Limitations of potentiostatic control in stress corrosion crack growth measurements
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The e l e c t r o d e potential d i s t r i b u t i o n along a c r a c k in a p o t e n t i o s t a t i c a t l y p o l a r i z e d s p e c i m e n has been d e r i v e d a n a l y t i c a l l y by including p o l a r i z a t i o n b e h a v i o r and solution conductance c o n s i d e r a t i o n s . The a n a l y s i s has b e e n applied to the s t r e s s c o r r o s i o n c r a c k s within low alloy s t e e l s in an 8M s o d i u m hydroxide solution at 373 K and shows that the e l e c t r o d e potential at the tip falls to the n o r m a l e q u i l i b r i u m c o r r o s i o n potential as the c r a c k length i n c r e a s e s . T h e s e r e s u l t s show that p o t e n t i o s t a t i c c o n t r o l at the tip of a s t r e s s c o r r o s i o n c r a c k is s u b j e c t t 9 l a r g e v a r y i n g s y s t e m a t i c e r r o r s . C o n s e q u e n t l y the validity of s t r e s s c o r r o s i o n m e c h a n i s m s b a s e d on p o t e n t i o s t a t i c a l l y c o n t r o l l e d c r a c k growth m e a s u r e m e n t s which do not take into a c c o u n t such e r r o r s should be r e e x a m i n e d .
1. INTRODUCTION S T R E S S c o r r o s i o n c r a c k growth m e a s u r e m e n t s u n d e r c o n t r o l l e d e l e c t r o d e p o t e n t i a l conditions u s i n g e i t h e r p o t e n t i o s t a t i c c o n t r o l o r galvanic coupling has b e e n widely used to i n v e s t i g a t e the m e c h a n i s m s and the k i n e t i c s of s t r e s s c o r r o s i o n c r a c k growth. 1,2 Such i n v e s t i g a t i o n s have b e e n c a r r i e d out on both smooth and p r e c r a c k e d s p e c i m e n s . Smooth s p e c i m e n s have b e e n tested for d i f f e r e n t t i m e s to give c r a c k length v s time data which has allowed some e s t i m a t e of both t i m e for c r a c k i n i t i a t i o n and m e a n c r a c k growth rate.* More r e c e n t l y , f a t i g u e - p r e c r a c k e d s p e c i m e n s have b e e n used to evaluate c r a c k growth r a t e s in t e r m s of the applied s t r e s s i n t e n s i t y . ~ S t r e s s c o r r o s i o n c r a c k growth r a t e s have b e e n m e a s u r e d with the s p e c i m e n m a i n t a i n e d u n d e r potent i o s t a t i c c o n t r o l and these r a t e s have then b e e n c o r r e l a t e d with other e l e c t r o c h e m i c a l p r o p e r t i e s , such as hydrogen evolution r a t e , anodic d i s s o l u t i o n r a t e , p a s s i v a t i o n b e h a v i o r and ' b a r e ' s u r f a c e d i s s o l u t i o n k i n e tics 3-~ obtained at the s a m e e l e c t r o d e potential. Such c o r r e l a t i o n s have then b e e n used to either r e i n f o r c e or d e t r a c t f r o m one of the v a r i o u s l y p r o p o s e d models of s t r e s s c o r r o s i o n c r a c k growth. A f r e q u e n t a s s u m p tion in these p r o c e d u r e s is that the e l e c t r o d e potent i a l 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 is equal to or close to that which e x i s t s at the
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