CORRELATION BETWEEN LONG TIME AND SHORT TIME TESTS TO DETECT PITTING AND EXFOLIATION CORROSION IN Al-Li (8090) ALLOYS

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CORRELATION BETWEEN LONG TIME AND SHORT TIME TESTS TO DETECT PITTING AND EXFOLIATION CORROSION IN Al-Li (8090) ALLOYS

J. Garcia, P. Ponthiaux, M. Habashi, J. Galland

To cite this version:

J. Garcia, P. Ponthiaux, M. Habashi, J. Galland. CORRELATION BETWEEN LONG TIME AND SHORT TIME TESTS TO DETECT PITTING AND EXFOLIATION CORROSION IN Al-Li (8090) ALLOYS. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-861-C3-870.

�10.1051/jphyscol:19873101�. �jpa-00226535�

JOURNAL DE PHYSIQUE

Colloque C3, suppl6ment au n 0 9 , Tome 4 8 , septembre 1 9 8 7

CORRELATION BETWEEN LONG TIME AND SHORT TIME TESTS TO DETECT PITTING AND EXFOLIATION CORROSION IN A1-Li (8090) ALLOYS

J. GARCIA, P. PONTHIAUX, M. HABASHI and J. GALLAND

Ecole Centrale des Arts et Manufactures, F-92295 ChStenay-Malabry Cedex, France

We have studied t h e behaviour of an A1-Li a l l o y (8090) t o p i t t i n g corrosion and exfol i a t i o n a s a f u n c t i o n of ageing temperature. Three methods have been employed : I ) visual e s t i m a t i o n using EXCO-test (ASTM recommandations), 2) potenti a1 - s o l u t i o n 1 eve1 measurements i n EXCO, CH30HICC14, CH30H/CC14/CuC12 and NaC1 /Hz02 s o l u t i o n s , 3 ) s t r e s s corrosion cracking by slow s t r a i n r a t e method.

Correlation has been found between t h e s e methods, especi a1 1 y i n t h e ageing temperatures range 125O t o 190°C.

INTRODUCTION

The b e s t known a p p l i c a t i o n of high s t r e n g t h aluminium a l l o y s i s i n t h e aero- space industry. The hi ghest-strength a1 umi ni um a1 1 oys a r e those based on A1 -Cu-Mg (2000 s e r i e s ) , A1 -Zn-Mg-Cu (7000 s e r i e s ) a1 1 oy systems and A1-Li -Cu-Mg (8090 and 2091 s e r i e s ) . These a1 loys, especial 1 y 7000 s e r i e s , may under s p e c i f i c c o n d i t i o n s , corrode by p i t t i n g o r by e x f o l i a t i o n processes and may f a i 1 a t stresses f a r below t h e y i e l d s t r e n g t h , due t o s t r e s s - c o r r o s i o n cracking (S.C.C. ) - Corrosion by e x f o l i a t i o n i s a s e l e c t i v e a t t a c k t h a t proceeds along mu1 t i p l e narrow paths para1 l e l t o t h e s u r f a c e of metal.

I t i s believed t h a t t h e corrosion products f o r c e t h e l a y e r s a p a r t and cause t h e metal t o swell ( 1 ) . Flaks of metal may be pushed up from t h e surface. This type of corrosion i s associated with a marked d i r e c t i o n a l i t y of g r a i n s t r u c t u r e . I f t h e g r a i n s t r u c t u r e i s equiaxed, exfol i a t i o n corrosion does not usual 1 y occur (1 1.

To determinate t h e r e s i s t a n c e t o t h e e x f o l i a t i o n , ASTM TASK GROUP 601.05.02.- T.G8 ( 2 ) had developped a t e s t method, designated EXCO, which specify t o t a l immersi,on of f r e s h l y etched specimens i n a s o l u t i o n of t h e following composition : 4.0 M/1 NaCl + 0.5MI1 KNO3 + 0.1 MI1 HNO3.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19873101

C3-862 J O U R N A L D E PHYSIQUE

Testing times required are 48 hr for 7000 series a1 loys and 96 hr for 2000 series a1 1 oys. Specimens having high resistance to exfoliation should show no visible bl i steri ng or delamination after exposure to thi s test.

Another simple test to perform which requires less than one hour to run is based on measurement of solution potential of an unstressed specimen in a mixture of absolute methyl alcohol (CH30H) and carbon tetrachloride (CC14) (3). It was found that the potential level varies with ageing time of 2219-T851 alloy plate and there was a good correlation between the potential values and the performance obtained in stress-corrosion cracki ng test (3). This method has been extended to 7000 series a1 loys as 7178 and 7075 and has shown to offer promise as a rapid qua1 ity control to estimate the exfoliation or S.C.C. resistances of alloys studied (3). This method was modified by adding to the precedent solution more than 5.5 grams per litre of anhydrous cupric chloride (C12Cu) (4).

On the other hand, W.W. BINGER and al. (5) have measured the solution potenti a1 1 eve1 s of different a1 umi nium a1 loys and second-phase-precipi tates in aqueous solution containing the following composition :

53 g/l NaCl + 3 g/l H202 .

This type of test allows to detect the presence of different elements and precipitates according to their potential values given in table I.

Traditionally, S.C.C. data for a1 uminium a1 loys have been obtained by exposing smooth, stressed specimens to a corrosive environment (6).

The most widely used testing procedure involves alternate immersion in aqueous 3% NaCl solution. Specimens are immersed 10 min. followed by a 50 min. drying period in air. Test times range from 30 to 180 days. An alternative S.C.C. test is based on measurement of percent loss of ductility and/or percent loss of tensile strength using low deformation rate (S.S.R. test) (7). Test time varies then from few minutes to less than one month.

Table I : Potential levels measured in NaCl/H202 for elements and phases within A1

alloys.

This paper describes t h e r e s u l t s obtained, on the resistance o f e x f o l i a t i o n and o f S.C.C. using S.S.R. t e s t , on an aluminium-1 ithium a l l o y (8090) as a function o f ageing temperature (20' t o 210°C) f o r 12 hours ageing. EXCO, CH30H/CC14, CH30H/CC14/C12Cu and NaCl /Hz02 solutions are employed. Attempt has been done t o c o r r e l a t e the resistance o f e x f o l i a t i o n i n the d i f f e r e n t solutions mentionned above and the performance o f the A1-Li a l l o y i n S.C.C. t e s t w.r.t. EXCO-solution.

EXPERIMENTAL

The experimental material was supplied by CEGEDUR-PECHINEY i n the form o f 0,8 mm r o l l e d p l a t i n the s o l u t i o n t r e a t e d (535OC during 1 - h r and then c o l d water quenched) and stretched (3%) condition (T351) .

The chemical composition o f the a l l o y used i n t h i s i n v e s t i g a t i o n i s given i n t a b l e II.

TABLE I 1 : Chemical composition o f the a1 loy 8090 employed (wt. %).

...

I ^{L i} I ^I.lg I ^Cu 1 ^Zr I ^Si I ^{T i} I ^Fe I ^{Na, ppml} I---____--- I

Heat treatments were c a r r i e d out i n a laboratory furnace under vacuum ( I Pa).

The ageing temperatures chosen were 20°, 50°, 100°, 125O, 1 50°, 170°, 190°, 200"

and 210°C f o r an ageing time o f 12 hours. Another heat treatment was employed, i .e.

heating t h e a1 l o y i n T-351 c o n d i t i o n t o 525OC during one hour and then water quenched (T4 condition). The enviroments used i n the S.C.C. were a i r o f 1 aboratory and EXCO s o l u t i o n w i t h a s t r a i n r a t e equal t o 7.5-10-5 s-1. Potential l e v e l s were measured i n EXCO, CH30H/CC14, CH30H/CC14/C12Cu and NaCl /Hz02 solutions as a function o f ageing temperature. Test time was chosen as 30 min. I n addition, t e s t s were achieved on unstressed specimens having d i f f e r e n t heat treatments i n EXCO s o l u t i o n during 96 h r t o estimate t h e i r resistance t o p i t t i n g and e x f o l i a t i o n . A1 1 t e s t s were c a r r i e d out a t room temperature and were doubled.

EXPERIMENTAL RESULTS AND DISCUSSION

Tensile properties are very s e n s i t i v e t o heat treatment condition, especial 1 y

ageing temperature. R, ( y i e l d strength), Rm CR,

U.T.S.(I + 6 m a x ) l and 6, C Sm =

I n (1 + Emax)] were measured. € max. i s t h e maximum homogeneous p l a s t i c

deformation. Figure l a shows t h a t Re and Rm increase s l i g h t l y when the ageing

C3-864 J O U R N A L DE PHYSIQUE

t e m p e r a t u r e i n c r e a s e s from 20° t o 125°C and r a p i d l y when t h i s t e m p e r a t u r e i n c r e a s e s from 125" t o 210°C, Re v a l u e s a r e n o t a f f e c t e d by EXCO s o l u t i o n a s e x t e r n a l e n v i r o r m e n t , b u t Rm v a l u e s measured i n a i r , i n t h e t e m p e r a t u r e r a n g e 20° t o 100°C,

are h i g h e r t h a n t h o s e o b t a i n e d i n EXCO s o l u t i o n . A t h i g h e r t e m p e r a t u r e t h a n 1 OO°C, EXCO s o l u t i o n has no e f f e c t on Rm l e v e l s . F i g u r e l b shows t h e v a r i a t i o n o f t h e stress l e v e l T i a t which PORTEVIN-LE CHATELIER phenomenon (P.L.C.) i s observed a s a f u n c t i o n of a g e i n g t e m p e r a t u r e . When t h e environment i s a i r , P.L.C. phenomenon d i s a p p e a r s a t t e m p e r a t u r e s h i g h e r t h a n 190°C, w h i l e t h i s phenomenon i s n o t observed a t ageing t e m p e r a t u r e s h i g h e r t h a n 1 50°C when EXCO s o l u t i o n i s employed. On t h e o t h e r hand, i n a i r , 6, i s c o n s t a n t i n t h e range 20' t o 1 OO°C, t h e n d e c r e a s e s a t f i r s t r a p i d l y from 100" t o 125OC and s l i g h t l y from 125O t o 210°C, f i g u r e 2 . T h i s f i g u r e shows a l s o t h a t bm v a l u e s measured i n EXCO s o l u t i o n a r e lower t h a n t h o s e o b t a i n e d i n a i r e x c e p t i n g a t 125°C. As shown above, t h e p a r a meters FL,

Qi(P.L.C.), R, and 6 , v a r y i n d i f f e r e n t manners a s a f u n c t i o n of ageing t e m p e r a t u r e ( f i g . l a , I b and f i g . 2) and of t h e n a t u r e of t h e environment employed.

Knowing t h a t t h e absorbed s p e c i f i c energy t i l l f r a c t u r e (A.S.P.E.F.) ( 8 , 9 , 1 0 ) i n c l u d e s , a t once, a l l t h e parameters i s s u e d from t e n s i l e t e s t , t h i s energy can be used t h e n a s a f r a c t u r e c r i t e r i o n .

A.S.P.E.F. i s t h e i n t e g r a l o f a t e n s i l e diagram p l o t t e d i n true s t r e s s - s t r a i n s c a l e :

A.S.P.E.F.= Wc 6 A : ( =

0

With = k . & n where k i s m a t e r i a l c o n s t a n t and n s t r a i n hardening exponent.

P l o t t i n g t h e c a l c u l a t e d v a l u e s of Wc a s a f u n c t i o n o f a g e i n g t e m p e r a t u r e TA, f i g u r e 3 , r e s u l t s a r e i n good agreement w i t h t h o s e o b t a i n e d whi t h sm ^{= f (TA) ,}

f i g u r e 2 . Moreover, f i g u r e 4 shows t h e v a r i a t i o n o f Sc % r e s u l t e d f r 0 m 6 ~ v a r i a t i o n S c ( h m ) %

[ ( S m a i r - GmEXCO)/6111airl x I 0 0 and from Wc v a r i a t i o n

Sc(Wc)%

[(Wcair - W c ~ ~ ~ 0 ) / W c a i r 1 x 100 a s a f u n c t i o n Of a g e i n g temperature. The two c u r v e s a r e i d e n t i c a l and Sc % ( 6 , ) c r i t e r i o n may b e t a k e n a l r e a d y a s r e p r e s e n t a t i v e o f t h e s u s c e p t i b i l i t y of 8090 a l l o y t o S.C.C. i n a i r and i n EXCO s o l u t i o n . F i g u r e 4 shows a1 so t h e r e s u l t s o b t a i n e d by a p p l y i n g EXCO-test f o l l o w i n g ASTM recommandations e s t a b l i s h e d by v i s u a l e s t i m a t i o n . So t h e f o l l o w i n g remarks c o u l d be made :

a ) w i t h o u t stress, no c o r r o s i o n i s d e t e c t e d i n t h i s a1 Toy i n t h e a g e i n g t e m p e r a t u r e r a n g e 20' t o 125"C, b u t i n t h i s r a n g e , S.C.C. i s a d e c r e a s i n g f u n c t i o n a s TA i n c r e a s e s .

b) when t h e ageing t e m p e r a t u r e i n c r e a s e s from 125O t o a b o u t 170°C, t h e importance o f p i t t i n g i s i n c r e a s e d , and a l s o t h e S.C.C.

c ) maximum S.C .C. i s o b t a i n e d when e x c e s s of p i t t i n g i s observed a t 170°C by

applying EXCO-test.

d ) a t 1 90°C ageing temperature, the exf 01 i a t i on and S.C.C. a r e important.

e ) a t higher temperatures than 190°C, the two parameters Sc (bm) and Sc (Wc) a r e not equivalent : the former decreases whereas the l a t e r does not change with t h e importance of exfol iation. This i s due probably t o 1 ocal i zed segregation preci pi tates more complex in the conditions of increasing ageing temperatures.

Measurement of potenti a1 1 eve1 s in EXCO, CH30H/CC14, CH30H/CC14/C12Cu, and NaCl/H202 solutions shows t h a t the maximum potential variation i s obtained i n t h e range of ageing temperature 150" t o 1 90°C, figure 5. W e can conclude t h a t a t ageing temperature higher than 150°C, magnesi um and copper a r e present e i t h e r , within t h e

oxide film ( 1 1 ) or i n the matrix (12, 13). The most probable phase formed in the matrix could be S ' (A12CuMg). Potential variation measured e i t h e r i n CH30H/CC14/Cl ~ C U , or i n NaC1 /H202 solutions confirm t h i s hypothesis (wr igth scale i n t h e figure 5 f o r the l a t e r solution) . The increase of S.C.C., p i t t i n g and tendency t o e x f o l i a t i o n i n the range of 125" t o 190°C i s then due t o the segregation of magnesium and copper in the oxide film and S t p r e c i p i t a t e i n t h e matrix.

For lower ageing temperatures, i .e. from room temperature (14 and T-351 conditions) up t o 150°C, i t i s assumed t h a t the existence of coherent 6 phase (A13Li), i t s precipitation and i t s coarsening in p a r t i c l e s shape when the ageing temperature increases ( 1 4 ) , contribute t o the change in the mechanical properties of t h e A1-Li alloy e i t h e r in a i r or in EXCO-sol ution.

Figure 6 points out the variation of f a i l u r e surface as a function of ageing temperature and of environment. In a i r , the type of rupture changes from d u c t i l e

(TA = 20°C) t o d u c t i l e with f l a t regions containing s l i p l i nes when the ageing temperature increases from 20" t o 150°C. Coarsening of s l i p character i s observed a t 150°C proving t h a t shearing of 6 ' p a r t i c l e s by moving dislocations (15) i s operative even a t t h i s temperature. A t 1 90°C, the type of rupture i s sub-granular with d u c t i l e t e a r i n g s on t h e sub-grain boundaries. When the t e n s i l e test i s achieved i n EXCO solution, f a i l u r e by shearing mechanism i s favoured even a t TA equal t o 20°C. A t 1 90°C, d u c t i l e tearings on the sub-grain surfaces seems t o be reduced, figure 6 .

On the other hand, P.L.C. phenomenon observed i n p o l y c r i s t a l l i n e A1-Li alloy

i s a t t r i b u t e d t o the interactions between moving dislocations and substitutional

magnesi urn atoms ( 1 6 , 17). P .L.C. phenomenon disappears when S ' phase begins t o

p r e c i p i t a t e ( 1 7). T h e disappareance of P.L.C. phenomenon a t 150°C when the

enviroment i s EXCO-solution instead of 190°C i n the case of a i r as external

C3-866 JOURNAL DE PHYSIQUE

envirorment, p o i n t s out t h a t during t e n s i l e t e s t i n EXCO-solution, two events may be produced :

a) I n the range o f ageing temperature 20' t o 150°C, the density o f defects i s increased which accelerate 6 ' p a r t i c l e s p r e c i p i t a t i o n ( 1 8).

b) A t higher ageing temperatures than 1 50°C, the increased i n dislocations density promotes s i t e s t o i n i t i a t e S' p r e c i p i t a t i o n . The two mechanisms could be t h e r e s u l t a n t o f species d i f f u s i o n from the external s o l u t i o n t o t h e matrix ; the most probable species may be hydrogen atoms which are well known t o increase t h e densities o f defects and o f dislocations (1 9, 20).

CONCLUSION

I n t h i s investigation, whe have studied the behaviour o f an A1-Li a l l o y (8090 series), aged i n d i f f e r e n t conditions, t o t h e corrosion by p i t t i n g and by e x f o l i a t i o n . Three methods were employed :

a) visual estimation o f the degree o f p i t t i n g and e x f o l i a t i o n by applying EXCO-test, recommended by ASTM, during 96 h r (long time);

b) short time t e s t (30 min);

establ i shed on p o t e n t i a l s o l u t i o n 1 eve1 s measurement i n EXCO, CH30H/CC14, CH30H/CCl 4/C1 2Cu and NaC1 /Hz02 solutions,

c ) stress-corrosion cracking t e s t i n EXCO-sol u t i o n achieved by t e n s i l e t e s t using low deformation r a t e (7.5 10-5 s-1).

An attempt has been made t o c o r r e l a t e between the r e s u l t s obtained by these three methods. These r e s u l t s suggest the f o l lowing conclusions :

1 ) t h e r e l a t i v e v a r i a t i o n o f absorbed s p e c i f i c energy ti 11 f r a c t u r e (A.S.P.E.T. measured i n a i r and i n EXCO s o l u t i o n i s the same as t h a t o f maximum t r u e s t r a i n 5 m. The two parameters are obtained by t e n s i l e t e s t w i t h low deformation r a t e (7.5 10-5 s - I ) ,

2) EXCO-test visual estimation (long time t e s t ) , p o t e n t i e l level measurement ( s h o r t time t e s t ) i n the d i f f e r e n t solutions mentionned above, may be correlated together when the ageing temperature i s higher than 150°C. I n t h i s range o f temperatures, S.C.C. r e s u l t s are i n good agreement w i t h the previous two tests.

The s e n s i t i v i t y o f t h e a l l o y t o corrosion by p i t t i n g and by e x f o l i a t i o n i s due t o S ' p r e c i p i t a t e s ;

3) a t lower temperature than 150°C, EXCO-test and potential level measurement show t h a t the A1-Li a l l o y i s immune against the corrosion. However, S.C.C. t e s t shows t h e contrary. P r e c i p i t a t i o n o f 6 ' p a r t i c l e s i s thought t o be the o r i g i n o f t h i s behaviour ;

4) k i n e t i c s o f 6 ' and S' p a r t i c l e s i n i t i a t i o n i s thought t o be promoted by

species d i f f u s i o n , name1 y hydrogen atoms, which increase d i s l o c a t i o n density.

REFERENCES

( 1 ) M.O. SPEIDEL, M.V.HYATT - " S t r e s s c o r r o s i o n c r a c k i n g o f h i g h - s t r e n g t h a1 m i n i urn a1 1 oys" Advances i n c o r r o s i o n science and technology - E d i t e d by M.G. FONTANA, R.W STAEHLE, Plenum press, New-York - London, v o l 2, 1972, p. 116.

(2) ASTM Designation : G 34 - ASTM (1916 RACE S t r e e t P h i l a . Pn. 19103, U.S.A.

(3) R.A. SCHULTZ, Alcoa A1 l o y s 7075-T76 and 7178-T76, Alcoa Green l e t t e r GL-214 D6c. (1969).

(4) R.L. HORST, Jr., B.W. LIFKA - Corrosion Nace, Vol. 26, n03 (1970), p. 111.

(5) W.W. BJNGES, E.H. HOLLINGS WORTH, D.O. SPROWLS - A1 uminium (KENT R. VAN HORN Ed.) - ^{A.S.M. 1,} (1967), p.209.

( 6 ) D.O. SPROWLS, R.H. BROWN - Metals Prog. 81, 5, (19621, p. 77.

(7) A.M. LOGINOW, J.F. BATE - Corrosion, Vol. 25, NO1, (1969), p.15.

(8) L. GILLEMOT - Proc. lst I n t . Conf. on Fracture. T. YOKOBORI and C o l l . Ed.

Sendai, Japon (19651, p.47.

( 9 ) L. GILLEMOT - Engin. Fract. Mech. 8, (1976), p.239.

(10) F. GILLEMOT, A. KATITANY - ASTM - STP 819, (1983), P.74.

(11) F. DEGREVE, Ph. MEYER, N. THORNE - l i t h i u m D e p l e t i o n i n A l - L i - X - a l l o y s "

PECHINEY I n t e r n a l Report.

(12) S.J. HARRIS, B. NOBLE, K. DINSDALE, M. PRIDHAM - Proc. A l - A l l o y s , t h e i r p h y s i c a l and mechanical p r o p e r t i e s . E d i t o r s E.A. STARKE J r . and T.H. SANDERS Jr., U n i v e r s i t y o f V i r g i n i a , C h a r l o t t e s v i l l e , USA, June (1 986), p.755.

(13) B. NOBLE, G.E. THOMSON - Metal S c i J.S. (1971), p.114.

JOURNAL DE PHYSIQUE

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Fig Re and Rra v a r i a t i o n versus F i g I b - 6; (P.L.C. 1 v a r i a t i o n s versus ageing temperature i n two ageing temperature i n two e n v i r o n ments : a i r and environ ments : a i r and

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environ ments : a i r and two environ ments ; a i r and

Exco-sol ution. Exco-solution.

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C H 3 0 ~ / ~ ~ 1 4 / ~ 1 2 C u and NaCl/H2%.

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C3-870 JOURNAL D E PHYSIQUE

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