The corrosion behavior of some Fe-Cr alloys
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m e l t e d f r o m high p u r i t y m a t e r i a l s . T h e i r total C +N content was g e n e r a l l y l e s s than 150 ppm, and the final a n n e a l i n g a f t e r hot and cold r o l l i n g was 30 min. at l l 0 0 ~ followed by a water quench. Although this is a s a t i s f a c t o r y t r e a t m e n t for high p u r i t y f e r r i t i c s t a i n l e s s s t e e l s , it will be shown l a t e r that for l e s s p u r e a l l o y s it is u n s a t i s f a c t o r y . To avoid the pitting p r o b l e m , we shifted our e m p h a s i s to F e - 3 5 Cr alloys which proved r e l i a b l y r e s i s t a n t to FeCls.
RESISTANCE TO HYDROFLUORIC ACID S e a s t r o m s studied the anodtc p o l a r i z a t i o n b e h a v i o r of c h r o m i u m and s t a i n l e s s s t e e l s in h y d r o f l u o r i c acid. In his s t u d i e s both c h r o m i u m and the s t a i n l e s s s t e e l s showed some r e s p o n s e to anodic p a s s i v a t i o n in h y d r o f l u o r i c acid, but only in the case of c h r o m i u m did p a s s i v a t i o n s i g n i f i c a n t l y r e d u c e the c o r r o s i o n r a t e . Our work on the addition of c h r o m i u m to F e - C r alloys to i m p r o v e t h e i r r e s i s t a n c e to c h l o r i d e s led us to i n v e s tigate if s i m i l a r r e s u l t s could be obtained for the fluorides. Fig. 1 shows the p o l a r i z a t i o n b e h a v i o r of an e x p e r i m e n t a l F e - 3 2 Cr alloy in 10 pet HF at r o o m t e m p e r a t u r e . This c u r v e is s i m i l a r to the r e s u l t s that Seas t r o m obtained for c h r o m i u m . The lower c u r r e n t s in the p a s s i v e r e g i o n show that the c o r r o s i o n r a t e of the e x p e r i m e n t a l F e - C r a l l o y s in HF can be r e d u c e d to t o l e r a b l e l e v e l s by p a s s i v a t i o n . P a s s i v i t y can be induced e i t h e r by the application of e l e c t r i c c u r r e n t or m o r e s i m p l y by the addition of oxidizing agents to the c o r r o d i n g solution. 6 We used Fe2(SO4)3"XH20 (22 pet Fe) as the p a s s i v a t i n g agent in our s t u d i e s . I m m e r s i o n c o r r o s i o n t e s t s were r u n at r o o m t e m p e r a t u r e tn polyethylene c o n t a i n e r s . R e a g e n t - g r a d e acid and d i s t i l l e d water made up the t e s t s o l u t i o n s . The f e r r i c sulfate was d i s s o l v e d in the s o l u t i o n s b e f o r e the s a m p l e s w e r e i m m e r s e d . A s a m p l e was c o n s i d e r e d p a s s i v e if it showed no weight loss in a 3 0 - m i n . e x p o s u r e to the t e s t s o l u t i o n s . R e s u l t s of the i m m e r s i o n t e s t s a r e shown in F i g s . 2 and 3. In Fig. 2 the weight of f e r r i c ion r e q u i r e d for inhibition is plotted a g a i n s t the c h r o m i u m content of the alloys for s e v e r a l acid c o n c e n t r a t i o n s . Even in 2 pet HF it was not p o s s i b l e to p a s s i v a t e alloys with l e s s than 25 pet Cr with any addition of
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