An investigation of hot corrosion morphology by color X-ray distribution imaging
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by Color X-Ray Distribution Imaging D. W. McKEE, M. McCONNELL, R. B. BOLON, AND G. ROMEO The s t r u c t u r e of c o r r o s i o n l a y e r s on n i c k e l - b a s e a l l o y s r e s u l t i n g f r o m r e a c t i o n with s u l f u r dioxide and s o d i u m s u l f a t e at e l e v a t e d t e m p e r a t u r e s (hot c o r r o s i o n ) has b e e n s t u d i e d b y the m e t h o d of c o l o r X - r a y d i s t r i b u t i o n i m a g i n g . This technique i n v o l v e s the p r e p a r a t i o n of c o l o r c o m p o s i t e p r i n t s b y s u p e r i m p o s i n g p r i m a r y c o l o r i m a g e s of the individual e l e m e n t X - r a y d i s t r i b u t i o n s on a single s h e e t of c o l o r f i l m . By a p r o p e r c h o i s e of p r i m a r y t i n t s , the d i s t r i b u t i o n of r e a c t i o n p r o d u c t s and o t h e r f e a t u r e s of th~ c o r r o s i o n m o r p h o l o g y can be c l e a r l y defined a s a r e s u l t of c o l o r m i x i n g in the c o m p o s i t e m i c r o g r a p h . The d i s t r i b u t i o n of c o r r o s i o n p r o d u c t s in the s u r f a c e l a y e r s of N i - C r and Ni-A1 a l l o y s and an IN 738 s p e c i men following s u l f i d a t i o n was i n v e s t i g a t e d using this technique. A simple technique for preparing color composite m i c r o g r a p h s f r o m b l a c k and white X - r a y d i s t r i b u t i o n i m a g e s o b t a i n e d with the e l e c t r o n m i c r o p r o b e has b e e n d e s c r i b e d r e c e n t l y . 1 P o s i t i v e i m a g e s f o r up to t h r e e separate elements are first obtained by photographing the c a t h o d e - r a y tube d i s p l a y of the m i c r o p r o b e in the u s u a l way. T h e s e i m a g e s c o n s i s t of white dots, one f o r each d e t e c t e d X - r a y p u l s e , on a b l a c k b a c k g r o u n d and denote the d i s t r i b u t i o n of e a c h e l e m e n t in the s e l e c t e d f i e l d of view. A c o m p o s i t e c o l o r i m a g e is then o b t a i n e d b y s e q u e n t i a l l y photocopying each X - r a y i m a g e on c o l o r f i l m t h r o u g h a p r i m a r y c o l o r f i l t e r . C o l o r m i x i n g then r e s u l t s in t i n t e d r e g i o n s of high c o n t r a s t which identify the v a r i o u s p h a s e s p r e s e n t in the s a m p l e m i c r o s t r u c ture. It has b e e n s u g g e s t e d I that t h i s technique m a y p r o v e u s e f u l in m e t a l l u r g i c a l and p e t r o g r a p h i c a l a p p l i c a t i o n s and, m o r e r e c e n t l y , 2 it h a s b e e n u t i l i z e d in an i n v e s t i gation of the f r a c t u r e m e c h a n i s m of f i r e c l a y c o n d e n s e r s u s e d in a z i n c - r e d u c t i o n s m e l t e r . The p r e s e n t p a p e r i l l u s t r a t e s the a p p l i c a t i o n of the c o l o r c o m p o s i t e t e c h nique to the s t u d y of hot c o r r o s i o n of n i c k e l - b a s e a l l o y s . This type of c o r r o s i o n , which a r i s e s f r o m the c o m
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