High temperature behavior of superalloys exposed to sodium chloride: I. mechanical properties
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W. M. ROBERTSON,
AND
F. MANSFELD
An i n v e s t i g a t i o n of the c o r r o s i o n b e h a v i o r of s u p e r a l l o y s in the p r e s e n c e of s o d i u m c h l o r i d e at e l e v a t e d t e m p e r a t u r e s has been conducted and the e f f e c t of c o r r o s i o n on m e c h a n i c a l p r o p e r t i e s i n v e s t i g a t e d . R e s i d u a l m e c h a n i c a l p r o p e r t i e s as d e t e r m i n e d by a p o s t e x p o s u r e t e n s i l e t e s t w e r e found to be s e r i o u s l y d e g r a d e d , the a m o u n t of d e g r a d a t i o n being dependent upon the p a r t i a l p r e s s u r e of oxygen in the e n v i r o n m e n t . C r e e p r a t e s w e r e i n c r e a s e d by as much as t h r e e o r d e r s of m a g n it u d e in the p r e s e n c e of NaC1.
T H E hot c o r r o s i o n and s u l f i d i z a t i o n of s u p e r a l l o y s f o r t u r b i n e blade a p p l i c a t i o n s in a s i m u l a t e d j e t engine e n v i r o n m e n t has been studied for s o m e y e a r s now. Th e d e g r a d a t i o n of m e c h a n i c a l p r o p e r t i e s of s u p e r a l l o y s at e l e v a t e d t e m p e r a t u r e s in a i r e x p o s e d to NaC1 p o s e s a r a t h e r d i f f e r e n t p r o b l e m in that the s a l t is d e p o s i t e d on the s a m p l e r a t h e r than c o n ta in e d in the f u e l / a i r m i x t u r e . T h i s p r o b l e m has r e c e i v e d only a l i m i t e d a m o u n t of a t t e n t i o n in the p a s t . T h e only r e l a t e d s t u d i e s a r e the w o r k of P i c k e r i n g e t al.,1 M o s k o w it z and R e d m e r s k i , 2 and Seybolt, 3 none of whom studied the e f f e c t on m e c h a n i c a l p r o p e r t i e s in any d e t a i l , but found r a p i d c o r rosion. T h e o p e r a t i o n of p r o p o s e d r e u s a b l e s p a c e v e h i c l e s w i l l s u b j e c t the s t r u c t u r e s to conditions c o n d u ci v e to hot s a l t c o r r o s i o n . E x p o s u r e to s e a a i r or s e a s a l t p r i o r to launch w i l l p r o v i d e the n e c e s s a r y c o n d i t i o n s . Such v e h i c l e s m ay be s u b j e c t e d to a s e a a i r e n v i r o n m e n t f o r s o m e t i m e p r i o r to launch, and w i l l p r o b a b l y have s o m e s a l t d e p o s i t e d in t h e i r o u t e r s u r f a c e . A f t e r launch, and on s u b s e q u e n t r e e n t r y into the E a r t h ' s a t m o s p h e r e , k i n eti c heating w i l l r a i s e the heat s h i e l d t e m p e r a t u r e to 1200~ or a b o v e , depending on location on the v e h i c l e . It is this e x p o s u r e of high t e m p e r a t u r e m a t e r i a l s to s e a s a l t and e l e v a t e d t e m p e r a t u r e in a i r that p r o m p t e d the p r e s e n t w o r k . Th e o p e r a t i n g conditions a r e such that t h e r e a r e two c r i t i c a l r e g i m e s w h e r e p r o p e r t y d e g r a d a t i o n m ay be i m p o r t a n t . T h e f i r s t is during the h i g h - s t r e s s p e r i o d s of launch w h e
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