8/12/2019 J. Electrochem. Soc. 1990 Mansfeld 78 82

1/5

Pitting and Passivation of A I A l loy s and A I B ase d

Meta l Mat r ix Composi tes

F . M a n s f e l d , * S . L i n , S . K i m , * a n d H . S h i h *

C o r r os i on a n d E n v i r o n m e n t a l E f f e ct s L a b o r a t o r y , D e p a r t m e n t o f M a t e r i a l s S c i e n c e a n d E n g i n e e r i n g ,

U n i v e r s i t y o f S o u t h e r n C a l i f o r n i a , L o s A n g e l e s , C a l i f o r n i a 9 00 89 -0 24 1

ABSTRACT

Pitting and crevice corrosion of A1 alloys and Al-based metal mat rix c omposites can be detect ed by characteristic

changes of the impedan ce spectra in the low frequenc y region. A pitting model has b een dev eloped which is in agreement

with the expe rimenta l data. A fitting procedure has been used to analyze a large amount of data which hav e been obtained

for as-received samples and samples whic h had been passivated in CeC13 solutions. This chemical passivation process pro-

duces surfaces which are very resistant to localized corrosion. The c hemical passivation pr ocess provide s a valuable alter-

nativ e to ch roma te conve rsion coatings. A16061, A1/SiC, and A1/graphite whic h had been passiv ated in CeC13 for one week

did not pit in 0.5N NaC1 for at least one month. El ectroch emical imped ance spectroscopy EIS) is a conveni ent tool for

monito ring of the passivation and the corrosion processes.

The application of electrochemical impedance spectros-

copy EIS) as a new tool in corrosion research has resulted

in a wealth of information con cerning met hods of corro-

sion protection which were difficult to study with tradi-

tional dc techniques. This includes corrosion protection

by conversion coatings 1), polyme r coatings, and anodic

films 2). EIS has also provide d information concerni ng

corrosion protection by inhibitors 3), a subject which has

been discussed more recently in connection with a new

model of corroding surfaces which takes into account the

inhomoge neities of most real surfaces 4). In this paper it

will be demonstrated that EIS is also useful for detection

and mon itoring of localized corrosion phenomena. The

corrosion behavior of A1 alloys and Al-based metal matrix

comp osit es in a erated 0.5N NaC1 will serve to illustrate this

concept 5). It will be shown that chemical passivati on--a

process which consists of immersio n in CeC13 soluti ons--

produces surfaces which possess excellent resistance to

localized corrosion 5-7). EIS has been used to monitor the

passivation process and will be used in the future to deter-

mine the optimum passivation parameters. Recording of

EIS data during the corrosion test allows continuous mon-

itoring of the paramete rs which characterize the properties

of the surface and their changes as localized corrosion

occurs.

In order to describe the events which occur on a cor-

roding surface before and after localized corrosion has

been observed, the mod el shown in Fig. 1 has been de-

veloped. This model is in agreement with a large number

of EIS data whi ch have been collected by the authors for

A1 alloys and Al-based M1VICs 5-7). The parameters of this

model are defined as follows: R, is the solution resistance,

Rp is the polariza tion resista nce of the passive surface, an d

Cp is the corresponding capacitance. Rpit s the polarization

resista nce of the pitted area and Cpit is its capacita nce. F is

the area frac tion of the pit ted surf ace 0 =< F =< 1). W de-

scribes the transmission line behavior which is observed

at the lowest frequencies when pitting occurs and is ex-

pressed as W = K j~ )n /F , where n and K are constants. A

fitting procedure has been developed for the analysis of

experimental impedance data which is described else-

where 8).

Impe danc e spectra wh ich correspond to F = 0 and

F = 0.005 are shown in Fig. 2. Even for this very smal l area

at which pitting is assumed to occur, very significant

changes in the EIS data are observed. The pronounce d in-

crease of the capacitance Ct = Cp 1- F ) r cp i t , the

change in the frequency dependence of the impedance at

the lowest freque ncies and t he oc currence of a low fre-

quen cy max imu m of the phase angle are characteristic of

the pitting process.

Hinton et al. 9-12) have used rare earth metal chlorides

as inhib itors for A1 alloys in NaC1. It was a ssum ed that the

surface films formed in solutions such as 0.1M NaC1 and

*Electrochemical Society Active Member.

1000 ppm of CeC13 redu ce t he rate of the ox ygen redu cti on

reactiorL thereby shifting the corrosion potential in the

negative direction, while the pitting potential remained

unchanged. In the present approach, surface modification

is achie ved by i mme rsi on in CeC13 for seven days or

longer. The corrosion resistance of the resulting surface

films is then teste d b y im mer sio n in 0.5N NaC1. The CeC13

is therefore not used as an inhibitor, but as a substance

which forms a conversion coating in the A1 alloy. Passiva-

tion in CeC13 prod uce d ve ry corrosion resi stant surfaces

for most materials studied. As a result the imped ance was

very high and did not show a dc limit at the lowest fre-

quenci es measu red usually 1-10 mHz). For these cases,

wher e F = 0, a new fitting proc edu re was devel ope d 13)

which allows determination of Rp despite the fact that the

experimental data show mainly capacitive behavior. For

cases where a dc limit of the imp edanc e was obser ved in

the measu red freque ncy range and localized corrosion did

not occur, the integration techni que 14) was used.

E x p e r i m e n t a l R e s u l t s a n d D is c u s s i o n

The m ateria ls stu died were AI 7075 in the T6 and the T73

condi tion, A1 6061, A1 6061/SIC, and A1 6061/ graph ite

A1/Gr). The A1/SiC MMC DWA Com pos ite Speci alti es)

cont aine d 25 vol ume pe rcen t v/o) of 10 I~m SiC particu-

lates which were m ixed with A1 6061 powder and pro-

cessed with an extrusion method. The A E G r MMC DWA)

cont aine d 55 v/o of P 100 graphite fibers with eight al-

tern atin g layers of graphite an d A1 6061 and was clad

with an A1 6061 face sheet of 50 ~m thickness. Surfaces

were degreased, alkaline cleaned and deoxidized in a chro-

mate HNO3 bath deoxidizer 17, Amchem) before exp osure

to the test solution. After ex posu re to CeC13, the sample s

R s

1 ....

R p / [ 1 - F ]

R p i t W

F

O ~ F ~ < 1 , W - - - - ( K / F )( J r n

F ig . 1 . M o d e l fo r th e im p ed an c e o f th e p i t t in g p r o cess o n AI -b ased

mater ials.

78

J . E l e c t r o c h e m . S o c .

Vo l. 13 7, No. 1, Ja nu ar y 1990 9 The Electrochemical Society, Inc.

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8/12/2019 J. Electrochem. Soc. 1990 Mansfeld 78 82

2/5

J . E l e c t r o c h e m . S o c .

V o l . 137 , No . 1 , Janua ry 1990 9

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

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F i g. 4 . C o m p u t e r f i t o f e x p e r im e n t a l d a t a . 1 , A I 7 0 7 5 - T 6 a f t e r 2 h i n

0 . 5 N N a C I , p o s s i v at i on f o r o n e w e e k i n ] 0 0 0 p p m C e C I s , A = 2 0 c m 2 ;

2 , fi t d a t a R , = 2 . 9 ~ , R p = 1 6 3 ~ , C p = 1 6 2 ~ F , c~ = 0 . 9 5 .

w e r e r i n s e d s e v e r a l t i m e s w i t h d e i o n i z e d w a t e r b e f o r e i m -

m e r s i o n i n Na C1 . S a m p l e s w e r e e x p o s e d h o r i z o n t a l l y a n d

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

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

T h e c o r r o s i o n b e h a v i o r d u r i n g e x p o s u r e t o C eC 13 o r

N a C 1 w a s m o n i t o r e d u s i n g E I S . I n m o s t t e s t s t h e s a m p l e

w a s f i r s t i m m e r s e d i n 1 00 0 p p m C eC 13 f o r o n e w e e k a n d

t h e n t r a n s f e r r e d t o 0 . 5 N N a C 1 ( o p e n t o a i r) . T h e s e r e s u l t s

w e r e c o m p a r e d w i t h E I S d a t a fo r s a m p l e s w h i c h w e r e e x -

p o s e d t o N aC 1 w i t h o u t t h e c h e m i c a l p a s s i v a t i o n t r e a t m e n t

i n C eC 13. T h e e x p o s e d a r e a o f t h e t e s t e l e c t r o d e w a s 2 0 c m 2.

T h e i m p e d a n c e s p e c t r a w e r e o b t a i n e d a t th e c o r r o s i o n p o-

t e n t i a l , E . . . . A s a t u r a t e d c a l o m e l e l e c t r o d e ( S C E ) w a s u s e d

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

P t w i r e t o r e d u c e t h e p h a s e s h i f t a t h i g h f r e q u e n c i e s ( 1 5).

T h e m e a s u r e m e n t s w e r e c a r ri e d o u t w i t h a S o l a r t r o n

M o d e l 1 25 0 f r e q u e n c y r e s p o n s e a n a l y z e r a n d a M o d e l 1 2 8 6

p o t e n t i o s t a t . T h e a p p l i e d a c s i g n a l w a s 1 0 m V . T h e s p e c t r a

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

i m u m s e n s i t iv i t y a t a ll fr e q u e n c i e s . U s u a l l y t h e c u r r e n t

m e a s u r i n g r e s i s t o r w a s c h a n g e d a t 1 H z to a v a l u e w h i c h

w o u l d m a t c h t h e i m p e d a n c e e x p e c t e d i n t h e lo w f re -

q u e n c y r a n g e . F o r t h e a n a l y s i s o f t h e E I S d a t a t h e l o w f r e-

q u e n c y r e g i o n c o n t a i n s v e r y i m p o r t a n t i n f o r m a t i o n s i n c e

b o t h t h e p o l a r i z a ti o n r e s i s t a n c e a n d / o r th e i m p e d a n c e e le -

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

t h e s e d a t a .

F i g u r e 3 s h o w s a t y p i c a l r e s u l t o b t a i n e d f o r a s - r e c e i v e d

A 1 6 0 61 f o r w h i c h s o m e p i t s h a d a l r e a d y i n i t i a t e d a f t e r 2 4 h

e x p o s u r e t o 0 .5 N N aC 1 . A l s o s h o w n i n F ig . 3 a r e t h e r e s u l t s

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

i n t h e v e r y l o w f r e q u e n c y r a n g e t o t h e d c l i m i t o f t h e i m -

p e d a n c e . T h i s l i m i t o c c u r s o n l y a t 1 0 -6 H z a n d i s t h e r e f o r e

6 . 9 0

b e y o n d t h e r a n g e o f t h e i n s t r u m e n t a t i o n a n d r e a l is t ic

t i m e s f o r t h e m e a s u r e m e n t . F i g u r e 3 al s o s h o w s w h y t h e

K r a m e r s - K r o n i g ( K -K ) r e la t i o n s c a n n o t b e u s e d f o r t h e v a l -

i d a t i o n o f th i s t y p e o f d a t a i n th e f r e q u e n c y r e g i o n w h i c h i s

d o m i n a t e d b y t h e t r a n s m i s s i o n l i n e e l e m e n t (1 6, 17). S i m u -

l a t i o n s u s i n g t h e m o d e l i n F i g. 1 h a v e s h o w n t h a t f o r e x -

p e r i m e n t a l d a t a w i t h a l o w - f r e q u e n c y l i m i t o f 1 0 -3 H z

a g r e e m e n t w i t h t h e K - K t r a n s f o r m s w i l l o c c u r o n l y f o r fr e-

q u e n c i e s e x c e e d i n g a p p r o x i m a t e l y 1 0 -~ H z . F o r t h i s r e a-

s o n , t h e K - K t r a n s f o r m s c a n n o t b e u s e d i n t h e p r e s e n t c a s e

t o c h e c k t h e v a l i d i ty o f t h e e p x e r i m e n t a l d a t a i n t h e l o w -

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

p r o c e s s .

F i g u r e 4 s h o w s r e s u l t s f o r A 1 7 0 7 5- T 6 w h i c h h a d b e e n

p a s s i v a t e d i n 1 0 00 p p m C eC 13 f o r o n e w e e k a n d t h e n e x -

p o s e d t o a e r a t e d 0 .5 N N a C1 f o r 2 h . T h e E I S d a t a ( c u r v e 1)

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

o b s e r v e d w i t h t h e f i t d a t a ( c u r v e 2 ), w h i c h r e s u l t i n a v e r y

h i g h v a l u e o f R p = 3 . 2 6 1 06 ~ - c m 2. F i g u r e 5 i l l u s t r a t e s t h e

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

c r e a s i n g e x p o s u r e t i m e . F o r t h e u n t r e a t e d A 1 7 0 7 5- T 6 p i t s

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

n o u n c e d i n c r e a s e o f C t i n t h e c a p a c i ti v e r e g i o n a n d t h e a p -

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

e n c e i n t h e l o w - f r e q u e n c y r e g i o n i n a g r e e m e n t w i t h t h e

p i t t i n g m o d e l ( F i g . 1 a n d 2 ).

T h e d a t a i n c u r v e 1 o f F i g . 5 s h o w s o m e e r r a t i c c h a n g e s

o f t h e im p e d a n c e m o d u l u s / Z / a n d t h e p h a s e a n g l e a t t h e

l o w e s t f r e q u e n c i es . A n a t t e m p t t o u s e t h e K - K t r a n s f o r m s

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

i n t h e t r a n s m i s s i o n l i n e r e g i o n d u e t o t h e r e a s o n s d i s-

c u s s e d a b o v e . I t i s m o s t l i k e l y t h a t t h e d r i f t o f t h e c o r r o -

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

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