INTERGRANULAR AND EXFOLIATION CORROSION STUDY OF Al-Li-Cu-Mg-Zr ALLOYS

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INTERGRANULAR AND EXFOLIATION

CORROSION STUDY OF Al-Li-Cu-Mg-Zr ALLOYS

M. Reboul, P. Meyer

To cite this version:

M. Reboul, P. Meyer. INTERGRANULAR AND EXFOLIATION CORROSION STUDY OF Al-Li-Cu-Mg-Zr ALLOYS. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-881-C3-889.

�10.1051/jphyscol:19873103�. �jpa-00226537�

JOURNAL D E PHYSIQUE

Colloque C3, suppl6ment au n 0 9 , Tome 48, septembre 1987

INTERGRANULAR AND E X F O L I A T I O N CORROSION STUDY O F A l - L i - C u - M g - Z r ALLOYS

M. REBOUL and P . MEYER

Cegedur-Pechiney, Centre de Recherche et Developpement, B.P. 27, F-38340 Voreppe, France

ABSTRACT

The 8090 type AI-Li alloys are prone to intergranular and exfoliation corrosion in the peak aged temper. In marine atmosphere exfoliation corrosion occurs within 2 years. The 48 H EXCO test give a good prediction of the corrosion susceptibility of 8090 type alloys in marine atmosphere.

Under-ageing and over-ageing are two possibilities to suppress exfoliation susceptibility. Exfoliation corrosion is always intergranular on specimens examined in this study.

lntergranular corrosion is usually explained by the existence of a continuous anodic path at grain boundaries. The anodic path in Al-Li-Cu-Mg alloys is first a Cu depleted zone. lntergranular and exfoliation corrosion only occur in Cu containing AI-Li alloys. The corrosion potential Eo of AI-Li alloys reflect the Cu content in solid solution and is not affected by Li addition. The pitting potential also reflect the Cu content of the solid solution, it is shifted 30 mV in the active direction by Li addition at the PA temper. Li addition prevent intergranular corrosion desensitization observed for 2XXX AA, in the PA temper.

INTRODUCTION

Al-Li-Cu-Mg-Zr alloys e.g. 8090 are prone to intergranular and exfoliation corrosion, particularly in the peak aged temper. Corrosion of Al Li alloys have been extensively studied during the last 5 years (1-6) but the intergranular corrosion mechanism for Al-Li-Cu-Mg alloys is not ye1 clearly understood. lntergranular corrosion only dissolve grain boundaries.

This particular form of corrosion which may be encountered with differents metallic materials, generally results from the existence of a continuous anodic path (anodic vs the bulk alloy) at grain boundaries. In a corrosive environnement this micro galvanic coup:e will dissolve the anodic path and produce intergranular corrosion.

For conventional aluminium alloys, two very different anodic paths may be encountered :

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19873103

C3-882 J O U R N A L DE PHYSIQUE

- precipitation of an anodic intermetallic compound e.g. A13 Mg2 in the 5XXX series which form a continuous "film" around grains. The corrosion potential of this phase -1.15 Vsce* is very anodic vs -0.78 Vsce for the bulk alloy (7).

- depletion of a cathodic element e.g. Cu in the intergranular precipitate free zone of 2XXX series alloys (8,9).

In sheets, forged and extruded products, the flat elongated grain shape associated with these anodic paths results in exfoliation corrosion, in a corrosive environnement, such as marine atmosphere. AI-Li-CU-Mg alloys containing at the same time anodic (Li,Mg) and cathodic (Cu) alloying elements vs aluminum could form both types of anodic paths.

The aim of this paper is to study the intergranular and the exfoliation-corrosion mechanisms for Al-Li-Cu-Mg alloys. The influence of alloying elements and microstructure on corrosion of t h e s e a l l o y s w i l l b e studied. C o m p a r i s o n s between exfoliation-corrosion laboratory tests and marine atmosphere behaviour will be also discussed.

EXPERIMENTAL

We have focused this corrosion study on the 8090 AI-Li alloy, first AI-Li-Cu-Mg alloy studied by PECHINEY. Exfoliation corrosion of 8090 alloy in different tempers has been tested in the marine atmospheres of Salin de Giraud France on the Mediterranean Sea shore and Ostende Belgium on the North Sea. Results have been compared with laboratory EXCO test (ASTM G34) for 48H and 96H and MASTMAASIS (ASTM G85 annex 2) for 2 and 4 weeks, performed by two different laboratories lab 1 and lab 2.

The 8090 alloy and simplified alloys have been cast into 150 kg billets.

Table 1 8090 and simplified alloys studied.

The billets have been machined and extruded to produce 100X13 mm bars.

The bars were solution heat treated, water quenched, some of them were 2%

stretched (TX51) to relieve internal stresses. Further ageing was performed in conventional air furnace.

Cast no ALLOY

1299 8090

11 56 AI-Li 151 8 AI-Li-Zr 1522 Al-Li-Mg-Zr 1600 Al-Cu-Mg-Zr

*- mVsce all the potentials are vs the standard calomel electrode (saturated calomel in a saturated KC1 solution)

COMPOSITION % Li

2,4 1,7 2,4 2,4

Fe 0,03 10,02 0,02 0,03 0,03

S i 0,03 10,02 0,02 0,02 0,03 Cu

1,3

1,3

Mg 1,0

0,90 0,97

Zr 0,lO 0,10 0,12 0,11

Ti 0,03

<0,02 0,03 0,03

Extruded 100 X 13 mm specimens Marine atmosphere (Salin de Giraud) and exfoliation laboratory test results TABLE II

-

⁸⁰⁹⁰ T6

S= Surface M= Midsection

OK= same degree in 96H EXCO (lab 1 vs SG) + means that fhis test is more severe than marine test for fhis specimen TABLE Ill - 8090 T 651

48H 160°C M

12H 190'C M S 3H 220%

M S 48H 220°C

M

EXCO PlASTPlAAS I S

EB

EB EA ENEB

EB

48H 160°C M S 1ZH 190%

M S 3H 220°C

M S 48H 220°C

M

F i g . 1

-

E x f o l i a t i o n c o r r o s i o n o f 8090 T6 (48H

a

160°C) A1-Li a l l o y observed a f t e r l a b o r a t o r y t e s t s .

EC

EC P EB

ENEB

EB P

P EA

EB

€4

S E A P P P E A P

EA P EA

P P P P

P

EC PIEA

N EA

P N P

EB

EB

EB P EB

P

EA P E A E A E A E A

P P

P

ENEB

€NEB ENEB ENEB

P P

EBIEC

EA EA

P PlEA

P P

I

P

P P P

P

EC EA EA ENEB ENEB P EBIEC

EB P EA P EA P P

P P P P P P P

EB P EA P P P P

EB P EA P EA P P

CK

EB P

EA P P P P

+

CK

CK

C K + C K + - C K

C K + C K + C K -

C K C K + + a C K

+

CK

+ + + C K -

+ O K + -

C K C K C K W - C K

+ + + +

C K O K C K C K

CK CK

C3-884 JOURNAL DE PHYSIQUE

Corrosion specimens 50x1 00x1 3 and 100X100X13 were sawed and machined to test both the extruded surface and midsection in the different corrosive environments.

- lntergranular corrosion susceptibility of AI-Li-Cu-Mg alloys is generally tested for 6h in the NaCl IM+H202 0,3O/0 solution (MIL-H-6088, ASTM B597, AIR 9050) but because some simplified alloys do not contain cathodic elements, this neutral test will not attack these specimens. To check if the distribution of anodic elements is a determining factor of the intergranular corrosion of AI-Li alloys, we also used two more aggressive tests :

-

intergranular acid test, 24h immersion in NaCl 3%

+

HCI 0,5%. This test is used in our laboratory for corrosion resistant Al alloys such as 5XXX series.

- INTERANO which is an anodic test 6h ImA/cm2 in NaC104 2M

+

AIC13 0.03M

+

Cr04 (NH4)2 0.01M, to impress a corrosion attack. We use this test for corrosion resistant alloys such as 6XXX series.

At completion of attack specimens are metallographically examined on a cross section to check the penetration mode of corrosion and to measure the maximum depth of corrosion penetration.

- Electrochemical measurements

Corrosion potentials have been measured in N a C l l M

+

H202 0.3%

(ASTM-G69).

Pitting potentials have been measured in NaCl 3% with a galvanostaircase technic described in reference (1 0).

RESULTS

-

Exfoliation corrosion results are given in tables II and Ill.

The most severe corrosion degree obtained in marine atmosphere is ED for 8090 T6 in UA and PA tempers Appendix I. Exfoliation is observed first in the marine exposition under shelter (a roof prevent these specimens to be washed by rains), then on the ground ward face of the specimens exposed in open atmosphere.

Exfoliation obtained during atmospheric and EXCO tests have the same feature (Fig. 1 and Appendix I). Microscopic cross section examinations show (fig 2) that both MASTMAASIS and EXCO tests only produce interganular Exfoliation attacks fig.2.

-

lntergranular corrosion results are given on table IV. lntergranular corrosion and exfoliation corrosion (EXCO 48H) only occur in copper containing alloys, regardless of the corrosion test. We will compare these results in more details in the discussion section.

- Electrochemical results : The average potentials obtained with 2 specimens minimum are given in table V. The corrosion potentials are given on figure 3, the pitting potentials are given on figure 4.

Fig. 2

-

8090 T6 Exfoliation corrosion

-

Cross section examination X 200

TABLE IV - AlLi 8090 and simplified alloys - Exfoliation (EXCO 48H) and lntergranular corrosion results

S = Sunace M = MlaSeCllOn

Tempers AQ = T351 - PA = T851 12H190°C - ^{UA T851 3H 160°C} - ^OA = T851 48H 220°C intergranular Corrosion - l(300) means lntergranular attack 300 pm maxi pdnbtralian P pits - P+I = pits + lntergranular ramifications

1156 S AIL1 2% M

1518 S lL12,5%Zr

1522 S AIL12 5Mg 1Zr M

1600 S AICuI,35Mg

1Zr M . .

1.7

12,4 M

2.4

0

TABLE V - CORROSION AND PITTING POTENTIALS (mVsce) FOR 8090 AND SIMPLIFIED ALLOYS 0

0

0

1,3

1 1 5 6 Li 2%

1518 P LI 25%.Zr

1522 T LI 2.5%, Mq 1%.Zr

1600 T Cu 1.35%.

Mq 1%.Zr

Eo Midsection mVsce

Ep Midsection mVsce Alloy

S = Surface M = M~dsection -749

-768 -754 -755 -752 -730 -752 -739 -740 -766 -788 -682 -696 -709 -714 0

0

0.9

1

UA P A OA AQ UA P A OA AQ UA P A OA A UA PA OA

A Eo M - S Ageing

-7 -7 -9 -11

- 3 -10 -7 - 3 - 3 -8 - 4 1 7 5 - 3 0

0 , l

0.1

0.1

Ep - ^Eo

M EoS

mvsce

- 7 7 3 - 7 7 2 -772 -771 - 7 6 5 -774 -781 - 7 8 9 - 7 9 2 -786 - 8 0 5 - 7 0 5 -721 -761 - 7 5 4

31 11 27 27 16 54 3 6 53 55 28 2 1 22 1 9 47 43 - 7 8 0

- 7 7 9 - 7 8 1 -782 - 7 6 8 - 7 8 4 -788 - 7 9 2 - 7 9 5 - 7 9 4 -809 -704 -715 -756 - 7 5 7 P

PlEA P P

N N N

P EB

N N N

EB EE

P P

N N N N N N N N N N

N N N N N N N N N N

N N N N N N N N N N

N N N N N N N N N N

11300) 11300)

N N N

P P

N N N - N

P P

N N

i(400) l(300)

N

N N

N N

P+1(200) P+1(200)

N

N N

N N

11150) l(100)

N N N P P P

-

P P

P P

P P+1(200)

P P

P P

P P

11200) l(300)

P P

P P

P P

P

P P

p P

P+1(300) N

C3-886 JOIJRNAL DE PHYSIQUE DISCUSSION

The first 8090 specimens tested in marine atmosphere have now been exposed for 46 months. The exfoliation corrosion results are given in appendix I. These results show that for susceptible specimens only pits appear during the first 6 months in marine atmosphere. Exfoliation corrosion only appears between 6 and 12 months, exfoliation is completely developed within 2 years. The degrees of exfoliation do not change any more during 2 additional years of exposure (Appendix I).

Results given in tables II an Ill have been obtained after 24 months.

Table II and Ill also compare the results for each specimen. Comparisons ol exfoliation corrosion results are summinarized in table VI.

Table VI summarized compararisons of exfoliation results.

OK means same degree between both tests ; 2+ in the MASTMAASIS 4W means that this test is too severe (vs Marine atmosphere ) for 2 specimens ;

1- that this test is not severe enough for 1 specimen.

The comparaison between lab 1 and lab 2 results for the 48H EXCO test, shows different ratings for 3 specimens of one rating letter over 1 0 quotations. These differences are considered as normal, in the precision section of ASTM G34 for a rating obtained by visual inspection. Because exfoliation corrosion results are based on visual examination of corroded specimens compared with standard photographs, we cannot hope to obtain perfect agreement betwsen inspectors and between different tests. From these results, taking this difficulty into account, we think that both the 4 weeks MASTMAASIS and the 48H EXCO tests may be used to predict exfoliation corrosion of 8090 type AI-Li alloys in marine atmospheres.

However the 48H EXCO test which is more simple, more rapid and a little bit too severe, appears as a realistic accelerated test.

These results also confirm that EXCO 96H is definitely too severe, and in our opinion, that MASTMAASIS 2W is not severe enough.

-

Intergranular and exfoliation corrosion obtained on table IV show that only Cu containing alloys are susceptible to both types of corrosion. Taking into account only the neutral intergranular test, alloy Al-Cu-Mg-Zr is only

Lab 1 vs Lab 2 U(C0

48H 7J10 OK 2

+

1 -

Lab 1 vs Marine Atmosphere (24 months) MASTMAASIS

2W 311 0 OK

7+

EXCO 48H 7"0 OK

3+

EXCO 96H 2Il0 OK

8+

MASTMAASIS 2W 7fI0 OK

3-

MASTMAASIS 4W 7J1 OK

2+

1

-

susceptible in the UA temper. The 8090 alloy is susceptible in both UA and PA tempers

.

So even if Li cannot generate by itself intergranular corrosion in alloys without Cu, it increases the susceptibility of Cu containing alloys, or prevents desensitization in the PA temper (as compared with conventional 2XXX alloys e.g. 2024).

A more detailed study on the influence of ageing conditions on intergranular, exfoliation and corrosion (Pechiney Unpublished works) has shown that the susceptibility areas to these different intergranular corrosion types largely overlap, but the borderlines are differents. So exfoliation corrosion of Al-LilCu-Mg alloys is not fully and completely understood by the electrochemical mechanism used to explain intergranular corrosion.

- Electrochemical potentials results of fig. 3 for different alloys and tempers, show that only Cu in solid solution significantly affects the corrosion potential Eo of Al. For both 8090 and Al-Cu-Mg-Zr alloys the corrosion potential in the as quenched condition is approximately -700 mVsce. The Cu precipitation during ageing, producing the S' phase in the matrix and S at grain boundaries (ll), shifts the corrosion potential in the more active direction from -700 to -760 mVsce. No futher evolution of potential is observed between peak-aged and over-aged tempers. Alloys containing copper stay 10 to 20 mV more cathodic versus alloys without Cu.

No significant difference appears on corrosion potentials with lithium addition (2.4%). Unpublished Pechiney results obtained with homogenized and quenched binary AI-Li alloys have shown that Li do not affect the corrosion potential of Al as far as the Li content is below 3%. The non influence of Li addition on Eo. is also shown when comparing the midsection potential with the surface potential of the Li depleted surface (12) table V.

FIG. 3

-

Corrosion Potentials mV/sce

-

^{8090 and}

simplifleld alloys Midsection.

FIG. 4

-

Pitting Potentials m V s c e

-

^{8090 and}

simplifleld alloys midsection.

Fig. 4 shows pitting potentials Ep of the same alloys and tempers. Copper in solid solution has again the strongest influence. The relative positions of curves for Cu containing alloys with or whithout Li, show that Li addition

C3-888 J O U R N A L D E PHYSIQUE

shifts Ep from -680 mV to -655 mV in the as quenched temper and from -710 to -740 mV in PA and OA tempers. A similar (but less pronounced)

effect is observed with Mg addition ( E ~vs E~ I ~~ ~~ IHowever no ~ ~ ~ ~ ) . correlation was found between Ep or Ep-Eo and intergranular corrosion

susceptibility of these alloys (table V).

CONCLUSIONS

-

The 8090 type AI-Li alloys in the peak aged temper are prone to exfoliation corrosion. In marine atmospheres exfoliation needs 6 to 10 months to appear and reach its final quotation degree within 2 years.

-

The 48H EXCO test give a good prediction of the corrosion susceptibility of 8090 and similar alloys in marine atmosphere except in OA tempers.

-

Underageing and overageing are two efficient means to decrease or suppress exfoliation corrosion of 8090 type alloys.

-

Exfoliation corrosion is always intergranular for the Al-Li-Cu-Mg alloys examined in this study.

-

The 8090 type Al-Li alloys in the peak age temper are prone to intergranular corrosion. This corrosion type is generally explained by the presence of a continuous anodic path at grain boundaries. The anodic path in Al-Li-Cu-Mg alloys is first a Cu depleted zone, near the grain boundaries.

Only Cu containing alloys are susceptible to intergranular and exfoliation corrosion in the PA temper. Only Cu affect the corrosion potential of these alloys and shift it in the positive direction.

- Lithium has a detrimental effect on intergranular corrosion, it increases the susceptibility of Cu containing alloys, or prevent desensitivation in the PA temper.

-

The corrosion potentials of Al-Gu-Mg alloys reflect the Cu content in solid solution for each temper, and are not affected by Li additions. The pitting potential also reflect the Cu content in solid solution and is shifted 30 mV in the active direction by Li addition for PA and OA tempers.

BIBLIOGRAPHY

1 ) NISKANEN P., SANDERS T.H., RlNKER J.G., MAREK M.

Corrosion Science 22 (4), 283-304, 1982 2 ) PlZZO P.P., TSAO C.T.

NACE annual Conference, Corrosion 85, paper no 69, Boston March 25-29 (1985)

3 ) CIESLAK S.J., HART R.M., MEHR P.L., MUELLER L.N.

Alcoa allthalrte alloy 2090

-

Technical information

17th Internat. SAMPE Tech. Conf. Kiamesha Lake NY Oct. 22-24, (1985) 4) HOLROYD N.J.H., GRAY A , SCAMANS G.M.

3rd Internat. AI-LI Conf., OXFORD July 8-15 (1985) 5) COLVlN E.L., CAHEN G.L., STONER G.E., STARKE E.A.

Corrosion, 42 (7). 416-421, July 1986

6) MORAN J.P., STARKE E.A., STONER G.E., CAHEN G.L.

NACE annual conf. Corrosion 86, paper no 203, Houston March 17-21, (1 986)

7 ) LIFKA B.W., SPROWLS D.O.

F r o m " L o c a l i z e d C o r r o s i o n C a u s e of m e t a l f a i l u r e "

ASTM-STP 51 6, (1 972), 120-144.

8 ) KETCHAM S.J., HAYNIE F.H.

Corrosion 17 (7), 242t, 1963 9 ) SPROWLS D.O., BROWN R.H.

From "Fundamental Aspects of Stress Corrosion Cracking" The Ohio State Univ. (1 967), 466-51 2.

1 0) REBOUL M.C., BOUVAIST J.

Werkstoffe und Korrosion 30, 700-712, (1979) 1 1 ) SAINFORT P., DUBOST B., MEYER Ph.

"Advanced Materials R & D for Transport"

Symposium on light metals - Strasbourg Nov. 26-28, (1985) Les Editions de Physique (1986)

1 2 ) THORNE N., DUBUS A., LANG J.M., DEGREVE F., MEYER PH.

4th Intern. AI-Li Conference PARIS, June 10-12, 1987.

APPENDIX I

E x f o l i a t i o n c o r r o s i o r l o f A1 alloys i n the marine atmosphere of Ostende

~ e l g i u m on the ~ o r t i , s e a s h o w (tlidsection).

EXFOLIATION CORROSION

8090 T B L l 1" 190. (1 8090 T l l l 2 " y l 160. I.,

w e -

8090 T6 UA 4H a t 190°C 8090 T6 PA 12H at 190°C

Surface P Midsection = ED S = E A M = E C