LOCAL CHEMISTRY OF STRESS-CORROSION
CRACKING IN Al-Li-Cu-Mg ALLOYS
J. Craig, R. Newman, M. Jarrett, N. Holroyd
To cite this version:
J. Craig, R. Newman, M. Jarrett, N. Holroyd. LOCAL CHEMISTRY OF STRESSCORROSION CRACKING IN Al-Li-Cu-Mg ALLOYS. Journal de Physique Colloques, 1987,
48 (C3), pp.C3-825-C3-833. <10.1051/jphyscol:1987397>. <jpa-00226629>
HAL Id: jpa-00226629
https://hal.archives-ouvertes.fr/jpa-00226629
Submitted on 1 Jan 1987
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JOURNAL DE PHYSIQUE
C o l l o q u e C 3 , s u p p l 6 m e n t a u n 0 9 , Tome 48
, septembre 1987
LOCAL CHEMISTRY OF STRESS-CORROSION CRACKING I N A l - L i - C u - N g
J.G.
CRAIG, R.C.
NEWMAN, M . R .
JARRETT*
and N . J . H .
ALLOYS
HOLROYD*
Corrosion and Protection Centre, UMIST, P.O. BOX 88,
GB-Manchester M60 lQD, Great-Britain
* ~ l c a nInternational Ltd, Southam Road, Banbury,
GB-Oxon OX16 7SP, Great-Britain
ABSTRACT
alloys is subtly
The i n i t i a t i o n of s t r e s s - c o r r o s i o n c r a c k i n g i n A1-Li-Cu-Mg
a f f e c t e d by the c h e m i s t r y of t h e environment. Aerated 3.5% NaCl s o l u t i o n i s unable
t o i n i t i a t e c r a c k s under t o t a l immersion c o n d i t i o n s , but removal from t h e s o l u t i o n
a f t e r a couple of days can l e a d t o a r a p i d f a i l u r e . This "pre-exposure" e f f e c t i s
c o n s i s t e n t with a b s o r p t i o n of CO i n t o a l k a l i n e i n t e r g r a n u l a r f i s s u r e s , c r e a t i r g a
c r i t i c a l balance between a c t i v e $ i s s o l u t i o n and p a s s i v a t i o n by l i t h i u m aluminate.
The proposed l o c a l environment h a s been simulated m a c r o s c o p i c a l l y and shown t o cause
r a p i d SCC i n simple immersion t e s t s .
Sulphate a d d i t i o n s i n h i b i t t h e i n t e r g r a n u l a r c o r r o s i o n seen i n NaC1 s o l u t i o n s , and
0.012M i n 0.6M NaCl).
In d i l u t e
can induce SCC a t v e r y low c o n c e n t r a t i o n s (e.g.
chloride solutions, simulating natural fresh waters, e i t h e r bicarbonate or sulphate
a t an a p p r o p r i a t e l e v e l is r e q u i r e d f o r SCC i n i t i a t i o n .
I n a l l t h e s e s o l u t i o n s , both a c i d i c and a l k a l i n e c r a c k c h e m i s t r i e s a r e p o s s i b l e ,
depending on t h e degree of s e p a r a t i o n of anode and cathode. The a l k a l i n e chemistry
i s unique t o a l l o y s c o n t a i n i n g l i t h i u m .
INTRODUCTION
The SCC behaviour of wrought Al-ti-Cu-Mg
a l l o y s d i f f e r s from t h a t of c o n v e n t i o n a l
h i g h - s t r e n g t h aluminium a l l o y s (1). The d i f f e r e n c e s a r e n o t p a r t i c u l a r l y n o t i c e a b l e
when long c r a c k s a r e p r e s e n t , but a r e v e r y marked d u r i n g t h e i n i t i a t i o n s t a g e of
SCC. I n p a r t i c u l a r , smooth peak-aged specimens of a l l ~ y8090 a r e immune t o SCC i n
3.5% NaCl s o l u t i o n under t o t a l immersion c o n d i t i o n s , b u t can f a i l r & p i d l y under
s h o r t - t r a n s v e r s e l o a d i n g i f removed from the s o l u t i o n a f t e r a few days and -posed
t o l a b o r a t o r y a i r . T h i s pre-exposure e f f e c t could i n v o l v e both i n t e r n a l hydrogen
(absorbed d u r i n g immersion) and hygroscopic s a l t s on the s u r f a c e and/or w i t h i n g r a i n
boundary c o r r o s i o n f i s s u r e s (2).
i s t h e i r a b i l i t y t o generate alkaline solutions
shavings a r e
w i t h i n c r a c k s , c r e v i c e s o r i n t e r g r a n u a l f i s s u r e s (1). I f A1-Li-Cu-Mg
exposed t o a minimal volume of de-aerated NaCl s o l u t i o n , t h e pH r i s e s t o about 11.3
accompanied by vigorous hydrogen e v o l u t i o n and a c t i v e c o r r o s i o n ( 1 ) . T h i s r e s u l t ,
shown i n F i g u r e 1, s u g g e s t s t h a t a l k a l i n e c o n d i t i o n s a r e t o be expected w i t h i n
c a v i t i e s t h a t do not have a s u f f i c i e n t l y l a r g e remote cathode t o support l o c a l
a c i d i f i c a t i o n by aluminium d i s s o l u t i o n . The pre-exposure' phenomenon i s l i k e l y t o
come i n t o t h i s c a t e g o r y , u n l e s s aqueous s o l u t i o n w i t h i n t h e g r a i n boundary f i s s u r e s
have e l e c t r o l y t i c c o n t a c t w i t h t h e s u r f a c e of t h e m a t e r i a l .
A unique f e a t u r e of A1-Li a l l o y s
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987397
JOURNAL DE PHYSIQUE
12
w
11
>
4
5
10
5
r
a 9
-
z
0
8
r
2
w
NO A I R CONTACT
5s A I R
7
CONTACT
6
1
10
100
1000
10,000
TIME(m~nutes)
F i g u r e 1 V a r i a t i o n of pH w i t h time f o r a l l o y 8090 shavings i n 3.5% NaCl s o l u t i o n
without a i r c o n t a c t ( 1 ) .
A second unique f e a t u r e a r i s e s from t h e thermodynamics of t h e A1-Li-H20
system.
J i n g Gui and Devine (3) showed t h a t anodic p a s s i v a t i o n was p o s s i b l e i n Li2C03
s o l u t i o n b u t n o t i n Na2C03 o r K2C03, and t h a t t h i s was a s s o c i a t e d with enrichment of
Li i n t h e s u r f a c e oxide f i l m . I n s p e c t i o n of s o l u b i l i t y d a t a ( 4 ) s u g g e s t s t h a t t h i s
o b s e r v a t i o n can be i n t e r p r e t e d i n terms of t h e low s o l u b i l i t y of l i t h i u m
meta-aluminate (LiA10 *).
I n t h e p r e s e n t work, t h e pre-exposure e f f e c t discovered by Holroyd e t a 1 (1) h a s
been explored f u r t h e r , e s p e c i a l l y with regard t o t h e e f f e c t of atmospheric Cop.
Simple-immersion SCC t e s t s have been c a r r i e d o u t i n chloride/carbonate/bicarbonate
s o l u t i o n s of v a r i o u s i o n i c s t r e n g t h s , i n c l u d i n g s i m u l a t i o n s of n a t u r a l f r e s h waters
F i n a l l y t h e e f f e c t of s u l p h a t e has been explored, s i n c e t h i s is a n o t h e r u b i q u i t o u s
i 6 n i n r e a l environments. I n each c a s e , t h e behaviour of a l l o y s h a v i n g s and
' a r t i f i c i a l c r e v i c e s h a s been s t u d i e d t o i n t e r p r e t t h e SCC behavirous i n terms of
c r a c k chemistry, e s p e c i a l l y pH.
EXPERIMENTAL PROCEDURE
Material
An experimental h e a t of a l l o y 8090 was used, with t h e f o l l o w i n g composition i n wt.%:
L i 2.5, Cu 1.3, Mg 0.7, Z r 0.12, balance A l . It was s u p p l i e d a s solution-annealed
( u n r e c r y s t a l l i z e d ) one-inch p l a t e , and was peak-aged a t 190°C f o r 16 hours. The
y i e l d s t r e n g t h i n MPa was 450(L), 425(LT) and 350(ST).
SCC T e s t i n g
M i n i a t u r e specimens, measuring 25 x 4 x lmm, were c u t from t h e h e a t - t r e a t e d p l a t e s o
t h a t bending would a p p l y a ST t e n s i l e s t r e s s and permit c r a c k growth i n t h e LT
d i r e c t i o n . They were abraded t o a 600-grit f i n i s h and t e s t e d i n perspex r i n g s ,
u s i n g a nylon b o l t t o apply a bending s t r e s s j u s t above y i e l d . The b o l t and r i n g
gave some degree of spring-loading, so t h a t any s u s t a i n e d c r a c k growth i n v a r i a b l y
l e d t o f a s t f r a c t u r e . Times t o f a i l u r e were recorded and p l o t t e d a g a i n s t
a p p r o p r i a t e environmental parameters.
A l l t h e t e s t media were n a t u r a l l y a e r a t e d s o l u t i o n s , a t room temperature (15-19°C)
o r a t a c o n t r o l l e d 25'C, prepared from a n a l y t i c a l - g r a d e r e a g e n t s and d e i o n i z e d
water.
Pre-exposure t e s t s were c a r r i e d o u t by immersion of s t r e s s e d specimens i n
3.5% NaCl s o l u t i o n f o r v a r i o u s p e r i o d s , followed by exposure t o l a b o r a t o r y a i r of
v a r i o u s h u m i d i t i e s with o r without C02 (removed by NaOH s o l u t i o n s of v a r i o u s
s t r e n g t h s ) . The e f f e c t of v a r i a t i o n s i n washing procedure was a l s o examined i n
these tests.
E l e c t r o c h e m i c a l Measurements
P o t e n t i a l - t i m e curves were recorded f o r ~ o l i s h e d , u n s t r e s s e d specimens of peak-aged
a l l o y 8090 (25 x 2 5 m ) i n v a r i o u s n a t u r a l l y a e r a t e d s o l u t i o n s . Anodic p o l a r i z a t i o n
c u r v e s were measured on s i m i l a r specimens i n d e a e r a t e d s o l u t i o n s , u s i n g a
p o t e n t i o s t a t and v o l t a g e scan g e n e r a t o r .
Shavings Experiments
Alloy shavings were corroded i n minimal volumes of s o l u t i o n a s d e s c r i b e d e a r l i e r
(1). The pH w i t h i n t h e shavings was recorded c o n t i n u o u s l y f o r up t o 20 days. T e s t s
were c a r r i e d out with and without a i r (excluded by p a r a f f i n ) o r atmospheric C02
(removed by sodium hydroxide p e l l e t s ) .
Crevice Experiments
The b a r r e l of a pH e l e c t r o d e was used t o form a c y l i n d r i c a l c r e v i c e 6 m i n d i a m e t e r ,
0.5mm i n width and 25mm i n depth, w i t h i n a d r i l l e d h o l e i n t h e h e a t - t r e a t e d a l l o y
p l a t e . The pH was recorded f o r up t o 20 days, w i t h o r without a cathode s u r f a c e of
0.72 cm2 o u t s i d e the c r e v i c e , and with o r without a i r o r CD2.
RESULTS AND DISCUSSION
Pre-exposure T e s t s and t h e Role of CO.,
The pre-exposure t e s t s , u s i n g 3.5% NaCl s o l u t i o n , reproduced q u a l i t a t i v e l y t h e
e a r l i e r d a t a of Holroyd e t a 1 (1).
For a pre-exposure period of 6 days a t room
temperature (15-lg°C), t h e f a i l u r e time i n l a b o r a t o r y a i r of -50% RH was about 1
day. I f t h e whole t e s t was c a r r i e d out a t 25*C, then t h e f a i l u r e time was a few
Changing t h e IUi'to 100% had no a p p a r e n t e f f e c t on t h e r e s u l t s ,
hours (Table 1).
but no c r a c k i n g occurred over s i l i c a g e l .
Figure 2
I n i t i a t i o n of c r a c k s from i n t e r g r a n u l a r f i s s u r e s i n l a b o r a t o r y a i r a f t e r 6
days i n 3.5% NaCl s o l u t i o n a t room temperature.
JOURNAL DE PHYSIQUE
C3-828
Table 1 Times t o f a i l u r e f o r s e l e c t e d pre-exposure
NaCl s o l u t i o n .
Immersion Time
( d a y s ) i n 3.5%
NaCl s o l u t i o n
Temperature
("c)
t e s t s , a f t e r immersion i n 3.5%
Post-Treatment
Environment
Lab. A i r
Mean time
to f a i l u r e
hours
[no. of t e s t s ]
,*
1
6
20 5
Blot Dry
6
20 5
Lab. A i r
Rinse i n
De-ionised Water
20[21
6
2025
Rinse/Blot Dry
Lab. Air
20 [2
1
6
20 f 5
Rinse/Dry i n
Warm A i r
Lab. Air
15 [2
I
6
20 5
Rinse/Blot Dry
100% RH Air
20[3
1
6
20 5
Rinse/Blot Dry
100% RH
c o 2 - f r e e Air
6
6
25fl
10f5
20[2
N o failures
[3]
Rinse/Blot Dry
Lab. Air
5[3I
R i n s e / B l o t Dry
Lab. Air
No f a i l u r e s
f31
*
To t h e n e a r e s t 5 hours
-
Six-day pre-exposure t e s t s were c a r r i e d out with v a r i o u s procedures immediately
a f t e r removal from t h e NaCl s o l u t i o n : ( a ) no washing ( i . e . allowing r e s i d u a l
s o l u t i o n t o dry o u t ) ; ( b ) b l o t t i n g dry; ( c ) r i n s i n g i n d i s t i l l e d water and b l o t t i n g
d r y . A l l the r e s u l t s l a y w i t h i n one s c a t t e r band
i . e . macroscopic amounts of
NaCl d e p o s i t a r e not r e q u i r e d f o r c r a c k i n g t o occur.
-
The s u r f a c e s of t h e specimens a f t e r pre-exposure were s l i g h t l y g r e e n i s h , covered
w i t h a l o o s e hydrous t a r n i s h , and showed numerous i n t e r g r a n u l a r f i s s u r e s ( p i t - l i k e
i n c r o s s - s e c t i o n ) which were packed w i t h a d a r k hydrous m a t e r i a l .
The c r a c k s
i n i t i a t e d from t h e s e f i s s u r e s , a s shown i n F i g u r e 2. Both m e t a l l i c ( a t t a c h e d ) and
o x i d i z e d (green, unattached) copper p a r t i c l e s were v i s i b l e i n and around t h e
fissures.
None of t h e pre-exposed specimens t e s t e d above NaOH s o l u t i o n ( i . e . without C02)
showed any evidence of c r a c k i n g a f t e r p e r i o d s of up t o 30 days. The pre-exposure
t r e a t m e n t s included room temperature f o r 6 days, and 25°C f o r 6-20 days ( t h e l a t t e r
normally g i v i n g a f a i l u r e w i t h i n 2 hours).
The NaOH c o n c e n t r a t i o n s i n c l u d e d 0.5M
(RH=97%), 2M (RHn80X) and s a t u r a t e d ( v e r y low RH). A l l t h e specimens were b l o t t e d
d r y a f t e r removal from the NaCl s o l u t i o n , then put i n t o the C02 f r e e environment
w i t h i n 30 s ; even a few minutes' exposure t o warm a i r c o n t a i n i n g C02 was enough t o
cause f a i l u r e *en s u b s e q u e n t l y deprived of C02.
These r e s u l t s s t r o n g l y s u p p o r t t h e f o l l o w i n g model of t h e pre-exposure
phenomenon:
1.
The NaCl s o l u t i o n i s too a g r e s s i v e ( d e p a s s i v a t i n g ) towards t h e g r a i n boundaries
t o permit SCC i n i t i a t i o n .
2.
On removal from t h e NaCl s o l u t i o n , t h e (now i s o l a t e d ) g r a i n boundary f i s s u r e s
behave l i k e the shavings i n F i g u r e 1, becoming a l k a l i n e and a c t i v e with a v e r y
low c o r r o s i o n p o t e n t i a l .
3.
Absorption of C02 c r e a t e s an a l k a l i n e c h l o r i d e / c a r b o n a t e s o l u t i o n of g r a d u a l l y
d e c r e a s i n g pH, which e v e n t u a l l y allows spontaneous p a s s i v a t i o n (by LiA102)
within the fissures.
4.
SCC i n i t i a t e s and propagates i n t h i s i d e a l b o r d e r l i n e a c t i v e / p a s s i v e c o n d i t i o n ,
a s s i s t e d by hydrogen absorbed along g r a i n boundaries d u r i n g immersion.
D i r e c t v e r i f i c a t i o n of t h i s model r e q u i r e s l o c a l chemistry measurements, which a r e
i n progress.
I n d i r e c t evidence can be obtained by s i m u l a t i n g the f i s s u r e
environment, a s f o l l o w s
.
Immersion T e s t s i n 3.5% NaCl/carbonate/bicarbonate S o l u t i o n s
These t e s t s l e d t o r a p i d SCC f a i l u r e s over a range of pH, which changed w i t h t o t a l
c a r b o n a t e c o n c e n t r a t i o n ( F i g u r e 3). The upper l i m i t of pH f o r SCC was roughly
c o r r e l a t e d with a p a s s i v e - a c t i v e t r a n s i t i o n , a s shown i n F i g u r e 4- t e s t s c a r r i e d
out a t pH 11.65 and 0.1M Li2C03 but with a n imposed p o t e n t i a l of -720mV (SCE), gave
extremely r a p i d f a i l u r e s . F i g u r e 5 shows anodic p o l a r i z a t i o n curves which
reproduce t h e p a s s i v i t y shown by J i n g Gui and Devine ( 3 ) .
-C
Figure 3
I
.
-~
S~mpl! Immersion
Times t o f a i l u r e f o r SCC t e s t s i n 3.5% NaCl c o n t a i n i n g 0.1M o r 0.01M
(Li2COi + LiHC03).
3 5 % NaCl t O l M ILi2C03 t L l H t O 3 1
1E
-600
-
v.
"0
t
1
-*zr
.,
--
Y1
25
- 'i -1000 - 2
----.___+___
...................... L - = 1 1
+-------
35
3 5 % N o C l t O O l M l L 1 2 C O j t LIHCOJI
-
r,*-.-.----.-----.I05
,_--.- - - - - - - - - - - - - - - - -.l1°
:I5
t
PY
f
,
1
2
TIME Ihoursi
Figure 4
3
-
_
-
1
2
TINE (hours1
P o t e n t i a l - t i m e c u r v e s f o r t h e a e r a t e d s o l u t i o n s used i n t h e SCC t e s t s of
l i m i t of pH f o r SCC corresponds roughly t o a
Fig.3, shoving t h a t the upper
-passive-activc t r a n s i t i o n :
i 0+
3 LiHC03); ( b ) 0.01M (Li 2C03
( a ) 0.1M ( ~2 ~
+ LiHCO3)'
JOURNAL DE PHYSIQUE
C3-830
The reduction in the upper limit of pH for SCC with reduced carbonate concentration
Now we recognize that
was at first thought to be a crack chemistry effect (2).
this is partly due to the reduced ~ i +concentration and consequent reduction in the
passivating effect of LiA102 precipitation, so that the passive-active transition
pH is reduced. Use of Na2C03 instead of Li2C03 gave even lower passive-active
transition pH values.
The appearance of a lower limit of pH for SCC at the lower carbonate level (Figure
3) tends to argue for an alkaline crack nucleus.
CURRENT
DENSITY l r n ~ j c r n ~ l
Figure 5 Anodic polarization curves for deaerated solutions, showing the critical
current density for passivation and its variation with pH.
Dilute S o l u z
A lOppm chloride solution (2.5 x ~o-'M
NaC1) gave no SCC failures, but failure
occurred within 2 days at 2S°C if 3 x ~ o - ~NaHC03
M
was added (Table 2). A series
of tests carried out at 10f5°C showed a strong requirement for sulphate, but this
vanished at 25°C; further investigation of the importance of sulphate in dilute
solutions is continuing.
Table 2 Times to failure for SCC tests in dilute (10 ppm) chloride solutions
containing bicarbonate and/or sulphate ions.
Cl-(M)
HCO 3-(M)
SO~'-(M)
Temp. ( "C)
Time to failure,
hours
[no. oi tests]
2.5 x
3 x
10f5
2.5 x
3 x loe4
2525
<48
[3]
2.5
x
3 x 10'~
5
10f5
(48
[3]
2.5 x
3 x 10'~
5 x
2525
(96
[3]
lo-=
No failures [3]
These results show that natural fresh waters can be more aggressive, in terms of
simple-immersion SCC, than 3.5% NaCl solution. This probably has some engineering
significance.
ChlorideISulphate S o l u t i o n s
Since the i n i t i a t i o n of SCC seems t o r e q u i r e a s u b t l e i n h i b i t i o n of i n t e r g r a n u l a r
c o r r o s i o n , it is of i n t e r e s t t o e x p l o r e t h e e f f e c t s of n a t u r a l l y o c c u r r i n g p i t t i n g
i n h i b i t o r s such a s s u l p h a t e . T e s t s i n c h l o r i d e / s u l p h a t e s o l u t i o n s ( F i g u r e 6 )
immediately showed a s t r o n g tendency f o r SCC, and t h e amount of s u l p h a t e r e q u i r e d
l e s s t h a n 0.012M i n 0.6M NaC1. This r e s u l t suggested t h a t
was s u r p r i s i n g l y low
simple-immersion SCC should occur i n a r t i f i c i a l s e a w a t e r , which proved t o be s o ,
although t h e f a i l u r e time i n seawater was l o n g e r than.would have been a n t i c i p a t e d
from i t s s u l p h a t e c o n t e n t . Probably t h i s is due t o some e f f e c t of magnesium o r
calcium ions.
-
L
v
I
3
a
a , , ,
!
b
3
,
n ! ,
8
8090- PEAK
10000
AGED
or tificiol
seo woter
0.6M NoCl
I
I
0.6M No2SOL
+c-t-)+
varlable NoZSOk 1 vor~oble NaCl
Figure 6
Times t o f a i l u r e f o r SCC t e s t s i n n e u t r a l c h l o r i d e / s u l p h a t e s o l u t i o n s .
The e l e v a t i o n i n c o r r o s i o n p o t e n t i a l by s u l p h a t e was l e s s t h a n lOmV, and i t s e f f e c t
was apparent i n p o t e n t i o s t a t i c t e s t s a t -720mV (SCE), s o t h a t i t s e f f e c t must be
a s c r i b e d t o c r a c k chemistry m o d i f i c a t i o n r a t h e r than a change i n p o t e n t i a l . Some
i n h i b i t i o n of p i t t i n g and i n t e r g r a n u l a r c o r r o s i o n by s u l p h a t e was observed,
a l t h o u g h not r e a d i l y seen a t t h e lowest l e v e l s t h a t induced SCC.
Recent t e s t s , n o t d i s c u s s e d here, have a l s o r e v e a l e d SCC i n c h l o r i d e i n i t r a t e
solutions.
Shaving T e s t s
A s shown i n Figure I , t h e c h a r a c t e r i s t i c behaviour of shavings i n de-aerated NaCl
s o l u t i o n i s t o i n c r e a s e the pH t o 11.3.
The presence of a i r c o n t a i n i n g C02 causes
a g r a d u a l c a r b o n a t i o n and f a l l i n pH (1).
A shavings t e s t with a i r c o n t a c t but without W 2 gave even higher pH v a l u e s , up t o
12.5.
The presence of s u l p h a t e a f f e c t e d t h e behaviour of t h e shavings t e s t , but not t o
such an e x t e n t a s t o e x p l a i n r e a d i l y t h e o n s e t of SCC. F i g u r e 7 shows t h e s e
r e s u l t s , which r e q u i r e more i n t e r p r e t a t i o n .
A r t i f i c i a l Crevice T e s t s
Whenever t h e s e t e s t s used a p a s s i v e a l l o y s u r f a c e coupled t o t h e c r e v i c e ,
a c i d i f i c a t i o n occurred i n t h e c r e v i c e ( F i g u r e 8). T h i s was observed even a t 25ppm
(6 x 10'~m) Cl'm and i s c o n s i s t e n t both with t h e a c i d i c pH v a l u e s recorded i n deep
JOURNAL DE PHYSIQUE
C3-832
s t r e s s - c o r r o s i o n c r a c k s , and with t h e a b i l i t y of NaCl s o l u t i o n s t o s u s t a i n t h e
growth of long c r a c k s ( 1 ) .
I f no a l l o y s u r f a c e was coupled t o t h e c r e v i c e , then t h e c r e v i c e became m i l d l y
a l k a l i n e . I f Cop was a l s o removed, then a s u s t a i n e d a l k a l i n e pH was observed ( n o t
s u r p r i s i n g l y , a s t h i s should be s i m i l a r t o t h e corresponding shavings t e s t s shown
i n F i g u r e 1 ) . These r e s u l t s a r e a l s o shown i n F i g u r e 8.
5
j
2
10
/
>
AI-Li-Cu-Mg
SHAVINGS
3l/2% NaCl
NO AIR CONTACT
V)
1
Figure 7
10
100
T I M E (minutes1
1000
10000
V a r i a t i o n of pH w i t h time f o r s h a v i n g s t e s t s i n c h l o r i d e / s u l p h a t e
s o l u t i o n s without a i r c o n t a c t .
Ai-ii-Cu-Mg
Ic)
6mm. CREVICE
0
20
10
1
30
TIME (hr.1
Figure 8
V a r i a t i o n of pH with time f o r a r t i f i c i a l c r e v i c e s :
( a ) with a i r c o n t a c t ,
coupled t o a 0.72 a n 2 cathode; (b) with a i r c o n t a c t but without an
e x t e r n a l cathode; ( c ) w i t h a i r c o n t a c t , w i t h o u t a cathode and with C 0 2
removed by NaOH p e l l e t s .
CONCLUSIONS
1.
Pre-exposure "embrittlement" of A1-Li-Cu-Mg alloys is a form of
stress-corrosion cracking. The need for atmospheric C02 availability has been
attributed to the carbonation of an alkaline condensate generating a local
environment providing a critical balance between activity and passivation
necessary for SCC.
2. Alkaline chloride/carbonate/bicarbonate solutions can be severe SCC
environments for AL-Li-Cu-Mg alloys.
3.
Passivation by precipitation of lithium aluminate (LiA102) is an important
aspect of corrosion and SCC in A1-Li alloys.
4.
Small additions of Na2S04 to NaCl solution can induce SCC in simple immersion
tests.
5.
Both HC03- and
are able to induce SCC in dilute (e.g. 10 ppm) chloride
solutions. Natural fresh waters could be aggressive or benign, depending on
composition.
6.
Acidification is normal in long cracks, but in crack nuclei alkaline
chemistries may occur. This is a new type of "short crack" effect.
REFERENCES
1.
N.J.H. Holroyd, A. Gray, G.M. Scamans and R. Hermann, Aluminium-Lithium 111,
p.310, Institute of Metals, London (1986).
2. J.G. Craig, R.C. Newman, M.R. Jarrett and N.J.H. Holroyd, Environmental
Degradation of Engineering Materials 111, eds. M.R. Louthan, R.P. McNitt and
R.D. Sisson, p.313, Pennsylvania State University (1987).
3.
Jing Gui and T.M. Devine, Scripta Met., Vo1.21, p.853 (1987).
4.
Handbook of Chemistry and Physics, ed. R.C. Weast, p.Bl01, CRC Press Inc., 67th
Edition (1986).