Stress-Corrosion Cracking - American Chemical Society

10

Stress-Corrosion Cracking J. C. SCULLY

Downloaded by CHINESE UNIV OF HONG KONG on March 9, 2016 | http://pubs.acs.org Publication Date: January 31, 1979 | doi: 10.1021/bk-1979-0089.ch010

Department of Metallurgy, University of Leeds, Leeds LS2 9JT, England

S t r e s s c o r r o s i o n c r a c k i n g i s t h e phenomenon by which alloys fail by c r a c k i n g when s i m u l t a n e o u s l y s t r e s s e d and exposed t o certain e n v i r o n m e n t s . Failures o c c u r a t s t r e s s l e v e l s w e l l below t h a t which would cause failure in air. While the a p p l i c a t i o n o f the s t r e s s may be multi-axial, it is n e c e s s a r y f o r it t o have a tensile component and c r a c k i n g will u s u a l l y occur p e r p e n d i c u l a r l y to it. Stress corrosion cracking r e p r e s e n t s the most h i g h l y localized form o f c o r r o s i o n t h a t is e v e r e n c o u n t e r e d . W h i l e t h e d i s c u s s i o n in this c h a p t e r i s c o n f i n e d to metallic alloys, s t r e s s c o r r o s i o n c r a c k i n g is p a r t o f a l a r g e r range o f phenomena, s i n c e s i m i l a r failures o c c u r in n o n m e t a l l i c m a t e r i a l s , e.g., g l a s s in H2O, o r g a n i c polymers in p o l a r s o l v e n t s , a l u m i n a in H2O. F u r t h e r m o r e , w h i l e most o f the d i s c u s s i o n is c o n f i n e d t o a l l o y s c r a c k i n g in aqueous e n v i r o n m e n t s , s i m i l a r c r a c k i n g in some alloys o c c u r s in o r g a n i c liquids, steam, d r y gases and in b o t h liquid and solid m e t a l s . The amount o f c o r r o s i v e may be q u i t e s m a l l . Failures have been c a u s e d , f o r example, by t h e perspiration residue of a single fingerprint. Within the confines o f a s h o r t c h a p t e r , it is n o t p o s s i b l e t o d i s c u s s e v e r y example o f such failures. The d e s c r i p t i o n s b e low a r e c o n f i n e d m a i n l y t o 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 aqueous m e d i a . As an industrial problem s t r e s s c o r r o s i o n c r a c k i n g is o f c o n s i d e r a b l e i m p o r t a n c e . There is a l o n g h i s t o r y o f major and minor failures, particularly i n the c h e m i c a l i n d u s t r y and in t h e t r a n s p o r t i n d u s t r y , particularly o f components i n s h i p s and p l a n e s . It is a major potential s o u r c e o f failure in t h e n u c l e a r power i n d u s t r y in w h i c h , f o r example, a u s t e n i t i c s t a i n l e s s s t e e l s may fail i n h i g h purity water c o n t a i n i n g oxygen and c h l o r i d e i o n s a t t h e level o f p p b .

0-8412-0471-3/79/47-089-321$07.50/0 © 1979 American Chemical Society

Brubaker and Phipps; Corrosion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CORROSION C H E M I S T R Y

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A l a r g e amount o f r e s e a r c h e f f o r t has been d e v o t e d t o t h e s u b j e c t and t h e p u b l i s h e d l i t e r a t u r e i s q u i t e immense. Many c o n f e r e n c e s have been h e l d . Anyone c o n c e r n e d t o l e a r n more about t h e s u b j e c t can f i n d h i s way by t a k i n g (1_/> tp, and (b) be cause i t would seem self-evident that what cannot i n i t i a t e cannot propagate. In p r a c t i c e , however, such considerations are too simple. I t i s generally well recognized that engineering structures composed o f a number o f metal components always contain a m u l t i p l i c i t y o f surface cracks and flaws. These a r i s e during f a b r i c a t i o n and assembly. I f any of these i s an i n c i p i e n t stress corrosion crack, the considerations of i n i t i a t i o n are unimportant because they are i r r e l e v a n t . What then becomes important i s whether the crack w i l l propagate and a t what rate. Such considerations have given r i s e to the use of precracked notched specimens, l i k e those that are used i n measuring a m e t a l l u r g i c a l f a c t o r known as Fracture Toughness (5). The dimensions of such specimens are determined Ey considerations of Fracture Mechanics. This approach tends to be l i m i t e d to high strength a l l o y s since these often have mechanical properties that are c l o s e s t to the i d e a l required and because of t h e i r engineering importance. The type of specimem employed takes into account the stress concentration a r i s i n g from the presence of a crack i n a specimen and employs a measured component K, the stress i n t e n s i t y factor, which i s obtained from the applied stress σ χ d / 2 , where c i s the crack depth. I t has units ΜΝ·π\~3/2. I f such specimens are now tested as a func t i o n of t i m e - t o - f a i l u r e , the r e s u l t s obtained are of the kind shown i n Figure 2. A g a i n the question a r i s e s of a threshold which i s such specimens i s termed K - £ where the subscript I r e f e r s to the loading mode(5) . The whole term represents that value of Κ below r

SCC



10.

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Figure 1.

Stress-Corrosion

Cracking

327

Relationships between time-to-failure (t ) and applied stress (σ) com monly observed in stress-corrosion cracking f

Klc

CO

ζ

LU

H

?

κιsec

CO CO LU CC

Η ώ

TIME TO FAILURE

Figure 2. Retàionship between t and initial value of stress intensity factor (K)


f

328


which f a i l u r e does not occur. Since the i n i t i a t i o n time i s very small i n r e l a t i o n to t f (the reverse of the type of experiment to which Figure 1 a p p l i e s ) , t h i s t e s t gives a good i n d i c a t i o n of stress corrosion crack propagation r e s i s t a n c e . The r a t i o of K j . to K j (the fracture toughness i n a i r ) i s a u s e f u l r a t i o of r e l a t i v e s u s c e p t i b i l i t y . For T i a l l o y s , f o r example, i t v a r i e s between 0.2-0.9, which could be said to describe a l l o y s that are very susceptible to those that are scarcely susceptible. Since t h i s type of t e s t i s mainly concerned with propagation, a t t e n t i o n was subsequently focused upon measuring v e l o c i t y ν as a function of K. A general schematic p i c t u r e of r e s u l t s obtained i s shown i n Figure 3. A maximum of 3 stages of cracking may be observed. Stage I (the extent of which w i l l be deter mined by the d i f f e r e n c e between K and the Κ value at the Stage I / I I t r a n s i t i o n ) shows a logarithmic depen dence of crack v e l o c i t y ν upon K. For A l and T i a l l o y s an a c t i v a t i o n energy f o r t h i s stage of 112 kJ*mol~1 has been determined (2). Stage I I , sometimes referred to as the plateau v e l o c i t y , i s generally interpreted as being caused by the chemical or electrochemical reac t i o n at the crack t i p being l i m i t e d by d i f f u s i o n of a c r i t i c a l reactant or product within the s o l u t i o n . The idea i$ supported by observations (2) that increasing the s o l u t i o n v i s c o s i t y lowers the plateau v e l o c i t y . An example of t h i s i s shown i n Figure 4 (2). An a c t i v a t i o n energy of 12-21 kJ»mol" f o r t h i s region has been obtained both f o r A l and T i a l l o y s . Stage I I I i s r a r e l y observed and a r i s e s mainly f o r mechanical reasons. For many a l l o y s , therefore, Figure 3 would c o n s i s t of Stages I and II only. This type of experimental data has been of con siderable value. I t has become possible to examine a l l the major v a r i a b l e s , one at a time, f o r t h e i r e f f e c t upon crack v e l o c i t y . As a r e s u l t , i t becomes p o s s i b l e , i f required, to determine i n d e t a i l the e f f e c t of minute v a r i a t i o n s i n composition, changes i n heat treatment, electrochemical v a r i a b l e s and changes to the environment. Typical examples are shown i n Figures 5, 6, and 7 (2). When such data are a v a i l a b l e , they can be used i n a l l o y s e l e c t i o n . Stress corrosion s i t u a t i o n s cannot always be avoided. Under such circumstances a l l o y s which have a low plateau maximum v e l o c i t y can be chosen i n preference to a l l o y s with a high plateau v e l o c i t y under circumstances where the same a l l o y s are metallurg i c a l l y equivalent or interchangeable. Examples of t h i s have been given i n the l i t e r a t u r e (5).


s c c

l s c c

1


c


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ο

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II

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oc

DC !=

ο ο Co

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CO LU LU > DC

hco

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Klc

STRESS INTENSITY, Κ Figure 3.

General relationship between stress-corrosion-crack velocity (v) and stress intensity factor (K)


330


STRESS INTENSITY, (kg.mm" ) 3/2

0 ίο"


icf

ι

3

4

100

20 40 60 80 ι 1 1 1 1 1 1 ALLOY 7079-T651 2.5 cm THICK PLATE CRACK ORIENTATION TL 2 MOLAR KI SOLUTION (HoO+GLYCEROL) TEMPERATURE 23 *C POTENTIAL-450 mVvsE + u

ι

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/u

VISCOSITY η (CENTIPOISE) 10"

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68

7

417

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8

9

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0

5

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1 1

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10

15

20 25

30

STRESS INTENSITY, (MN/m ) 3/2

N.A.T.O. Figure 4.

The effect of solution viscosity upon the log v:K relationship of an aluminium alloy (2)


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Cracking

STRESS INTENSITY (kg-mm 20 40 60 80 J ι ι 1 J 1 1

3/2


1 0

, 0 -7i

£

1

~

0

10"

13

I 0

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) 100 1

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A L L O Y 7178 - T651 + O V E R A G E D AT 1 6 0 ° C 2.5 cm THICK PLATE CRACK ORIENTATION T L SATURATED AQUEOUS NaCI SOLUTION OPEN CIRCUIT TEMPERATURE 2 3 ° C I 5

I I I I 10 15 20 25 STRESS INTENSITY (MN/m )

I 30

3/2

N.A.T.O. Figure 5.

Effect of overaging on stress-corrosion-crack velocity in an aluminum alloy (2)




• ACIDIC SOLUTION (pH

ο INCREASING pH

POTENTIAL N.A.T.O. Figure 6.

Schematic of the influence of pH on stress-corrosion-crack velocity in titanium alloys (2)


SCULLY

Stress-Corrosion


-3 10

20 Π

333

Cracking

STRESS INTENSITY (kg. mm 40 60 I Γ

-3/2, 100

10

8mNaI 5mKI 3mKI 2mKI

10

0.5 m Κ Ι tz 10

0.2 m Κ Ι 0.1 m Κ Ι 0.05 m Κ Ι

r7 10

0.02 m Κ Ι 0.002m Κ Ι and DISTILLED WATER

10

10

10

ALLOY 7079-T651 2.5 cm THICK PLATE CRACK ORIENTATION TL AQUEOUS IODIDE SOLUTIONS POTENTIAL- 450 mV vs Ε ,„+ TEMPERATURE 23 'C 2 pH* 6 μ

n

•10 10

15

20

25

30

3/2 STRESS INTENSITY (MN/m) 4

N.A.T.O. Figure 7.

Effect of solution concentration upon stress-corrosion-crack velocity in an aluminum alloy (2)



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The

Importance o f

Repassivation

I n many o f t h e a l l o y systems shown i n T a b l e I , t h e s t a b l e c o n f i g u r a t i o n of the a l l o y s u r f a c e i s t h a t i t i s f i l m e d . Many o f t h e a l l o y s , e.g., s t a i n l e s s s t e e l s , A l , T i , Zr and Mg a l l o y s a r e o n l y u s a b l e i n such a c o n d i tion. Such a c o n s i d e r a t i o n a p p l i e s n o t o n l y t o t h e s e a l l o y s covered w i t h a t h i n p a s s i v e f i l m but a l s o t o t h o s e on w h i c h r e l a t i v e l y t h i c k f i l m s a r e formed. The p o s s i b l e mechanisms by w h i c h s t r e s s c o r r o s i o n c r a c k i n g o c c u r s a r e c o n c e r n e d w i t h r e a c t i o n s between u n f i l m e d m e t a l and t h e e n v i r o n m e n t . B e f o r e c o n s i d e r a t i o n o f t h e s e i t i s n e c e s s a r y t o c o n s i d e r how t h e s e v a r i o u s t y p e s o f f i l m b r e a k down i n i t i a l l y . W h i l e many o f t h e a l l o y s e x h i b i t p i t t i n g , i t i s not necessary f o r p i t t i n g as such t o p r e c e d e c r a c k p r o p a g a t i o n . P i t t i n g i s assoc i a t e d w i t h s t a t i c u n s t r e s s e d m e t a l s whereas c r a c k i n g i s a s s o c i a t e d w i t h a m e t a l whose s u r f a c e i s s t r e s s e d . The a p p l i c a t i o n o f a s t r e s s c r e a t e s a s u r f a c e s t r a i n r a t e (or c r e e p s t r a i n - r a t e , s i n c e i t i s v e r y l o w ) . Of f u n d a m e n t a l i m p o r t a n c e i s t h e i n t e r a c t i o n o f t h e def o r m i n g s u r f a c e and t h e r e a c t i o n s on t h e m e t a l s u r f a c e . The s u b j e c t i s n o t a s i m p l e one, b u t t h e main p o i n t s can b e s t be e n v i s a g e d by a t t e m p t i n g t o draw t h e r e p e t i t i v e segment o f e v e n t s o c c u r r i n g a t a c r a c k t i p o r a t t h e base o f a s u r f a c e f l a w . T h i s i s a t t e m p t e d i n F i g u r e 8. The a p p l i c a t i o n o f a s t r e s s c r e a t e s a s l i p step which breaks the p a s s i v e f i l m which i s i n e q u i l i b r i u m w i t h t h e s o l u t i o n . There i s abundant e x p e r i m e n t a l e v i d e n c e t h a t such e v e n t s t a k e p l a c e (e.g., 6). The s l i p s t e p h e i g h t w i l l u s u a l l y be v e r y much g r e a t e r t h a n t h e f i l m t h i c k n e s s and t h e f i l m i s usually relatively b r i t t l e . B o t h t h e s e would appear t o be n e c e s s a r y c o n d i t i o n s f o r t h e s i t u a t i o n d e p i c t e d i n F i g u r e 8. A f t e r f i l m f r a c t u r e , r e a c t i o n t h e n o c c u r s between t h e newly c r e a t e d m e t a l s u r f a c e and t h e e n v i ronment. The s p e c i f i c t y p e o f r e a c t i o n w i l l be c o n s i d e r e d below s i n c e i t i s t h e i m p o r t a n t p r o c e s s t h a t determines crack propagation. But t h e t i m e a v a i l a b l e f o r t h i s r e a c t i o n t o o c c u r w i l l be d e t e r m i n e d by t h e r e p a i r t i m e o f t h e f i l m , commonly r e f e r r e d t o as t h e r e p a s s i v a t i o n t i m e (7). As soon as t h e f i l m f r a c t u r e s , i t w i l l s t a r t t o reform s i n c e the e q u i l i b r i u m c o n d i t i o n o f t h e s u r f a c e i s t h a t i t i s f i l m e d . I t has been argued (7) t h a t t h e d e l a y i n r e p a s s i v a t i o n i s o f c r u c i a l importance. I f r e p a s s i v a t i o n occurs too r a p i d l y , there w i l l n o t have been t i m e f o r enough r e a c t i o n t o o c c u r t o cause an i n c r e m e n t o f c r a c k growth. I f r e p a s s i v a t i o n o c c u r s t o o s l o w l y , t h e n t o o much r e a c t i o n may have o c c u r r e d , g i v i n g r i s e t o c r a c k b l u n t i n g and p i t t i n g .



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A

Β

C

D

Figure 8. Schematic of the sequence of events occurring at the tip of a propagat ing stress-corrosion crack. The film is fractured (B) and immediately starts to repair (C) while dissolution is occurring. Complete repassivation occurs at D by which time the crack has extended.


CORROSION


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CHEMISTRY

The i m p o r t a n t f a c t o r i s t h e r e p a s s i v a t i o n time s i n c e i t d e t e r m i n e s f o r how l o n g t h e m e t a l and e n v i r o n m e n t c a n react together. F i g u r e 8 c a n be c o n s i d e r e d i n a more g e n e r a l sense. The p l a s t i c d e f o r m a t i o n t h a t c r e a t e s t h e s l i p s t e p c a n be c o n s i d e r e d as a s t r a i n - r a t e t h a t , i n t h e most gene r a l way, i s c r e a t i n g new, u n f i l m e d , m e t a l s u r f a c e . I t w i l l be governed by m e c h a n i c a l and m e t a l l u r g i c a l f a c t o r s . The p r o c e s s t h a t c a u s e s f i l m f o r m a t i o n i s e l e c t r o c h e m i c a l and i t w i l l be dependent upon t h e p o t e n t i a l and a l l o t h e r e l e c t r o c h e m i c a l f a c t o r s . It is possible to consider that stress corrosion cracking o c c u r s when t h e r e i s a c r i t i c a l imbalance between t h e s e two r a t e p r o c e s s e s — one c r e a t i n g f r e s h m e t a l a r e a , t h e o t h e r f i l m i n g t h a t a r e a . Thus, t h e s i t u a t i o n dep i c t e d s c h e m a t i c a l l y i n F i g u r e 8 i s n o t merely t h a t o f a f i l m b r o k e n by an emergent s l i p s t e p s i n c e t h a t c o u l d be e x p e c t e d , and i s i n d e e d o b s e r v e d , o f n o n s u s c e p t i b l e a l l o y s and m e t a l s i n e n v i r o n m e n t s t h a t a r e known n o t t o cause s t r e s s c o r r o s i o n c r a c k i n g . What i s e n v i s a g e d i n F i g u r e 8 i s a p a r t i c u l a r s e t o f circumstances i n which p a s s i v a t i o n i s d e l a y e d f o r a narrow range o f t i m e i n t e r vals. A good example o f t h e i m p o r t a n c e o f r e p a s s i v a t i o n i s shown i n F i g u r e 9 (8). A t i t a n i u m a l l o y , T i - 5 A l 2.5 Sn, i n t h e form o f t e n s i l e specimens i s s t r a i n e d d y n a m i c a l l y i n two d i f f e r e n t s o l u t i o n s . I n aqueous NaCl s o l u t i o n T i does n o t c o r r o d e and r e p a s s i v a t i o n c a n be e x p e c t e d t o o c c u r . A t h i g h c r o s s h e a d speeds t h e t e s t ' i s o v e r i n a v e r y s h o r t t i m e and t h e r e i s no t i m e for crack i n i t i a t i o n . A f r a c t u r e i s observed f r a c t o graphically with the elongation t o fracture and t h e t e x t u r e t h e same as i n a i r . A t l o w e r c r o s s h e a d speeds s t r e s s c o r r o s i o n c r a c k propagation occurs w i t h a consequent r e d u c t i o n i n £f and c h a r a c t e r i s t i c c l e a v a g e f r a c t o g r a p h y . A t t h e l o w e s t c r o s s h e a d speeds t h e r e l a t i v e l y s l o w s t r a i n - r a t e i n d u c e d on t h e s u r f a c e i s such t h a t r e p a s s i v a t i o n p r e d o m i n a t e s o v e r c r a c k i n g and no c r a c k p r o p a g a t i o n o c c u r s . F r a c t o g r a p h i c a l l y , a i r f r a c t u r e i s seen and e i s h i g h . Thus, i n t h e aqueous s o l u t i o n c r a c k i n g i s c o n f i n e d t o a narrow range o f speeds. T h i s b e h a v i o r c a n be c o n t r a s t e d w i t h t h a t obs e r v e d f o r t h e same a l l o y exposed t o a CH3OH + 1 v o l . % cone HC1 m i x t u r e w h i c h i s c o r r o s i v e t o T i and i n w h i c h , t h e r e f o r e , no r e p a s s i v a t i o n m i g h t be e x p e c t e d . At high c r o s s h e a d speeds t h e same b e h a v i o r i s seen as t h a t o b s e r v e d i n t h e aqueous s o l u t i o n . A t t h e l o w e s t c r o s s head s p e e d s , however, because no r e p a s s i v a t i o n i s p o s s i b l e , c r a c k i n g i s o b s e r v e d and because t h e t e s t t a k e s i n c r e a s i n g l y l o n g t i m e s as t h e c r o s s h e a d speed i s f



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Cracking

16h

Corrosion Science Figure 9. The rehtionship between elongation-to-failure (e ) and crosshead speed for a Ti-5Al-2.5Sn alloy exposed to (1) aqueous NaCl, and (2) a CH OH/HCl mixture (8) f

3


CORROSION

338

l o w e r e d , the €f f a l l s c o n t i n u a l l y . I t can be e x p e c t e d t h a t t h e c r a c k v e l o c i t y would f a l l u n t i l i t became s i m i l a r t o the c o r r o s i o n r a t e o f the u n s t r e s s e d a l l o y w h i c h , i n t h i s example, i s s e l e c t i v e l y i n t e r g r a n u l a r . I n t h e CH3OH s o l u t i o n no K j o r any o t h e r t h r e s h o l d can be a n t i c i p a t e d and i t i s not found. I n the aqueous solution a K i can be e x p e c t e d , c o r r e s p o n d i n g t o t h a t v a l u e o f Κ t h a t r e s u l t s i n so low a s u r f a c e s t r a i n r a t e t h a t r e p a s s i v a t i o n can o c c u r . This s t r a i n - r a t e has been c h a r a c t e r i z e d έ (SO . K j i s commonly o b s e r v e d i n T i a l l o y s exposed t o aqueous c h l o r i d e s o l u t i o n s . From F i g u r e 9 and a l l t h a t has been d e s c r i b e d , i t can be a p p r e c i a t e d t h a t K i i s not a m a t e r i a l constant. F o r a g i v e n a l l o y i t v a r i e s a c c o r d i n g t o the e n v i r o n m e n t i n w h i c h i t i s measured. I t i s not an a l l o y t h a t has a t h r e s h o l d ; i t i s the a l l o y + e n v i r o n ment w h i c h may e x h i b i t one. The t y p e o f e x p e r i m e n t encompassed i n F i g u r e 9 has become i n c r e a s i n g l y i m p o r t a n t as a method o f t e s t i n g . The c o n d i t i o n s a r e s e v e r e , t h e t e s t s a r e r a p i d and t h e imposed c o n d i t i o n s o f a s l o w s t r a i n - r a t e a r e s i m i l a r t o those o c c u r r i n g at a crack t i p . For reasons d i s c u s s e d below, e x p e r i m e n t s s h o u l d be done p o t e n t i o s t a t i c a l l y . A r e c e n t c o n f e r e n c e (1_0) was d e v o t e d t o t h e c o n s t a n t e x t e n s i o n r a t e t e s t , o r g a n i z e d by A.S.T.M. The i m p o r t a n c e o f r e p a s s i v a t i o n and t h e i n t e r a c t i o n of t h i s process with a s t r a i n i n g metal surface p r o b a b l y c o n s t i t u t e s t h e e s s e n s e o f many s t r e s s c o r r o s i o n c r a c k i n g mechanisms. S i n c e r e p a s s i v a t i o n i s an e l e c t r o c h e m i c a l phenomenon, i t m i g h t be e x p e c t e d t h a t the necessary imbalance to achieve s t r e s s c o r r o s i o n , r e f e r r e d t o above, w i l l o c c u r o n l y o v e r a narrow range o f p o t e n t i a l , c o r r e s p o n d i n g t o a narrow range o f r e p a s s i v a t i o n r a t e s . Such r a t e s can be e x p e c t e d t o change m a r k e d l y i n t h o s e r e g i o n s o f p o t e n t i a l where t h e p a s s i v e range changes t o a p i t t i n g range o r t o an a c t i v e r a n g e , i . e . , where the f i l m i s c h a n g i n g from b e i n g t h e s t a b l e s u r f a c e c o n f i g u r a t i o n t o where i t i s an u n s t a b l e c o n f i g u r a t i o n . The zones o f p o t e n t i a l where c r a c k i n g m i g h t be e x p e c t e d a r e shown i n F i g u r e 10. There a r e examples i n t h e l i t e r a t u r e o f c r a c k i n g o c c u r r i n g i n such r e g i o n s (2^) . The s i t e o f t h e s e r e g i o n s depends upon a number o f e l e c t r o c h e m i c a l f a c t o r s . S t r e s s c o r r o s i o n f a i l u r e o c c u r s under openc i r c u i t c o n d i t i o n s when t h e c o r r o s i o n p o t e n t i a l o f an a l l o y l i e s w i t h i n the p o t e n t i a l range f o r c r a c k i n g o f t h a t a l l o y i n the p a r t i c u l a r e n v i r o n m e n t . Systems de s c r i b e d i n Table I f a l l i n t o t h i s category. I t must be emphasized t h a t under c o n t r o l l e d p o t e n t i a l c o n d i t i o n s s t r e s s c o r r o s i o n f a i l u r e may o c c u r i n a l l o y s exposed t o s

s

c

c

c

c

s


CHEMISTRY

s

c

c

c

c



10.

SCULLY

Stress-Corrosion

Cracking

339

e n v i r o n m e n t s w h i c h do n o t cause f a i l u r e under open c i r c u i t c o n d i t i o n s , α-brass f a i l s i n ammonia under open c i r c u i t c o n d i t i o n s , as i s i n d i c a t e d i n T a b l e I . When a n o d i c a l l y p o l a r i z e d , i t a l s o c r a c k s i n n i t r a t e and s u l f a t e s o l u t i o n s . Thus, the u n i q u e n e s s o f ammonia l i e s o n l y i n i t s a b i l i t y t o cause c r a c k i n g under open c i r c u i t c o n d i t i o n s under a wide range o f s o l u t i o n pH c o n d i t i o n s ( o t h e r s o l u t i o n s , e.g., t a r t r a t e s o l u t i o n s , can a l s o cause c r a c k i n g o v e r a narrow range o f pH). T h i s t y p e o f o b s e r v a t i o n a p p l i e s t o o t h e r systems too and i t s i m p o r t a n c e c a n n o t be e x a g g e r a t e d . I t has a b e a r i n g b o t h on m e c h a n i s t i c i v e s t i g a t i o n s and on t e s t i n g p r o c e u d r e s . I t i s f o o l i s h t o t e s t o n l y under open c i r c u i t conditions s i n c e , i f cracking occurs i n a poten t i a l range n o t i n c l u d i n g the open c i r c u i t p o t e n t i a l , s u s c e p t i b i l i t y w i l l be m i s s e d i n l a b o r a t o r y c o n d i t i o n s . I f i n s e r v i c e use the open c i r c u i t p o t e n t i a l moves i n t o the c r a c k i n g r a n g e , c r a c k i n g w i l l o c c u r . T h i s may appear t o be a s i m p l e p o i n t , but such problems do a r i s e and cause c o n f u s i o n . T e s t i n g s h o u l d a l w a y s be done o v e r a range o f p o t e n t i a l l a r g e r t h a n t h a t l i k e l y t o be e n c o u n t e r e d i n s e r v i c e under the most extreme c o n d i tions . R e p a s s i v a t i o n p r o c e s s e s have become an i m p o r t a n t s u b j e c t i n s t r e s s c o r r o s i o n s t u d i e s and a l s o i n o t h e r forms o f c o r r o s i o n , e.g., p i t t i n g c o r r o s i o n and c o r r o s i o n f a t i g u e . A range o f s c r a t c h i n g and s t r a i n i n g e l e c t r o d e t e c h n i q u e s have been employed. W h i l e i t i s n o t p o s s i b l e t o go i n t o d e t a i l , t h e r e s u l t s have t o be examined i n r e l a t i o n t o t h e t e c h n i q u e s employed, e.g., has r e p a s s i v a t i o n s t a r t e d b e f o r e t h e s c r a t c h i n g o r s t r a i n i n g has stopped? I t i s i m p o r t a n t a l s o t o know whether t h e c u r r e n t measured under p o t e n t i o s t a t i c c o n d i t i o n s i s a c o m p l e t e anode c u r r e n t o r t h e d i f f e r ence between an anode c u r r e n t and a c a t h o d e c u r r e n t (most commonly due t o H i o n r e d u c t i o n ) . T y p i c a l r e p a s s i v a t i o n r a t e s c o r r e s p o n d t o an e q u a t i o n (11) o f the t y p e : +

i - i

Q

=

(i - i )t~ 1

b

0

(2)

where i = c u r r e n t a f t e r t i m e t , ÎQ = c u r r e n t a f t e r comp l e t e r e p a s s i v a t i o n , i-j = c u r r e n t a f t e r one second and b i s a constant. The c o n s t a n t b v a r i e s w i t h p o t e n t i a l . Under c e r t a i n c o n d i t i o n (11) the r e l a t i o n s h i p s i s g i v e n by — i = i

max

exp(-gt)


(3)

340


where 3 i s a c o n s t a n t , and i i s t h e i n i t i a l maximum value of the c u r r e n t a f t e r d e s t r u c t i o n of the f i l m . When a f i l m i s b r o k e n , t h e i n i t i a l monolayer a d s o r b s w i t h i n 20-50 ms ( 1 2 ) . The measured c u r r e n t i s made up p a r t l y o f d i s s o l u t i o n and p a r t l y o f f i l m f o r m a t i o n , t h e r a t i o between them f a l l i n g w i t h t i m e . m

a

x


S t r e s s C o r r o s i o n Mechanisms The c i r c u m s t a n c e s under w h i c h t h e f i l m i s b r o k e n and r e p a i r e d have been d e s c r i b e d a t l e n g t h s i n c e t h e y c o n t r o l the subsequent r e a c t i o n t i m e d u r i n g w h i c h t h e c r a c k a c t u a l l y grows. I t has been s t r o n g l y emphasized s i n c e u n l e s s t h i s p o i n t i s u n d e r s t o o d c o n f u s i o n can a r i s e because t h e a l t e r a t i o n o f an e x p e r i m e n t a l v a r i a b l e , e.g., p o t e n t i a l o r pH, may have a g r e a t e r e f f e c t on r e p a s s i v a t i o n t h e n on t h e c r a c k i n g r e a c t i o n . Unl e s s t h a t p o s s i b i l i t y i s appreciated, ambiguity i n i n t e r p r e t a t i o n i s l i k e l y t o a r i s e on o c c a s i o n . An example i s g i v e n below w i t h r e s p e c t t o t h e e f f e c t o f c a t h o d i c p o l a r i z a t i o n on 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 T i a l l o y s i n aqueous c h l o r i d e s o l u t i o n s . I t i s a g e n e r a l p o i n t and i s not o f t e n emphasized. The Mechanism o f

Cracking

Why does c r a c k i n g o c c u r ? T h i s i s t h e most d i f f i c u l t q u e s t i o n t o answer. Many e x p l a n a t i o n s have been put f o r w a r d and w i t h s t r e s s c o r r o s i o n c r a c k i n g o c c u r r i n g i n so many a l l o y s i t i s not u n l i k e l y t h a t some v e r y s p e c i f i c p o i n t s a p p l y h e r e and t h e r e . An example might be t h e a c t i o n o f v o l u m i n o u s c o r r o s i o n p r o d u c t i n wedging open a c r a c k f o r w h i c h t h e r e i s e v i d e n c e i n i s o l a t e d cases. O v e r r i d i n g many such t i n y phenomenol o g i c a l p o i n t s , c r a c k i n g mechanisms can be d i v i d e d i n t o (1) a c t i v e p a t h , (2) hydrogen e m b r i t t l e m e n t , (3) s t r e s s s o r p t i o n and (4) c o r r o s i o n f i l m f r a c t u r e . I t i s now i n t e n d e d t o d i s c u s s t h e e s s e n t i a l f e a t u r e s o f t h e s e mechanisms. I t must be emphasized t h a t t h e s e a r e g e n e r a l c a t e g o r i e s and much s u b d i v i s i o n i s p o s s i b l e , p a r t i c u l a r l y i n t h e c a s e o f hydrogen emb r i t t l e m e n t . There i s no r e a s o n t h a t c r a c k i n g s h o u l d not o c c u r by a mechanism t h a t t r a v e r s e s more t h a n one o f t h e above, e.g., a c o m b i n a t i o n o f a n o d i c d i s s o l u t i o n and hydrogen e m b r i t t l e m e n t . These f o u r c a t e g o r i e s a r e i n no way e x c l u s i v e from each o t h e r . A c t i v e Path. Since the term s t r e s s c o r r o s i o n c r a c k i n g i m p l i e s c o r r o s i o n , i t i s not s u r p r i s i n g t h a t t h e f i r s t mechanism e v e r p r o p o s e d was t h a t c r a c k i n g



10.

SCULLY

Stress-Corrosion

341

Cracking

o c c u r r e d by p r e f e r e n t i a l d i s s o l u t i o n o v e r a narrow p r e f e r r e d f r o n t a l o n g a p a t h t h a t was p r e - e x i s t i n g ( 1 3 ) . T h i s was p r o b a b l y t r u e f o r t h e p a r t i c u l a r system Eëing i n v e s t i g a t e d , but s i n c e c r a c k i n g occurs i n a l l o y s which most c e r t a i n l y do n o t have a p r e - e x i s t i n g p a t h , more r e c e n t i d e a s have a t t e m p t e d t o e x p l a i n t h e l o c a l i z e d c o r r o s i o n by a p r o c e s s i n v o l v i n g r e p a s s i v a t i o n o f the crack s i d e s , thus c o n c e n t r a t i n g the c u r r e n t a t the t i p where v a r i o u s forms o f d i r e c t i o n a l l o c a l i z e d d i s s o l u t i o n occurs. The p o s s i b l e p r e f e r e n t i a l c o r r o s i o n o f d i s l o c a t i o n s p i l e d up a t t h e c r a c k t i p as a r e s u l t o f t h e a c t i n g s t r e s s has been c o n s i d e r e d a t l e n g t h o v e r many y e a r s . The s t r a i n energy a s s o c i a t e d w i t h such pile-ups of d i s l o c a t i o n s provides l i t t l e a d d i t i o n a l driving force for dissolution · As a r e s u l t a t t e n t i o n has f o c u s e d on m i n u t e c o m p o s i t i o n a l changes i n a m e t a l l a t t i c e o c c u r r i n g around d i s l o c a t i o n p i l e - u p s w h i c h may cause s i g n i f i c a n t d i r e c t i o n a l d i f f e r e n c e s i n d i s s o l u t i o n on an a t o m i s t i c s c a l e (1_5) · These i d e a s were o r i g i n a l l y a s s o c i a t e d w i t h e l e c t r o n m i c r o s c o p e o b s e r v a t i o n s o f t h e edges o f e l e c t r o p o l i s h e d t h i n f o i l s where r e g i o n s o f d i s l o c a t i o n p i l e - u p s showed, v e r y d i r e c t i o n a l d i s s o l u t i o n ( 1 6 ) . Such p r o c e s s e s g i v e r i s e t o u n u s u a l m o r p h o l o g i c a l e f f e c t s ( V7) and may c a u s e t u n n e l c o r r o s i o n (1_8) . A t some s t a g e i t i s n e c e s s a r y t o know whether t h e c u r r e n t f l o w i n g i s s u f f i c i e n t to account f o r the maximun v e l o c i t y o f t h e c r a c k , a r e q u i r e m e n t sometimes r e f e r r e d t o as f a r a d a i c e q u i v a l e n c e . From t h e a p p l i c a t i o n o f F a r a d a y ' s laws and assuming t h a t d i s s o l u t i o n a c c o u n t s f o r 10095 o f t h e c r a c k p r o p a g a t i o n t h e n : f

1

where ν = c r a c k v e l o c i t y , J - c h e m i c a l e q u i v a l e n t , F = the F a r a d a y , ρ = a l l o y d e n s i t y and i = d i s s o l u t i o n current density. F o r a number o f s t r e s s c o r r o s i o n c r a c k i n g s y s t e m s , e.g., m i l d s t e e l , a u s t e n i t i c s t a i n l e s s s t e e l s , measured b a r e s u r f a c e c u r r e n t d e n s i t i e s appear t o be o f t h e r i g h t o r d e r o f magnitude (_3) · F o r o t h e r a l l o y s , e.g., T i a l l o y s , i n w h i c h t h e c r a c k v e l o c i t i e s a r e v e r y much h i g h e r t h a n i n s t e e l s , much h i g h e r c u r r e n t d e n s i t i e s are r e q u i r e d , of the order r e q u i r e d f o r e l e c t r o c h e m i c a l m a c h i n i n g . T h e i r e x i s t e n c e has been c l a i m e d (2). R e f e r e n c e t o F i g u r e 8, t o g e t h e r w i t h t h e c o n c e p t of r e p a s s i v a t i o n , i n d i c a t e s d u r i n g the propagation of c r a c k s t h e c u r r e n t s h o u l d c o n s i s t o f a number o f s u c c e s s i v e t r a n s i e n t s . One i s shown i n F i g u r e 11, w h i c h m i g h t be t a k e n t o c o r r e s p o n d t o t h e sequence o f e v e n t s




342

t Figure 11. A general schematic of the current transient during the sequence of events drawn in Figure 8. The hatched area represents the total charge flowing.


10.

SCULLY

Stress-Corrosion

Cracking

343

d e s c r i b e d i n F i g u r e 8. What i s i m p o r t a n t i s t h e t o t a l amount o f c o r r o s i o n , r e p r e s e n t e d by t h e c h a r g e , w h i c h i s h a t c h e d i n F i g u r e 11. I t has been argued (9) t h a t s t r e s s c o r r o s i o n c r a c k p r o p a g a t i o n o c c u r s as a r e s u l t o f t h e passage o f a c o n s t a n t c h a r g e Q i / w h i c h t h e n i n i t i a t e s t h e n e x t i n c r e m e n t o f c r a c k growth. Crack a r r e s t occurs i f r e p a s s i v a t i o n occurs before Q i has passed. From such c o n s i d e r a t i o n s , w i t h i n t e g r a t i o n o f the a p p r o p r i a t e r e p a s s i v a t i o n r a t e equation over the t i m e l i m i t s between s u c c e s s i v e s l i p s t e p e v e n t s , i t has been p o s s i b l e t o d e r i v e t h e shape o f t h e c o r r o s i o n c u r r e n t : t i m e c u r v e s o f Stages I and I I shown i n F i g ure 3 (9) . m

n


m

n

Hydrogen E m b r i t t l e m e n t . A t t h e c r a c k t i p i n many a l l o y s l o c a l a c i d i t y and low p o t e n t i a l s e n s u r e t h a t hydrogen i o n d i s c h a r g e o c c u r s . T h i s i s t r u e f o r A l , T i , Zr a l l o y s and f o r s t a i n l e s s and h i g h s t r e n g t h p l a i n c a r b o n s t e e l s and a l s o f o r Mg a l l o y s i n w h i c h the c r a c k t i p s o l u t i o n i s a l k a l i n e . The q u e s t i o n i s whether any o f t h e s e a l l o y s c r a c k as a r e s u l t o f H a d s o r p t i o n w h i c h o c c u r s a f t e r n e u t r a l i z a t o n has occurred H +

+

e

H

( 5 )

= ads

and b e f o r e d e s o r p t i o n has o c c u r r e d by H

ads

+

H +

H

ads

+

H

+

e

either

= 2 H

( 6 )

or ads - 2 H

( 7 )

i n a c i d i c s o l u t i o n s and t h e e q u i v a l e n t r e a c t i o n s a p p r o p r i a t e f o r a l k a l i n e s o l u t i o n s i n t h e c a s e o f Mg a l l o y s . Some p r o p o r t i o n o f Hads w i l l be absorbed by t h e a l l o y i n any s i t u a t i o n . An i m p o r t a n t r a t i o i s H (absorbed/ H (evolved) and t h i s i s l i k e l y t o be v e r y s e n s i t i v e t o c a t h o d i c p o i s o n s i n t h e s o l u t i o n and t o c e r t a i n e l e ments w i t h i n t h e a l l o y . C r a c k i n g i n A l and T i a l l o y s and i n h i g h s t r e n g t h s t e e l s i s a c c e l e r a t e d by t h e presence of c a t h o d i c poisons. T h i s i s very s t r o n g evidence t h a t H p l a y s a r o l e i n the c r a c k i n g process. A f t e r e n t e r i n g t h e m e t a l i t may c o l l e c t i n s i d e t r a p s and cause d e c o h e s i o n . I t may combine around i n c l u s i o n s and cause l o c a l r e g i o n s o f v e r y h i g h p r e s s u r e , r e s u l t i n g i n b l i s t e r i n g and p o s s i b l y even f i s s i o n i n g . I n T i and Zr a l l o y s i t may form h y d r i d e s w h i c h i n t e r a c t w i t h t h e l a t t i c e , p r o m o t i n g c l e a v a g e by impeding


344

CORROSION CHEMISTRY


t h e movement o f d i s l o c a t i o n s . The e f f e c t s o f H on t h e m e c h a n i c a l p r o p e r t i e s o f m e t a l s i s a v e r y complex subj e c t and no a t t e m p t has been made o t h e r t h a n t o i n d i cate s e v e r a l p o s s i b l e g e n e r a l e f f e c t s . Another area o f some d i f f i c u l t y i s t h a t t h e d i f f u s i o n r a t e s o f H o f t e n appear t o be t o o s l o w t o a c c o u n t f o r t h e obs e r v e d c r a c k p r o p a g a t i o n r a t e s . The r e c o n c i l i n g o f these c o n t r a r y observations c a l l s f o r c o n s i d e r a b l e care. S t r e s s S o r p t i o n . T h i s mechanism supposes t h a t t h e r e a c t i o n between a s p e c i e s i n t h e environment and t h e m e t a l atoms a t t h e c r a c k t i p can cause a r e d i s t r i b u t i o n o f e l e c t r o n s i n t h e o r b i t s o f t h e atoms so t h a t t h e bond between them i s weakened ( 1 9 ) . I t i s n o t p o s s i b l e t o c i t e e x p e r i m e n t a l d a t a t K a t would s u p p o r t t h i s concept f o r the f r a c t u r e of metals although the absence may m e r e l y r e f l e c t t h e d i f f i c u l t i e s o f o b t a i n i n g such d a t a . C o r r o s i o n F i l m F r a c t u r e . A t v a r i o u s t i m e s i t has been s u g g e s t e d t h a t t h e f r a c t u r e o f c o r r o s i o n p r o d u c t f i l m s a t t h e c r a c k t i p w i t h t h e i r subsequent r e f o r m a t i o n c o n s t i t u t e s t h e main f o r w a r d movement o f t h e c r a c k . The e v i d e n c e f o r such a mechanism i s n o t t e r r i b l y c l e a r . T h i c k f i l m formed on s t e e l s and b r a s s e s , f o r example, may form by p r e c i p i t a t i o n from s o l u t i o n r a t h e r t h a n by d i r e c t c o r r o s i o n o f t h e m e t a l t o a s o l i d compound. Where t h i s o c c u r s , t h e i n i t i a l d i s s o l u t i o n t h a t p r e c e d e s the p r e c i p i t a t i o n would appear to f i t i n t o the a c t i v e path category. I f f r a c t u r e of t h e p r e c i p i t a t e d f i l m i s n e c e s s a r y , t h e n i t becomes a m a t t e r o f c h o i c e whether t h i s c o n s t i t u t e s a s e p a r a t e c a t e g o r y o r a s p e c i a l c a s e o f t h e a c t i v e p a t h mechanism. F o r m i l d s t e e l and b r a s s e s t h i s t y p e o f e x p l a n a t i o n i s commonly a s s o c i a t e d w i t h i n t e r g r a n u l a r c r a c k i n g . What needs t o be e x p l a i n e d i s why p r e f e r e n t i a l c o r r o s i o n o f t h e g r a i n boundary o c c u r s . O f t e n t h i s i s a t t r i b u t e d t o t h e p r e s e n c e w i t h i n t h e g r a i n boundary o f an u n i d e n t i f i e d element i n s o l i d s o l u t i o n w h i c h a l t e r s t h e dissolution kinetics. I t i s a requirement t h a t the f i l m i s s u f f i c i e n t l y p r o t e c t i v e t o reduce t h e r e a c t i o n r a t e t o an i n s i g n i f i c a n t v a l u e so t h a t i t s f r a c t u r e i s n e c e s s a r y f o r t h e c r a c k p r o p a g a t i n g r e a c t i o n t o be r e initiated. On b r a s s , f o r example, i n ammoniacal s o l u t i o n s t h i c k f i l m s form o v e r a wide range o f pH b u t i t i s o n l y o v e r a narrow range t h a t t h i s t y p e o f mechanism may a p p l y (2), c o r r e s p o n d i n g t o t h e f o r m a t i o n o f a relatively protective film.


10.

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Cracking

D i s t i n g u i s h i n g between t h e v a r i o u s mechanisms and how t h e y a p p l y t o v a r i o u s a l l o y systems i s n o t an easy matter. I t i s d i f f i c u l t t o design experiments t h a t g i v e c l e a r unambiguous answers. R a t h e r e l e m e n t a r y p o i n t s c a n be made i n o r d e r t o show some o f t h e d i f f i culties. Anodic p o l a r i z a t i o n w i l l f r e q u e n t l y shorten t f and a l s o r a i s e t h e c r a c k v e l o c i t y t o a maximum a s s o c i ated w i t h the onset o f simultaneous p i t propagation. These e f f e c t s a r e n o t n e c e s s a r i l y e v i d e n c e t h a t an a c t i v e p a t h mechanism i s o p e r a t i v e . I f t h e p o t e n t i a l o f t h e c r a c k t i p i s such t h a t H i o n d i s c h a r g e c a n o c c u r t h e n t h e g r e a t e r ease o f a c i d i f i c a t i o n a t t h e c r a c k t i p as a r e s u l t o f t h e a p p l i c a t i o n o f a n o d i c p o l a r i z a t i o n may i n c r e a s e t h e amount o f hydrogen a b s o r p t i o n and t h e r e b y i n c r e a s e t h e c r a c k v e l o c i t y i f a hydrogen e m b r i t t l e m e n t mechanism i s o p e r a t i v e . Con v e r s e l y , a n o d i c p o l a r i z a t i o n when a p p l i e d t o b r a s s r e s u l t s i n a lower crack v e l o c i t y i n n e u t r a l s o l u t i o n s (20) when t h e p o t e n t i a l i s r a i s e d i n t o t h e r e g i o n wEere t h e f i l m t h a t forms under open c i r c u i t c o n d i t i o n s i s u n s t a b l e . The same c u r r e n t i s t h e r e b y s p r e a d o v e r a much w i d e r a r e a , g i v i n g r i s e t o a much h i g h e r c r a c k d e n i s t y . The c r a c k v e l o c i t y i s t h e r e f o r e r e duced. I n t h i s system an a c t i v e p a t h mechanism i s o p e r a t i v e , y e t c r a c k v e l o c i t y i s reduced by a n o d i c polarization. Such c o n t r a r y i n t e r p r e t a t i o n s need n o t be c o n f u s i n g . They s e r v e t o u n d e r l i n e t h a t i t i s n e c e s s a r y t o l o o k c l o s e l y a t t h e consequences o f a g i v e n e x p e r i m e n t a l t e c h n i q u e . T h i s c a n be p a r t i c u l a r l y important f o r workers attempting t o i n t e r p r e t r e s u l t s o b t a i n e d w i t h one t e c h n i q u e on two q u i t e different alloys. Cathodic p o l a r i z a t i o n w i l l g e n e r a l l y shorten t f f o r a l l o y s t h a t a r e s u s c e p t i b l e t o hydrogen e m b r i t t l e ment. T h i s i s n o t u n i v e r s a l l y t r u e , however. I f c r a c k i n g i s o c c u r r i n g j u s t below Ε £ ( F i g u r e 1 0 ) , then c a t h o d i c p o l a r i z a t i o n w i l l a c c e l e r a t e t h e r e p a s s i v a t i o n p r o c e s s . T h i s i s n o t a case o f c a t h o d i c pro tection lowering the corrosion p o t e n t i a l to the e q u i l i b r i u m p o t e n t i a l o f t h e anode r e a c t i o n . F o r T i a l l o y s t h e a b s o r p t i o n o f hydrogen o c c u r s r e a d i l y on u n f i l m e d s u r f a c e s . On f i l m e d s u r f a c e s hydrogen e n t r y i s v e r y much reduced because t h e p a s s i v e f i l m has a low hydrogen p e r m e a b i l i t y . A l s o , t h e p a s s i v e f i l m i s n o t r e a d i l y reduced c a t h o d i c a l l y . Thus, c a t h o d i c polarization of T i alloys i n neutral chloride solu t i o n s a r r e s t s c r a c k p r o p a g a t i o n and p r e v e n t s c r a c k i n i t i a t i o n even though t h e mechanism o f c r a c k i n g i s t h a t o f hydrogen e m b r i t t l e m e n t . T h i s i s an example i n


+

t



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w h i c h t h e r o l e o f t h e p o t e n t i a l on t h e r e p a s s i v a t i o n p r o c e s s i s more i m p o r t a n t t h e n i t s r o l e on t h e mecha n i s t i c r e a c t i o n o c c u r r i n g on t h e u n f i l m e d m e t a l s u r face. I n very strong a c i d s o l u t i o n s , i n which t h e f i l m i s u n s t a b l e and r e p a s s i v a t i o n w i l l n o t t h e r e f o r e o c c u r , c a t h o d i c p o l a r i z a t i o n has no e f f e c t on c r a c k v e l o c i t y (2J_, 2) . Where an a c t i v e p a t h mechanism i s o p e r a t i v e , c a t h o d i c p o l a r i z a t i o n w i l l always lengthen t f and l o w e r t h e c r a c k v e l o c i t y , e v e n t u a l l y c a u s i n g crack a r r e s t . Some a l l o y s e x h i b i t r e v e r s i b l e e m b r i t t l e m e n t , e.g., Mg (22), A l (22), T i (24) and Zr (25) a l l o y s . F o r each a l T o y , e x p e r i m e n t s H â v e been done i n w h i c h specimens were exposed u n s t r e s s e d , t o s o l u t i o n s t h a t can cause c r a c k i n g , under a v a r i e t y o f c o n d i t i o n s . I f b r o k e n i n a i r i m m e d i a t e l y a f t e r removal from t h e s o l u t i o n , specimens e x h i b i t e d a l o w v a l u e o f €f and fracture surfaces c h a r a c t e r i s t i c of stress corrosion cracking. I f l a p s e o f t i m e o c c u r s between removal from s o l u t i o n and s t r e s s i n g , specimens e x h i b i t e d i n c r e a s e d v a l u e s o f Zf and d e c r e a s e d amounts o f s t r e s s corrosion-type fracture with increasing length of lapse o f time. T h i s b e h a v i o r i s c h a r a c t e r i s t i c o f hydrogen e m b r i t t l e m e n t f r a c t u r e and has been i n t e r p r e t e d as such f o r t h e f o u r t y p e s o f a l l o y s d e s c r i b e d . These are simple b u t very c l e a r experiments. Preventative

Measures

I t i s c l e a r l y i m p o r t a n t t o know how t o a v o i d , o r at l e a s t minimize the incidence o f the occurrence o f such a w i d e s p r e a d t y p e o f f a i l u r e . The c h o i c e s a r e r e l a t i v e l y few i n number and c a n be c o n v e n i e n t l y c o n s i d e r e d under t h e h e a d i n g s o f t h e words t h a t make up name o f t h e problem. S t r e s s . From F i g u r e s 1 and 2 i t has a l r e a d y been s t a t e d t h a t s t r e s s c o r r o s i o n c r a c k i n g c a n be r e d u c e d o r a b o l i s h e d a l t o g e t h e r by r e d u c i n g t h e s t r e s s l e v e l , d e p e n d i n g upon whether t h e system e x h i b i t s a t h r e s h o l d stress or Κ value. I n p r a c t i c e t h i s may mean e n s u r i n g t h a t components a r e s t r e s s r e l i e f a n n e a l e d , e.g., brass tubes a f t e r drawing, o r t h a t welds a r e given post-weld heat treatements, since i t i s r e s i d u a l s t r e s s t h a t i s o f t e n t h e p r o b l e m . To m i n i m i z e f a i l u r e s i n s t e e l tube assemblies h a n d l i n g sour o i l w e l l s , f o r example, i t i s recommended t h a t a l l w e l d s be k e p t be low a c e r t a i n h a r d n e s s l e v e l (26). The h a r d n e s s c a n be measured i n s i t u . Design can help i n reducing o p e r a t i n g s t r e s s l e v e l s a l s o . C r i t i c a l f l a w d e p t h has


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347

a l s o been m e n t i o n e d . C o n t r o l l i n g f l a w d e p t h i s e a s i e r t o d e s c r i b e t h e n t o a c h i e v e b u t c a r e s h o u l d be t a k e n t o p r e v e n t l a r g e f l a w d e p t h s , p a r t i c u l a r l y where a K l e v e l exists. There a r e examples o f t a n k s b e i n g operated c o n t a i n i n g l i q u i d s t h a t cause s t r e s s c o r r o s i o n c r a c k i n g w i t h r e s i d u a l and o p e r a t i n g s t r e s s e s c o n t r o l l e d so t h a t K j i s n o t exceeded. F o r h i g h strength a l l o y s overtempering o r overaging w i l l often g i v e a much more a c c e p t a b l e l i f e t i m e a t t h e c o s t o f a lower a l l o y s t r e n g t h . Such c o n s i d e r a t i o n s c a l l f o r a b a l a n c e between t h e r e q u i r e m e n t s o f t h e p l a n t o p e r a t o r and t h e demands o f t h e p l a n t d e s i g n e r . I

s

c

c


s

c

c

Corrosion. R e d u c i n g c o r r o s i o n by t h e u s e o f i n h i b i t o r s w i l l commonly r e d u c e o r even e l i m i n a t e s t r e s s c o r r o s i o n c r a c k i n g , p o s s i b l y by moving t h e c o r r o s i o n p o t e n t i a l o u t s i d e t h e range o f c r a c k i n g . Sometimes t h e s e a r e r e f e r r e d t o a s dangerous i n h i b i t o r s s i n c e , i f t h i s i s t h e i r sole function, cracking w i l l occur i f t h e c o r r o s i o n p o t e n t i a l moves back i n t o t h e c r a c k i n g r a n g e . The c o n t r a s t i s made w i t h s a f e i n h i b i t o r s w h i c h reduce o r p r e v e n t c r a c k i n g even i f t h e c o r r o s i o n p o t e n t i a l i s i n the c r a c k i n g range. I n p r a c t i c e t h e use o f i n h i b i t o r s o f e i t h e r c a t e g o r y may be r e s t r i c t e d by (1) s o l u b i l i t y p r o b l e m s , (2) economic a s p e c t s and (3) p r a c t i c a l i t y l i m i t s . Many f a i l u r e s o c c u r i n steam o r under c o n d e n s a t i o n c o n d i t i o n s . I n both o f these cases the t r a n s p o r t o f i n h i b i t o r s t o s i t e s o f crack i n i t i a t i o n i s n o t f e a s i b l e . W i t h o t h e r s y s t e m s , how e v e r , q u i t e s m a l l a d d i t i o n s o r changes may e l i m i n a t e p r o b l e m s , e.g., 1-2% H 0 t o CH3OH/HCI m i x t u r e s , u s i n g impure Ν 0/| r a t h e r t h a n pure N2O4, b o t h f o r T i a l l o y s . C a t h o d i c p r o t e c t i o n may a l s o a c h i e v e t h e same e f f e c t o f a l o w e r e d c o r r o s i o n r a t e . As w i t h i n h i b i t o r s i t s use i s l i m i t e d and f o r much t h e same r e a s o n s . M e n t i o n must be made o f c u r r e n t s t r e s s c o r r o s i o n c r a c k i n g problems i n gas t r u n k t r a n s m i s s i o n l i n e s i n t h e U.S.A. and e l s e w h e r e . A number o f f a i l u r e s have o c c u r r e d i n these l i n e s as a r e s u l t o f c e r t a i n carbonate/bicarbo nate mixtures being generated i n the a l k a l i n e l i q u i d a d j a c e n t t o t h e p i p e by t h e a p p l i c a t i o n o f c a t h o d i c p r o t e c t i o n (*0 . T h i s i s i n no way an argument a g a i n s t the u s e o f s u c h a p r o t e c t i o n method. I t i s m e r e l y a w a r n i n g about p o s s i b l e e f f e c t s . These f a i l u r e s a l s o u n d e r l i n e some o f t h e d i f f i c u l t technical/economic d e c i s i o n s a t t e n d a n t upon such f a i l u r e s . There a r e two q u i t e d i f f e r e n t q u e s t i o n s t o be s e t t l e d : (1) what c a n be done t o r e d u c e t h e i n c i d e n c e o f c r a c k i n g i n e x i s t i n g p i p e l i n e s ? and (2) what c a n be done t o r e d u c e t h e p o s s i b i l i t y o f c r a c k i n g i n p i p e l i n e s t o be i n s t a l l e d i n the future? B o t h t e c h n i c a l l y and e c o n o m i c a l l y t h e 2

2

American Chemical Society Library 1155 16th St. N. W. Washington, D. C. 20036 Brubaker and Phipps; Corrosion Chemistry

ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


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q u e s t i o n s pose a number o f d i f f e r e n t p r o b l e m s . T h i s i s i n some ways t y p i c a l o f a l l p r e v e n t a t i v e measures. There a r e many f a c t o r s t o be c o n s i d e r e d b e f o r e s e t t l i n g upon an a c c e p t a b l e s o l u t i o n . The use o f p a i n t i n g s and c o a t i n g s must be ment i o n e d . They a r e u s u a l l y a p p l i e d f o r o t h e r r e a s o n s — t o r e d u c e the r a t e o f g e n e r a l c o r r o s i o n and the c o s t o f c a t h o d i c p r o t e c t i o n . To the e x t e n t t h e y a r e s u c c e s s f u l , they w i l l f r e q u e n t l y minimize the i n c i d e n c e of s t r e s s c o r r o s i o n f a i l u r e s . C o a t i n g s a l w a y s have f a u l t s i n them and depending upon c o a t i n g s a l o n e i s u s u a l l y unw i s e . I t i s i n t h e i r use w i t h systems o f c a t h o d i c p r o t e c t i o n t h a t t h e y a r e t o be j u d g e d , t o g e t h e r w i t h a b i l i t y t o w i t h s t a n d s e r v i c e c o n d i t i o n s , e.g., c o n t i n ual temperature f l u c t u a t i o n s . I n c r e a s i n g the c o r r o s i o n r a t e might appear t o be a r a t h e r d r a s t i c c o u n t e r measure a g a i n s t s t r e s s c o r r o sion cracking. S i n c e i t i s a form o f h i g h l y l o c a l i z e d c o r r o s i o n , e x t e n d i n g c o r r o s i o n o v e r t h e whole o f the s u r f a c e w i l l u s u a l l y l e s s e n t h e p r o b a b i l i t y o f such failures. T h i s a p p r o a c h i s u n l i k e l y e v e r t o be a permanent remedy. I t i s employed i n making up m i x t u r e s c o n t a i n i n g HC1 t o c l e a n a u s t e n i t i c s t a i n l e s s s t e e l p a r t s i n c h e m i c a l p l a n t : t h e c o r r o s i o n r a t e i s maint a i n e d > 10 mpy (27). Cracking. P o s s i b l e e f f e c t s of plane s t r e s s / p l a n e s t r a i n t r a n s i t i o n s have a l r e a d y been i n d i c a t e d . The e f f e c t i v e n e s s o f t h e s t r a i n - r a t e i n p r o m o t i n g c r a c k i n g can be i m p o r t a n t where e n g i n e e r i n g s t r u c t u r e s a r e s u b j e c t t o p e r i o d i c s t r a i n i n g , e.g., p i p e l i n e s . I t i s p o s s i b l e t o have i n t e r r u p t e d l o a d i n g 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 w h i c h c a s e the s t r e s s combines t o produce a s t r a i n t r a n s i e n t t h a t g e n e r a t e s a c r a c k i n c r e m e n t . T h i s i s d i f f e r e n t from c o r r o s i o n f a t i g u e and t h e i n t e r f a c e between the two has been d i s c u s s e d (28). I n t e r r u p t e d l o a d i n g s t r e s s c o r r o s i o n c r a c k i n g can be o f p a r t i c u l a r i m p o r t a n c e where t h e system exh i b i t s a t h r e s h o l d under c o n s t a n t l o a d l a b o r a t o r y t e s t i n g conditions. A small f l u c t u a t i o n i n load (±1-2%) can reduce t h e t h r e s h o l d by 50% (3). Unless laboratory t e s t s simulate accurately service condit i o n s , c o n f u s i o n and a l a c k o f c o n f i d e n c e a r e l i k e l y t o ensue. S i n c e a t e n s i l e component o f s t r e s s i s r e q u i r e d , s t r e s s c o r r o s i o n c r a c k i n g can be p r e v e n t e d by p u t t i n g t h e s u r f a c e o f a component i n t o c o m p r e s s i o n , e.g., by short-peening. Where t h i s i s p r a c t i c a b l e , i t i s d e s i r a b l e . The t r e a t m e n t needs t o be a p p l i e d u n i f o r m ly. I t w i l l n o t be e f f e c t i v e i f p i t t i n g o c c u r s on t h e compressed l a y e r .


10. SCULLY

Stress-Corrosion Cracking

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Cracking may be avoided by using a nonsusceptible or less susceptible alloy. This usually entails using a more expensive alloy or a greater volume of a lower strength alloy. From what has been discussed above, such a choice depends upon a reliable test having been made in the environment under consideration.


Concluding Remarks Stress corrosion cracking i s a complicated subject and i t i s necessary to be very careful in making simplifications. Certain points have been emphasized in the description presented above. The simple idea of an imbalance between creep strain-rate and repassivation rate does seem to be an accurate description of an essential process occurring during the propagation of a stress corrosion crack. To appreciate the subtleties of this interaction does require both a metallurgical and electrochemical approach. In this subject a dual approach is always required. To do accurate tests, potential control is necessary and the investigation of a range of potentials is always required. Commercial metallurgical alloys are complicated heterogeneous assemblies of atoms. Some of these heterogeneities are responsible for cracking. Some points have been neglected because of the limitations of space and time. Why is the chloride ion so prevalent in Table I, for example? The answer almost certainly lies in i t s effect upon delaying repassivation as a result of metal ion hydrolysis and the low pH of many metal chlorides. In addition, i t promotes the i n i t i a l breakdown, probably in much the same way. With this and other points completely clear answers and explanations are not yet arrived at. Progress is being made, however, and eventually a good i f not perfect, predictive capacity w i l l be achieved over what remains a facinating and i r r i t a t i n g phenomenon. Literature Cited 1. 2. 3.

Staehle, R.W., Forty, A.J. and Van Roogen, D., Editors, "Fundemental Aspects of Stress Corrosion Cracking," N.A.C.E., Houston, Texas, 1969. Scully, J.C., Editor, "The Theory of Stress Corrosion Cracking in Alloys," N.A.T.O. Brussels, 1971 . McCright, R.D., Slater, J.E. and Staehle, R.W., Editors, "Stress Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys," N.A.C.E.,


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Swann, P.R., Ford, F.P. and Westwood, A.R.C., "Mechanisms of Environment Sensitive Cracking of Materials," Metals Society, London, 1977. 5. Brown, B.F., Editor, "Stress Corrosion Cracking in High Strength Steels and in Titanium and Aluminium Alloys," N.R.L., Washington, D.C., 1972. 6. Engseth, P. and Scully, J.C., Corrosion Science (1975) 15, 504. 7. Scully, J.C., Corrosion (1975) 15, 505. 8. Scully, J.C. and Powell, D.T., Corrosion Science (1970) 10, 371. 9. Scully, J.C., Corrosion Science (1975) 15. 207. 10. Scully, J.C., "Stress Corrosion Cracking — The Constant Strain Rate Technique," A.S.T.M., Philadelphia, in press. 11. Barbosa, M. and Scully, J.C., "Environment Sensitive Fracture of Engineering Materials," A.I.M.E., to be published. 12. Ambrose, J.R. and Kruger, J., Corrosion (1972) 28, 30. 13. "Symposium on Stress Corrosion Cracking of Metals," A.S.M./A.I.M.E., Philadephia, 1945. 14. Tromans, D. and Nutting, J., Corrosion (1965) 21, 146 15. Swann, P.R., Corrosion (1963) 19, 102. 16. Swann, P.R. and Nutting, J., J. Inst. Met. (1959/ 60) 80, 478. 17. Scamans, G. and Swann, P.R., Corrosion Science, in press. 18. Swann, P.R. and Embury, J.D., "High Strength Materials," Editor Zackey, V., Wiley, New York, 1965, p. 327. 19. Coleman, E.G., Weinstein, D. and Rostoker, W., Acta. Met. (1961) 9, 491. 20. Kermani, M. and Scully, J.C., Corrosion Science, in press. 21. Powell, D.T. and Scully, J.C., Corrosion (1968) 24, 151. 22. Chakrapani, D.G. and Pugh, E.N., 6th International Congress on Metallic Corrosion, N.A.C.E., Houston, to be published. 23. Scamans, G.M., Alani, R. and Swann, P.R., Corrosion science (1976) 16, 443. 24. Adepoju, T. and Scully, J.C., Corrosion Science (1976) 16, 789. 25. Majumdar, P. and Scully, J.C., Corrosion Science, in press. 26. N.A.C.E. Standard RP-04-72. 27. Laque, F.L. and Copson, H.R., "The Corrosion Resistance of Metals and Alloys," Reinhold, New York, 1963, p. 392. 28. Parkins, R.N. and Greenwell, B.S., Met. Sci. (1977) 21, 405. RECEIVED September21,1978.


Stress-Corrosion Cracking - American Chemical Society

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