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Additional kinetic and X - r a y diffraction d a t a will be obtained t o e s t a b l i s h the t e m p e r a t u r e at which the change i n the p a r a b o l i c r e a c t i o n m e c h a n i s m o c c u r s . - Diffusion e x p e r i m e n t s have been m a d e at 1200, 1300, and 1400 C using a concentration-gradient technique i n which a n a l y s i s i s m a d e of a concentration g r a d i e n t p r e p a r e d i n a cylindrical specimen. The t e r m i n a l or m a x i m u m solubility of n i t r o g e n i n the m e t a l w a s calculated f r o m the concentration gradient.

It c a n be s e e n that the diffusion coefficients of t h i s study a r e i n the r a n g e of those obtained f r o m i n t e r n a l - f r i c t i o n m e a s u r e m e n t s . (', 2, In c o n t r a s t , the solubility ( f r o m i n t e r n a l - f r i c t i o n m e a s u r e m e n t s ) r e p o r t e d by Ang and Wert( 3, is m u c h lower than value f r o m this r e s e a r c h f o r the s a m e t e m p e r a t u r e . Solubilities will be checked by m e t a l l o g r a p h i c examination of s t r u c t u r e s r e s u l t i n g f r o m d i r e c t s a t u r a t i o n of niobium.

The four unconnected s p e c i m e n s f r o m the batchdissolution r u n w e r e l e f t with b r i g h t , etched s u r f a c e s which a p p r o a c h being chemically polished. The liquid-phase s p e c i m e n which had b e e n i n contact with the s t a i n l e s s s t e e l had a much l o w e r c o r r o s i o n r a t e , was dull i n a p p e a r a n c e , and showed no etching o r s e l e c t i v e attack. Dissolution conditions , t h e r e f o r e , can be concluded to give higher c o r r o s i o n r a t e s i n the vapor phase than might be predicted f r o m reflux-type s t u d i e s , but do afford cathodic protection to s u b m e r g e d m e t a l .

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Tracking and Data Relay Satellite System NASA Facts


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