filmes oxidos 2.pdf

8/11/2019 filmes oxidos 2.pdf

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JO U RN A L O F M A T E RI A L S SCI E N CE L E T T E RS 2 1983) 304- - 306

P o t e n t i a l d e p e n d e n c e o f o x i d e f i l m c o m p o s i t i o n o n F e C r N i

a ll o y s in e l e v a t e d t e m p e r a t u r e s o lu t io n s

W . B O G A E R T S A . V A N H A U T E

Univers ity o f Leuven KU Leuven) , E lectrochemistry and Corrosion Lab oratory, de Croylaan 2,

B-3030 Hev erlee, Belgium

The u nde rs t and i ng of t he pa ss iv i ty and cor ros i on

behavi our o f me t a l s a t e l eva t ed t empe ra t ure s be -

c o m e s m o r e a n d m o r e i m p o r t a n t s in c e o p e r a t i o n

t empe ra t ure s have been ra i sed i n many i ndus t r i a l

processes . Espec ia lly in the chem ica l indu st r ies and

power gene ra t i ng p l an t s many a t t empt s have been

made t o unde rs t and and so l ve cor ros i on prob l ems

encount e red i n h i gh t empe ra t ure and h i gh pre s sure

aqueous envi ronment s .

U n f o r t u n a t e l y , n u m e r o u s e x p e r i m e n t a l d i f f i -

cu l ti e s have i mp eded a de t a i l ed s t udy o f t he mech-

anisms and e lec t ro chem ica l reac t ions cont rol l ing

t he com pos i t i on and s t ruc t ure o f anodi c passi ve

f il ms unde r t he se c i rcums t ances .

Some prev i ous i nves t i ga t i ons conce rn i ng t he

e l e ct r o c h e m i ca l b e h a v i o u r o f d i f f e re n t F e - C r - N i

a ll o ys i n d i c a te d t h e p r o b a b l e i m p o r t a n t e x t e n t o f

se l ec t i ve d i s so l u t i on and enr i chment phenomena

of individual a l loying e lements a t var ious e lec t rode

pot en t i a l s i n h i gh t empe ra t ure aqueous so l u t i ons

[1 ,2] . No s t ud i e s seem, howeve r , t o have been

und e r t aken t o a s se ss t he i n f l uence o f t he app l i ed

p o t e n t i a l o n t h e p r o p e r t i e s a n d c o m p o s i t i o n o f

ox i de f i lms form ed a t d i f fe ren t temp e ra t ure s . Onl y

sporadic informat ion i s ava i lable in the l i t e ra ture .

Some re su l t s o f Scanni ng Auge r M i c roprobe i n -

ves t i ga t i ons have , fo r i ns t ance , been publ i shed by

the present authors for s ta inless s tee l AISI 304 a t

a f i xed po t en t i a l and a t d i f fe ren t t emp e ra t ure s

ranging from 65 to 275 C [3] .

These resul ts rev ea led, for ins tance , Cr-deple t ion

processes in the passive fi lm and possible Ni-

e n r i c h m e n t p h e n o m e n a i n t h e d e e p e r o x i d e l a y er s

o r a t t h e o x i d e - m e t a l i n t e rf a c e .

In t h i s work we repor t more de t a i l s about t he

i nf l uence of t he e l ec t rode po t en t i a l and t empe ra -

t u r e o n t h e pa ss iv e f il m c o m p o s i t io n f o r F e - C r -

Ni a l loys , such as Type 304 (18Cr-8Ni) s ta inless

s te e l, I n c o l o y 8 0 0 ( 2 0 C r - 3 0 N i ) a n d I n c o n e l 6 0 0

(16 C r -75 N i ) i n a l kaf ine ch lor i de -cont a i n i ng sol u-

t ions .

Sampl e s have been t r ea t ed a t d i f fe ren t con-

t ro l l ed anod i c po t en t i a ls i n an au t oc l ave sys t em a t

t empe ra t ure s rang i ng f rom 65 t o 275 C . Deoxy -

gena t ed t e s t so l u t i ons , con t a i n i ng va r ious amoun t s

o f C 1- ( 4 7 5 t o 3 5 5 0 p p m ) t o g e t h e r w i t h H C O ~

and/ o r O H- (0 .1 M ) we re used . A f t e r exposu re ,

t he sampl e s we re t r ans fe r red t o a Scanni ng Auge r

Microprobe (SAM) wi th sub-micron spa t ia l reso-

hi t iono Depth prof i les or surface di s t r ibut ions of

di ffe rent a l loying or envi ronmenta l spec ies in the

sur face ox i de l aye r we re de t e rm i ned . M ore de t ai l s

about t he measur i ng i ns t rument a t i on and t he

adopt ed expe r i ment a l p rocedure have been de s -

c r i bed e l sewhe re [1 ,3 ] .

At l ower t empe ra t ure s on l y mi nor re l a t i ve

e n r i c h m e n t p h e n o m e n a o f d i f f e r e n t a l l o y i n g

cons t i t uen t s have been fou nd i n t he pa ss ive po t en-

t ia l range . Typica l resul t s show ra ther smal l oxide

thicknesses and an ini t i a l pass ive f i lm composi t ion

whi ch i s g rea t l y i n f l uenced by prev i ous sur face

t r e a t m e n t p r o c e d u r e s a n d th e b u l k c o m p o s i t i o n o f

the a l loy.

At somewha t more e l eva t ed t empe ra t ure s (e . g .

150 C ) i nc reas i ng en r i chment s o f chrom i um and

nicke l , due to a se lect ive di ssolut ion of i ron, occ ur

a t e lec t rode potent ia l s in the pr imary pass ive

e lec t ro de-p oten t ia l range . S t rong t ranspass ive di s-

so l u t i on beh avi our o f chrom i um cause s , how eve r ,

a sha rp dec rea se of t he chromi um cont en t and a

cor re spondi ng re l at ive n i cke l enr i chm ent a t h i ghe r

potent ia l va lues . Secondary pass iva t ion phen-

omena may be obse rved i n some of t he t e s t so l u-

t i ons ( ra t he r l ow C 1- concent ra t i ons and / or

h i ghe r pH) and may be pr i mar i l y a sc r i bed t o a

favourab l e anodi c behavi our o f n i cke l unde r t he se

expe r i ment a l condi t i ons .

Wi th increas ing tempera ture , the t ranspass ive

3 0 4 0 2 6 1 - 8 0 2 8 / 8 3 0 3 .0 0 . 12 ] 983 Chapman and Hail Ltd.


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800 , i nc one l 600 a nd pure i ron a nd n i c ke l . The

d i f fe re n t po t e n t i a l s a t whi c h pa ss i va t i on t r e a t -

me n t s a nd subse q ue n t A ES a na l yse s ha ve be e n c a r -

r ied out , a re indica ted a t the le f t .

I n t h e lo w e r p o t e n t i a l r an ge ( e . g . - - 3 0 0 m V ) ,

s t rong e nr i c hme nt s o f c hromi um a nd n i c ke l a re

found . Th i s i s i n a gre e me nt wi t h t he h i gh a nodi c

d i s so l u t i on c ur re n t o f i ron unde r t he p re se n t

e xpe r i me n t a l c ondi t i ons (c f . F i g. 2 : pure i ron) . At

e le c tr o d e p o te n ti al s h ig h er t h a n a b o u t - 1 0 0 m V

t ra nspa ss i v i t y o f c hromi um a nd a c or re spondi ng

de pl e t i on o f t h i s e l e me nt oc c urs . Th i s poss i b l y

resul t s in a genera l ized pass ivi ty breakdown i f

e xpe r i me nt a l c ondi t i ons a re unfa voura b l e fo r

es tabl i shing secondary pass ivi ty . Secondary pass-

i v i ty i s ma i n l y gov e rne d by t he be ha v i ou r o f n i c ke l

a nd i s f a voure d by l owe r C l - -c onc e n t ra t i ons a nd

higher va lues of solut io n pH (e .g. 10.5) .

T h e s i tu a t io n m i g h t , h o w e v e r , b e c o m e m o r e

c ompl i c a t e d be c a use o f t he a dd i t i ona l s e l e c t i ve

d i s so l u t i on p roc e sse s o f n i c ke l ( e . g . a bout - -200

mV), resul t ing in an increased anodic reac t ivi ty .

Thi s is pa r t i c u la r l y a ppa re n t f o r i nc one l 60 0 (F i g .

2) and o f course for pure N i -samples . Th e combi-

na t i on o f t he se p roc e sse s mi gh t p ro v i de a n e xp l a-

na t i on fo r ma ny a spe ct s o f t he c o r ros i on be ha v i ou r

o f F e - C r - N i a ll oy s i n t h e p r e se n t t y p e o f s olu -

t ions .

eferences

1. W. BOGAERTS A. VAN HAU TE M.J. BRABERS

and P. VANSLEMBROUCK Research Sympo sium

Corrosion 8 2, Ho uston (1982) p.81.

2. D. D. MACDONALD B. C. SYRET T and S. S. WING

Corrosion

35 (1979) 1.

3. W. BOGAERTS A. VAN HAUTE and M.J.

BRABERS

Proceedings of the 8th International

Congress on Metallic Corrosion, Mainz (1981) p.31.

Received 29 December 982

and accepted 6 January 983

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