Stress corrosion cracking of alpha brass in a tarnishing ammoniacal environment: Fractography and chemical analysis
- PDF / 1,873,293 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 97 Downloads / 194 Views
e x p o s e d by m e c h a n i c a l l y o p e n i n g s p e c i m e n s through which SCC c r a c k s had p r o p a g a t e d p a r t way. A proposed alternate rate-controlling mechanism is y i e l d - a s s i s t e d anodic d i s s o l u t i o n of b a r e m e t a l at the l e a d i n g edge of the p r o p a g a t i n g c r a c k . 1,14 H o a r and B o o k e r 2 c o n s i d e r e d that s i n g l e - s t e p r a p i d c r a c k e x t e n sion m a y p r o c e e d by t h i s m e a n s . L e i d h e i s e r and K i s s i n g e r 15 c l a i m e d that the a b s e n c e of an obvious t a r n i s h at the l e a d i n g edge and a high c o n c e n t r a t i o n of Zn ~+ ions in the liquid within the SCC c r a c k s u p p o r t s such a m o d e l . H o w e v e r , s t u d i e s of b r a s s s u r f a c e s by J e n k ins and D u r h a m 5 d e m o n s t r a t e d that thin oxide f i l m s can f o r m a f t e r o n l y b r i e f e x p o s u r e to a m m o n i a c a l e n v i r o n m e n t s , and that such f i l m s a r e t r a n s p a r e n t . S i m i l a r l y , B i r l e y and T r o m a n s 16 d e t e r m i n e d that c u p r o u s o x i d e e x i s t s on SCC f r a c t u r e s u r f a c e s which a r e not o b v i o u s l y t a r n i s h e d , but y e l l o w in c o l o r . D i r e c t s t u d i e s of the a c t i v e p r o c e s s e s within the propagating crack are required for a detailed unders t a n d i n g of SCC. T h e r e i s a high c o n c e n t r a t i o n of Zn ++ ions in s o l u t i o n within the p r o p a g a t i n g SCC c r a c k , a s d e s c r i b e d above and, a s L e e s 13 found by EDAX a n a l y s i s of the SCC f r a c t u r e s u r f a c e , the c o r r o s i o n p r o d u c t f i l m within the SCC c r a c k is quite r i c h in c o p p e r and d e p l e t e d of zinc. This a n a l y s i s is c o n s i s t e n t with the c o m p o s i t i o n of t h e f i l m found on the s p e c i m e n s u r f a c e . B i r l e y and T r o m a n s 18 d e t e r m i n e d by e l e c t r o n d i f f r a c tion t h a t c u p r o u s o x i d e d o e s e x i s t upon the s t r e s s c o r rosion fracture surfaces. These authors demonstrated a s well that o u t e r s u r f a c e c u p r o u s o x i d e l a y e r s on b r a s s a r e p o w d e r y d e p o s i t s h a v i n g v i r t u a l l y no m e c h a n i c a l s t r e n g t h , and thus do not b e h a v e a s b r i t t l e f i l m s . They p r o p o s e d a m o d i f i e d t a r n i s h - r u p t u r e m o d e l in which c r a c k i n g p r o c e e d s b y f o r m a t i o n and p a r t i n g of the w e a k c u p r o u s o x i d e . I n f o r m a t i o n about the depth of p e n e t r a t i o n of the f i l m with r e s p e c t to the SCC l e a d i n g edge i s l a c k i n g . Also, a s pointed out by Pugh, 6 the r a t e of f i l m p e n e t r a t i o n a l o n g the u n s t r e s s e d g r a i n b o u n d a r y has not b e e n m e a s u r e d . The r a t e c o n t r o l l i n g s t e p in SCC r e m a i n s u n i d e n t i f i e d . O b j e c t i v e s of the p r
Data Loading...
