STATISTICAL STUDY OF THE PITTING CORROSION OF THE 8090 ALUMINIUM LITHIUM ALLOY

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STATISTICAL STUDY OF THE PITTING

CORROSION OF THE 8090 ALUMINIUM LITHIUM ALLOY

S. Schnuriger, G. Mankowski, Y. Roques, G. Chatainier, F. Dabosi

To cite this version:

S. Schnuriger, G. Mankowski, Y. Roques, G. Chatainier, F. Dabosi. STATISTICAL STUDY OF THE PITTING CORROSION OF THE 8090 ALUMINIUM LITHIUM ALLOY. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-851-C3-859. �10.1051/jphyscol:19873100�. �jpa-00226534�

JOURNAL DE PHYSIQUE

Colloque C3, supplement au n09, Tome 48, septembre 1987

STATISTICAL STUDY OF T H E PITTING CORROSION OF THE 8090 ALUMINIUM LITHIUM ALLOY

S. SCHNURIGER, G. MANKOWSKI, Y. ROQUES, G. CHATAINIER and F. DABOSI

Ecole Nationale Superieure de Chimie, Laboratoire de MBtallurgie Physique, CNRS-UA 445, 118, route de N a r b o m e , F-31077 Toulouse Cedex, France

ABSTRACT

The p i t t i n g c o r r o s i o n o f t h e 8090 Aluminium L i t h i u m a l l o y has been s t u d i e d i n 0.01 M and 0.1 W Na SO s o l u t i o n s h a v i n g v a r i o u s c h l o r i d e c o n t e n t s . I n t h i s media, p i t s a r e induced i t ?andom p o t e n t i a l s ; t h e r e f o r e a s t a t i s t i c a l s t u d y o f p i t i n d u c t i o n t i m e s i n p o t e n t i o s t a t i c c o n d i t i o n s and p i t t i n g p o t e n t i a l s i n p o t e n t i o d y n a m i c c o n d i t i o n s has been performed.

The i n d u c t i o n t i m e s obey a l o g normal law. I n s o l u t i o n s w i t h h i g h c h l o r i d e c o n t e n t s , t h e d i s t r i b u t i o n s o f p i t t i n g p o t e n t i a l s a r e w e l l d e s c r i b e d b y a normal law, b u t a more and more i m p o r t a n t d i v e r g e n c e f r o m t h i s l a w i s observed i n s o l u t i o n s w i t h low c h l o r i d e c o n c e n t r a t i o n s . T h i s phenomenom r e s u l t s f r o m t h e f a c t t h a t w i t h a p p l i e d p o t e n t i a l i n c r e a s i n g , t h e mean i n d u c t i o n t i m e f i r s t l y decreases t h e n t e n d t o a s t a b l e value. The p i t t i n g p o t e n t i a l s d i s t r i b u t i o n s have been c a l c u l a t e d f r o m t h e e x p e r i m e n t a l p i t i n d u c t i o n t i m e s d i s t r i b u t i o n s . The c a l c u l a t e d d i s t r i b u t i o n s a r e c l o s e t o t h e e x p e r i m e n t a l ones, showing t h a t t h e s c a t t e r i n g o f t h e p i t t i n g p o t e n t i a l s r e s u l t s f r o m t h e s c a t t e r i n g o f t h e p i t i n d u c t i o n t i m e s .

The p o t e n t i o d y n a m i c and p o t e n t i o s t a t i c t e s t s l e a d t o a same c h l o r i d e c o n t e n t t h r e s h o l d under which no p i t appears. I n each s u l p h a t e s o l u t i o n , i n c r e a s i n g t h e c h l o r i d e c o n t e n t l e a d s t o a d e c r e a s i n g o f t h e s c a t t e r i n g o f t h e p i t t i n g p o t e n t i a l s . The mean p i t t i n g p o t e n t i a l v a l u e decreases towards a s t a b l e p i t t i n g p o t e n t i a l v a l u e o b t a i n e d p a r t i c u l a r l y i n NaCl s o l u t i o n s w i t h o u t Na2S04.

INTRODUCTION

By comparison w i t h t r a d i t i o n a l Aluminium a l l o y s , t h e Aluminium L i th i u n a l l o y s show many advantages. Every w e i g h t p e r c e n t o f added L i t h i u m reduces t h e d e n s i t y by 3 % and i n c r e a s e s t h e e l a s t i c modulus by 6 %. However, t h e s e a l l o y s p r e s e n t l o w d u c t i l i t y and l o w f r a c t u r e toughness r e s u l t i n g f r o m t h e p r e c i p i t a t i o n o f t h e o r d e r e d 6 ' phase ( A 1 3 L i ) . The s t r u c t u r a l a s p e c t and t h e mechanical p r o p e r t i e s o f t h e s e new aluminium a l l o y s have been t h e s u b j e c t o f numerous s t u d i e s . On t h e o t h e r hand, t h e i r c o r r o s i o n behaviour i n a g r e s s i v e media was l e s s s t u d i e d . L i t h i u m a d d i t i o n i s u n f a v o u r a b l e on t h e c o r r o s i o n r e s i s t a n c e p o i n t o f view, because L i t h i u m i s a v e r y r e a c t i v e element. f u r t h e r m o r e , p r e c i p i t a t i o n o f t h e 6 (A1Li ) phase on t h e g r a i n boundaries have d e l e t e r i o u s e f f e c t on t h e s t r e s s c o r r o s i o n r e s i s t a n c e (1,2). L i t h i u m a d d i t i o n s may a l s o i n c r e a s e t h e s u s c e p t i b i l i t y t o l o c a l i z e d c o r r o s i o n . blany a u t h o r s ( 3 ) found t h a t L i t h i u n c o n t a i n i n g aluminium a1 l o y s a r e inore s e n s i t i v e t o p i t t i n g c o r r o s i o n t h a n u n a l l o y e d aluminium o r 7050 a1 l o y .

The purpose o f t h i s s t u d y i s t o i n v e s t i g a t e t h e s u s c e p t i b i l i t y o f an

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19873100

J O U R N A L D E P H Y S I Q U E

AlLiCuibIgZr (8090) a1 l o y t o p i t t i n g c o r r o s i o n , u s i n g s t a t i s t i c a l methods i n p o t e n t i o d y n a m i c and p o t e n t i o s t a t i c c o n d i t i o n s .

EXPERIHENTAL

The 8050 a l l o y used i n t h i s i n v e s t i g a t i o n i s s u p p l i e d by Cegedur Pechiney under t h e f o r m o f 13 mm t h i c k p l a t e s , a f t e r T 651 h e a t t r e a t m e n t . I t s chemical c o m p o s i t i o n ( w t % ) i s as f o l l o w s :

2.55 L i - 1.30 Cu - 0.99 i'4g - 0.09 Z r - 0.025 T i - 0.025 Fe - 0.025 S i - b a l . A1 F o r t h e s t a t i s t i c a l t e s t s , t h e specimens were machined i n t o 10 mn d i a m e t e r c y l i n d e r s . Ten o f t h e s e specimens were embedded i n epoxy r e s i n on t h e same h o l d e r . With t h i s s e t t i n g , t h e p o l i s h i n g o f t h e t e n samples i s performed a t t h e same t i n e and t h e sane way. The f i n a l p o l i s h i n g was made w i t h 1000 g r i t S i c paper.

The a g r e s s i v e s o l u t i o n s used f o r t h i s s t u d y a r e 0.01;.1 o r O.li.1 sodium s u l p h a t e s o l u t i o n s w i t h v a r i o u s sodium c h l o r i d e a d d i t i o n s , a t room temperature.

S t a t i s t i c a l s t u d i e s o f p i t t i n g c o r r o s i o n were performed u s i n g a m u l t i c h a n n e l p i t t i n g d e t e c t o r connected t o t h e 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 equipment. As t h e p i t f o r m a t i o n l e a d s t o a sudden i n c r e a s e o f t h e anodic c u r r e n t , t h e p i t s a r e d e t e c t e d on each specimen when t h e c o r r e s p o n d i n g anodic c u r r e n t reaches a t h r e s h o l d v a l u e o f A. S i m u l t a n e o u s l y f o r . t e n specimens, t h e d e t e c t o r a l l o w s t h e determi n a t i o n o f :

- p i t i n d u c t i o n t i m e s , i n p o t e n t i o s t a t i c c o n d i t i o n s . F o r t h i s t y p e o f t e s t s , t h e specimens were f i r s t p r e p o l a r i z e d i n 0.1?4 Na2S04 s o l u t i o n s , f o r 33 minutes a t a p o t e n t i a l h i g h e r t h a n t h e r e p a s s i v a t i o n p o t e n t i a l . Then, c h l o r i d e i o n s were added i n t o t h e s o l u t i o n and t h e p i t d e t e c t i o n was s t a r t e d .

- p i t t i n g g e n e r a t i o n p o t e n t i a l s , i n p o t e n t i o d y n a m i c c o n d i t i o n s . F o r t h e s e t e s t s , c h l o r i d e i o n s were p r e s e n t i n t h e Na2S04 s o l u t i o n a t t h e b e g i n n i n g o f t h e p o l a r i z a t i o n , i n t h e c a t h o d i c range ( - 1000 mV/SCE). The p o t e n t i a l scan r a t e i s 1 V/h.

RESULTS AND DISCUSSION

The p o l a r i z a t i o n c u r v e i n 0.1 M Na2S04 w i t h o u t NaCl shows a p a s s i v e p l a t e a u (i

-

l o - ? -

1 0 - 3 -

- ^{Figure 1} : Potentiodynamic polariza-

tion curves o f the 8090 alloy (scan rate : 500 mV/h).

I I t

,,

- 500 0

potentla1 lmV/SCE)

C h l o r i d e i o n s a d d i t i o n s i n t h i s s o l u t i o n may i n d u c e t h e breakdown o f t h e p a s s i v e f i l m and generate p i t s . The observed p i t g e n e r a t i o n p o t e n t i a1 s have v e r y s c a t t e r e d values, over a 1 V range, as i t can be seen i n f i g u r e 1 ( c u r v e s 2,3,4,5) f o r 2 . 1 0 - ~ i ~ l c h l o r i d e c o n c e n t r a t i o n .

I n NaCl s o l u t i o n s w i t h o u t Na2S04, t h e p i t g e n e r a t i o n p o t e n t i a l (Ep) v a l u e i s r e p r o d u c i b l e ( 4 ) and v a r i e s l i n e a r l y w i t h t h e c h l o r i d e c o n t e n t a c c o r d i n g t o t h e r e l a t i o n :

Ep(mV/SCE) = - 740 - 64 l o g [ ~ l - ] [ 1 1

The b e t t e r r e s i s t a n c e o f t h e o x i d e f i l m i n t h e l e s s a g g r e s s i v e s u l p h a t e s o l u t i o n i s assumed t o be t h e reason f o r t h e p i t g e n e r a t i o n p o t e n t i a l s c a t t e r i n g . Furthermore, t h i s s c a t t e r i n g may r e s u l t f r o m t h e e x i s t e n c e o f p i t i n d u c t i o n t i n e s , themselves a r e s c a t t e r e d t o a l a r g e e x t e n t .

The s t u d i e s i n t h e s e weakly a g g r e s s i v e media a r e i n t e r e s t i n g because t h e i n f l u e n c e o f many parameters on t h e v a r i a t i o n o f p i t i n d u c t i o n t i m e s can be s t u d i e d ; f o r example, one o f t h e parameters i s t h e a p p l i e d p o t e n t i a l i n p o t e n t i o s t a t i c c o n d i t i o n s . B u t f o r such s t u d i e s , i t i s necessary t o use s t a t i s t i c a l methods.

. P i t induction times d i s t r i b u t i o n s

The p i t t i n g p r o b a b i l i t y P ( t ) i s determined by t h e 100(n/N) percentage, where n i s t h e number o f specimens i s i t t e d a t t h e t i m e t, and N t h e t o t a l number o f measures ( i n o u r t e s t s , N = 100). U s i n g a l o g a r i t h m i c s c a l e f o r t h e t i m e and a normal s c a l e f o r t h e p r o b a b i l i t y , t h e d i s t r i b u t i o n i s v e r y w e l l r e p r e s e n t a t e d by a s t r a i g h t l i n e . The f i g u r e 2 g i v e s an example o f p i t i n d u c t i o n t i m e s d i s t r i b u t i o n s .

Figure 2 : P i t induction times d i s t r i b u t i o n s obtained from p o t e n t i o s t a t i c t e s t s a t various applied potentials, i n 0.1M Na2S04 solutions with d i f f e r e n t chloride contents : 1- 0.1M NaCl, + 500 mV/SCE 2- 0.014M NaC1, - 100 mV/SCE 3- 0.03M NaC1, - 400 mV/SCE

The i n d u c t i o n t i n e s obey a l o g normal l a w :

I' :

^[

P ( t ) = - 2] d t [ 2 1

mo 2 o 2

where 0 i s t h e s t a n d a r d d e v i a t i o n ( u n i t : t i m e decade) and tS0 i s t h e mean

JOURNAL DE PHYSIQUE

i n d u c t i o n time, corresponding t o a 0.5 p i t t i n g probabi 1 i t y . F o r each p o t e n t i o s t a t i c t e s t , and f o r any experimental c o n d i t i o n ( i . e , f o r any a p p l i e d p o t e n t i a l , any c h l o r i d e c o n t e n t ,

...

a = 0.187 l o g ( t / ~ ) [ 3 1

where T i s t h e minimum i n d u c t i o n t i m e ; i t s v a l u e o b t a i n e d f o r a = 0, i s c l o s e t o 2 seconds.

The r e l a t i o n between t50 and o a l l o w s us t o use o n l y t50 t o c h a r a c t e r i z e t h e i n d u c t i o n t i n e s d i s t r i b u t i o n s .

T h i s t y p e o f d i s t r i b u t i o n s has a l s o been observed f o r t h e p i t t i n g c o r r o s i o n o f a z i r c o n i u m a l l o y i n c h l o r i d e c o n t a i n i n g aqueous and m e t h a n o l i c s o l u t i o n s (5,6).

F i ure 3 : Variation o f the standard

2-3-

l o 2 lo3 lo4 lo5

Mean induction time ( s )

* I n f l u e n c e o f t h e a p p l i e d p o t e n t i a l

F i g u r e 4 shows t h e t50 parameter v a r i a t i o n w i t h t h e a p p l i e d p o t e n t i a l f o r many c h l o r i d e c o n t e n t s . These c u r v e s show two p o t e n t i a l ranges, w i t h two p i t t i n g c o r r o s i o n behaviour :

- F o r E < -100 mV/SCE : t h e mean i n d u c t i o n t i m e t50 decreases s t e e p l y as t h e a p p l i e d p o t e n t i a l i n c r e a s e s and reaches a minimum value.

- For E > -100 mV/SCE : t50 i n c r e a s e s s l o w l y as t h e a p p l i e d p o t e n t i a l i n c r e a s e s and s t a b i 1 iz e s o v e r a 1 arge range o f p o t e n t i a1 s.

T h i s p l a t e a u may r e s u l t f r o m t h e c o m p e t i t i o n between two processes :

- t h e p i t i n d u c t i o n t i m e decreases as t h e a p p l i e d p o t e n t i a l increases,

- t h e growth o f t h e o x i d e f i l m w i t h t h e a p p l i e d p o t e n t i a l i n c r e a s e s t h e p i t i n d u c t i o n t i m e .

F i g u r e 4 : I n f l u e n c e o f t h e a p p l i e d poten- t i a l on t h e mean i n d u c t i o n t i m e i n O.1M NazSO4 s o l u t i o n s w i t h d i f f e r e n t c h l o r i d e c o n t e n t s :

1 : 4.10-3# NaCl 2 : 1 4 . 1 0 - ~ ~ NaCl 3 : 3 0 . 1 0 - ~ ~ NaCI

.

1-500 1

n . ~ . l . . . . , . . . . I

Applied p o t e n t i a l (mVISCE)

E f f e c t i v e l y , t h e c o n c e n t r a t i o n p r o f i 1 s obtained by Auger E l e c t r o n Spectroscopy show a d i f f e r e n c e i n t h e passive f i l m thickness f o r two samples p o l a r i z e d d u r i n g 30 minutes i n 0.1 M Na2S04, one a t - 100 mV/SCE and t h e o t h e r a t + 700 mV/SCE. The s p u t t e r i n g time t o remove t h e passive f i l m f o r t h e f i r s t sample i s h a l f as much important than f o r t h e o t h e r sample.

* E f f e c t o f t h e c h l o r i d e c o n t e n t

The f i g u r e 4 a l s o shows t h e c h l o r i d e content i n f l u e n c e on t h e t50 v a r i a t i o n f o r many a p p l i e d p o t e n t i a l s . It can be seen t h a t t h e parameter decreases when t h e c h l o r i d e c o n c e n t r a t i o n decreases. A d e t a i l e d study a t + 500 mV/SCE has been performed, i .e. i n t h e p o t e n t i a l range where t50 i s independent o f t h e a p p l i e d p o t e n t i a l . The v a r i a t i o n o f tS0 w i t h t h e c h l o r i d e content i s shown i n F i g u r e 5 and can be described by t h e r e l a t i o n :

tS0 = T + k (C-Co) -1

1 4 1

The c a l c u l a t e d curve i n F i g u r e 5 represents t h e r e l a t i o n [ 4 ] w i t h :

- r = 1.4 S. The minimum i n d u c t i o n time i n t h i s r e l a t i o n i s c l o s e t o t h e value found by t h e r e l a t i o n [ 3 ] .

- Co : the c h l o r i d e content t h r e s h o l d under which no p i t can appear. I t s value i s 1.47 l o m 3 mole/l.

- k a constant, ( k = 2.951).

.

The p i t t i n g p r o b a b i l i t y P ( E ) i s determined by t h e 100 (n/N) percentage where n i s t h e number o f specimens p i t t e d a t t h e p o t e n t i a l E and N t h e t o t a l number o f specimens (N = 100). Using a normal scale f o r t h e p r o b a b i l i t y and an a r i t h m e t i c scale f o r t h e p o t e n t i a l , t h e d i s t r i b u t i o n i s very we1 l represented by a s t r a i g h t l i n e as i t can be seen i n f i g u r e 6 ( c u r v e 1 ) .

C3-856 JOURNAL DE PHYSIQUE

* " " ' " " " """' ' ' " ' " 1

I

m -

- .

Figure 5 : Influence o f the c h l o r i d e

? content on the mean i n d u c t i o n time a t the

i applied p o t e n t i a l of + 500 mV/SCE.

0 : experimental p o i n t s

_. . calculated curve

ul . . ^{. .} . . ^,.'.

10-I Chloride c o n t e n t imole/l)

Figure 6 : P i t t i n g p o t e n t i a l s d i s t r i b u t i o n s obtained from potentiodynamic t e s t s ( 1 V/h) i n 0.1M Na2S04 w i t h various c h l o r i d e contents : 1 : 3 0 . 1 0 - ~ ~ NaCl 2 : 6 . 1 0 - ~ ~ NaCl 3 : 4 . 1 0 - ~ ~ NaCl 4 : 2 . 1 0 ~ ~ ~ MaCl

P i t t i n g p o t e n t i a l s a r e shown t o obey a normal law :

where o i s t h e s t a n d a r d d e v i a t i o n ( u n i t : mV) and E50 t h e mean p i t t i n g p o t e n t i a l c o r r e s p o n d i n g t o a 0.5 p i t t i n g p r o b a b i l i t y .

T h i s t y p e o f d i s t r i b u t i o n s has a l s o been observed f o r s t a i n l e s s s t e e l s ( 7 ) .

T h i s normal law o f d i s t r i b u t i o n i n p o t e n t i o d y n a n i c c o n d i t i o n s i s observed o n l y f o r t h e h i g h e s t c h l o r i d e c o n t e n t , i .e. 3 . 1 0 - ~ A . A more and more i m p o r t a n t d i v e r g e n c e

f r o m t h i s law i s observed when t h e c h l o r i d e c o n t e n t decreases as i t can be seen i n F i g u r e 5 ( c u r v e s 2, 3, 4 ) . T h i s phenomenon i s r e l a t e d t o t h e s t a b i l i z a t i o n o f t h e mean i n d u c t i o n t i n e , when t h e a p p l i e d p o t e n t i a l s a r e h i g h e r t h a n - 100 mV/SCE, i n p o t e n t i o s t a t i c node. I t i s p o s s i b l e f r o n t h e d a t a o b t a i n e d i n p o t e n t i o s t a t i c c o n d i t i o n s t o c a l c u l a t e t h e d i s t r i o u t i o n o f p i t g e n e r a t i o n p o t e n t i a l s u s i n g d i f f e r e n t mathematical r e 1 a t i ons :

. F o r t h e p o t e n t i o d y n a m i c t e s t s , t h e a p p l i e d p o t e n t i a l depends on t h e scan r a t e , a c c o r d i n g t o t h e r e l a t i o n :

i - = c ^{t t x v}

0 [ 6 1

where : - E

0 i s t h e a p p l i e d p o t e n t i a l , under which p i t s a r e n o t induced. From f i g u r e 4, to i s c l o s e t o - 500 mV/SCE.

- v i s t h e scan r a t e ; I n o u r case, v = 1 V/h.

- t i s t h e t i m e i n seconds, s i n c e t h e p o t e n t i a l Eo.

i r o n t h e r e l a t i o n [ 6 1 , dE/dt = v

~ h & e f o r e , t h e f o l l o w i n g r e l a t i o n can be w r i t t e n : dP(E) - d P ( t ) . dt - 1 dP(t)

- dE d t dE - v ' d t

dP0 can be c a l c u l a t e d f r o m t h e r e l a t i o n s [ 2 ] and [ 3 ] . d t

B u t t h e c a l c u l a t i o n o f t h e p i t t i n g p o t e n t i a l d i s t r i b u t i o n s , i .e. - d::E) , f r o m t h e d a t a o b t a i n e d i n p o t e n t i o s t a t i c node i s p o s s i b l e i f t h e t50 parameter i s d e s c r i b e d i n r e l a t i o n w i t h t h e p o t e n t i a l . T h i s can be done by u s i n g t h e r e l a t i o n s h i p t50 = ( E a p p l i e d ⁾ ( f i g u r e 4 ) f o r each c h l o r i d e c o n c e n t r a t i o n . The two p o t e n t i a l ranges o f t h e s e e x p e r i m e n t a l curves a r e m a t h e m a t i c a l l y d e s c r i b e d by t h e f o l l o w i n g r e l a t i o n s :

- F o r t h e 3 . 1 0 - ~ 1.1 NaCl : E < - 300 mV/SCE

I

- F o r t h e 4 . 1 0 - ~ 1.1 NaCl : E < - 100 mV/SCE

I

I t can be concluded t h a t t h e p i t g e n e r a t i o n p o t e n t i a l s s c a t t e r i n g depends d i r e c t l y on t h e p i t i n d u c t i o n t i m e s s c a t t e r i n g .

* I n f l u e n c e o f t h e c h l o r i d e c o n c e n t r a t i o n on t h e p i t t i n g p o t e n t i a l

I n NaCl s o l u t i o n s w i t h o u t Na2S04, t h e p i t g e n e r a t i o n p o t e n t i a l s a r e f o u n d t o be r e p r o d u c i b l e and dependent on t h e c h l o r i d e c o n t e n t , as i t can be shown by t h e

C3-858 JOURNAL DE PHYSIQUE

- M

-

I.' X

.-

'

T A m ' d calculated ( A 1 , B ' ) .Z p i t t i n g p o t e n t i a l s d i s t r i bu- OZ .- t i o n s f o r two c h l o r i d e

2 contents, i n 0.1M Na2S04

a

- ^I A,AS : 3 0 . 1 0 - ~ ~ NaCl B,BV : 4 . 1 0 - ~ ~ NaCl

r e l a t i o n [I]. It can be seen i n F i g u r e 8, ( c u r v e 1 ) . I n s o l u t i o n s c o n t a i n i n g Na2S04, when t h e c h l o r i d e c o n c e n t r a t i o n i n c r e a s e s t h e p i t t i n g p o t e n t i a l s s c a t t e r i n g decreases, i . e . t h e s t a n d a r d d e v i a t i o n decreases and t e n d t o a n u l l value. Then, t h e mean p i t t i n g p o t e n t i a l v a l u e decreases towards t h e s t a b l e v a l u e o b t a i n e d i n NaCl s o l u t i o n s w i t h o u t Na2S04.

The f i g u r e 8 shows t h e o v e r a l l e f f e c t o f c h l o r i d e and s u l p h a t e c o n c e n t r a t i o n s on t h e p i t t i n g p o t e n t i a l , o b t a i n e d by p o t e n t i o d y n a n i c t e s t s . I t can be seen t h e e x i s - t e n c e o f a c h l o r i d e c o n t e n t t h r e s h o l d under w h i c h no p i t appears. I n 0.1;1 Na2S04, t h i s t h r e s h o l d i s c l o s e t o t h e one o b t a i n e d i n p o t e n t i o s t a t i c mode ( F i g u r e 8, c u r v e 3 and F i g u r e 5 ) . The i n c r e a s e o f t h e Na2S04 c o n c e n t r a t i o n l e a d s t o an i n c r e a s e o f t h i s t h r e s h o l d value, which was f o u n d t o be 1.5 IO-~;*I and 5.10-~!,1 r e s p e c t i v e l y f o r 0.1;? and O.0li.l s u l p h a t e s o l u t i o n s .

r"-*

-":

+I000 -

:

- mean p i t t i n g p o t e n t i a l and the p i t gene-

- r a t i o n p o t e n t i a l i n NaCl without Na2S04

- (scan r a t e 1 V/h).

- 1 : without Na2S04 . 2 : 0.01M Na2S04

- ¹ 3 : 0.1M Na2S04

u~ .... ^l . . . ( * . . . I . ^{1 .,...*I} . ^, . ( . . . . I . . ^l.lL,,,

10-2 10'' Chloride content (mole/l)

CONCLUSIONS

- I n c h l o r i d e c o n t a i n i n g Na2S04 s o l u t i o n s , p i t i n d u c t i o n t i n e s obey a l o g normal l a w and p i t g e n e r a t i o n p o t e n t i a l s obey a normal law.

- I n p o t e n t i o s t a t i c c o n d i t i o n s , t h e mean i n d u c t i o n t i n e remains c o n s t a n t o v e r a l a r g e range o f p o t e n t i a l s , when t h e a p p l i e d p o t e n t i a l i n c r e a s e s beyond t h e v a l u e - 100 mV/SCE.

- I n p o t e n t i o d y n a m i c c o n d i t i o n s , i t appears, w i t h t h e c h l o r i d e c o n t e n t decreasing, a more and more i m p o r t a n t d i v e r g e n c e f r o m t h e normal d i s t r i b u t i o n o f t h e p i t g e n e r a t i o n p o t e n t i a l s . T h i s r e s u l t i s r e l a t e d t o t h e i n v a r i a n c e o f tS0 w i t h t h e a p p l i e d p o t e n t i a l . The c a l c u l a t e d p i t t i n g p o t e n t i a l d i s t r i b u t i o n s u s i n g t h e e x p e r i m e n t a l d a t a o b t a i n e d i n p o t e n t i o s t a t i c c o n d i t i o n s show a c l o s e r e l a t i o n s h i p between t h e p i t t i n g p o t e n t i a1 s s c a t t e r i n g and t h e i n d u c t i o n t i m e s s c a t t e r i n g .

- The mean p i t i n d u c t i o n t i m e and t h e mean p i t t i n g p o t e n t i a l depends on t h e c h l o r i d e c o n c e n t r a t i o n . I n 0.1M Na2S04 s o l u t i o n s , t h e p o t e n t i o d y n a m i c and p o t e n t i o s t a t i c t e s t s show t h e same c h l o r i d e c o n t e n t t h r e s h o l d , under which no p i t appears. T h i s t h r e s h o l d v a l u e decreases when t h e s u l p h a t e c o n t e n t decreases. F o r each s u l p h a t e c o n t e n t and w i t h t h e c h l o r i d e c o n t e n t i n c r e a s i n g t h e p i t t i n g p o t e n t i a l s c a t t e r i n g decreases and t h e mean p i t t i n g p o t e n t i a l v a l u e s t e n d t o t h e s t a b l e v a l u e s Ep o b t a i n e d i n NaCl s o l u t i o n s w i t h o u t Na2S04.

REFERENCES

1. T.H.Sanders J r , P.W.Niskanen, Res. Mechanica L e t t e r s , 1 (1981 1, p. 370.

2. P. Niskanen, T.H. Sanders Jr., J. G. Rinker, H. Marek, C o r r o s i o n Science, v o l . 22, 4 (19821, p. 283.

3. R.E. D i c k e r , D.J. Duquette, Proceeding o f t h e 2" d l - L i conference, Ed. T.H.

Sanders J r . , E.A.Starke J r . , The tvletal l u r g i c a l S o c i e t y o f AIME, Warrendale, (19831, p. 581.

4. S. S c h n u r i g e r , G. Mankowski, Y. Roques, G. C h a t a i n i e r , F. Dabosi, P r e s e n t e d a t t h e l o t h I n t e r n a t i o n a l Congress on m e t a l 1 i c c o r r o s i o n , Madras, Nov. 1987.

5. G. Mankowski, P. E y g a z i e r , Y. Roques, G. C h a t a i n i e r , F. Dabosi, P a s s i v i t y o f m e t a l s and semi conductors, Ed. M. Froment, E l s e v i e r , Amsterdam, 1983, p. 299.

6. Y.Roqlres, G.Mankowski, G.Chatainier, F.Dabosi, E l e c t r o c h e m i c a l Methods i n C o r r o s i o n Research, Ed. M. Duprat, N a t e r i a l Science Forum, vo1.8 (1986), p.105.

7. T. S h i b a t a , T. Takeyama, C o r r o s i o n , 33, 7 (19771, p. 243.