5

- 1 1 0

- 1 3 5

- -1 6 0

- 1 8

l 8 0 ~ C

.:

l

50 .

:...-

....

2 0 = C

3 0 C

r ~

-2 1( l i i I

- 0 . 1 0 - 0 . 0 6 -0 . 0 2 0 . 0 2 0 . 0 6

I ( # A c m -2 )

P o l a r i z a t i o n r e s is ta n c e c u r v e s fo r A I S [ 3 1 6 L i n 1 0 0 0 p p m C I - s o l u t i o n s o f p H 4

a n d a t d i f f e r e n t te m p e r a t u r e s


11/19

FIG. 12.

0 . 1 6

- 1 4 5

1

- 1 6 3

- 1 8 1

E

w

- 1 9 9

- 2 1 7

- 2 3 5 ~ i

- 0 0 3 0 - 0 . 0 2 2 - 0 . 0 1 4 - 0 . 0 0 6

I ( ,Am 2 )

p H 7

pH4

p H 9

0,002

P o l a r i z a t io n r e s i s ta n c e c u r v e s f o r A I S I 3 1 6 L i n 1 00 0 p p m C l - s o l u t i o n s o f v a r io u s

p H v a l u e s a t 3 0 C .

0 . 1 5 -

0 . 1 4 -

0 . 1 3 -

0 . 1 2 -

0 . 1 1

-

. 1 1 . 4

A

E

o

8

o

(.~

Fro . 13 .

I I I I I I

1 .8 2 , 2 2 . G 3 . 0 3 , 4 3 , 8

L o g C l -

C o r r o s i o n r a t e v s lo g [ C I - ] a t 20 C a n d p H 4 .

p H a n d [ C I - ] a n d t h e c o r r o s i o n o f A I S I 3 1 6 L s t e el 1 81 9


12/19

8 20 A . U . M A LIK e t a l .

0 . 3 0

0 . 2 6

A

> , 0 . 2 2

E

0 . 1 8

o

0.14.

8

~

0 . 1 0

L_

g , -

0.06

0 . 0 2

FIG. 14.

13 p H 4

,a, pH 9

zx

/x

~,

i I I

2 0 4 0 6 0 8 0 1 0 0

T e m p e r e t u r e ( C )

C o r r o s i o n r a t e v s t e m p e r a t u r e a t 2 0 C a n d p H v a l u e s 4 a n d 9 .

F1G. 15.

9 0 0

6 0 0

3 0 O

>

E

, , , 0

- 3 0 0

' 1 ' ' I ' ' 1 ' ' ' ~ . . . . . . . . I ' '~ . .. .

J

,

. - "

. . . . . . . . . . . . . - _ ~ . "

10 -3 10 -2 10 1 100 101 102 103

I ( /~A cm 2)

C y c l i c p o l a r i z a t i o n c u r v e f o r A I S I 3 1 6 L i n 1 0 0 p p m C I - s o l u t i o n o f p H 4 a t 3 0 (


13/19

p H a n d [ C I - ] a n d t h e c o r r o s i o n o f A I S I 3 1 6 L s t ee l 1 82

FIG. 16.

6 0 0

4 0 0

2 0 0

E

U.I

- 200

' " i ' " , l ' " i ' " ' l ' " i ' " ' l ' " 1 ' " 9 ' " i ' " ' i " l '

7

J

- 4 0 0 , , ;, , , l < , : A . . . . . . [ , ~ l l , , , , ,

1 0 - 3 1 0 - 2 1 0 - 1 1 0 0 1 01 1 0 2 1 0 3

I ( / ~A

c m - 2 )

C y c l i c p o l a r i z a t i o n c u r v e f o r A I S 1 3 1 6 L i n 1 0 00 p p m C I - s o l u t i o n o f p H 4 a t

3 0 C .

F I~ . 17 .

E

W

1 5 0

5 0

- 5 0

- 1 5 0

2 5 0

. Z -

- i

a r - '

c

--- _

- 3 5 o . . . . . . J . . . . . i . i

. . . . . . . .

1 0 - 2 1 0 - 1 1 0 0 1 01 1 0 2

I (/.LA cm 2)

C y c l i c p o l a r i z a t i o n c u r v e f o r A I S 3 1 6 L in 1 0 0 0 p p m C I s o l u t i o n o f p H 4 at 8 0 C .


14/19

1822 A . U . MALIKe t a l .

TABLE 2. RESULTS FROM CYCLIC POLARIZATIONSTUDIES

T e m p [ C l - ] p H g p i t g p r o t E p it - gp r ot E r

C ppm mV mV mV mV

20 100 4 759 -1 99 958 - -

20 300 4 502 -82 584 300

20 500 4 438 -100 538 280

20 1000 4 425 -77 502 250

20 5000 4 322 -140 462 200

30 1000 4 260 -62 322 67

50 1000 4 177 -29 206 20

80 1000 4 - 4 -38 34 0

20 1000 7 217 -49 266 233

30 1000 7 294 -4 298 66

50 1000 7 234 -4 8 282 82

80 1000 7 111 -27 138 133

20 1000 9 862 -186 1058 - -

30 1000 9 611 -2 79 990 - -

50 1000 9 389 -27 3 662 - -

80 1000 9 467 -37 5 842 - -

25 24153* 7.3 304 -230 534 - -

20 1000t 4 504 -79 578 200

20 1000t 7 427 -10 7 634 87.5

20 1000t 9 814 -216 1030 67

20 24153* 7.3 304 -230 534 75

* Art i f i c ia l sea water .

t

De -ae r a t e d .

c o r r o s i o n r a t e g e n e r a l l y i n c r e a s e s l i n e a r l y w i t h i n c r e a s i n g [ C I - ] i n t h e r a n g e

1 0 0 - 5 0 0 0 p p m ( F i g . 1 3 ) ; ( i i ) w i t h i n c r e a s i n g p H , t h e c o r r o s i o n r a t e d e c r e a s e s , b e i n g

h i g h e s t a t p H 4 . T h e r a t e s a t p H 7 a n d 9 a r e s i m i l a r i n m a g n i t u d e ; ( i i i ) i n g e n e r a l ,

c o r r o s i o n r a t e i n c r e a s e s w i t h i n c re a s i n g t e m p e r a t u r e b e i n g h i g h e s t a t 8 0 C a n d

l o w e s t a t 3 0 C ( F i g . 1 4 ) .

Cyc l i c po la r i za ti on

S o m e r e p r e s e n t a t i v e c y c l ic p o l a r i z a t i o n c u r v e s f o r 3 1 6 L i n

C l - - c o n t a i n i n g s o l u t i o n s a r e s h o w n i n F i g s 15 - 1 7 . A h y s t e r e si s l o o p is t r a ce d d u r i n g

r e v e r s e s c a n i n d i c a t i n g t h e p o s s i b i l i t y o f p i t ti n g . T a b l e 2 li s ts t h e v a l u e s o f p i t ti n g

p o t e n t i a l ( Ep it ) a n d p r o t e c t i o n p o t e n t i a l ( Ep ro t ), w h i c h a r e d e f i n e d a s t h e p o t e n t i a l

w h e r e t h e f o r w a r d a n d r e v e r s e s c a n s c r o s s . R e p a s s i v a t i n g p i t t i n g p o t e n t i a l ( E r ) i s

a l s o c o n s i d e r e d , w h i c h i s d e f i n e d a s t h e m o s t a c t i v e p o te n t i a l a t w h i c h t h e n u c l e a t i o n

o f u n s t a b l e ( i . e . r e p a s s i v a t i n g ) p i ts c a n o c c u r a n d i s c h a r a c t e r i z e d b y a r e v e r s i b l e

i n c r e a s e i n c u r r en t d e n s i t y ( C D ) . T h e s t a b l e n u c l e a t i o n p o t e n t i a l ( E p i t ) i s a s s e s s e d a s

t h a t a t w h i c h a c o n s i s t e n t i n c r e a s e i n C D o c c u r s , i n d i c a t i n g t h e i n i t i a t i o n o f n o n -

r e p a s s i v a t i n g p i ts . Epr ot i s t h e m o s t a c t i v e p o t e n t i a l a t w h i c h p i t p r o p a g a t i o n c a n

O c c u r .

I n g e n e r a l , p i t ti n g p o t e n t i a l , E pi t i s s h i f t e d t o a m o r e n e g a t i v e ( o r a c t i v e ) v a l u e

w i t h in c r e a s i n g [ C I - ] o r t e m p e r a t u r e .

A t a p a r t i cu l a r t e m p e r a t u r e a n d [ C I - ]

E p i t

s h i ft s to a m o r e n o b l e p o t e n t i a l w i t h

i n c r e a si n g p H . T h e e l e c t r o c h e m i c a l l y m e a s u r e d p i t p o t e n t i a l ,

Epi t

i s f o u n d t o b e a


15/19

p H a n d [ C 1 - ]

a n d t h e c o r r o s i o n

o f A I S I 3 1 6 L

s t e e l

1823

900

8 0 0

7 0 O

6 0 O

E 5 0 0

~

4oo

3OO

2 0 0

1 0 0

FIG. 18.

I I

2 4

L o g C I - ( p p m )

P i t t i n g p o t e n t i a l

v s l o g [ C 1 - ] a t 2 0 C a n d p H 4 .

l in e a r f u n c t i o n o f t h e lo g a r i t h m o f [ C I - ] ( F i g . 1 8 ) . Th is li n e a r d e p e n d e n c e o f

E p i t o n

l o g [ C 1 - ] c a n b e r e p r e s e n t e d a s:

g p i t = A + B log [C1-] .

Bo t h t h e A a n d B c o e f fi c i e n t s a r e t e m p e r a t u r e d e p e n d e n t . F i g u r e 1 9 s h o w s t h e

d e p e n d e n c e o f

E p i t o n

t e m p e r a t u r e a t d i f f er e n t v a l u e s o f p H a t a fi x e d [ C l - ] . A s h i ft

o f E p i t i n t h e n e g a t i v e d i r e c t i o n u s u a l l y o c c u r s a s th e t e m p e r a t u r e i n c r e a s e s .

E

t.u

FIG. 19.

9 0 0

o o

7 0 0

6 0 0 [ ] p H 4

p H 7

5 0 0 o p H 9

4 O 0

2OO

100

0

- 1 0 0 I I I I

o 2 0 4 0 6 0 8 0 ~ o o

Tempereture ( C )

P i t t i n g po t e n t i a l v s t e m p e r a t u r e f o r

A I S I 3 1 6 L in 1 00 0 p p m C I s o l u t i o n o f

v a r i o u s p H v a l u e s a t

3 0 C .


16/19

1 8 2 4 A . U . M A LI K e t a l .

D I S C U S S I O N

The immersion tests carried out on 316L coupons at different [C1-] concentra-

tions, pH and time intervals and under static and dynamic conditions show extremely

low weight losses (10-50/~g in typical 4-month runs). At a particular chloride

concentration and immersion time, the weight losses were more or less independent

of pH and immersion conditions (static or dynamic). The number and depth of the

pits increase with increasing immersion time. The maximum number of pits were

found on specimens immersed in solutions of pH 4. At pH 7 and 9, the pits were

generally shallow and the number was smaller, though some of them were deep.

Under dynamic conditions, the number of pits observed on the surface of the

specimen was much smaller than under stagnant conditions.

In the presence of C l- , austenitic stainless steels are subjected to local attack in

the form of pitting or crevice corrosion due to breakdown of the protective Cr20 3

film at random sites. 6 The pitting on the passive surface has been explained by the

competitive adsorption mechanism 15'16 in which chloride ions move into metal/oxide

film interface at the metal surface. At a particular chloride concentration, a critical

potential E p i t ) develops which is sufficient to displace oxygen from the protective

oxide layer. It appears from the present study that the low pH and stagnancy would

provide most favourable conditions for pit growth. In a typical case, at 30C and 4-5

ppm dissolved oxygen, the pits grow to maxima of 450 and 325 #m at pH 4 under

static and dynamic conditions, respectively, when 316L specimens were immersed in

300 ppm chloride solutions for 4 months. At higher pH values (7 and 9) the depth

rarely exceeded 70/~m. The pit depth has been found to be a parabolic function of

CI- concentration and therefore, with increasing [C1-] the rate of pit growth appears

to slow down.

The electrochemical studies on pitting corrosion were carried out at pH values of

4, 7 and 9. At a particular [C1-] and a given temperature, the corrosion rates were

3 2 0

300

280

260

I:: 24O

I,~ 220

200

180

160

0

FIG. 20.

1 I

2 4 6

Log Cl - (ppm)

R e p a s s i v a t i n g p o t e n ti a l v s l og [ C 1 -] f o r A I S I 3 1 6 L i n a q u e o u s s o l u t i o n o f p H 4

a n d a t 2 0 C .


17/19

p H a n d [ C l - ] a n d t h e c o r r o s i o n o f A I S I 3 1 6 L s t e e l 1 82 5

900

8 0 0

7 0 0

600

E

uj~ 500

4 O O

30O

2O0

20

Flo . 21 .

I I

2 2 2 4 2 6

t i

P i t t in g p o t e n t i a l v s i n d u c t i o n t i m e f o r p it in i t ia t i o n in 1 0 00 p p m C I s o l u t i o n o f p H

4 and a t 20C.

f o u n d t o b e h i g h e s t at p H 4 a n d l o w e s t a t p H 9 . C o n s i d e r i n g t h e e f f e c t o f p H o n

p i t ti n g p o t e n t i a l , E p i , t h e p o t e n t i a l w a s f o u n d t o b e s h i ft in g to m o r e p o s i ti v e v a l u e s

w i t h i n c r e a s i n g p H . T h e c o r r o s i o n r a t e o f 3 1 6 L in a r ti fi c ia l s e a w a t e r ( [ C 1 -] 2 4 1 53

p p m a n d p H = 7 . 3 ) w a s s i m i la r t o t h a t i n 10 00 p p m C I - s o l u ti o n s o f p H 7 a n d 9 b u t

m u c h l o w e r t h a n t h a t i n a s o lu t io n o f p H 4 o f t h e s a m e c o n c e n t r a t i o n . T h e e f f e c t o f

c h l o r i d e c o n c e n t r a t i o n o n t h e s t a b l e p i t n u c l e a t i o n p o t e n t i a l , Epi t is c o n s i s t e n t w i t h

t h a t o b s e r v e d w i t h o t h e r s y s t e m s w h e r e b y t h e p o t e n t i a l v a r ie s l in e a r l y w i t h t h e

l o g a r i t h m o f C l - a c t i v i ty . L e c k i e a n d U h l i g 15 r e p o r t e d a 8 8 m V s h i f t i n t h e c r i ti c a l

n u c l e a t i o n p o t e n t i a l b y a 1 0 - f o l d i n c r e a s e in C I - a c t iv i ty in th e c o n c e n t r a t i o n r a n g e

0 . 0 1 -1 M ( 3 5 0 - 3 5 , 0 0 0 p p m ) . I n th e p r e s e n t s t u d y a s h if t o f a b o u t 2 0 0 - 3 0 0 m V w a s

o b s e r v e d b y a s i m i l a r i n c r e a s e i n t h e c o n c e n t r a t i o n r a n g e 1 0 0 - 50 0 0 p p m a t p H 4 . T h e

r e p a s s i v a t in g p o t e n t i a l , E r a l so v a r i e s l in e a r l y w i t h t h e l o g a r i t h m o f t h e c o n c e n -

t r a t i o n o f c h l o r i d e ( F ig . 2 0 ). H o w e v e r , n o s y s t e m a t i c v a r i a t i o n in p r o t e c t i v e

potent ial ,

E pro t , w a s f o u n d . A t [ C l - ] l e v e l s o f 1 0 0 p p m o r b e l o w n o r e p a s s i v a t i n g

p o t e n t i a l w a s o b s e r v e d . T h e i n d u c t i o n t i m e , ti, f o r p i t i n it i a ti o n m e a s u r e d u n d e r

o p e n c i r c u i t c o n d i t i o n s h a s b e e n f o u n d t o b e a l i n e a r f u n c t i o n o f p i t ti n g p o t e n t i a l ,

E p it , a n d f o l l o w s t h e r e l a t i o n s h i p :

Epi t = A + B log t

w h e r e A a n d B a r e t e m p e r a t u r e d e p e n d e n t c o e f f ic i e nt s ( Fi g. 2 1 ).

A t [ C 1 -] c o n c e n t r a t i o n s a t w h i c h s ta b l e n u c l e a t i o n w a s e v i d e n t ( 1 0 0 - 3 0 ,0 0 0 p p m )

e x t e n s i v e h y s t e r e s i s w a s o b s e r v e d u p o n s c a n r e v e r s a l w i th r e p a s s i v a t i o n o c c u r r i n g in

t h e v i c i ni t y o f th e O C P . T h e p i t t in g p o t e n t i a l , g p i t , w a s i n v a r i a b l y m o r e p o s i t iv e t h a n

t h e p r o t e c t i v e p o t e n t i a l ,

Eprot.

T h e d i f f e r e n c e b e t w e e n p i tt in g a n d p r o t e c t iv e

p o t e n t i a l s d e c r e a s e s w i t h i n c r e a s i n g [ C I - ] a n d is a l i n e a r f u n c t i o n o f lo g a r i t h m o f

[ C 1 -] ( F i g . 2 2 ). T h e o c c u r r e n c e o f p i t i n i ti a t io n p o t e n t i a l s i n th e t r a n s p a s s i v e r a n g e

c a n b e e x p l a i n e d i n te r m s o f t h e l o c a l i z e d b r e a k d o w n o f a s u r f a c e f il m o n 3 1 6 L th a t i s


18/19

1826 A .U . MALIK t a l .

640

600

; 560

5 2 0

~ 480

FIG. 22.

440

4 0 0 I I

0 2 4

L o g C I - ( p p r n )

Difference of pitting potential and protective potential vs log [C I-] at 20C and

pH 4.

s o m e w h a t l es s p r o t e c t i v e ( i. e . s h o w s a h i g h e r r a t e o f g e n e r a l d i s s o l u t i o n ) t h a n t h a t

p r e v a l e n t i n th e p a s s iv e r e g i o n . A t p H 4 , u n d e r s i m i l ar c o n d i ti o n s o f t e m p e r a t u r e a n d

[ C l - ] , t h e r e p a s s i v a t i n g p o t e n t i a l , E r , a p p e a r s a t a m u c h h i g h e r p o s i ti v e p o t e n t i a l

t h a n a t p H 7 . N o r e p a s s i v a t i n g p o t e n t i a l w a s o b s e r v e d a t p H 9 . I t l e a d s t o t h e

c o n c l u s i o n t h a t a t p H 4 , th e n u c l e a t i o n o f r e p a s s i v a t i n g p it s o c c u r s a t o v e r p o t e n t i a l s

f a r b e l o w t h a t r e q u i r e d f o r s t a b l e p it t in g ; r e s u l t in g t h e r e b y in t h e o n s e t o f a d y n a m i c

p r o c e s s o f p it i n it ia t io n a n d r e p a s s iv a t i o n p r i o r t o t h e d e v e l o p m e n t o f p r o p a g a t i n g

p i t s .

C O N C L U S I O N

T h e p i t t i n g b e h a v i o u r o f A I S I 3 1 6 L in c h l o r i d e - c o n t a i n i n g s o l u t io n s i s g r e a t l y

i n f l u e n c e d b y t h e v a r i a t i o n in [ C 1 - ], p H , d i s s o l v e d o x y g e n , t e m p e r a t u r e a n d f l ow

c o n d i ti o n s . T h e e l e c t r o c h e m i c a l p o l a ri z a t io n e x p e r i m e n t s c a r r i e d o u t s h o w t h a t t h e

c o r r o s i o n r a t e , p i t t i n g p o t e n t i a l , E p i t , a n d r e p a s s i v a t i n g p o t e n t i a l , E r , a r e l i n e a r

f u n c t i o n s o f [ C l - ] . T h e i n d u c t i o n t i m e , ti, m e a s u r e d u n d e r o p e n c i r c u it c o n d i t i o n s ,

h a s a l so b e e n f o u n d t o b e a l i n e a r f u n c t i o n o f p i tt i n g p o t e n t i a l , E p i .

A n a n a l ys i s o f th e e x p e r i m e n t a l r e s u lt s in d i c a t e s t h a t in g e n e r a l , l o w p H , h i g h

[ C l - ] a n d s t a g n a n c y a r e t h e m o s t f a v o u r a b l e c o n d i t i o n s f o r i n i ti a t io n a n d p r o p a g a -

t i o n o f p i ts i n A I S I 3 1 6 L s t e e l .

R E F E R E N C E S

1. T. HODGKIESS,A. MACIVER nd P. Y.

CHONG,

D e s a l i n a t i o n 66, 147 (1987).

2. T . HODGKIESS nd N . G.

ARY,

D e s a l i n a t i o n

55,229 (1985).

3. E. H. NEWTON, . O. BIRKETT, . A. HUNTER,E. W. SIEDERand P. G. TOMALIN,R & D Progress

Report No. 278, U.S . Officeof Saline W ater, Arthur D. Little Inc. Report, Cam bridge, MA (1967).

4. J. W . OLDF1ELD nd B. TODD ,D e s a l i n a t i o n 55,261 (1985).

5. W . S. LEE , J. W . OLDFIELD nd B . TODD,D e s a l i n a t i o n 44, 209 (1983).

6. H. H.

UHLIG,

C o r r o si o n H a n d b o o k . W iley, New Y ork (1948).


19/19

p H a n d [ C 1 - ] a n d t h e c o r r o s i o n o f A I S I 3 1 6 L s t e e l 1 82 7

7. M. G. FONTANA, Corros ion Eng ineer ing . M c G r a w H i l l, N e w Y o r k ( 1 9 8 6 ).

8 . A .J . SEDmCKS,Corros ion o f S ta in les s S tee ls . Wi l ey , N ew Y o r k ( 1 9 7 9 ) .

9. R. STACKLE, B. BROWN, J . KRUCER an d A . A6RAWAL (ed s) , L o c a l i z ed Co r r o s i o n , 3 r d ed n , p .2 5 2 .

N A C E , H o u s t o n , T e x a s ( 1 97 4 ).

10. J. R. GALVELE, P a s s iv i t y o f M e t a l s ( eds R. P . FRANKENTnAL an d J . KRUGER), p . 285 . E lec t roc he m

So c . , N ew J e r s ey ( 1 9 7 8 ) .

1 1 . B . B A R OU X , Pas s i v a t i o n an d l o ca l i zed co r r o s i o n o f s t a i n l e s s s t ee l s , i n P a s s i v i t y o f M e t a l s a n d

S em i c o n d u c t o r s ( ed . M . FORMENT), p . 531 . E lsv ier , Am ste rda m (1983) .

12. Z.

SZKLARSKA-SMIALOWSKA,

i t t ing , C orros ion o f Meta ls . N A C E , H o u s t o n ( 1 9 8 6 ) .

13. Z.

SZKLARSKA-SM1ALOWSKA,

orros ion 2 7 ,2 2 3 ( 1 9 7 1 ) .

14. Z.


n d u s t r i a l P r o b l em s T r ea t m en t a n d Co n t r o l T ec h n i q u es . P e r g a m o n

P r e s s , O x f o r d ( t 9 8 7 ) .

15 . H . P . LECKIE and H . H . UHUG, J . e lec trochem. Soc . 113, 1262 (1967) .

16. J . HORVATH an d H . H . U HLIG, J . e lec trochem. Soc . 115 ,791 (1968) .

17. R. NISHMURA, M . ARAKI an d K . KUDO , Corros ion 40 ,465 (1984) .

18. R. NISHMURA, Corros ion 4 3 ,4 8 6 ( 1 9 8 7 ) .

1 9 . J . H . W A N G , C . C . Su an d Z .


orros . Sci . 44,

732 (1988) .

20. H. E. DEVERALL an d J . R . M AURER,M a t er . P er fo r m a n c e 17, 3 (1978) .

21. H. P. HACK, M a t er . P er fo r m a n c e 22 , 24 (1983) .

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