Corrosion fatigue and stress corrosion cracking of type 304 stainless steel in boiling NaOH solution
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T H E p r i m a r y p u r p o s e of this work was to a s s e s s the effect of cyclic s t r e s s , s u p e r i m p o s e d on a high m e a n s t r e s s , on the c r a c k i n g of F e - C r - N i alloys in hot conc e n t r a t e d c a u s t i c solution. By s t u d y i n g the s t r e s s c o r r o s i o n c r a c k i n g (SCC) of this alloy at the s a m e time the a c c e l e r a t i v e effect of c y c l i c a l loads over s u s tained loads could be d e t e r m i n e d . T h i s study is p a r t of a m a j o r p r o g r a m to c h a r a c t e r i z e the i n t e r a c t i o n of fatigue loading with SCC; on the l a t t e r t h e r e has b e e n s i g n i f i c a n t work r e p o r t e d on F e - C r - N i a l l o y s in hot c a u s t i c e n v i r o n m e n t s .1-9 The high c o n c e n t r a t i o n of caustic (17.5M or 46 wt pct) was chosen b e c a u s e at the a t m o s p h e r i c b o i l i n g point p r e v i o u s work in our l a b o r a t o r y 1~ has shown that for SCC to o c c u r , the c o n c e n t r a t i o n of NaOH m u s t exceed about I5M to c r a c k Type 304. The p r e s e n t work also e m p h a s i z e d the i n i t i a t i o n stage of c o r r o s i o n fatigue (CF) and SCC.
E X P E R I M E N T A L PROCEDURE C o m m e r c i a l grade Type 304 s t a i n l e s s s t e e l was obtained in the f o r m of cold r o l l e d 12.7 m m (-~ in.) round b a r s . The heat n u m b e r is C-17882, U n i v e r s a l Cyclops Specialty S t e e l s . The c h e m i c a l c o m p o s i t i o n is given in T a b l e I. 23 cm (9 in.) long s p e c i m e n s were m a c h i n e d n e a r the c e n t e r to 6.35 m m (0.250 in.) r e duced d i a m e t e r on a guage length of 25.4 m m (1.0 in.), with a n e c k r a d i u s of 38 m m ( l ~ i n . ) . Heat t r e a t m e n t was done in e v a c u a t e d q u a r t z c a p s u l e s . Solution a n n e a l i n g was done at 1093~ for 2 h, followed by w a t e r quenching; s e n s i t i z e d s p e c i m e n s were i n i t i a l l y given this solution a n n e a l before holding at 649~ for 4 h, then w a t e r - q u e n c h e d . After heat t r e a t m e n t the g r a i n ANTWI BOATENG is Research Associate, Fontana Corrosion Center, and JAMES A. BEGLEY is Professor, both with the Department of Metallurgical Engineering, Ohio State University, Columbus, OH 43210, and R. W. STAEHLE, formerly Professor of Metallurgical Engineering, and Director of The Fontana Corrosion Center, Ohio State University, is now Dean, Institute of Technology, University of Minnesota, Minneapolis,MN 55455. Manuscript submitted July 6, 1978. METALLURGICALTRANSACTIONSA
size was about 80 t~m (ASTM 4 ~ - 5 ) . M e c h a n i c a l p r o p e r t i e s after heat t r e a t m e n t a r e given in T a b l e II. C o r r o s i o n fatigue t e s t s were r u n in b o i l i n g (140~ s t a t i c s o l u t i o n s of 17.5M (46 wt pct) NaOH contained in a 600 ml capacity heated teflon test ceil; a s c h e m a tic is shown in F i g . ! . E l e c t r o c h e m i c a l p o t e n t i a l was m e a s u r e d with a Hg/H
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