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PLEXIGLASS II

f r a c t u r e c h a r a c t e r i s t i c s of Inconel 718 d u r i n g c a t h o d i c c h a r g i n g under monotonic loading c o n d i t i o n s . The i m p l i c a t i o n s of t h e s e r e s u l t s will b e d i s c u s s e d in t e r m s

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of mechanistic differences between the classic problem of i n t e r n a l h y d r o g e n e m b r i t t l e m e n t (IHE), i.e., e m b r i t t l e m e n t c a u s e d b y c a t h o d i c c h a r g i n g ; and the m o r e r e cent p r o b l e m of g a s e o u s h y d r o g e n e m b r i t t l e m e n t (GHE), i.e., e m b r i t t l e m e n t c a u s e d b y a g a s e o u s h y d r o g e n e n vironment. Smooth t e n s i l e s p e c i m e n s , with 3.8 c m gage s e c t i o n s and 0.48 c m d i a m , w e r e m a c h i n e d f r o m 0.80 c m r o d m a t e r i a l . M e c h a n i c a l p r o p e r t i e s t e s t i n g , with and w i t h out c a t h o d i c c h a r g i n g , w e r e conducted u s i n g a 90 k N I n s t r o n u n i v e r s a l t e s t i n g m a c h i n e . The c a t h o d i c c h a r g Ing e q u i p m e n t is s c h e m a t i c a l l y shown in Fig. 1. The e n v i r o n m e n t was a 3.5 p c t NaC1 s o l u t i o n a c i d i f i e d with c o n c e n t r a t e d s u l f u r i c a c i d to a pH 0.3. D u r i n g t e n s i l e t e s t i n g , the p o t e n t i a l was m a i n t a i n e d at - 0.80 V v s s a t u r a t e d c a l o m e l e l e c t r o d e with a Wenking p o t e n t i o s t a t . The i n i t i a l c u r r e n t d e n s i t y was --230 A/m2; once y i e l d ing o c c u r r e d , the c u r r e n t d e n s i t y s l o w l y i n c r e a s e d to -~ 620 A / m 2. The c r o s s h e a d s p e e d r a n g e d f r o m 25 to 508/~m/min. R e s u l t s f r o m the m e c h a n i c a l p r o p e r t i e s t e s t s a r e p r e s e n t e d in T a b l e I. F o r s p e c i m e n s that w e r e c a t h o d i c a l l y c h a r g e d , a d e c r e a s e is shown in both e l o n g a t i o n and r e d u c t i o n of a r e a . The c r o s s h e a d s p e e d a p p e a r s to have i t s m o s t s i g n i f i c a n t effect on the r e d u c t i o n of a r e a , Fig. 2. R e s u l t s f r o m 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 y and o p t i c a l m i c r o s c o p y a r e p r e s e n t e d in F i g s . 3 to 5. The e f f e c t of c r o s s h e a d s p e e d on c r a c k i n i t i a t i o n is shown in Fig. 3. While the n u m b e r of i n i t i a t i o n s i t e s that r e s u l t e d f r o m c a t h o d i c c h a r g i n g a p p e a r s to r e m a i n c o n s t a n t , the s l o w e r c r o s s h e a d s p e e d a l l o w e d f o r g r e a t e r opening of s u r f a c e c r a c k s . This c a u s e d a d e c r e a s e in the r e d u c tion of a r e a . A v i e w of a s u r f a c e c r a c k i n i t i a t i n g p r e m a t u r e t e n s i l e f a i l u r e d u r i n g c a t h o d i c c h a r g i n g i s shown in F i g . 4. Such s u r f a c e c r a c k i n g was not o b s e r v e d on t e n s i l e s p e c i m e n s that w e r