High temperature oxidation of tungsten wires in water vapor-argon mixtures
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two p r e v i o u s papers, x'z the high temperature o x i d a t i o n of t u n g s t e n w i r e s in n a t u r a l c o n v e c t i o n was m e a s u r e d in d i l u t e O2-argon and CO2-argon a t m o s p h e r e s in t h e t e m p e r a t u r e r a n g e b e t w e e n 2450 and 3000 K. T h e s e m e a s u r e m e n t s a r e a p a r t of a s e r i e s of i n v e s t i g a t i o n s on the effect of the l a m p a t m o s p h e r e on the o p e r a t i n g c h a r a c t e r i s t i c s of i n c a n d e s c e n t l a m p f i l a m e n t s . The r a t e s of t h e s e two o x i d a t i o n r e a c t i o n s gen'e r a l l y d e c r e a s e with i n c r e a s i n g t e m p e r a t u r e at c o n s t a n t bulk oxidant p a r t i a l p r e s s u r e . In both c a s e s , the o v e r a l l r e a c t i o n is g a s t r a n s p o r t c o n t r o l l e d and a v o l a t i l e o x i d e - c o u n t e r d i f f u s i o n m o d e l i s in r e a s o n a b l e a g r e e m e n t with the data. The c o u n t e r diffusion model, which i s b a s e d on a s t a g n a n t f i l m m a s s t r a n s f e r t h e o r y of thin w i r e s in n a t u r a l convention, s a s s u m e s no h o m o g e n e o u s r e a c t i o n s in the b o u n d a r y l a y e r . In the p r e s e n t p a p e r , the r a t e of v o l a t i l i z a t i o n of t u n g s t e n w i r e s in f r e e c o n v e c t i o n b y high t e m p e r a t u r e o x i d a t i o n in H 2 0 - a r g o n a t m o s p h e r e s was m e a s u r e d in the t e m p e r a t u r e r a n g e b e t w e e n 2450 and 3000 K and in the H20 p a r t i a l p r e s s u r e r a n g e b e t w e e n 3.39 • 10 -s and 1.82 • 10 -4 a t m at 0.789 a t m (600 mm) t o t a l p r e s s u r e . A d d i t i o n a l m e a s u r e m e n t s w e r e a l s o c a r r i e d out at 0.395 a t m (300 mm) t o t a l p r e s s u r e f o r s e l e c t e d c o n d i t i o n s . The s u p p l e m e n t a l e x p e r i m e n t s d e m o n s t r a t e that the r e a c t i o n b e t w e e n s m a l l c o n c e n t r a t i o n s of w a t e r v a p o r in a r g o n and t u n g s t e n w i r e s in n a t u r a l c o n v e c tion i s g a s t r a n s p o r t c o n t r o l l e d . EXPERIMENT The e x p e r i m e n t a l p r o c e d u r e that was u s e d in the p r e v i o u s i n v e s t i g a t i o n s t'2 had to b e m o d i f i e d f o r the w a t e r v a p o r - a r g o n s t u d i e s . In the p r i o r m e a s u r e ments, a copper filament holder containing a number of 1.52 • 10 -~ c m d i a m , 5 c m long s t r a i g h t tungsten w i r e s * w e r e p l a c e d into an u l t r a - h i g h v a c u u m s y s t e m . *The tungstenwiresare so-calleddopedtungstencontaininga total of 100 ppm impuritiesconsistingmainlyof aluminum,potassium,and silicon. The v a c u u m c h a m b e r was b a c k f i l l e d with an a n a l y z e d F. J. HARVEY is Senior Scientist, Westinghouse Research and Development Center, Churchill Boro, Pittsburgh, Pa. 15235 Manuscript submitted October 1, 1973. METALLURGICAL TRANSACTIONS
O2-argon o r CO2-argon g
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