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W H E N high t e m p e r a t u r e n i c k e l - o r c o b a l t - b a s e s u p e r a l l o y s a r e u s e d a s v a n e s and b l a d e s in g a s t u r b i n e e n g i n e s , they a r e found s u s c e p t i b l e to an a c c e l e r a t e d a t t a c k , much f a s t e r than n o r m a l o x i d a t i o n . The r e s u l t i n g c a t a s t r o p h i c d e t e r i o r a t i o n has b e e n r e l a t e d 1'2 to the p r e s e n c e of s a l t d e p o s i t s , p a r t i c u l a r l y Na2SO4, on the hot s e c t i o n c o m p o n e n t s . T h i s c o r r o s i o n p h e n o m enon h a s b e e n v a r i o u s l y r e f e r r e d to a s " h o t c o r r o s i o n " o r " s u l f i d a t i o n " , s i n c e the p r e s e n c e of s u l f i d e s i s f r e q u e n t l y o b s e r v e d 2-5 b e n e a t h the o x i d e s c a l e s . A l a b o r a t o r y technique which is r e l a t i v e l y s i m p l e to p e r f o r m and i n t e r p r e t , and which s i m u l a t e s in p a r t the a c c e l e r a t e d d e g r a d a t i o n j u s t d e s c r i b e d , c o n s i s t s of c o a t ing a l l o y s with Na2SO4 and s u b s e q u e n t l y o x i d i z i n g t h e s e c o a t e d s p e c i m e n s at e l e v a t e d t e m p e r a t u r e s in e i t h e r a i r o r oxygen. The r e s u l t i n g a t t a c k is r e f e r r e d to a s Na2SO4-induced o x i d a t i o n . I n c r e a s e d s t r e n g t h and r e d u c e d c r e e p r a t e s at e l e v a t e d t e m p e r a t u r e s can be p r o d u c e d by d i s p e r s i o n s t r e n g t h e n i n g , in which the m e t a l c o n t a i n s a u n i f o r m d i s t r i b u t i o n of a f i n e l y d i s p e r s e d s e c o n d - p h a s e o x i d e . I n i t i a l l y , this m e c h a n i s m w a s l i m i t e d to s i m p l e m e t a l s , but r e c e n t a d v a n c e s in f a b r i c a t i o n t e c h n i q u e s have e n a b l e d the p r o d u c t i o n of c o m p l e x a l l o y s . The c o m m e r c i a l l y a v a i l a b l e a l l o y Ni-20 wt pct C r - 2 wt pct ThO2 ( T D - N i C r ) is b a s e d on the highly o x i d a t i o n r e s i s t a n t n i c h r o m e c l a s s of h e a t e r a l l o y s . R e c e n t s t u d i e s 6-s of the o x i d a t i o n b e h a v i o r of T D - N i C r have e s t a b l i s h e d that the p r e s e n c e of ThO2 in the a l l o y p r o m o t e s the s e l e c t i v e o x i d a t i o n of c h r o m i u m , and that the g r o w t h r a t e of the Cr203 l a y e r is a p p r o x i m a t e l y one o r d e r of m a g n i t u d e l e s s than for the g r o w t h of Cr203 on a s i m p l e N i - C r a l l o y . The p u r p o s e of the p r e s e n t i n v e s t i g a t i o n is to study the Na2SO4-induced o x i d a t i o n of T D - N i C r and to d e t e r m i n e 1) w h e t h e r hot c o r r o s i o n type r e a c t i o n s a r e p r o d u c e d on this a l l o y and 2) w h e t h e r o r not the ThO2 p a r t i c l e s affect such reactions. EXPERIMENTAL

19.2 wt pct Cr and 1.98 wt pct ThO2 with the p r i m a r y i m p u r i t i e s b e i n