Mechanisms of corrosion fatigue crack propagation in Al-Zn-Mg alloys
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STOLTZ
AND
R. M. PELLOUX
C o r r o s i o n fatigue c r a c k p r o p a g a t i o n t e s t s w e r e p e r f o r m e d on c o m m e r c i a l 7075 a l l o y s . T e s t i n g w a s done in a 3.5 p c t s o d i u m c h l o r i d e s o l u t i o n u n d e r c o n s t a n t i m p r e s s e d p o t e n t i a l and u n d e r r e v e r s e d a n o d i c - c a t h o d i c c u r r e n t c o n d i t i o n s . R e s u l t s i n d i c a t e d that a c a t h o d i c p o t e n t i a l of - 1 . 4 0 0 V v s SCE w a s s u f f i c i e n t to r e d u c e c o r r o s i o n fatigue c r a c k g r o w t h r a t e s to the l e v e l o b s e r v e d in d r y a r g o n . By a l t e r n a t e l y i m p r e s s i n g anodic and c a t h o d i c c u r r e n t s , it w a s shown that anodic p o t e n t i a l s enhance the c r y s t a l l o g r a p h i c d e p e n d e n c e of the f r a c t u r e m o d e , r e s u l t i n g in b r i t t l e s t r i a t i o n s , while c a t h o d i c p o t e n t i a l s r e s u l t in d u c t i l e s t r i a t i o n s f o r m e d by s h e a r . M o d i f i c a t i o n of the a l l o y c h e m i s t r y and l o w e r i m p u r i t y c o n tent r e s u l t e d in a t w o - f o l d r e d u c t i o n in c r a c k g r o w t h r a t e s . T h e r m o m e c h a n i c a l t r e a t m e n t of t h e s e a l l o y s to r e f i n e the g r a i n s i z e p r o v e d d e t r i m e n t a l . Adding an i n h i b i t o r to the s o d i u m c h l o r i d e s o l u t i o n w a s found to be the m o s t e f f e c t i v e m e a n s f o r r e d u c i n g c o r r o s i o n fatigue c r a c k g r o w t h r a t e s . A m o d e l f o r the e n v i r o n m e n t - s u r f a c e i n t e r a c t i o n i s s u g g e s t e d .
FORSYTH,
Stubbington, and C l a r k 1'2 w e r e the f i r s t to r e p o r t that the A I - Z n - M g a l l o y s e x h i b i t two d i f f e r e n t t y p e s of fatigue s t r i a t i o n s . Stubbington 3 f u r t h e r c h a r a c t e r i z e d t h e s e s t r i a t i o n s and d i f f e r e n t i a t e d t h e m into two t y p e s : T y p e A o r d u c t i l e s t r i a t i o n s and T y p e B or brittle striations. Type A striations form during crack p r o p a g a t i o n in d r y a i r while T y p e B s t r i a t i o n s a r e u s u a l l y o b s e r v e d on f r a c t u r e s u r f a c e s p r o d u c e d in a c o r r o s i v e e n v i r o n m e n t such a s s e a w a t e r . T h e T y p e B s t r i a t i o n s c o n t a i n "a b r i t t l e c o m p o n e n t and r i v e r m a r k i n g s c h a r a c t e r i s t i c of c l e a v a g e f r a c t u r e " . 3 Stubbington a l s o noted that T y p e B s t r i a t i o n s o c c u r on a f r a c t u r e s u r f a c e that is on o r n e a r a {100} p l a n e in a l u m i n u m a l l o y s . F i g . 1 shows the b a s i c d i f f e r e n c e s b e t w e e n the two t y p e s . T h i s f r a c t o g r a p h r e p r e s e n t s the t r a n s i t i o n c o r r e s p o n d i n g to a sudden change in the e n v i r o n m e n t , f r o m d r y a i r to s e a w a t e r , d u r
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