Aluminum-rich alloy layers formed during the hot dip galvanizing of low carbon steel
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Aluminum-Rich Alloy Layers Formed During the Hot Dip Galvanizing of Low Carbon Steel G. J . HARVEY AND P . D. MERCER T H E addition of a l u m i n u m to c o m m e r c i a l g a l v a n i z i n g baths (---0.15 wt pct) to produce ductile coatings by s u p p r e s s i n g the growth of b r i t t l e F e - Z n i n t e r m e t a l l i c s is well known. It is now g e n e r a l l y a g r e e d that s u p p r e s s i o n is due to the f o r m a t i o n of a thin a l u m i n u m - r i c h l a y e r on the s u r f a c e of the s t e e l . Some c o n t r o v e r s y , however, e x i s t s on w h e t h e r the l a y e r is an Fe-A1 b i n a r y alloy, 1'2 a n F e - A 1 - Z n t e r n a r y alloy, 3'4 or a c o m b i n a t i o n of both S-7 Most of this data has b e e n obtained f r o m the e x a m i n a t i o n of l a b o r a t o r y p r e p a r e d s p e c i m e n s , which G. J. HARVEY and P. D. MERCER are Superintendent, Projects, and Senior Research Officer, respectively, Corporate Research and Technology, John Lysaght (Australia) Limited, Newcastle, N.S.W., Australia. Manuscript submitted May 31, 1972. METALLURGICALTRANSACTIONS
(a)
(b)
Fig. 1--Electron mierographs of shadowed replicas of the top surface of aluminum-rich layers formed during galvanizing at 450~ and an A1 level of 0.18 pet. (a) immersion time 2 s, mean layer thickness 0.180 ~m, (b) immersion time 4 s, mean layer thickness 0.305 Urn. Magnification 5000 times.
differ f r o m those produced on m o d e r n continuous g a l v a n izing l i n e s in that they r e q u i r e r e l a t i v e l y long dipping t i m e s and wet fluxing methods. The i n v e s t i g a t i o n of p r o d u c t i o n s p e c i m e n s has not been e x t e n s i v e , n o r has the e x a m i n a t i o n of the i n t e r m e t a l l i c d r o s s p h a s e s which f o r m in the g a l v a n i z i n g bath and which a r e c l o s e l y r e l a t e d to the p h a s e s f o r m e d on the s t e e l s u r f a c e . Our p r e s e n t i n v e s t i g a t i o n is c o n c e r n e d with these neglected a r e a s , and this note r e p o r t s some i n i t i a l r e s u l t s which we feel a r e of i m m e d i a t e i n t e r e s t . The a l u m i n u m - r i c h l a y e r s f o r m e d in coatings p r o duced on high speed g a l v a n i z i n g l i n e s a r e l e s s than 0.4 ~tm thick. T h u s optical m i c r o s c o p y is an u n s u i t a b l e method of i n v e s t i g a t i o n . X - r a y d i f f r a c t i o n and m i c r o p r o b e t e c h n i q u e s a l s o have s e r i o u s l i m i t a t i o n s as the p e n e t r a t i o n a n d / o r the d i a m e t e r of the X - r a y b e a m is s e v e r a l t i m e s g r e a t e r than the t h i c k n e s s of the alloy layers. Transmission electron microscopy, electron d i f f r a c t i o n , and s e n s i t i v e c h e m i c a l a n a l y s i s w e r e chosen in this i n v e s t i g a t i o n a s being the p r e f e r r e d t e c h n i q u e s for alloy l a y e r e x a m i n a t i o n . B r i e f l y , s a m p l e p r e p a r a t i o n c o
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