166 perio,dically.
The corrosion rate increased and some selective g r a i n
boundary a t t a c k occurred, b u t t h e g r a i n boundaries d i d not crack when t h e corrosion samples from t h e loop were deformed. I
Thus, we conclude t h a t corrosion alone cannot account f o r t h e ob-
I
served cracking.
However, chromium depletion by corrosion could make t h e
material more s u s c 6 p t i b l e t o cracking by t h e inward d i f f u s i o n of f i s s i o n products, and t h i s w i l l be discussed later i n more d e t a i l . I
Exposure t o Fission Products Three types of experiments have been run i n which samples are exposed t o f i s s i o n products: (1) exposure of mechanical property samples t o vapors of f i s s i o n products, (2) e l e c t r o p l a t i n g t e l l u r i u m on mechanical property samples, and ( 3 ) studying t h e mechanical p r o p e r t i e s of a l l o y s t h a t contain
small a l l o y i n g a d d i t i o n s of t h e f i s s i o n products.
S u l f u r has been included
i n t h i s work even though i t is not a f i s s i o n product, since it is i n t r o duced i n a r e a c t o r by l u b r i c a n t inleakage and i s reputed t o be detrimental t o nickel-base a l l o y s .
The experiments with f i s s i o n product vapors have included S, Se, T e , 12, As, Sb, and Cd. These materials have s u f f i c i e n t vapor pressure a t 65OOC t o t r a n s p o r t t o mechanical property samples.
The samples were
annealed i n a q u a r t z capsule w i t h each f i s s i o n product f o r various t i m e s a t 65OoC, deformed a t 25OC, and sectioned f o r metallographic examination. ,
Examination of the samples exposed i n t h i s way has revealed several important f a c t s . 1. Of t h e samples of INOR-8,
type 304L stainless steel, and nickel-
200 exposed t o a l l seven elements f o r 2000 h r a t 650째C, only INOR-8 and n i c k e l 200 exposed t o t e l l u r i u m had i n t e r g r a n u l a r cracks a f t e r deformation
at 25OC. 2.
The cracks produced i n INOR-8 by exposure t o t e l l u r i u m look q u i t e
s i m i l a r t o those formed i n t h e material from t h e MSRE (Fig. 113). 3.
Exposure t o t e l l u r i u m over t h e temperature range of 550 t o 7OOOC f
and concentration range of two orders of magnitude showed t h a t t h e s e v e r i t y of cracking i n INOR-8 and n i c k e l 200 increased with i n c r e a s i n g temperature and t e l l u r i u m concentration.
Type 304L s t a i n l e s s steel d i d not crack
under any of t h e conditions i n v e s t i g a t e d .
bd