Effects of zinc coatings on the stress corrosion cracking and hydrogen embrittlement of low-alloy steel
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and Hyler in work with h i g h - s t r e n g t h s t r u c t u r a l f a s t e n e r s found that h o t - d i p zinc coatings caused a r e duced r e s i s t a n c e of s t e e l bolts to s t r e s s c o r r o s i o n c r a c k i n g and hydrogen e m b r i t t l e m e n t . ~ These a u t h o r s suggested that the d e t r i m e n t a l effect of the zinc coating on s t r e s s c o r r o s i o n r e s i s t a n c e r e s u l t s f r o m e n hanced hydrogen r e d u c t i o n and a b s o r p t i o n by the s t e e l at b r e a k s in the zinc coating where the s t e e l functions as the cathode of a Z n - F e galvanic couple, and that the r e s u l t i n g hydrogen e m b r i t t l e m e n t acts to a u g m e n t the s t r e s s - c o r r o s i o n - c r a c k i n g p r o c e s s . Boyd and H y l e r ' s finding is in g e n e r a l a g r e e m e n t with that of Biefer and Syrett e who found that Ksc , the t h r e s h o l d s t r e s s i n t e n s i t y for s t r e s s c o r r o s i o n c r a c k i n g of HY-140 (140 ksi YS) steel, is r e d u c e d by coupling to a zinc anode, and that of Sandoz 3 who found that the Ksc of s e v e r a l quenched and t e m p e r e d alloy s t e e l s is d e c r e a s e d by coupling to an anode of a n o t h e r s a c r i f i c i a l metal, m a g nesium. In c o n t r a s t , Brown r e p o r t e d that the Ksc of 4340 s t e e l is only slightly affected by cathodic p o l a r i z a t i o n , 4 Wacker r e p o r t e d that the Ksc of HY-130 (146 ksi UTS) is unaffected by cathodic p o l a r i z a t i o n , 5 and McDonald r e p o r t e d that the s t r e s s - c o r r o s i o n - c r a c k i n g r e s i s t a n c e of h a r d e n e d s t e e l is u n a l t e r e d by the application of a g a l v a n i z e d coating. 6 Studies of m a r a g i n g s t e e l by Stavros and Paxton showed that, although t i m e to f a i l ure above Ksc is s h o r t e n e d by e i t h e r anodic or c a t h odic p o l a r i z a t i o n , Ksc itself is not a l t e r e d . 7 The p r e s e n t work was u n d e r t a k e n to m o r e p r e c i s e l y d e t e r m i n e the effect of hot-dip zinc coatings on the s t r e s s c o r r o s i o n c r a c k i n g and h y d r o g e n e m b r i t t l e m e n t of l o w - a l l o y s t e e l , and to d e t e r m i n e the effects of other types of zinc coatings including e l e c t r o p l a t e d zinc and A1- Zn. H. E. TOWNSEND,JR. is Research Supervisor,Corrosion Prevention and Coatings Section, Homer Research Laboratories, Bethlehem Steel Corporation, Bethlehem, Pa. 18016. Manuscript submitted September 24, 1973. METALLURGICAL TRANSACTIONS A
EXPERIMENTAL P r e c r a c k e d c a n t i l e v e r b e a m s p e c i m e n s (Fig. 1), s i m i l a r to those employed in p r e v i o u s work, 8 were p r e p a r e d f r o m o n e - i n c h - s q u a r e c o l d - d r a w n and a n n e a l e d AISI 4140 b a r stock with the c h e m i c a l c o m p o s i t i o n given in Table I. Heat t r e a t m e n t c o n s i s t e d of a u s t e n i t i z i n g in a i r at 1500~ for 60 rain followed by
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