Carbon deposition and the role of reducing agents in hot-corrosion processes
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carbons and m e t a l l i c p a r t i c l e s have been detected in the carbon deposit at c o n s i d e r a b l e d i s t a n c e s from the bulk m e t a l s u r f a c e , e'~ In spite of much effort in this a r e a , the specific influence of v a r i o u s m e t a l s on the growth of carbon l a y e r s has not yet been completely clarified. This p a p e r is concerned with the r e a c t i o n of the oxides of nickel and chromium, oxidized nickel and c h r o mium, a n i c k e l - a l u m i n u m - c h r o m i u m alloy, sodium sulfate with c a r b o n deposited f r o m methane at t e m p e r a t u r e s in the range 800 to 1000oc, and the r o l e of c a r bon in the h o t - c o r r o s i o n p r o c e s s .
EXPERIMENTAL Weight changes of the v a r i o u s solid s u b s t r a t e s on heating in different gaseous environments were d e t e r mined in a Chevenard t h e r m o b a t a n c e , Model TH-59, fitted with a quartz sheath so that m e a s u r e m e n t s could be c a r r i e d out at constant gas flow r a t e s . 8 Powdered s a m p l e s w e r e p l a c e d in a p o r c e l a i n c r u c i b l e mounted on top of a fused alumina r o d attached v e r t i c a l l y to the t h e r m o b a l a n c e b e a m . Metal coupons were p l a c e d h o r i zontally on a 1 in. diam alumina p l a t f o r m attached to the t o r s i o n r o d . Weight changes in flowing gas at l i n e a r l y i n c r e a s i n g t e m p e r a t u r e s or under i s o t h e r m a l conditions were r e c o r d e d a u t o m a t i c a l l y by means of a drum r e c o r d e r . T e m p e r a t u r e s were m e a s u r e d with a Pt-P,h/Pt thermocouple positioned j u s t above the s a m ple. Linear t e m p e r a t u r e r i s e r a t e s of 150 and 300~ p e r h were g e n e r a l l y used with gas flow r a t e s of 250 ml p e r min. Weight changes of • mg could be detected by this method. Coupons of a Ni-A1-Cr alloy and pure chromium (1.5 by 0.7 by 0.1 cm) were cut from s l i c e s of a s - c a s t ingots. The s p e c i m e n s were wet ground through 600 g r i t silicon c a r b i d e p a p e r before use. The bulk composition of the alloy was 76.16 Ni, 13.46 A1, 10.38 Cr. R e agent Grade Nickel Oxide (green) and anhydrous C.P. chromium sesquioxide were obtained f r o m F i s h e r Scientific Company and Matheson, Coleman, and Bell, r e s p e c t i v e l y . The powdered carbon s a m p l e s used were T h e r m a x Carbon Black (R. T. Vanderbllt Company), Carbolac Lampblack (Cabot Corp.) and 325-mesh c r y s talline graphite (UltraCarbon Corp.). In addition to high p u r i t y oxygen and nitrogen a m i x t u r e of 10 pct methanenitrogen was supplied by the Linde Company. The a n hydrous sodium sulfate used was Mallinckrodt Analyticat Reagent Grade. VOLUME 4, AUGUST 1973-1877
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Fig. 1--Reduction of anhydrous sodium sulfate by solid c a r bon. T h e r m o g
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