The propagation of transgranular stress-corrosion cracks in admiralty metal
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1. INTRODUCTION ALPHA-phase Cu-Zn alloys undergo both intergranular and transgranular stress-corrosion cracking (I-SCC and T-SCC) in aqueous ammoniacal solutions, depending on the specific conditions. For example, annealed Cu-30 Zn cracks transgranularly in nontarnishing solutions but fails predominantly by I-SCC in tarnishing solutions; ~heavily cold worked Cu-30 Zn, on the other hand, undergoes T-SCC in both types of solution.2 While less prevalent than the intergranular form, t r a n s g r a n u l a r service failures occur frequently in these alloys. A d m i r a l t y Metal, in p a r t i e u I a r , c o m m o n l y falls by T-SCC, and this has been a t t r i b u t e d to the p r e s e n c e of Sn and As. 3 In c o n t r a s t to I-SCC, T-SCC in the Cu a l l o y s has r e c e i v e d l i t t l e s y s t e m a t i c attention. Pugh and G r e e n 4 studied the T-SCC of a n n e a l e d C u - 3 0 Zn in n o n t a r n i s h ing 15 N a m m o n i a c a l s o l u t i o n s , and c o n f i r m e d that, u n d e r o p e n - c i r c u i t conditions, a c r i t i c a l c o n c e n t r a t i o n of c u p r i c c o m p l e x ions is n e c e s s a r y for the o c c u r r e n c e of c r a c k i n g . However, p o t e n t i o s t a t i e e x p e r i m e n t s d e m o n s t r a t e d that c r a c k i n g o c c u r s in the a b s e n c e of the c u p r i c c o m p l e x e s if the s p e c i m e n is a n o d i c a l l y p o l a r ized so that the d i s s o l u t i o n r a t e e x c e e d s a c r i t i c a l value. The c u p r i c c o m p l e x is the m a i n cathodic s p e c i e s in this s y s t e m and thus it was concluded that its r o l e in o p e n - c i r c u i t t e s t s i s s i m p l y to p e r m i t r a p i d anodic d i s s o l u t i o n . T h e s e o b s e r v a t i o n s e l i m i n a t e the p o s s i b i l i t y that the c u p r i c c o m p l e x e s act as c r i t i c a l s p e c i e s in the a d s o r p t i o n model, a s has b e e n s u g g e s t e d by some workers. 5 P i n c h b a c k , Clough and Heldt ~ have studied the t r a n s granular fracture surfaces in Cartridge B r a s s tested in a n o n t a r n i s h i n g 15 N a m m o n i a c a l solution, u s i n g the
s c a n n i n g e l e c t r o n m i c r o s c o p e and A u g e r e l e c t r o n s p e c t r o s c o p y (AES). The f r a c t u r e s u r f a c e s w e r e o b s e r v e d to b e c l e a v a g e - l i k e , s i m i l a r to those p r o d u c e d by T-SCC in o t h e r s y s t e m s . 7'8 AES data i n d i c a t e d that the f r a c t u r e s u r f a c e s w e r e f r e e of oxide f i l m s , and that d e z i n c i f i c a t i o n had o c c u r r e d . However, AES s t u d i e s w e r e not c a r r i e d out on the e x t e r n a l s u r f a c e s o r on u n s t r e s s e d s a m p l e s exposed to the solution, so that it i s not c l e a r w h e t h e r d e z i n c i f i e a t i o n o c c u r s at the tip of the s t r e s s - c o r r o s i o n c r a c k as a r e s u l t of the s t r e s s - c o r r o s i o n p r o c e s s i t s e l f o r w h e t h e r it
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