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1. INTRODUCTION

2. E X P E R I M E N T A L PROCEDURE

T H E k i n e t i c s of the f o r m a t i o n of c h r o m i u m oxide on p u r e c h r o m i u m as well as on i r o n , n i c k e l , and cobalt a l l o y s with high c h r o m i u m contents has often been studied b e c a u s e gastight Cr2Os l a y e r s with p r o t e c t i v e p r o p e r t i e s a r e f o r m e d on the s u r f a c e s so that these m a t e r i a l s can be u s e d at high t e m p e r a t u r e s . B e c a u s e of its high f r e e e n e r g y of f o r m a t i o n Cr203 is f o r m e d m o r e e a s i l y than many other oxides. The d e s t r u c t i o n of p r o t e c t i v e s c a l e s has, however, b e e n studied v e r y r a r e l y . In addition to m e c h a n i c a l d e s t r u c t i o n it is i n d i s p e n s a b l e to c o n s i d e r c h e m i c a l d e c o m p o s i t i o n s by r e d u c t i o n or by t r a n s f o r m a t i o n into other compounds like c a r b i d e s or n i t r i d e s , in p a r t i c u l a r d u r i n g p r o c e s s e s i n v o l v i n g changing gas a t m o s p h e r e s . It is known f r o m l i t e r a t u r e that CrzO3 can be r e d u c e d only by pure dry hydrogen above a p p r o x i m a t e l y 1300 K; 1-7 f a i l u r e s to achieve that, however, have also b e e n p u b l i s h e d , s-~4 The d i f f i c u l t i e s a r e b a s e d on the t h e r m o d y n a m i c s of the r e a c t i o n : Cr203 + 3 H2 = 2 Cr + 3 H20

[1]

with the e q u i l i b r i u m c o n s t a n t : 3

9a 2

PH~O

K p - p3

.

Cr

[2]

H2 a c h o 3 and the free e n e r g y of r e a c t i o n A G ~ :15 A G ~ = 3 7 3 , 4 2 2 - 89.25 T J ' m o l -~

[3]

F r o m E q s . [2] and [3] it follows, that the r a t i o P H 2 0 / P H ~ at the e q u i l i b r i u m is v e r y s m a l l , e . g . 2.8 x 10 4 at 1273 K and 3.2 • 10 -~ at 1073 K. T h e r e f o r e a r e d u c tion is only p o s s i b l e in oxygen f r e e d r y h y d r o g e n with continuous r e m o v a l of w a t e r v a p o r f r o m the r e a c t i o n zone. The papers published so far deal mainly with the production of chromium metal. The kinetics and, in p a r t i c u l a r , the r a t e c o n t r o l l i n g step of the r e d u c t i o n have not yet b e e n d e t e r m i n e d . W. F. CHU, formerly Research Associate with Dechema-Institut, is now with BBC Forschungszentrum, D-6900 Heidelberg, Germany. A. RAHMEL is Head of the High Temperature Corrosion Department, Dechema-lnstitut, D-6000 Frankfurt/Main-97, Postfach 97 01 46, Germany. Manuscript submitted December 28, 1978. METALLURGICAL TRANSACTIONS B

The k i n e t i c m e a s u r e m e n t s were c a r r i e d out in a closed loop, F i g . 1. The r e a c t i o n tube ( a l u m i n a , 15 m m ID, 1000 m m length) was p l a c e d in a h o r i z o n t a l f u r n a c e , which allowed r e d u c t i o n t e m p e r a t u r e s up to 1473 K. The length of the constant t e m p e r a t u r e zone in the f u r nace was a p p r o x i m a t e l y 4 cm, and the t e m p e r a t u r e f l u c t u a t i o n s did not exceed