Galvanostatic vs potentiostatic Control in corrosion fatigue testing of aluminum alloys
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M c G U ~ N E S S A N D O . F . DEVEREUX
R e c e n t work r e p o r t e d by t h i s l a b o r a t o r y z'2 has d e a l t with the effect of p o t e n t i o s t a t i c p o l a r i z a t i o n on both the fatigue life and the r a t e of fatigue c r a c k growth in a l u m i n u m a l l o y s in a s a l i n e e n v i r o n m e n t . It was noted d u r i n g t h e s e t e s t i n g p r o g r a m s that the c u r r e n t p a s s e d t h r o u g h the fatigue s p e c i m e n v a r i e d d u r i n g the c o u r s e of a t e s t . In s o m e c a s e s the c u r r e n t was even o b s e r v e d to change sign, e.g., a t e s t i n i t i a l l y i n t e n d e d to be p e r f o r m e d u n d e r anodic p o l a r i z a t i o n by v i r t u e of s e l e c t i n g a s p e c i m e n potential g r e a t e r t h a n the e x p e r i m e n t a l l y o b s e r v e d c o r r o s i o n o r r e s t p o t e n t i a l , E R , might s u b s e q u e n t l y b e c o m e cathodic a s s e e n by the d i r e c t i o n of c u r r e n t flow t h r o u g h the s p e c i m e n . 3 This c l e a r l y i m p l i e d that e i t h e r the anodic or cathodic p o l a r i z a t i o n curve, or both, was b e i n g s u b s t a n t i a l l y d i s p l a c e d as fatigue p r o g r e s s e d , p r e s u m a b l y as a d i r e c t r e s u l t of fatigue r a t h e r t h a n s i m ple e n v i r o n m e n t a l e x p o s u r e . It was p r e v i o u s l y noted 2 that, in p a r t i c u l a r , the cathodic p o l a r i z a t i o n b e h a v i o r of the 7075 alloy was s i g n i f i c a n t l y d i s p l a c e d t o w a r d s h i g h e r c u r r e n t s when m e a s u r e d u n d e r cyclic s t r e s s , and the T a f e l slope was i n c r e a s e d . Other f a c t o r s a s i d e , this would c a u s e the r e s t potential to move i n the n o b l e d i r e c t i o n 4 i n a c c o r d with the v a r i a t i o n s e e n d u r i n g fatigue t e s t i n g . Without w i s h i n g to r a i s e the q u e s t i o n a s to w h e t h e r t e s t s p e r f o r m e d at c o n s t a n t c u r r e n t w e r e m o r e o r l e s s s i g n i f i c a n t f r o m e i t h e r a t h e o r e t i c a l viewpoint o r a p r a g m a t i c one, it was decided to p e r f o r m a fatigue life t e s t p r o g r a m u n d e r g a l v a n o s t a t i c conditions to d e t e r m i n e if the c o n s i s t a n c y of such t e s t s could be i m p r o v e d . The t e s t s w e r e p e r f o r m e d on s h e e t 7075T6 a l u m i n u m in an a e r a t e d s o l u t i o n of 3.5 pct NaC1 i n d i s t i l l e d water at r o o m t e m p e r a t u r e (~21~ The p r o c e d u r e was s i m i l a r to that p r e v i o u s l y d e s c r i b e d , ~ t e s t i n g b e i n g p e r f o r m e d in a constant a m p l i t u d e , r e minimum stress v e r s e b e n d i n g m a c h i n e with R = m a x i m u m s t r e s s = 0. The s p e c i m e n s w e r e unnotched, but a s t r e s s - r a i s e r in the f o r m of a c i r c u l a r hole was employed to l o c a l i z e f a i l u r e . M a x i m u m s t r e s s at t
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