Corrosion of Stainless Steel in Chloride Solution

7/25/2019 Corrosion of Stainless Steel in Chloride Solution

1/18

Appl . Phy s . A 38 , 1 -18 (1985)

A p p l i e d

Physics

urfaces

9 Springer-V erlag 1985

C o r r o s i o n o f S t a in l e ss S t e el s in C h l o r id e

A n X P S I n v e s t ig a t i o n o f P a s s iv e F i lm s

P. Brf iesch and K. Mfi l l e r

B ro wn -B o v e r i R e s e a rc h C e n t e r , C H-5 4 0 5 B a d e n , S wi t z e r l a n d

A. At rens

Unive rs i ty o f Que ens land , S t . Lucia , Quee ns land , 4067 Aus t ra l i a

H . N e f f

No r t h C a ro l i n a S t a t e Un i v e r s i ty , R a le i g h, N C 27 65 0, U S A

Rece ived 13 Feb rua ry 1985 /Accep ted 12 June 1985

S o l u t i o n

Abstract Five commerc ia l s tee l s rang ing f rom the mar tens i t i c s ta in less s tee l con ta in ing

1 2 c h ro m i u m t o t h e s u p e r fe r r it e c o n t a i n in g 2 9 c h ro m i u m , 4 m o l y b d e n u m , a n d 2

n i c k el h a v e b e e n s t u d i e d b y XP S . In a d d i t io n , a p u re i r o n -c h ro m i u m a l l oy c o n t a i n in g 7

c h ro m i u m h a s b e e n in v e s ti g a te d . Arm c o i ro n a n d p u re c h ro m i u m (9 9 .9 9 ) we re i n c l u d e d

a s r e fe r en c e s. T h e fo rm a t i o n o f th e p a s s i v e fi lm s (o r c o r ro s i o n )o c c u r r e d i n d e o x y g e n a t e d

0 .1 M NaC1 so lu t io n (pH = 5 .6 ) , f rom w hich the samples w ere trans fer red d i rec t ly to the X PS

c h a m b e r u n d e r c o n t ro l l e d a t m o s p h e re (Ar ). C o n c e n t r a t i o n p ro fi l es (a t .- ) o f t h e a l l o y

cons t i tue n ts in the i r ox id ized and meta l l i c s ta tes have been de te rm ined se para te ly f rom the

m e a s u re d X P S d e p t h p rof il e s. F o r c > 1 2 c h ro m i u m t h e p as s i v e f il m s h a v e th e fo l lo wi n g

s t ruc tu re : there i s a dep le t ion o f Cr in the inner reg ion , fo l lowed by an en r ichm en t

(conc en t ra t ion max imum ) in the cen t ra l reg ion o f the f ilms. The he igh t o f th i s ma x im um

increases , and i t s pos i t ion sh i f t s towards the su rface wi th increas ing ch romium con ten t in

t h e a l l o y . T h e o u t e rm o s t m o n o l a y e r s a r e r i c h i n wa t e r a n d h y d ro x y l g ro u p s . Va r i o u s

s ign if ican t p rope r t i es o f the f i lms change d ras t i ca l ly a t the c r i ti ca l ch rom ium conc en t ra t io n

o f a b o u t 1 2 . T h is b e h a v i o u r i s r a t h e r i n d e p e n d e n t o f t h e o t h e r c o m p o n e n t s (M o , N i , C u )

p resen t in the a l loys and i s d i scussed in t e rms o f a phase t rans i t ion in the f i lms which i s

c o n t ro l l e d b y t h e c h ro m i u m c o n c e n t r a t i o n .

PACS: 68.45.v , 81.60.Bn

De ta i led inves t iga t ions o f b lade fa i lu res in low-

p re s s u re s t e a m t u rb i n e s t a g e s h a v e i n d i c a t e d t h a t t h e

b re a k d o w n o f p a s s i v it y i s a c r it i ca l s t e p i n t h e d a m a g e

process [1 ] . As a f i r s t s t ep in unders tand ing pass iv i ty

b reakdown, i t i s necessary to ga in in s igh t in to the

s t ru c t u re a n d c h e m i c a l c o m p o s i t i o n o f t h e p a s s iv e

f i lms which fo rm on re levan t t echn ica l a l loys under

cond i t ions chosen to s imula te those found in p rac t ice .

T h e r e l e v a n t e n v i ro n m e n t w i t h i n t h e l o w-p re s s u re

s team tu rb ine s tages i s the Wi lson zone where f i r s t

c o n d e n s a t i o n o c c u rs . T h e e x a c t c o m p o s i t i o n o f t h e

c o r ro s i v e m e d i u m i s s o m e wh a t d i f f i c u l t t o m e a s u re ,

bu t i t i s cer ta in tha t w e are dea l ing w i th a deaera ted

c h l o r id e s o l u t i o n ( t e m p e ra t u re r a n g in g b e t we e n 5 0 ~

a n d 1 0 0 ~ T h e b l a d e a l l oy s i n m o s t c o m m o n u s e a t

p resen t a re essen t ia l ly mar ten s i t i c s ta inless s tee ls con-

t a i ni n g 1 2 c h ro m i u m , a l t h o u g h p re c i p i ta t i o n -h a r -

dened mar te f i s i t ic s ta in less s tee ls and dup lex s ta in less

s tee l s have a l so been used .

T h i s p a p e r r e p o r t s a n X P S i n v e s t i g a t io n o f c o m m e rc i a l

a l lo y s . T h e y r a n g e f ro m 1 2 - c h ro m i u m m a r t e n s i ti c

s ta in less s tee l to the superfer r i t e con ta in ing 29 Cr ,


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2 P. Briiesch et al.

4 % M o , a n d 2 % Ni . I n a d d i t i o n , a p u re i r o n -

c h r o m i u m a l lo y c o n t ai n in g 7 % c h r o m i u m h a s b e e n

i n v e s t i g a t e d . As e n d m e m b e r s , p u re Arm c o i ro n a n d

p u re c h ro m i u m h a v e a l s o b e e n s t u d i e d . I n o rd e r t o

s imula te the cond i t ions found in p rac t ice , the su rface

f i l m s wh i c h we re s t u d i e d b y XP S h a v e b e e n fo rm e d

u n d e r o p e n -c i r c u i t c o n d i t io n s a n d n o t u n d e r p o t e n t i o -

s t a ti c c o n t ro l a n d t h e c h l o r i d e e l e c t ro l y t e wa s d e a e ra -

t e d . F ro m XP S m e a s u re m e n t s a l t e rn a t i n g w i t h Ar - i o n

s p u t t e r i n g , d e p t h p ro f i l e s h a v e b e e n o b t a i n e d fo r t h e

m a i n a l l o y i n g e l e m e n t s i n b o t h t h e i r m e t a l l i c a n d

ox id ized s ta tes , as wel l as fo r oxygen . From a semi-

q u a n t i t a t i v e a n a l y s i s a b s o l u t e c o n c e n t r a t i o n s i n a t . -%

h a v e b e e n d e r i v ed . T h e d i s t o r t io n s o f t h e d e p t h p ro f il e s

o r i g i n a ti n g f ro m t h e f i n it e s a m p l in g d e p t h o f XP S h a v e

b e e n c o r r e c te d . B y t h is p ro c e d u re w e o b t a i n e d d i f-

f e r e n ti a l c o n c e n t r a t i o n s wh i c h a p p ro x i m a t e l y r e fl e ct

t h e t ru e c o n c e n t r a t i o n s a s a fu n c t i o n o f d e p t h . T o o u r

k n o w l e d g e o n l y i n t e g ra l c o n c e n t r a t i o n s o f t h e e l e-

m e n t s i n t h e p a s s i v e f i l m s h a v e b e e n o b t a i n e d i n t h e

l i te r a t ur e . S u c h i n t e g ra l c o n c e n t r a t i o n s c o m p r i s e

c o n t r i b u t i o n s f ro m a l l l e v e l s b e l o w t h e s u r f a c e ; t h e

p re s e n c e o f st e e p c o n c e n t r a t i o n g ra d i e n t s i n t h e p a s -

s ive fi lms impl ies tha t dep th p ro f i l es der ived o n the bas i s

o f i n t e g ra l c o n c e n t r a t i o n s a r e c o n s i d e ra b l y

d is to r ted .

Ad d i t i o n a l i n fo rm a t i o n a b o u t t h e c h e m i c a l n a t u re o f

t h e p a s s i v e f i l m s h a s b e e n o b t a i n e d b y a n g l e -

d e p e n d e n t XP S m e a s u re m e n t s . I n c o n t r a s t t o i o n

spu t te r ing , th is meth od i s nond es t ruc t iv e and serves to

check the re l i ab i l ity o f the resu l t s ob ta ine d by ion

s p u t te r i n g . T h e c o m b i n a t i o n o f t h e t wo m e t h o d s

y ie lds , no t on ly a de ta i l ed charac te r iza t ion o f the

su rface f ilms bu t a l so a c o rre la t ion o f f i lm p roper t i es as

a fu n c t i o n o f c h ro m i u m c o n t e n t i n t h e d i ff e r en t

al loys.

The pap er i s o rgan ized as fo l lows : Sect . 1 descr ibes the

exper imen ta l p rocedure , inc lud ing mater ia l s , e lec t ro -

c h e m i ca l m e a s u re m e n t s , X P S m e a s u r e m e n t s a n d d a t a

reduct ion . In Sec t s. 2 and 3 we p resen t the resu l t s o f the

Table 1. Designations and structures of materials studied

No . Com merc ia l Our

designation designa-

tion

1 Armco iron Fe

1A 7Cr

2 X2 0CrM oV1 2 12CrlMo

3 X 7 C r N i C u N b5 6 2 15Cr6Ni

4 X3C rNiM ol7 13 3 17Crl3Ni

5 X4CrMnNiMoN2646 26Cr6Mn

6 29-4-2 29Cr4Mo

7 99.99 Cr Cr

e l e c t ro c h e m i c a l a n d XP S m e a s u re m e n t s , r e s pe c t iv e l y.

The las t sec t ion i s devo ted to a d i scuss ion o f the

e x p e r i m e n t a l r e s u lt s i n t h e l i g h t o f p re s e n t k n o w l e d g e

from re la ted s tud ies o f pass ive f ilms. Spec ia l a t t en t io n

is g iven to the d ras t i c changes o f var ious f i lm p ro -

p e r t i e s o b s e rv e d n e a r t h e c r i t i c a l c h ro m i u m c o n -

c e n t r a t i o n o f a b o u t 1 3 a t . -% ( a b o u t 1 2 w t . -% ) , a n d

these su rface p roper t i es a re co rre la ted wi th the e lec t ro -

chem ica l p roPer t i es o f the a l loys .

1 E x p e r i m e n t a l

1 1 Ma ter ia ls

Tab le 1 con ta ins de s igna t ions and s t ruc tu res o f the

mater ia l s s tud ied .

T h e c o m p o s i t i o n s o f t h e s a m p l e s g i v e n in T a b l e 2 h a v e

b e e n d e t e rm i n e d b y o p t i c a l e m i s s i o n s p e c t ro s c o p y ,

x -ray f luo rescence and wet -chemica l ana lys i s . The

a l loys were s tud ied in a hea t - t rea ted cond i t ion cons i s -

ten t wi th no rma l use . The su rfaces were po l i shed in the

m a n n e r u s u a l l y p r a c t i c e d i n m e t a l l o g ra p h y . T h i s i n -

vo lves po l i sh ing them wi th ever f iner po l i sh ing

mater ia l , t e rm ina t ing wi th a lum ina o f l es s than 0 .3 gm

size and a mirror-qual i ty surface fin ish .

1 2 Electrochemical Setu p

T h e e l e c t ro ch e m i c a l s e t u p wa s u s e d ,

( i ) to fo rm f i lms fo r XPS s tud ies under open-c i rcu i t

cond i t ions ,

( ii ) t o m e a s u re t h e c o r r e s p o n d i n g o p e n -c i r c u it p o t e n -

t i a ls as fu nc t ions o f t ime,

( i i i ) to measure cu rren t -po ten t ia l cu rves .

The e lec t rochemica l se tup has been descr ibed in de ta i l

in a recen t pub l ica t ion [2 ] . He re we sha l l b r ie f ly g ive

the essen t ia l s (F ig . 1 ). The e lec t rochem ica l p rep ara t io n

cha mb er i s a ho r izon ta l g lass cy l inder d i rec t ly f l anged

t o t h e f a s t i n s e r t io n l o c k o f t h e KR AT OS E S 3 0 0 XP S

s p e c t ro m e t e r . T wo p o r t s a l l o w g a s a n d e l e c t ro l y t e

Structure

ferrite

ferrite

martensite

precipitation-

P H

hardened

austenite

duplex

ferrite

ferrite


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Corr osion o f Stainless Steels in Chloride Solution

Table 2. Wt.- (upper lines) and at.- (lower lines) of elements contained in the materials studied

No. Fe Cr Mo Ni Cu Mn V Nb C Si

1 99.85

1A 93.00 7.00 . . . . . . . .

92.52 7.48 . . . . . . . .

2 85.52 11.9 0.92 0.66 - 0.48 0.28 - 0.24

84.36 12.60 0.53 0.62 - 0.48 0.30 - 1.09 -

3 75.28 14.8 1.65 5.2 1.53 0.8 - 0.23 0.06 0.45

74.86 15.81 0.95 4.92 1.34 0.81 - 0.14 0.27 0.89

4 65.98 17.8 2.3 11,5 - 1.7 0.09 0.05 0.029 0.55

65.67 19.03 1.33 10,89 - 1.72 0.10 0.03 0.13 1.09

5 62.04 25.9 2.22 42 - 5.6 - - 0.039 -

61.42 27.53 1.27 3.95 - 5.63 - - 0.18 -

6 64.24 29.0 3.6 2,44 - 0,07 0.12 0.05 0.005 0.48

63.58 30.83 2.07 2.30 - 0,07 0.13 0.03 0.02 0.95

7 - 99.99 . . . . . . . .

99.99 . . . . . . . .

I N S E R T I O N L O C K

E L E C T R O C H E M I C A L

P R E P A R A T I O N C H A M B E R

A u x i l i a r y P o r t

S a m p l e h o l d e r

t o U H V - P r e p a r a t i o n

C h a m b e r a n d

P h o t o e l e c t r o n -

S p e c t r o m e t e r

P l a t i n u m

C o u n t e r - E l e c t r o d e

G l a s s T u b e ~ _ ~

E l e c t r o d e -

E l e c t r o l y t e

C o n t a c t

g E l e c t r o d e

R e f e r e n c e - E l e c t r o d e

S a t . C a l o m e l - E l e c t ro d e )

t o P o t e n t i o s t a t

IIIIIIIIIIIIIIIIII

Fig. 1. Schematic diagram of the electrochemical preparation chamber in combination with the fast insertion lock


4/18

4 P. Brfiesch eta l.

s o l u t i o n t o b e a d m i t t e d . T h e w o rk i n g e l e c t ro d e is

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

m o v e d h o r i z o n t a l l y f ro m t h e a t m o s p h e re t h ro u g h t h e

p r e p a r a t io n c h a m b e r i n t o t h e v a c u u m s y s te m ( U H V

p re p a ra t i o n , c l e a n - s p u t t e r i n g ) a n d b a c k t o t h e c h a m -

b e r . A g l a s s t u b e m o u n t e d w i t h i ts o p e n i n g b e n e a t h t h e

e l e c t ro d e h o l d s a c e r t a i n v o l u m e o f t h e e l e c t ro ly t e

(0 .1 M N a C 1 , p H a b o u t 5 . 6 , r o o m t e m p e ra t u r e , m a d e

w i t h B A K E R o rg a n i c - f re e w a t er ) . Th i s v o l u m e c a n b e

l i f ted so tha t the me n iscus o f the liqu id con ta c t s the

e lec t rode when des i red . The e lec t ro ly te i s deaera ted in

a d v a n c e b y a rg o n b u b b l i n g . A s l i gh t a rg o n o v e rp re s -

s u re i n t h e c h a m b e r k e e p s o x y g e n o u t d u r i n g t h e

e l e c t ro c h e m i c a l m e a s u re m e n t s a n d f i l m - fo rm in g e x p o -

su re . Sa id g lass tube a l so ho lds the re fe rence e lec t rod e

wi th Lugg in cap i l l a ry . The coun tere lec t rode i s a wi re

(P t ) w o u n d a ro u n d t h e Lu g g i n c a p i l l a ry ' s t h i c k p o r -

t io n . I n v i e w o f t h e v e ry s m a l l v o l u m e o f e l e c tro l y t e

u s e d , t h e d e a e ra t e d c o n d i t i o n m a y n o t m e e t t h e

m o s t r i g o ro u s c r i t e r i a b u t i s e x p e c t e d t o b e c o n s i s t e n t

wi th cond i t ions in the Wi l son zone .

Th e t h r e e e l e c t ro d e s w e re c o n n e c t e d t o a P A R 1 23

p o t e n t i o s t a t t o r e c o rd d y n a m i c c u r r e n t -p o t e n t i a l

c u rv e s. O p e n -c i r c u i t p o t e n t ia l s (O C P ) w e re m o n i t o r e d

v i a t h e s a m e s y s t e m a n d r e c o rd e d a s fu n c t io n s o f t im e ,

u p t o p e r i o d s o f 5 t o 1 0 k s .

A l l e l e c t ro c h e m i c a l m e a s u re m e n t s w e re p e r fo rm e d

a f t e r su r f a c e p r e p a ra t i o n o f th e m e c h a n i c a l l y p o l i s h e d

e l e c t ro d e s a m p l e s b y a rg o n - i o n s p u t t e r i n g i n U H V

(10ra in , 4k eV , 12 gA /cm 2 , inc idence ang le 45~ The

d y n a m i c c u r r e n t -p o t e n t i a l c u rv e s w e re r e c o rd e d w i t h a

s c a n r a t e o f 1 0 0 m V / s . Th i s u n u s u a l l y h i g h s p e e d w a s

se lec ted in o rder n o t to m ask the e f fec t o f the p re t rea t -

m e n t b y g ro w t h o f s e c o n d a ry p a s s i v a t i o n f il m s. I n t h e

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

i n g c o n c e n t r a t i o n s o f m e t a l i o n s i n t h e e l e c t ro l y t e

o r ig ina t ing f rom the ac t ive-d i s so lu t ion reg ion . Repea t

c u rv e s w e re a l w a y s r e c o rd e d a f t e r r e p e a t e d c l e a n -

s p u t t e r in g i n U H V . B y c o n t i n u o u s c y c li n g s t e a d y - s t a te

c u r r e n t -p o t e n t i a l c u rv e s c o u l d b e o b t a i n e d . A l l sw e e p s

w e re s t a r t e d f ro m t h e c a t h o d i c r e g i o n . Th e p o t e n -

t i o s t a t w a s s e t c l o s e t o t h e h y d ro g e n e v o l u t i o n p o t e n t i a l

befo re the c i rcu i t was c losed by the su rg ing e lec t ro ly te .

Th e s w e e p w a s t r ig g e re d o n e o r t w o s e c o n d s a f t e r

t h e m e n i s c u s t o u c h e d t h e e l e c t ro d e .

Th e r e s u l ts o b t a i n e d i n t h e s e p o t e n t i o d y n a m i c c u r r e n t

m e a s u re m e n t s w e re c h e c k e d i n s e p a ra t e e x p e r i m e n t s .

A m u c h l a rg e r v o l u m e o f t h e d e a e ra t e d e l e c t ro l y t e w a s

u s e d . Th e e l e c t ro d e s w e re ro t a t i n g d i s k s p r e t r e a t e d

m e c h a n i c a l l y a n d e l e c tro c h e m i c a ll y . Th e c u rv e s w e re

r e c o rd e d a t 1 0 0 m V / s , a s b e fo re , a n d a t t h e m u c h

s l o w e r s c a n r a t e s o f 20 a n d 2 m V / s . W i t h o u t d i s c us s in g

these resu l t s in fu r ther de ta i l a t th i s po in t , we wish to

po in t ou t tha t , ( i) a t 100 mV /s the shape o f a l l cu rves

(Fe , Cr , and s ix a l loys ) i s es sen t ia l ly the same as

repo r ted here fo r the spu t te r ed samples , ( ii ) a t s lower

scan ra tes , es sen t ia l new fea tu res do no t a r i se in the

fo rw ard scans ( there a re cases o f cons ide rab le hys te r -

es i s in the reverse scans fo l lowing t ranspass ive d i s so -

l u t i o n w h i c h a r e n o t r e l e v a n t t o t h e p r e s e n t w o rk ) , a n d

( ii i) mo s t consp icuo us ly , the typ ica l fea tu res o f pass iv -

a t ion cu rves , v iz ., a reg ion o f h igh cu rren t s (ac t ive

d i s s ol u t io n ) f o l l o w e d , b e y o n d a F l a d e p o t e n t ia l , b y a

reg ion o f d ras t i ca l ly lowe r cu rren t s , a re n o t ev iden t in

the cu rves reco rded in ch lo r ide so lu t ions ; sec t ions

where the charac te r i s t i c i s nega t ive a re e i ther to ta l ly

absen t o r qu i te in s ign i f i can t . Hence a l l d i scuss ion

base d on the resu l t s o f F ig . 9 wi l l be unaffec ted by the

u n c o n v e n t i o n a l p ro c e d u re a d o p t e d , b y n e c e s s i t y , i n

the ele ctroch em ical cel l of Fig . 1 .

P r i o r t o t h e X P S m e a s u re m e n t s t h e e i gh t s a m p l e s w e re

e x p o s e d t o t h e e l e c t ro l y t e fo r a p e r i o d o f 1 5 h . Th e

c o n d i t i o n s w e re e x a c t l y th e s a m e a s d e s c r i b e d a b o v e ,

w h e n m e a s u r i n g o p e n -c i r c u i t p o t e n t i a ls ; t h e e l e c t ro d e s

were t ru ly a t open c i rcu i t to s imula te p rac t i ca l con-

d i t ions . Af te r the exposure t ime the e lec t ro ly te was

s e p a ra t e d f ro m t h e e l e c t ro d e a n d t h e l a t t e r w a s

c a re fu l l y r i n s e d w i t h d e a e ra t e d B A K ER w a t e r w h i l e

keep ing the e lec t rochemica l ce l l under a rgon a tmo-

sphere . The e lec t rode wi th the su r face f i lm was then

t r a n s f e r r e d d i r e c t l y i n t o t h e X P S s p e c t ro m e t e r b y

us ing the fas t in ser t ion lock . Res idua l water rem ain ing

o n t h e e l e c t ro d e , a n d t h e a rg o n i n t ro d u c e d f ro m t h e

e lec t rochem ica l ce ll , were pu mp ed o f f a f t e r t rans fer in

t h e U H V s y s t e m .

1 .3 . X P S M e a s u r e me n t s a n d Da t a R e d u c ti o n

X P S m e a s u r e m e n t s w e re ca r r ie d o u t i n a K R A T O S E S

3 0 0 E l e c t ro n S p e c t ro m e t e r i n t h e F A T m o d e o f

ope rat ion using M gK~I,2 exci tat ion (1253.6 eV). The

b a s e p r e s s u re w a s a fe w ti m e s 1 0 -9 t o r t . B i n d i n g

energ ies o f e lec t rons were de te rm ined us ing a go ld

s t a n d a rd ; t h e o v e ra l l r e s o l u t i o n w a s a b o u t 1 . 3 e V .

S a m p l e s h a v i n g a s u r f a c e a r e a o f 5 1 6 m m 2 a n d a

t h i ck n e s s o f 2 r a m w e re u s e d. D e p t h p ro fi l es w e re

o b t a i n e d b y s p u t t e r i n g w i t h a rg o n i o n s (4 k e V ,

12 jxA/cm 2, angle of inciden ce 45~ Th e sput ter rate

depends on the compos i t ion and wi l l , in genera l ,

chang e w i th dep th . As wi ll be shown b e low, the pass ive

f i lms on the a l loys to a l a rge ex ten t cons i s t o f Cr203

W e t h e re fore d e t e rm i n e d t h e s p u t t e r r a t e o f C r2 0 3 b y

u s i ng a f i l m o f k n o w n t h i c k ne s s p r e p a re d b y r e a c t iv e

spu t te r ing in a Balzers r f /dc mach ine . T he th ickness o f

t h e fi lm h a s b e e n m e a s u re d b y a n A l p h a -S t e p T e n c o n

i n s t ru m e n t . Th e s p u t t e r r a t e w a s a b o u t 3 0 A / m i n ; a

s i m i l a r s p u t t e r r a t e h a s b e e n o b t a i n e d fo r F e 2 0 3 .

Th e a b s o l u t e c o n c e n t r a t i o n s ei z) o f a t o m s i a t d e p t h z

f ro m t h e s u r f a c e h a v e b e e n e v a l u a t e d b y m e a n s o f

a rg o n - i o n s p u t t e r i n g a l t e rn a t i n g w i t h X P S m e a s u re -


5/18

Corrosion of Stainless Steels in Chloride Solution 5

meri ts . B y defini t ion

c , ( z ) = n ~ ( ~ )

Z j n j ( z )

(1)

wh e re

n i ( z )

i s t h e n u m b e r d e n s i t y (n u m b e r o f a t o m s p e r

u n i t v o l u m e ) o f a t o m s o f t y p e i a t a d e p t h z a n d t h e s u m

ex tends o ver a l l types o f a tom s in the sam ple . I t i s

shown in the Append ix tha t under cer ta in s impl i fy ing

a p p ro x i m a t i o n s

n i ( z )

i s g iven by

h i z ) : R i X z ) -

2 , ,

d g ~ s z )

a T ( 2 )

wh e re

R i s ( z ) = n ~ 1 7 6 (3)

He re I~ i s the in tens i ty o f pho to e lec t ro ns o r ig ina t ing

f ro m a t o m s o f ty p e i i n s t a t e s o f t h e p u re e l e m e n t i, a n d

I ~ ( z ) i s the co rrespond ing in tens i ty f rom the sample

after sp utter ing off a laye r of th ickness z. 2 ,~ is a m ean

escape d ep th (spa t ia l average) and n o i s the num ber

dens i ty o f a toms i in the pu re e lemen t i ; n ~ 1 7 6

wh e re z ~ i s th e n u m b e r o f a t o m s i p e r u n i t v o l u m e v o .

C h o o s i n g vo = 100 A we hav e n~ = 8 .549, nor = 8 .372,

n ~ a n d n ~ F o r o x y g e n w e p u t

n ~ wh i c h c l o se l y c o r r e s p o n d s t o t h e n u m b e r

d e n s i t y o f o x y g e n i o n s i n C r2 0 3 o r F e 2 0 3. T h e

in tens it i es I~ fo r the meta l s have be en de te rmin ed f rom

the o bserv ed in tens i t ies o f the a l loys a f te r su ff ic ien t ly

long argon- ion spu t te r ing , un t i l sa tu ra t ion va lues o f

the in tensi t i es were a t t a ined , an d by normal iz ing these

in tens it i es to 100% of the co rres pond ing m eta l . In th is

c o n n e c t i o n i t s h o u l d b e m e n t i o n e d t h a t a p u re F e -C r

a l l o y d o e s n o t u n d e rg o p re f e r e n t i a l s p u t t e r i n g wh e n

s p u t t e r e d w i t h Ar i o n s [3 ] .

Assum ing a me an spu t te r ra te f such tha t z = g t wh ere t

i s the sp u t te r t imel (2 ) can a l so be w r i t t en in the fo rm

n , ( t ) = R i , ( t ) - e i , R i ~ ( t ) ,

(4)

wh e re

R i ~ ( t ) = n ~ 1 7 6

a n d

f i ~ = X i j f .

T h e e s c a p e

d e p t h s fo r N i 2 p , F e 2 p , C r2 p , O l s , a n d M o 3 d a r e

est imated to be 13, 15, 16, 17, and 19/k , respect ively

[4 , 5 ] . Fo r the p rac t ica l ca lcu la t ions we have assumed

a m e a n e s c a p e d e p t h 2 -= 1 5 A fo r a ll 2'~ . I n o u r

e x p e ri m e n ts ~ - 0 . 5 A / s g i v in g i , ~ 3 0 s . T h e s e c o nd

term in (2 ) and (4) co rrec t s fo r the def o rm at ion o f the

dep th p ro f i l e assoc ia ted w i th the f in i te sampl ing de p th

[6 ] (2 -ef fec t) . In X PS 95% of the to ta l ph o toe lec t ro n

pea k in tens i ty o r ig ina tes f rom a layer o f dep th

6 = 3 2 - s i n O , wh e re O i s t h e t a k e -o f f a n g l e o f t h e

e lec t rons ; in ou r case O = 90 ~ and f i = 45 A. N eg lec t ing

t h e s e c o n d te rm o n e o b t a i n s i n te g ra l c o n c e n tr a t i o n s ,

a s m e n t i o n e d i n t h e In t ro d u c t i o n . A t t = 0 th e s l o p e

/~(0) in (4) usua l ly is not w ell defined. F or th is re aso n

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

Cr20 ~ Cr

2 0 s

6 0 s

120s

l ~ 3 0 0 s

~ ' q ~ 6 0 0 s

6 7 0 6 7 5 6 8 0 6 8 5

Ek i n (e V )

Fig. 2. Evolution of the C r 2 p 3 / 2 XPS line of sample 2 as a

function of sputtering time

the su rface, the dep th p ro f i l es shown in F igs . 10 to 14

a r e un c e r ta i n f o r t < 1 0 s o r f o r z < 5 , a , .

C o n c e n t r a t i o n p ro fi le s o f F e , C r , M o , N i , a n d o x y g e n

h a v e b e e n d e t e rm i n e d b y o b s e rv i n g t h e i n t en s it i es o f

the Fe2p , Cr2p , M o3d , Ni2p , and O ls l ines. F igure 2

fo r in s tance sho ws the e vo lu t ion o f the Cr2p3 2 l ine o f

3+

sample 2 as a func t ion o f spu t te r ing time. On ly Cr i s

o b s e r v ed a t t = 0 a n d t = 2 0 s ; t h e s t ru c t u re t h en

gradual ly changes , and a f te r 600s the spec t rum i s

d o m i n a t e d b y m e t al l ic c h ro m i u m . An o t h e r e x a m p l e is

g i v en i n F i g . 3 wh i c h s h o w s t h e s p e c t r a o f th e M o 3 d -

and Ni2 p l ines o f sample 4 . These sp ec t ra wi l l be

d iscussed in Sec t . 3 . The eva lua t ion o f the in tens i t ies

i n v o lv e s b a c k g ro u n d s u b t r a c t io n , [ 7 ] d e c o n v o l u t i o n

of the com pos i te s t ruc tu res in to the i r meta l l i c and

o x i d ic c o m p o n e n t s , a n d m e a s u re m e n t s o f t h e a r e a s o f

t h e i n d i v i d u a l c o m p o n e n t s . F i g u re 4 fo r i n s t a n c e

shows the deco nvo lu ted spec t ra o f the Fe2p3 /2 ,

Cr2p3 /2 , and O l s peak s o f samp le 6 . In v iew o f poss ib le

spu t te r - ind uced ef fect s ( see be low) , the spec t ra re -

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

more qua l i t a t ive manner . The Fe2pa/2 s t ruc tu re , fo r

e x a m p l e , wa s d e c o m p o s e d o n l y i n t o t h e m e t a l l i c

c o m p o n e n t F e a n d a s in g le o x i d ic c o m p o n e n t

Fe ox = Fe 2 + + Fe a + .

In o rd e r t o o b t a i n s o m e q u a l i ta t i v e i n fo rm a t i o n a b o u t

d e p t h p ro f i le s w i t h o u t a rg o n - i o n s p u t t e ri n g w e h a v e

me asured the XPS in tens it i es o f the Fe2p , Cr2p , and

O l s l ines as func t ions o f the e lec t ron emiss ion ang le O.

F i g u re 5 fo r in s t a n c e s h o ws t h e a n g u l a r v a r i a t i o n o f

the Fe2pa/2 l ine of sam ple 6 . The s ignificance of these


6/18

6 P . B r i ie sch e t a l .

a M o 6 M o

f . . . . u o ~ u d f . . . . . . .

0 s

2 0 s

/

1 01 5 t 0 2 0 10 2 5 1 0 3 0

E k i n e V )

b N i ~ p

O s

2 0 s

6 0 s

1 8 0 s

5 9 5 4 0 0 4 0 5

E k i n e V )

F i g . 3 a a n d b . E v o l u t i o n o f ( a ) t h e M 0 3 d , a n d ( b ) t h e N i 2 p X P S

lines as a function of sputtering time

spec t ra wi l l be d i scussed in Sect s. 3 and 4 . At th i s po in t

we u s e t h e o b s e rv e d a n g l e -d e p e n d e n t i n te n s it ie s I ~ , ( O )

in o rde r to che ck the re l i ab i l i ty o f dep th p ro f i l es c i ( z )

o b t a i n e d b y a rg o n - i o n s p u tt e r in g . T h e p ro c e d u re

invo lves ca lcu la t ing the I ~ ( 0 ) f ro m t h e c o n c e n t r a t i o n s

c ~(z ) a s i n p u t d a t a , a n d c o m p a r i n g t h e I ~ , ( O ) c , ~ wi t h

Is(O)obs- S ince in (1) the to ta l num ber dens i ty

N = E j n j ( z ) i s near ly indep ende n t o fz (as can be ver i f i ed

numerica l ly ) we have

I ~ ( 0 ) ~ g K ~ s ( O ) o~ q ( z ) e - ~ /x i~ s in O d z (5)

o

F o rm i n g t h e r a t i o

I ~ (6)

R i ( O ) ~ = I ~ + I . ~ ( 0 ) '

wh e re I ~ i s the in tens i ty observed fo r the to ta l

o x i d e (C r 3 +, F e 2 + + F e 3 + ) a n d I . ~ ( 0 ) i s the in tens i ty

o b s e rv e d fo r t h e m e t a l (C r, F e ) we o b t a i n

c ~ e - ~ z/ z s i nO d z

R i ( O ) ~ , z ~ = o , (7)

c ~(z) e -~ : s i ~

F e 2 p 3 / 2 a

5 4 0 5 4 5 5 5 0

6 7 5 6 8 0

7 2 0 7 2 5

F: k in EV )

Fig . 4 . De con vo l u t io n o f the Fe2p3 /2, C r2p3 /2 , and O ls X PS l ines

of sample 6 before sputtering

Fe s* Fe a+ Fe

90*

6 0 *

4 5 *

5 0

t 5

5 4 O 5 4 5 5 5 0

E k i n e V )

F i g . 5 . A n g u l a r v a r i a t i o n o f t h e F e 2p 3 /2 X P S l i n e o f sa m p l e 6

b e f o r e s p u t t e r i n g

wh e re q ( z ) = c ~ + c m ( z ). I t s h o u l d b e m e n t i o n e d t h a t

Ri (O)obs i s independ en t o f spu t te r - indu ced ef fect s

because the in tens i t i es in (6 ) have been measured

befo re spu t te r ing ; on the o ther hand , R i ( O ) c , l c is

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

F i g u re 6 s h o ws a c o m p a r i s o n b e t we e n t h e R i(O)o bs a n d

Ri(O)~alc- The agre em ent is sat isfac tory. I t sh ould be

men t ioned , ho weve r , tha t l ess sa t i s fac to ry resu l ts have

been ob ta ine d espec ia l ly fo r the i ron p ro f i l es in the case

o f very th in f ilms. The dev ia t ions resu l t ing in these

cases a re re la ted to e f fect s such as p referen t ia l re duc-


7/18

Corro s ion of Sta in less Steels in Chlor ide Solut ion 7

R O)

4 O

0 . 9

0 . 8

O . i

R o)

t . 0

0 . 9 -

0 . 8 -

0 . 7

0

i o x I I I I I

i ~ m

l l I I l I

I I I I I I

I I I I I

1 5 3 0 4 5 6 0 7 5 9 0 0 0

Fig. 6. Com par ison o f experimenta l (x , o) and ca lcula ted ( - - )

R O)

values for the Fe2p3n and Cr2pa/2 XPS l ines sam ples 1 ,

2, and 6 (see text)

t i o n o f ir o n b y a rg o n - i o n s p u t te r i n g, n o n u n i fo rm f il m

th ickness and /o r spu t te r - ind uced io n mix ing [8 ] . All

these e f fec t s wi l l be espec ia l ly impO rtan t in very th in

f il m s. T h a t s o m e r e d u c t i o n o f i r o n o x i d e b y a rg o n - i o n

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

F ra n k e n t h a l [ 3 ] , wh o a l s o s h o we d th a t f o r C r2 0 3 t h e

reduct ion i s s ign i f ican t ly l ess than fo r Fe203 . The

in f luence o f spu t te r ing on the com pos i t io n o f the f ilms

is further d iscussed in Sect . 4 .1 .

2 E l e c t r o c h e m i s t r y

T h e s u r f ac e fi lm s s t u d i e d in t h e p r e s e n t wo rk b y X P S

we re p ro d u c e d a t o p e n c i r cu i t. F o r t h i s r e a s o n o p e n -

c i r cu i t p o t e n t ia l s (O C P ) o f t h e s a m p l e s we re m e a s u re d

under iden t ica l cond i t ions . Th is was done in o rder to

s i m u l a t e t h e c o n d i t i o n s e x i s t in g i n t h e W i l s o n z o n e a n d

a l s o i n o rd e r t o o b t a i n t h e d a t a r e q u i r e d fo r c o m -

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

o t h e r wo rk e r s a t f i l m s p ro d u c e d u n d e r p o t e n t i o s t a t i c

con t ro l .

T h e OC P - t i m e c u rv e s s h o w s p o n t a n e o u s p a s s i v a t i o n

fo r t h e c h ro m i u m -c o n t a i n i n g s a m p l e s . T h e v a l u e s o f

po ten t ia l and the t rends in the po ten t ia l - t ime cu rves

p rov id e a measu re fo r nob i l i ty (stab i l ity aga ins t co rro -

sive act ion) of a g iven al loy.

P o t e n t i o d y n a m i c c u rv e s we re r e c o rd e d i n o rd e r t o

have a f ram e o f re fe rence (ac t ive /pass ive reg ion) fo r the

open-c i rcu i t po ten t ia l s . The abso lu te va lues o f the

c u r r e n t s m e a s u re d c a n b e t a k e n a s a m e a s u re o f

reac t iv i ty (e lec t rochemica l d i s so lu t ion ) . I t shou ld be

p o i n t e d o u t t h a t t h e d o u b l e - l a y e r ch a rg in g c u r r e n t s i n

o u r s y s t e m s a r e o f t h e o rd e r o f 5 g A .

I I I I I I I

0

1 o o ~

-zoo ~ |

~ ' ~ ~ A 7

r

- 4 0 0 ~ 2

Cr

IMo

~ ' ~ I ~ I ~5 c , O N i

5 o o i - . / l " I 7 c ~ 1 5 N i

- ~ - / 5 126

C r

6Mn

% s 2 9

r

4 M o

-~ooF


8/18

8 P. Brfiesch et a l .

0

1 0 0

2 o o

3 0 0

>

E

... 40 0

o - 5 0 0

6 0 0

7 0 0

t e I I I

;A c ~

I

12 Cr tMo

I

__ t5 Cr 6 N i ++ J

4 1 7 C r , 3 N i i + ( ~ + + ~ / -

5 2 6 C r 6 M n i . . - ~

2 9 C r 4 M o I + + ~ / J ~ 7 ~

t 0 t t 02 t 03 t 04

T I M E

/

s

a

~/PA/r

~ t 5 0 0

[ / / . .

/ / - ; 5 0 0 / . . -

. . T f ~ 5 0 0 i 5 0 0 t 0 0 0 t 5 0 0

U / m Y v s S C E

5 0 0

b

i p A c m : 1 5 0 0

t 0 0 0

/(ig) = ~

5 0 0 i

- t O . . . . . . . . . . . . . J

. ~ ~ . . ~ : 5 6 o o 5 0 0 1 o oo t 5 o o

7

U / m V v s S C E

5 0 0

F i g . 9 a a n d b . P o t e n t i o d y n a m i c c u r r e n t - p o t e n t i a l ( v s s c e) c u r v e s

fo r samples 1 to 7 in 0 .1 M N aC1 so lu t ion (deae ra ted w i th At ) ,

r e c o r d e d a t a s c a n r a t e o f 1 0 0 m V / s . ( a ) A n o d i c s c a n u s i n g f r e sh

su r face ( spu t te red in U H V , t rans fe r red d i rec t ly to the ce ll (F ig . 1 ),

e l ec t ro l y te c o n t a c t e s ta b l is h e d a t - 1 0 0 0 t o - 1 1 0 0 m V . ( b ) T h e

same a f te r 10 anod ic -ca thod ic cyc le s

n o t t h e s a m e a s t h e s t a t e o f t h e s u r f a c e to w h i c h F i g . 9 b

refers, s ince in the l a t t e r case , a l t e rna t ing ox ida t ion a nd

re d u c t i o n o f th e s u r f a c e a r e i n d u c e d e x t e rn a ll y b y

curren t f low (wi th a cu rre n t o f 100 IxA, som e 0 .3 a tom ic

layers can be d i s so lved anod ica l ly per second) . F igu res

Fig . 8 . O pen-c i rcu i t po ten t ia l s (v s sce ) a s func t ions o f the

l o g a r i t h m o f t i m e f o r s a m p l e s 1 t o 7 i n 0 .1 M N a C 1

s o l u t i o n ( d e a e r a t e d w i t h A t )

7 and 9b s t i l l revea l para l l e l s in e lec t rochemica l

b e h a v i o u r o f t h e a l l o y s u n d e r t h e t w o s e ts o f c o n -

d i t ions . Dur ing cyc l ing aga in , a l loys a re more nob le

w h e n t h e c h ro m i u m c o n t e n t i s h ig h e r. A l l o y 2 d i s p la y s

t h e l a rg e s t c h a n g e i n t h e v o l t a m m o g ra m b e t w e e n t h e

f i rs t a n d t e n t h s w e e p . Th e O C P c o n d i t i o n o f th e a l l o y s

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

the l e f t o f the co ord in a te o r ig in in the p lo t s o f

Fig. 9.

The charge passed dur ing f i r st sw eeps (F ig . 9a)

a m o u n t s t o v a l u e s o f 1 -2 m C / c m 2 , a n d t h u s c o r r e -

s p o n d s t o s o m e t h i n g l i k e t h r e e m o n o l a y e r s o f d i - o r

t r i v a le n t c o m p o u n d fo rm e d o n t o p o f t h e s u r fa c e

s p u t t e r e d c l e a n i n U H V .

Th e o p e n -c i r c u i t p o t e n t i a l s a t t a i n e d a f t e r 4 k s a c c o rd -

ing to F ig . 7 and the cu rren t s reco rded , a ccord ing to

Fig . 9 , a t U = 0 V (sce) have been p lo t t ed in F ig . 16a

a g a i n s t t h e c h ro m i u m c o n t e n t o f t h e a ll o y s. Th e t r e n d s

seen in these p lo t s wi l l be d i scussed be lo w in con junc-

t ion wi th the X PS resu l t s o f F igs . 16b and c .

3 X P S R e s u l ts

F i g u re 1 0 s h o w s t h e d e p t h p ro f il e s o f c h ro m i u m fo r t h e

d i f fe ren t a l loys , as measured a f te r exposure to the

e l e c t ro ly t e u n d e r O C P c o n d i t i o n s d u r i n g 1 5 h . I n t h e

a l loy subs t ra tes (a t spu t te r t imes t > 600 s ) the c on-

cen t ra t ions a re re la t ive ly c lose to the bu lk va lues l i s t ed

in Tab le 2 . Fo r a l loy 1A (7 Cr) the p ro f i l e i s

qua l i t a t ive ly d i f fe ren t compared wi th the o ther a l loys :

o n l y a v e ry s m a l l a c c u m u l a t i o n o f C r , o f th e o rd e r o f

1 , is obse rved near z ~ 20 A. The s i tua t ion i s qu i te

d i f fe ren t fo r the o ther a l loys con ta in ing 1 2 o r mo re

c h ro m i u m : In t h e i n t e r f a c e r e g i o n b e t w e e n t h e a l l o y

a n d t h e p a s s i v e fi lm t h e r e i s a d e p l e t i o n o f c h ro m i u m

w h i c h i s f o l l o w e d b y a p ro n o u n c e d m a x i m u m w i t h i n

the pass ive f ilm . Fo r a l loy 2 (12 .6a t . - Cr) the

ma x im um of Oct(Z) i s a t a de p th o f z ~ 30 A, w h i le fo r

the o ther a l loys con ta in ing mo re than 12 .6 a t ; - Cr


9/18

C o r r o s i o n o f S t a i n le s s S t e e ls i n C h l o r i d e S o l u t i o n 9

C c r ( t )

( at )

5 0

/ k - - - -

. . . . . ._ + ; . - :.

. o _ , ; . / 2

'10

0 I I I I I I I I / / ~

0 4 0 8 0 t 2 0 t 8 0

S p u t t e r t i m e

(s ) 5 0 0 6 0 0

I I I I I I / I

~

0 2 0 4 0 6 0 9 0 Approx, depth ( ~) t 5 0 3 0 0

F i g . 1 0. D e p t h p r o f i l e s o f th e t o t a l c h r o m i u m c o n c e n t r a t i o n

C r ~ +

C r ) o f s a m p l e s 1 A t o 6 . @ 7 . 0 C r ; @ 1 2 . 6 C r , 0 . 6 M o ,

0 . 6 N i ; @ 1 5 . 8C r , 0 . 9 M o , 4 . 9 N i ; @ 1 9 C r , 1 . 3 M o , 1 0 . 9 N i ;

@ 2 7 . 5 C r , 1 . 3 M o , 4 N i , 5 . 6 M n ; @ 3 0 . 8 C r , 2 M o , 2 .3 N i

( T ab le 2 )

the max imum is located between 10 and 15 A below the

surface. With increasing chromium content in the bulk,

the maximum becomes considerably higher but the

increments become smaller at the higher chromium

concentrations; this indicates that a saturation value

of the peak concentration near about 35-40 at.-% Cr

will be reached (normalized with respect to all compo-

nents, metal plus oxygen). Note that the chromium

concent ration for pure

Cr2 3

s 40 at.-%. The decrease

of the chromium concentra tion in the outermost layers

is due to the presence of hydroxides and water in this

region of the passive films, as well as to some disso-

lution o f chromium into the electrolyte. That appreci-

able amount s of hydroxides and water are contained in

the films can be deduced from the shape of the Ols

XPS line (Fig. 4c). Angle-dependent measurements

actually reveal that hydroxides and water are ac-

cumulated in the outermost layers of the films (Fig. 15).

In addi tion to these species, the surface will hold some

unavoidable oxygen-containing contamination also

contributing to the decrease in chromium concen-

tration very close to the surface.

On the basis of deconvolution of the Fe2p and Cr2p

XPS lines it is possible to derive individual con-

centration profiles for the metals (Fe , Cr ) as well as

for the oxides (Fe ~ Cr~ These profiles are shown in

Figs. 11 to 13. In these figures Cr ~ stands for Cr 3+

while Fe ~ represents the sum of Fe 2+ and Fe 3+. In

pure Armco iron (sample 1, Fig. 11a) the broad con-

centration profile of Fe ~ reflects a relatively thick

oxide film of the order of 50 A. In this case the oxide

film contains abou t 90% Fe 3+ and 10% Fe z+ (Fig.

16c). For pure FezO3 the concentration of iron would

be 40at .-%, the 10% Fe 2+ would raise this figure to

40.8 at.-%. These values agree well with the mean iron

concent ration in the film of sample 1 (Fig. 11a). From

Figs. 11 to 13 it is seen that with increasing chromium

concentration in the alloy the iron and chromium

peaks become narrower and also shift towards the

surface. The coexistence of iron and chromium in the

passive films indicates that an iron-chromium oxide

exists at least in the inner part of the passive films, while

the outer part is enriched in hydroxides and water as

mentioned above. It should be emphasized, however,

that no sharp interface between the outer hydroxide-

water region and the inner oxide region exists; this

can be seen from the angle-dependent XPS measure-

ments (Fig. 15).

Figures 11 to 13 seem to indicate that metallic iron and

chromium coexist with the iron-chromium oxides.

Such an overlap between oxide and metal profiles

would be expected if the oxide contained metallic

clusters. Nothing is known about the stability of such

metallic clusters in very thin oxide films. On the other

C i ( t )

( a t )

7 0

6 0

5 0

C i

( a t

4 0 ~

I

oy

2O -

1 0

O

0

(t)

% ) - -

- g O -

6 O

5 O

4 O

3 O

2 O

l 0

i

L-

/ k - - - ~

, i

. . . . /

f

i

/ / / \ \ \ ~ / Fe

I I - . .we ] I I

t I ~ q l - - -

4 0 8 0 4 ~ . 0 t 8 0 S p u t t e r t i m e ( s ) 5 0 0 ]< 6 00 L --

I I I I 1 i / I ~ _

2 0 4 0 6 0 9 0 A p p r o x. d e p t h ( ~ ) 1 5 0 3 0 0

/ / - - ~

._..._.~/. ---~

Fg x

/ (

O r . .

. . . . c r T

- /

t----/r

0 4 0 8 0 t ~ -0 ~ 8 0 S p u t t e r t im e ( s ) 5 0 0 6 0 0

j I I I I l // I],,_

O 2 0 4 0 6 0 9 0 A p p r o x . d e p t h

~)

t 5 0 5 0 0

F i g . 1 a a n d b . D e p t h p r o f i l e s o f ( a) s a m p l e 1 ( p u r e A r m c o i r o n )

and ( b ) sample 2 ( 12 .6 Cr , 0 .5 M o , 0 .6 Ni ) . Fe m, Cr ~ : m e ta l l i c i r on

a n d c h r o m i u m , F e ~ = F e z + + F e 3 + , C r ~ = C r 3 +


10/18

10 P. Brfiesch et al.

c l ( t )

( a t % ) ' a

7 0

6 0

5 0

4 0

3 0

2 0

t 0

0

I

0

J

4O

I

2O

.Fe~

l k - ' - -

,,,,-.---

% . c o x m

~ ~ . . . ~ j . - - . C r

l , ~ ~ ' r - ~ I s - ~ r - -' I - = - / / ~

8 0 ' 1 2 0 t 8 0 S p u t t e r t i m e ( s ) ~ 0 0 6 0 0

I l I 1 I

4 . 0 6 0 9 0 A p p ro x . d e p th ( .~ ) ' ~50 3 0 0

. . . . . / / - - -

/ / - - - -

c l ( t )

( a t % )

7 0

6 0 t ~ . . . . ~ . - = = ~ - ~ ' ~ Fern

4 0

3 0 . c ~

r m

20 ..-----~--

r

I J L - : ~ _ t '-.+-.., J_ I I / F - -

0 4 0 8 0 4 2 0 t 8 0 S p u t t e r t i m e ( s) 3 0 0 6 0 0

l I I I I I l i

~

0 2 0 4 0 6 0 9 0 A p pr ox . d e p t h ( ~ ) 4 5 0 3 0 0

Fig. 12. Dep th profiles of(a) sam ple 3 (15.8 Cr, 0.9M o, 0,4 Ni)

and (b) samp le 4 (19 Cr, 1.3 Mo, 10.9 Ni). Fe ~, Cr : m etallic iron

and chromium, Fe~ 2+ +F e 3+, Cr~ 3+

h a n d , i t s h o u l d b e k e p t i n m i n d t h a t , a l t h o u g h w e h a v e

d e r i v e d d i f f e r e n t i a l d e p t h p r o f i le s , o u r d e p t h r e s o l u t i o n

s t il l i s li m i t e d d u e t o i n t r i n s i c e f fe c t s s u c h a s i n h o m o g e -

n e o u s f i lm t h ic k n e s s a n d s p u t t e r - i n d u c e d m i x i n g o f

t h e a t o m s a n d i o ns . T h e l i m i t a t io n s o f d e p t h r e s o l u t i o n

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

f o r t h e v e r y t h i n f il m s w h e r e t h e m e t a l - o x i d e o v e r l a p i s

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

p h a s e i s e x p e c te d t o b e b e t t e r s e p a r a t e d f r o m t h e o x i d e

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

i s un l i ke ly .

T h e d e p t h p r o f i l e o f o x y g e n i s s h o w n i n F i g . 1 4a . F o r

p u r e i r o n ( s a m p l e 1 ) t h e o x y g e n p r o f i l e d e c r e a s e s v e r y

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

A l r e a d y f o r th e a l l o y c o n t a i n i n g 1 2. 6 a t . - % C r ( s a m -

p l e 2 ) t h e o x y g e n c o n c e n t r a t i o n

C o ( Z )

d e c r e a s e s m o r e

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

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

c a t e s t h a t t h e m e a n f il m t h ic k n e s s d e c re a s e s c o n s i d e r -

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

b u l k . I t is a ls o w o r t h n o t i n g t h a t t h e s l o p e o f

C o ( Z

f o r

I/---

c i ( t ) ] _

( e l % } F

/

6 0 , ~ F = m , ' I ~

5 0

4 0

3 0 c , ~ i / - -

z o g ^ i

\ _

, , 1 I .

F l

I ~. w . f .

Io I - /~ .~-

VJ'~Y-,_ 3 -.,--..,_., , %%0

4 0 8 0 t 2 0 t 8 0 S p u l t e r t i m e

e )

3 ( ~ 0

I I I I I : I

2 0

40

6 0 9 0 A p p rox . d e p th (n )

t 5 0 300

I

0

c; I t }

l e t % )

7 0

6 0 ~ ~ ' F .

5 0

4 0

3 ~ / . / y c , ' . . . . . . . ~ . - - . . . . . . .

;v ;

: >_ . .

L 5 , ,4 , J I ' I * - - - ~ - . 1 ~

o 4 0

I

2 o

. . . . t . . .

- - , i

I I

8 0 4 2 0 t 8 0 S p u t t e r t i m e ( =1 3 0 0 6 0 0

I I i l i i i =

4 0 6 0 9 0 A p p ro x . d e p l h ( l ) 4 5 0 ] 0 0

. f

r, ~ cSx

1 / ' ~ - ~ . . 4 .- -- ,~ , . . . . . . 4

4 0 8 0 t 2 0 4 8 0 p u t t e r t im e ( e )

l I I I

2 0 4 0 6 0 9 0 A p p r o x . d e p t h ( ~ i

Fig. 13a~z. De pth p rofiles of(a) sa mple 5 (27.5 Cr, 1.3 Mo , 4N i,

5.6Mn), (b)sample 6 (30.8Cr, 2M o, 2.3N i), and (c) pure

chromium. Fem, Cr : metall ic iron and chromium.

Feox =F e z+ +Fe 3+, Crox =C r 3+

I

o

c; ( t )

I t '& )

9

8

7

6

5

4

3

2

1

I

s a m p l e s 1 a n d 2 c h a n g e s c o n s i d e r a b l y n e a r z ~ 1 0 A .

T h i s i n d i c a t e s e x c e s s o x y g e n v e r y c lo s e t o t h e s u r f a c e

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

w e l l a s o t h e r o x y g e n - c o n t a i n i n g c o n t a m i n a t i o n s . F i -

n a l ly , it s h o u l d b e n o t e d t h a t e v e n a f t e r lo n g s p u t t e r in g

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

5 - 8 a t . - % i s o b s e r v e d i n t h e a l l o y s a n d a l s o i n p u r e

c h r o m i u m . T h i s m i g h t b e s u r p r i s i n g a t f i r s t s i g h t ;

h o w e v e r , i t i s a w e l l - k n o w n f a c t t h a t t h e c o m p l e t e


11/18

Co rrosi on of Stainless Steels in Ch loride Solu tion 11

o

I

o

c . o ( t )

a t 1

C o I t ) / t t - - -

. ; 3 a

I i I i L t i

I ; ; _ _

4 0 B O t 2 0 t 8 0

S p u l i e r t i m e

( t ) 3 0 0

1 I I I I

2 0 4 0 6 0 9 0 A p p ro x. d ep th ~ ) ~ 5 0

t . 4

t . 2

t . 0

0 . 8

0 . 6

0 . 4

0 . 2

0

I

0

c ~ p ( t )

a t

%

4 4

t Z

t 0

8

6

4

2

0

I

0

b

/

: f @

/ f t . ~ |

/ I I I I I I

4 0 - 8 0 t 2 0

1 I I

2 0 4 0 6 0

6 0 0

3 0 0

# - - -

- / , - -

. m

, ' - -

I I / L _

1 8 0 S p u l te r t im e

I s) 3 0 0 6 0 0

I I I ; I _

9 0 A p p r o x . d e p t h

~ ) t 5 0 3 0 0

- - -

c \

/ . . . ~ > . . . . . . . . . . . . . . . . . . . l /_

I

_ ._ t i t __

' I I I I I I I I H- --

4 0 8 0 4 2 0 t 8 0

S p u t ter t im e

( e) 3 0 0 6 0 0

I I I I I l I ~

2 0 4 0 6 0 9 0 A p p r ox . d e p t h l l ) t 5 0 3 0 0

Fig. 14a-c. Dept h profiles of a) oxyg en, b) metall ic mo lyb-

denum, and (c) metallic nickel of selected samples @ pure

Armco ir on; @ 12.6Cr, 0 .5Me, 0 .6N i; @ 15.8Cr, 0 .9Me,

4.9Ni; | 1 .3M o, 10.9Ni; @27.5Cr, 1 .3M e, 4Ni,

5.6 M n; @__30.8Cr, 2 Me, 2.3 Ni; @ pure chromium

rem oval o f oxygen i s d if f icu l t o r e ven imposs ib le us ing

a rg o n - i o n s p u t t e r in g o f m e t a l s a n d a l l oy s w i t h h ig h

oxygen af f in i ty [10 ] .

F i g u re 3 a s h o ws t h e M o 3 d s p e c t r a o f s a m p l e 4 a t

different sputt ering t imes. Befo re sputtering, at t = 0 s ,

t h e s t ru c t u re o f t h e M o 3 d d o u b l e t i s c o m p l e x a n d

d i f fe ren t f rom the s t ruc tu res observed a f te r spu t te r ing

a t 20, 60 , and 180s . Befo re spu t te r ing the Mo3d3 /2

s t ruc tu re i s b ro ade r and s l igh tly sh i f ted tow ards

h igher b ind ing energ ies compared to the spec t ra a f te r

spu t te r ing ; in add i t ion a shou lder i s observed a t the

h igh-b ind ing-energy s ide o f th i s s t ruc tu re . These fea-

tu res imply tha t the meta l l i c Mo3d3 /2 l ine over laps

wi th l ines o f ox id ic 3ds/z s ta tes ind ica t ing the ex i s tence

o f sm a ll a m o u n t s o f m o l y b d e n u m o x id e s a n d / o r

oxyh ydrox ides [5 ] . The fac t tha t a f te r a spu t te r ing t ime

o f o n l y 2 0 s t h e s p e c t ru m a l r e a d y s h o w s t h e f e a t u re s o f

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

o x i d e s a r e r a p i d ly r e d u c e d b y a rg o n - io n b o m b a rd m e n t

[1 1 -1 3 ] .

In c o n t r a s t t o m o l y b d e n u m , e s s e n t ia l ly n o n i c k el i n t h e

ox id ic s ta te has been observed in the pass ive f i lms .

E v e n i n s a m p l e 4 c o n t a i n in g a s m u c h a s 1 3 % N i , o n l y

meta l l i c n icke l has been observed befo re and a f te r

spu t te r ing (F ig. 3b ) : F igures 14b and c show th e dep th

pro f i l es o f meta l l i c M e and Ni . There i s a s t rong

dep le t ion in the reg ion o f the pass ive fi lms . Conce rn ing

t h e c o e x i st e n c e o f m e t a l l ic M e a n d N i w i t h m i x e d i ron -

c h ro m i u m o x i d e s t h e s a m e r e m a rk s a p p l y a s m a d e

a b o v e fo r m e ta l li c C r a n d F e . F o r s a m p l e s 4 a n d 6 we

o b s e rv e a p ro n o u n c e d a c c u m u l a t i o n o f m e t a ll i c n ic k e l

in the reg ion o f the in te r face be tw een the pa ss ive f ilm

and the a l loy .

Whi le no n icke l ox ides and on ly re la t ive ly smal l

a m o u n t s o f m o l y b d e n u m o x id e s a n d / o r o x y h y d r o x -

ides have been observed in the pass ive f i lms , the

s i tua t ion i s d i f fe ren t fo r the manganese con ta ined in

samp le 5 (Tab le 2 ) . In th i s a l loy ma ngan ese i s obse rved

i n b o t h t h e m e t a ll i c a n d o x i d ic s ta t e (M n O, M n O 2 , a n d

poss ib ly oxyhyd rox ides ) , even a f te r 180 s o f a rgon- ion

sputtering.

The su rface sens i t iv i ty o f XP S w hich i s a conse quenc e

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

so l id , can be increased by lower ing the e lec t ron

emiss ion ang le O. F igure 15 shows the angu lar de-

p e n d e n c e o f th e r a t i o

I1 0) /I2 0) ,

w h e r e 11 0) is the

i n te n s it y d ue to O H - a n d H 2 0 a n d I 2 ( O ) i s t h e

in tens i ty o f 0 2 - bo un d to m eta l ions in the ox ide .

T h e s e i n te n s it ie s h a v e b e e n o b t a i n e d b y d e c o n v o l u t i o n

of the O ls l ines as show n in F ig . 4c , and the ang u lar

depen dence o f these in tens i t ies i s g iven by (5 ) . F igure 15

re fl e ct s t h e f a c t t h a t O H - a n d H 2 0 a r e e n r i c h e d i n t h e

ou te r par t o f the pass ive fi lms , the en r ichmen t be ing

m o s t p ro n o u n c e d i n t h e h i g h -a l l o y e d s a m p l e 6 c o n -

ta in ing 31 a t . -% chrom ium.

In o rd e r t o i n v e s t ig a t e c h l o r i d e p e n e t r a t i o n i n t o t h e

pass ive f i lms we have s tud ied two ex t reme cases ,

nam ely pure i ron (sample 1 ) and the a l loy con ta in ing

31 a t . -% chro miu m (sam ple 6 ). In pu re i ron we do f ind

s m a l l a m o u n t s o f c h l o r in e i n c o rp o ra t e d i n t h e o x i d e

f ilm, even a f te r a rgon- ion spu t te r ing o f 180 and 300 s ;

s i n c e a t t h e s e s p u t t e r t i m e s n o s o d i u m h a s b e e n

detec ted we can exc lude the poss ib i l i ty tha t the


12/18

12 P . Br t i e sch e t a l .

I

2

2 0

1 8

t 6

t 4

t 2

t 0

0 8

0 6

0 4

0 2

t s

[

0 t 5 3 0 4 5 6 0 7 5 9 0 O

F i g . 1 5. A n g u l a r d e p e n d e n c e o f l l O ) o f s a m p l e s 1 , 3 , a n d 6 . I 1 :

i n t e n si t y o f O H - a n d H 2 0 , 1 2 : i n t en s i ty o f 0 2 - b o u n d t o m e t a l

ions in the oxide

c h l o r i d e o r i g i n a te s f ro m r e s i d u a l t ra c e s o f N a C 1 p re -

sen t a t the su r face o f the sample . In samp le 6 , howe ver ,

n o d e t e c t a b l e i n c o rp o ra t i o n o f c h l o r id e s i n t o t h e

p a s s i v e f i l m h a s b e e n o b s e rv e d .

4 Di scuss i on

4 1 The Composit ion o f the Passive Films

In t h e fo l l o w i n g w e s u m m a r i z e t h e m o s t i m p o r t a n t

resu l t s concern ing the com pos i t io n o f the pass ive f ilms

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

l i terature.

A n g l e -d e p e n d e n t X P S m e a s u re m e n t s h a v e s h o w n t h a t

h y d ro x i d e s a n d w a t e r a r e a c c u m u l a t e d i n t h e o u t -

e rm os t l ayers o f the pass ive f ilms and tha t the c en t ra l

r e g i o n o f t h e f i lm s c o n s i s ts m a i n l y o f ir o n -c h ro m i u m

o x id e s . Th e t r a n s i ti o n f ro m t h e o u t e r h y d ro x i d e -

wa ter reg ion to the inner ox ide reg ion i s con t in -

u o u s . Th e s e r e s u l t s a g re e w i t h t h o s e o b t a i n e d b y

O l e f j o rd [4, 5 ]; O k a m o t o [1 4 ] a n d B o c k r i s e t a l. [ 1 5,

1 6 ] h a v e a l s o e s t a b l i s h e d t h a t h y d ro x i d e s a n d w a t e r

are es sen t ia l com pon en ts o f the su r face f ilms.

Th e p a s s i v e f i l m s a r e e n r i c h e d i n c h ro m i u m . In t h e

r e g i o n o f t h e c r it i ca l c h ro m i u m c o n c e n t r a t i o n o f a b o u t

1 3 % , t h e c h ro m i u m c o n c e n t r a t i o n i n t h e f i l m s i n -

c r e a s e s n o n l i n e a r l y w i t h t h e c h ro m i u m c o n t e n t i n t h e

b u l k . F o r a l l o y s c o n t a i n i n g m o re t h a n 2 0 % c h ro m i u m

the cen t ra l reg ion o f the pass ive f i lms cons i s t s ma in ly o f

C r~ O 3 . O u r d e p t h p ro f i l e s s h o w p ro n o u n c e d m a x i m a

i n t h e c h ro m i u m c o n c e n t r a t i o n w h i c h a r e l o c a t e d c l o se

t o t h e s u r f a c e . Th e s e m a x i m a a r e m o re p ro n o u n c e d

t h a n t h o s e fo u n d i n t h e l i t e r a t u r e [1 7 -2 0 ] a n d a r e

qual i t a t ive ly s imi la r to the p ro f i l es observed fo r a i r -

o x i d iz e d i r o n -c h ro m i u m a l l oy s [2 1 ]. O n l y v e ry fe w

ind ica t ions a re foun d in the l i te ra tu re fo r a min im um in

t h e c h ro m i u m c o n c e n t r a t i o n a t t h e i n t e r f a c e b e t w e e n

the pass ive f i lm and the under ly ing a l loy [18 ] . A

poss ib le exp lana t ion fo r th i s observa t ion i s d i scussed

at the end of Sect . 4 .3 .

S i n ce i n F i gs . 1 1 -1 4 w e s h o w a b s o l u t e c o n c e n t r a t i o n s

o f t h e e l e m e n t s, o n e m i g h t b e t e m p t e d t o d e r i ve t h e

s to ich iom et ry o f the pass ive f ilms as a func t ion o f dep th

be low the su r face . Th is i s, in p rinc iple , poss ib le , bu t i t

s h o u l d b e k e p t i n m i n d t h a t t h e o b s e rv e d s t o i c h io m e -

t ry m a y d e v i a t e c o n s i d e ra b l y f ro m t h e t r u e s t o i c h io m e -

t ry due to spu t te r - induced e f fec t s , in par t i cu la r to

se lec tive spu t te r ing o f oxygen . W e have es t ima ted the

s to ich iom et ry in the cen t ra l re g ion o f the su r face f i lms

( i.e. , a t the ma x im um of the Fe ~ o r Cr ~ p ro f i l es in

F i g s . 1 1 -1 3 ) a n d h a v e fo u n d a c o m p o s i t i o n M 2 0 ~

( M 2 = F e ~1 76 w h e r e x ~ 1 . 8 - 2 . 3 f o r % < 2 0 %

a n d x ~ 3 f o r % > 2 0 % . T h e s e r e su lt s a g re e q u a li ta -

t i ve l y w i t h F ra n k e n t h a l ' s s p u t t e r e x p e r i m e n t s o f F e 2 0 3

as d i scussed a t the e nd o f Sect . 1 .3 . To f ind the t rue

c o m p o s i t i o n m a y r e q u i r e t h e u s e o f si g n if i ca n t ly m i l d e r

s p u t t e r c o n d i t i o n s . O n t h e o t h e r h a n d , t h e e a s e o f

spu t te r - ind uced redu c t ion o f the f ilms wi th l es s than

20% Cr i s a fu r ther ind ica t ion fo r the i r in su ff ic ien t

chemica l s t ab i l ity .

N i c k e l a n d c o p p e r h a v e b e e n o b s e rv e d o n l y i n t h e

meta l l i c s t a te ; these e lemen ts a re s t rong ly dep le ted in

the pass ive f i lms . On the o ther hand , we observe an

accu mu la t ion o f meta l l i c n icke l in the in te r face reg ion

between the pass ive f i lm and the a l loy ; th i s accumu-

la t ion i s mos t p ronounced in the n icke l - r i ch a l loy 4

con ta in ing 13% n ickel . An accu mu la t ion o f n icke l in

t h e i n t e r f a c e r e g i o n h a s a l s o b e e n o b s e rv e d b y

Olef jo rd [22 ] .

A s e x p e c t e d , m e t a l l ic m o l y b d e n u m i s st r o n g l y d e p l e t e d

in the pass ive f i lms . Molybdenum in ox id ic s ta tes i s

p resen t befo re spu t te r ing , s imi la r to the resu l t s o f

O l e f j o rd a n d B ro x [5 ] . Th e d i s t r i b u t i o n o f t h e s e s p ec i e s

in the pass ive f i lms canno t be dedu ced s ince a rgon- ion

s p u t t e r in g l e a d s to i n s t a n t a n e o u s r e d u c t i o n [1 1 -1 3 ] .

A n e n r i c h m e nt o f m o l y b d e n u m w a s f o u n d i n th e

p a s s i v e f i l m b y G o e t z a n d La n d o l t [ 2 3 ] f o r a n a l l o y

c o n t a i n in g 1 1 % M o ; t h i s c o n c e n t r a t i o n i s c o n s i d e r-

a b l y h i g h e r th a n t h e m o l y b d e n u m c o n c e n t r a t i o n s i n

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

resu l t s i s no t poss ib le . In con t ras t to Olef jo rd [2 2 ] bu t

i n a g re e m e n t w i t h Lu m s d e n a n d S t a e h l e [2 4 ] , w e d o

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

the pass ive f ilm . Th is migh t b e due to the fac t tha t i t i s

d i f f i cu l t to de tec t such accumula t ions in nar row

reg ions by a m eth od invo lv ing spu t te r ing ; in add i t ion ,

in compar ing d i f fe ren t resu l t s i t shou ld be bo rne in

min d tha t the p referen t ia l d i s so lu t ion o f a l loy ing


13/18

C o r r o s i o n o f S t a i n le s s S t e el s i n C h l o r i d e S o l u t i o n 1 3

c o m p o n e n t s d e p e n d s t o s o m e e x t e n t o n t h e e l e ct ro l yt e

used fo r the e lec t rochemica l exper imen ts .

In sample 5 (dup lex s tee l) con ta in ing 5 .6 Mn , the

pass ive f i lm con ta ins manganese in the ox id ized s ta te .

S i n ce t h e M n 2 p 3 /2 b i n d i n g e n e rg ie s o f M n O , M n 2 0 3 ,

and M n3 0 4 a re o n ly s l igh tly d i f fe ren t [25 ] , i t i s no t

poss ib le to es tab l i sh the ac tua l ox ida t ion s ta te ; the

s p e c t r a a r e c o m p a t i b l e w i t h M n / a n d M n 3 b u t n o t

wi th M n 4+ .

S m a l l a m o u n t s o f c h l o r id e o r i g i n a t in g f ro m t h e

e lec t ro ly te cou ld be observed in the su rface f i lm on

p u re i ro n . Un d e r o u r e l e c t ro c h e m i c a l c o n d i t i o n s

(OC P) th i s f i lm i s no t a pass ive f ilm and a l lows ch lo r ide

to pene t ra te to a cons iderab le ex ten t . Th is i s no t the

c a s e fo r t h e c h ro m i u m - r i c h s a m p l e 6 c o n t a i ni n g 3 1

C r , a n d p ro b a b l y n e i t h e r fo r th e o t h e r a l lo y s c o n t a in -

i n g m o re th a n 1 2 C r ; t h e a b s e n c e o f c h l o r id e s f ro m

t h e p a s s i v e fi lm s h a s a l s o b e e n r e p o r t e d b y E l f s t r6 m

[1 9 ] a n d S z k l a r s k a -S m i a l o ws k a [2 6 ] .

4.2. Passive Fi lm s

and Cri t ical Chromium Concentrat ion

I t i s a wel l -know n fac t tha t be low a c r i t i ca l ch rom ium

conc en t ra t io n Corit o f abo u t 12 the co rro s ion ra te o f

i ron -ch romium s tee l s increases d ras t i ca l ly wi th de-

creas ing ch rom ium con te n t [27 ] . Al loys wi th c < co rlt

a r e n o n n o b l e , t h a t i s , t h e y n o rm a l l y s h o w s t ro n g

corr osion , w hile al loys w ith c > co,i t are noble , that is ,

they fo rm a pass ive f i lm which s t rong ly suppresses

corros ion . Th is c r i t i ca l l imi t i s observed , no t on ly in

p u re i ro n -c h ro m i u m a l lo y s b u t a l s o in t h e c o m m e rc i a l

s tee ls inves t iga ted in th i s s tudy . F igure 16a shows the

open-c i rcu i t p o ten t ia l s (OC P) a t 4 ks f rom Fig . 7 as

wel l as the cu rren ts a t U = 0 V (sce) f rom Fig . 9b . On e

c a n s e e i m p o r t a n t c h a n g e s o f b o t h t h e O C P a n d t h e

corro s ion cu rren ts a t c ~ co rit . The resu l t s fo r OC P are

c o n s i s te n t w i t h t h o s e o f Uh l i g [9 ] o b t a i n e d u n d e r

s imi la r cond i t ions (deaera ted NaC1 so lu t ion ) .

T h e q u e s t i o n s n o w a r i se t o w h a t e x t e n t t h e p ro p e r t i e s

o f the pass ive f i lms chan ge a t corot and how these

p ro p e r t i e s c a n b e c o r r e l a t e d w i t h t h e e l e c t ro ch e m i c a l

b e h a v i o u r .

F i g u re 1 6 b i l lu s t ra t e s th e c h ro m i u m a c c u m u l a t i o n ,

dccr as def ined in F ig . t 0 as w el l as the me an th ickness

d o f the su rface f i lms as func t ions o f the ch rom ium

conc en t ra t io n in the a l loys . The f i lm th icknesses d have

been es t im ated f rom the de p th p ro f i l es (F igs . 11 to 13

and 14a). Both quanti t ies change drast ical ly at Cc~it .

Accord ing to F ig . 16b co rros ion -res i s tan t a l loys

(c > co, it ) a re charac te r ize d by th in pass ive f i lms which

a re s t ro n g l y e n ri c h e d i n c h ro m i u m . O n t h e o t h e r h a n d ,

non nob le s tee ls (c < C ,~ t) a re ch arac te r ized by re la t ive ly

th ick ox ide o r hydrox ide f i lms , the ch romium enr ich -

me n t o f wh ich i s on ly smal l o r n il . As men t ioned in

, a

t . 5 - . ...... ......

I

I

. .

n

~ 0 . 5

Z

0 - - t i i

0 8 t 6 2 4 5 2

C c r ( a t ) l n a l l o y

kd b A )

6 0 .

5(> 9

/

4 g

3 0

d

o I j o o

0 ~ )

t o

2 0

3 0

C c r ( a t ) i n a g o y

c ,C : ~ 1 7 6 - - . I - - - :

0 4

0 , 2 - - , , 4

2

~ C c r i t

0 ~' I ~ I I

0 1 0 2 0 3 0

C c r ( a t

) l n a l l o y

A C c

a t %1-

+ 1 0 0

ul

0

I00

>

E

- 3 0 0 -

|

5 o o

0

70 0

~ 1 4

12

I0

8

6

4

2

t . O

F e 3 +

F e 2 + + F e 3 +

0 . 9

0 . 8

0 . 7

F i g . 1 6 . ( a ) O p e n - c i r c u i t p o t e n t i a l s f ro m F i g . 7 ( p o i n t s a t 4 k s ;

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

ipa)

f r o m F i g s .

9a (e ) a nd 9b (0 ) (po in ts a t 0 V, le f t -hand sca le ) a s func t io ns o f the

c h r o m i u m c o n t e n t i n t h e a l l o y s. T h e v e r t ic a l a r ro w s i n d i c a t e t h e

t e n d e n c y t o w a r d s 1 0 k s ; t h e l o w e r p a r t o f t h e O C P c u r v e is

i n t e r p o l a t e d a c c o r d i n g t o U h l i g [ 9 ] . ( b ) M e a n t h i c k n e s s d o f t h e

s u r f a c e l a y e r s a n d c h r o m i u m e n r i c h m e n t A c c r (Fig. 10) as

f u n c t i o n s o f th e c h r o m i u m c o n t e n t i n t h e a ll o y s . | a c c o r d i n g t o

E l f s t r 6 m [ 1 9 ] . ( c ) N o r m a l i z c d c o n c e n t r a t i o n o f C r 3 +

[ C r ~ ~ + F e ~ ( - - f r o m d e p t h p r o f i le s ; . . . . . f r o m i n t e g r a l

c o n c e n t r a ti o n s ) a n d n o r m a l i z e d c o n c e n t r a t i o n o f F e 3 +

[ F e 3 + / F e z + + F e 3 + )] a s f u n c t i o n s o f t h e c h r o m i u m c o n c e n -

t r a t i o n i n t h e a l l o y s


14/18

14 P. Brfiesch et al.

Sect. 4.1, the lat ter films do not const itute passive films,

since they were formed under OCP conditions not

leading to a point in the passive region at long

exposure times. This s tatement applies in particula r to

the films formed on pure iron and alloy 1A whose film

thicknesses are not well reproducible and probably

also depend on the electrolyte [28]. The film thick-

nesses of samples 1 and 1A plotted in Fig. 16b are

estimated mean thicknesses with an uncertainty of at

least __ 20 A. We do not believe, however, tha t these

films were formed only after the end of exposure of the

samples to the electrolyte, because all manipula tions

were carried out under controlled conditions (transfer

of the sample in argon atmosphere). We cannot rule

out that precipitation of corrosion products from the

solution has con tributed to the films found on samples

1 and 1A and is responsible for some of the irreproduci-

bility in film thickness. On the o ther hand, it is known

that the passive films (c>%it) formed under OCP

conditions after a long exposure time (about 15 h) are

quite stable [29], and for this reason their film

thicknesses are expected to be well reproducible and

unaffected by precipitation effects. At this point it

should be mentioned that a qualitatively similar

behaviour of film thickness as a function of chromium

concentration has also been found for pure iron-

chromium steels by means of ellipsometric

methods [30].

Figure 16c shows the normalized cation concentr ation

ofCr 3 +, tha t is the ratio R = Cr~ ~ + Fe~ as well

as the normalized concent ration of Fe 3+, that is

P = Fe 3+/(Fe 2+ + Fe 3 +), in the passive films, as func-

tions of chromium concentration c in the alloys. The

values of P have been determined by deconvolu tion of

the Fe2p3/2 spectra before argon i on sputtering

(Fig. 4a). Alloy 2 with c = 12.6 at.-% Cr is just at %~t

9 and R-~ 40%. For the true passive films with c > corotwe

obtain R > 50% in agreement with Fischmeister et al.

[31] and with Asami et al. [28]. A quali tatively similar

behaviour for P(c) has been found by Asami using

1N H2SO 4 as the e lectrolyte [32]; his absolute values

are, however, significantly different from our values,

possibly because of the different electrolyte or the

different method used for d ata evaluation. The fact that

alloy 2 is just at the limit between active and passive

behaviour is also reflected in Fig. 7 (compare the

corresponding remarks made in Sect. 2).

The general behaviour of the normalized concent-

ration of Fe a +, that is of P c ) , can be explained if one

assumes tha t Fe 3 + ions lost from the film by disso-

lution in the electrolyte are partially replaced by Cr 3

supplied from the alloy by diffusion. Note the rela-

tively large deviation of the data point for sample 5

containing 27.5 at.-% Cr a nd 5.6 at.-% Mn (Table 2).

This deviation would be expected under the assump-

tion that the substitution of Fe by Mn occurs mainly

in the divalent state. According to Pourba ix [33] the

system is just at the bound ary between Mn 2+ and

Mn 3 +.

It is very remarkable that the functions ACc~(C),d c),

R c ) ,

and

P c )

shown in Figs. 16b and c are largely

independent of alloy structure an d the presence of the

alloying elements Mo, Ni, and Cu (Table 2). This is

understandable if one remembers that Ni and Cu are

very strongly depleted in the passive films and that Mo

occurs only in very small quantities. The functional

dependences shown in Figs. 16b and c are therefore

almost completely determined by the main elements

iron and chromium. Deviations are expected to occur

only if the alloying elements are present in the oxidic

state in appreciable quantities, as is the case for

manganese in sample 5 (Fig. 16c).

4 . 3. T h e H y p o t h e s i s o f a P h a s e T r a n s it i on

in th e Pa ss ive F i lm

The properties of the surface films depicted in Figs. 16b

and c all show a drastic change near the critical

chromium concentration c=c~rit~12.5at.-%. This

strongly suggests that the transition from corrosive

behaviour at c < eerit o the passive behaviour at c > co,it

is due to a phase transition in the surface film which is

driven by the chromium concen tration c in the alloy, or

more precisely, by the corresponding chromium con-

centration x c ) in the films.

Transmission-electron-diffraction studies by McBee

and Kruger [-34] indicate that the anodically formed

surface films are composed of iron-chromium oxides

of the spinel type with the composition

2 3 3

Fe Fe2-x(c~Crx(c)O,. If this is the case the phase

transition is expected to occur at a critical con-

centration x = xcrit. The results obtained by XPS and

SAM show, however, that the composition of the

surface films is more complicated, because the films not

only contain oxides but also hydroxides and water, the

latter species being accumulated in the outermost

monolayers.

Consider now an alloy with chromium concentrat ion c

whose clean surface (such as ob tained after argon-ion

sputtering) is exposed to the electrolyte. In the first

monolaye rs certain reaction products, such as hydrox-

ides will be formed. Further oxidation of the alloy

below the surface film can continue only if the oxygen-

carrying species (O) can migrate from the electrolyte

throu gh the surface film already formed. In the station-

ary state the rate of oxidation at the alloy-film nterface

is equal to the dissolution rate at the film-electrolyte

interface so that the film thickness and composition

remain constant. The oxygen current is therefore

equivalent to a corresponding current of iron and


15/18

Corrosion of Stainless Steels in Chloride Solution 15

chromium in to the e lec t ro ly te . Le t Do be an e f fec t ive

d i f fus ion coeff ic ien t fo r oxygen (which inc ludes pos -

sible effects of electrical f ields acr oss the fi lm). Do wil l

i n g e n e ra l d e p e n d o n x ( a n d h e n c e o n c ) : Do = Do x).

Fr om the dep th p ro f i l es o f oxyge n (F ig. 14a) i t fo llows

t h a t

Do x)

is lar ge for x < x~rlt (slow ly falling profiles),

b u t t h a t

Do x)

is sm all for x >Xcr~t (rapidly fal l ing

pro f i l es ) . Thus , the phase t rans i t ion men t ioned above

man i fes t s i t se lf a l so in a s t rong change o f

Do x)

n e a r

x = xcr i r The na tu re o f th is p hase t rans i t ion i s no t

k n o wn ; i t m i g h t b e a s t ru c t u ra l p h a s e t r a n s i t i o n

(crys ta ll ine to amo rpho us , fo r example [34 ]) o r a phase

t r a n s i ti o n i n a m i x e d c o n d u c t o r i n wh i c h t h e e l e c t ro n ic

a n d i o n i c c o n d u c t i v i t y a r e s t ro n g l y c o u p l e d a n d

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

film.

In the case o f a s lowly fa l ling dep th p ro f i l e o f oxygen

(X


16/18

16 P. Br/iesch et al.

s ta in less s tee l num ber 2 be ing on the m arg ins o f

showing p i t t ing co rros ion [48 ] . Indeed , the e lec t ro -

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

we re fulfil led , w ith al l the s tainless s teels as well as the

p u re C r s h o wi n g p a s s i v e c o r ro s i o n b e h a v i o u r .

Un d e r t h e e x p e r i m e n t a l c o n d i t i o n s c o n s i d e re d i n t h is

s t u d y a l l o y 2 c a n b e c o n s i d e re d a t r a n s i t i o n b e t we e n

t h e n o n p a s s i v e i ro n a n d t h e i ro n -c h ro m i u m a l l o y

c o n t a i n i n g 7 C r o n t h e o n e h a n d a n d t h e fu l ly p a s s i v e

b e h a v i o u r e x h i b i t e d b y a l l o ys 3 t o 6 o n t h e o t h e r h a n d .

I f these a l loys 3 to 6 a re then sa id to exh ib i t s t ab le

pass iv i ty , it i s c lear tha t the s tab le pass ive f i lm i s la rge ly

i n d e p e n d e n t o f a l l o y c o m p o s i t io n . I t c a n b e c h a ra c t e r -

ized by a fou r- layer s t ruc tu re cons i s t ing in tu rn o f a

h y d ra t e d l a y e r c o n t a i n i n g m u c h o x y g e n a n d O H - i o n s

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

m i x e d i ro n -c h ro m i u m o x i d e , a l a y e r a p p ro x i m a t i n g

C r2 0 3 wh i c h i s i n i m m e d i a t e c o n t a c t w i t h a m e t a l l i c

l a y e r d e p l e te d i n c h ro m i u m a n d e n r i c h e d i n n ic k el . T h e

ou te r 3 layers a re on ly 1 .5 to 2 .0 nm in dep th and thus

re p re s e n t o n l y s e v e ral m o n o l a y e r s : t h e a c t u a l n u m b e r

o f m o n o l a y e r s d e p e n d s o n w h e t h e r o x y g e n o r C r 2 0 3 i s

t a k e n a s t h e s t a n d a rd o f m e a s u re m e n t . W i t h i n t h is

g e n e ra l s t ru c t u re t h e re a re t wo a d d i t i o n a l n o t e w o r t h y

fe a t u re s : n o c h l o r i d e i s i n c o rp o ra t e d i n t o t h e p a s s i v e

f il m ; a n d m e t a l li c c h ro m i u m a n d i ro n c o e x i s t w i t h i n

the ox ide layers. Th is las t observ a t ion can be in te rp re-

ted as e i ther meta l l i c c lus te rs wi th in the pass ive f i lm o r

a s i n h e re n t (p ro b a b l y l o c a l i z e d ) i n h o m o g e n e i t i e s i n

fi lm thickness .

These s imi la r i t i es in the s t ruc tu re and compos i t ion o f

the s tab le pass ive f i lm fo r a l loys 3 to 6 wou ld be

expec ted to l ead to s imi la r p roper t i es . Thus , fo r

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

e i ther the d i f fus ion ra te th rough the f i lm o r a l t e rna-

t i v e l y b y l e a k a g e s t h ro u g h l o c a l i z e d s p o t s wh e re t h e

f i lm is par t i cu la r ly th in . These exp ec ta t ions a re cons i s -

t e n d w i t h t h e e x p e r i m e n t a l o b s e rv a t i o n s t h a t t h e c o r ro -

s i o n r at e , a s m e a s u re d b y i p a i s v i r tua l ly the same fo r

al loys 3 to 6 .

On t h e o t h e r h a n d , wh e n t h e c o r ro s i o n r e s i s t a n c e i s

cons ider ed in te rms o f res i s tance to p i t ting co r ros ion in

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

p e ra t u re s, t h e n a n a d d i t i o n a l c o n s i d e ra t i o n c o m e s i n to

p lay . Th is i s the cons ide ra t ion o f the f i lm s tab i l i ty

under the more aggress ive cond i t ions . The d i f fe rences

in th is so r t o f co rro s ion res i s tance m us t res ide in

differing abi l i t ies o f the d ifferent s tainless s teels to

ma in ta in the s tab i l i ty o f the i r pass ive f ilm in env i ron-

me n ts o f d if fe ren t aggress iv ity . As th i s so r t o f co rro s ion

re s i s t a n c e i n c re a s e s w i t h b o t h c h ro m i u m a n d m o l y b -

denum con ten ts , i t appears tha t the ava i lab i l i ty o f

ch ro miu m in the a l loy i s one o f the c r i ti ca l fac to rs in

p a s s iv i ty , a n d t h a t t h e ro l e o f m o l y b d e n u m m u s t b e i n

fac i l i t a t ing ch romium ava i lab i l i ty e i ther by eas ing

c h ro m i u m t r a n s p o r t t o s i te s wh e re i t m a y b e o x i d i z ed

t o C rz O3 o r b y d e c re a s i n g t h e a c t i v it y o f e n v i ro n -

m e n t a l s p e c ie s wh i c h wo u l d t e n d t o s o l v a t e t h e

c h ro m i u m .

S i n ce t h e fi lm s t ru c t u re a n d c o m p o s i t i o n a s m e a s u re d

in th is s tud y is very s imi la r to tha t de te rm ined in o th er

studies [-4 , 5 , 14-24, 26, 56] i t appea rs t hat a s table

pass ive f i lm wi l l have s imi la r s t ruc tu re and compo-

s i ti o n u n d e r a l l c o n d i t i o n s o f a l l oy c o m p o s i t i o n a n d

env i ronmen ta l exposure . Th is f i lm s t ruc tu re wi l l then

p ro d u c e c o m p a ra b l e c o r ro s i o n r a te s . T h is is d o c u m e n t -

e d i n T a b l e 3 wh e re t h e p a s s i v e c u r r e n t d u r i n g

p o l a r i z a t i o n s t u d i e s h a s b e e n t a k e n a s a m e a s u re o f

the co rros ion ra te . The va lues in th i s t ab le do indeed

show th a t a w ide range o f s ta in less s tee ls under a wide

ra n g e o f e n v i ro n m e n t a l c o n d i ti o n s , h a v e p a s s iv e c u r -

ren t dens i t i es wi th in a re la t ive ly narro w range o f va lues .

Ho w e v e r , it s h o u l d a l s o b e n o t e d t h a t t h e re is a

sys temat ic increase in pass ive cu rren t dens i ty wi th a

d e c re a s e i n p H , a d e c re a s e i n m o l y b d e n u m c o n t e n t o r

an increase in t empera tu re . These re la t ive ly minor

d i f fe rences in pass ive cu rren t dens i ty mus t then b e

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

p a s s i v e f i l m s t ru c t u re a n d c o m p o s i t i o n . W h a t i s

n e e d e d i s a n i n -d e p t h s t u d y r e l a t in g p a s s i v e c o r ro s i o n

Table 3. Passive current densities for a wide range of stainless steels under a wide range of environmentalconditions

Material Environment Passive current Scan rate Ref.

[ga/c m

Corrosion of Stainless Steel in Chloride Solution

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