The role of tarnishing in the stress-corrosion cracking of alpha-brass in aqueous citrate and tartrate solutions
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A m e r i c a n B r a s s Company, and the solutions which Johnson and L e j a r e p o r t e d to c a u s e SCC, namely i) an aqueous c i t r a t e solution containing 0.6 M p o t assi u m c i t r a t e and 0.07 M copper, adjusted to pH 10.3 by the addition of sodium c a r b o n a t e , and if) an aqueous t a r t r a t e solution containing 0.4 M p o t a s s i u m t a r t r a t e and 0.03 M co p p er , adjusted to pH 13.5 by the addition of p o t ass i um hydroxide. E x p o s u r e of the alloys to e i t h e r solution led to the f o r m a t i o n of v i s i b l e s u r f a c e f i l m s . Th o se f o r m e d d u r ing i m m e r s i o n in the t a r t r a t e solution cl o sel y r e s e m b l e d the black t a r n i s h produced by a m m o n i a c a t solutions, while those f o r m e d in the c i t r a t e solution w e r e l i g ht e r in c o l o r . Identification of S u r f a c e F i l m s E l e c t r o n - d i f f r a c t i o n st u d i es w e r e c a r r i e d out on thin f o i l s of Cu-30 Zn which had been e l e c t r o p o l i s h e d in an o r t h o p h o s p h o r i c acid solution and then i m m e r s e d for s e v e r a l days in e i t h e r the c i t r a t e or t a r t r a t e solution. C o n c u r r e n t st u d i es w e r e m a d e on foils i m m e r s e d in 15 N aqueous a m m o n i a containing 8 g p er l i t e r copper, Th e diffraction p a t t e r n s w e r e s i m i l a r for the t a r t r a t e and a m m o n i a c a l solutions, s e e Fig. 1, and the l a t t i c e s p a c i n g s d e t e r m i n e d f r o m the p a t t e r n s w e r e in good a g r e e m e n t with e s t a b l i s h e d v a l u e s f o r cuprous oxide, T a b l e I. FoiIs exposed to the c i t r a t e solution did not f o r m a sufficiently thick f i l m to yield a c o m p l e t e diff r a c t i o n p a t t e r n , and only the two m o s t intense d i f f r a c tion r i n g s w e r e o b s e r v e d . However, the l a t t i c e s p a c ings calculated f r o m t h ese r i n g s also m at ch ed e s t a b lished values f o r cuprous oxide. Kinetics of F i l m Growth The kinetics of film growth w e r e investigated using the e l l i p s o m e t r i c technique e m p l o y e d p r e v i o u s l y to study t a r n i s h growth in a m m o n i a c a l solutions. 6 In this technique, outlined in d e t a i l in this p r e v i o u s paper, polished s u r f a c e s of w~stressed m a t e r i a l a r e exposed to the t e s t solution and ch an g es in the e l l i p s o m e t e r v a l u e s of the s u r f a c e a r e m o n i t o r e d /n situ as a funcVOLUblE 2, MAY 1971 --1379
Fig. 1 - - E l e c t r o n - d i f f r a c t i o n p a t t e r n o b t a i n e d f r o m C u - 3 0 Zn foil e x p o s e d to the t a r t r a t e s o l u t i o n .
Table I. Comparison of Calculated Lattice Spacings, A, for the Surface Films Formed in Tartrate and Ammoniacal Solutions with Established Values for Cuprous Oxide
Tartrate Solution 3.019 2.463 2,127 1,746 1.505 1,300 1.066 0.936 0.870
Ammoniacal Solution
Values for Cu2 O, ASTM Index
3,018 2.462 2,127 1,83
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