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NASA TECHNICAL MEMORANDUM NASA "P X-64516 INVESTIGATION OF THE TECHNICAL M I C R O N I C S CONTROL (TMC) PROCESS FOR ELECTROPOLISH ING VAR IOUS METALS By James R. Lowery Astronautics Laboratory April 15, 1970 NASA George C. Marshall Space Flight Center Marshall Spdce Flight Cmter, Alabama MSFC - F a m 3190 (September 1968)

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M a r s h a l l Space F l i g h t Center, Alabama 35812 I 12. SPONSORING AGENCY NAME AND ADDRESS T E C H N I C A L R E P O R T S T A N D A R D T I T L E P A G E 13. T Y P E OF REPORT & PERIOD COVERED ! ~ e c h n i c a lMemorandum 14. SPONSORING AGENCY CODE 1 A s t r o n a u t i c s Laboratory, Science and Engineering D i r e c t o r a t e I - 16. ABSTRACT The Technical Micronics C o n t r o l (TMC) p r o c e s s f o r e l e c t r o p o l i s h i n g a v a r i e t y of m e t a l s and m e t a l a l l o y s was i n v e s t i g a t e d . The e l e c t r o p o l i s h i n g s o l u t i o n produced l u s t r o u s s u r f a c e s on s t a i n l e s s steel, b r a s s , copper, n i c k e l , aluminum, and s t e e l ; it produced o n l y semi-bright s u r f a c e s on I n v a r and Rene' 41 b u t produced a c o n s i d e r a b l e amount of metal smoothing. <.' The p a s s i v a t i o n of s t a i n l e s s s t e e l was improved s u b s t a n t i a l l y b y ' t h i s method over conventional a c i d p a s s i v a t i o n methods. ' t h a t of TMC p o l i s h e d s u r f a c e s of s t a i n l e s s steel, b r a s s , and carbon s t e e l d i d n o t a c c e p t a n such s u r f a c e s were more r e s i s t a n t t o t a r n i s h o r e l e c t r o p l a t e w i t h good adherence but c o r r o s i o n t h a n mechanically p o l i s h e d s u r f a c e s . Based on r e s u l t s of t h i s s t u d y , t h e TMC p r o c e s s i s recommended f o r p o l i s h i n g and smoothing most m e t a l s and f o r p a s s i v a t i n g s t a i n l e s s s t e e l . 17. KEY WORDS MSFC - F o r m 3 2 9 2 (May 1969) 18. D I S T R I B U T I O N S T A T E M E N T STAR Announcement

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TABLE OF CONTENTS Page SUMMARY.................................................... 1 INTRODUCTION............................................... 1 EXPERIMENTAL PROCEDURE..................................... 3 DISCUSSION OF RESULTS...................................... 4 Appearance............................................ 4 P a s s i v a t i n g E f f e c t s................................... 6 T a r n i s h R e s i s t a n c e.................................... 6 E l e c t r o p l a t i n g on E l e c t r o p o l i s h e d S u r f a c e s............ 7 RECOWNDATIONS............................ 8 CONCLUSIONS AND REFERENCES................................................. 9 iii

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LIST OF ILLUSTRATIONS Figure T i t l e Page 1 Differences i n t h e Grain Size of Specimens Taken From 0.125 and 5.0 Inch Thick Sections of Aluminum Alloy 7075-T6............... .......................... 10 2 Differences i n t h e I n t e r m e t a l l i c Phases of 0.125 Inch Sheet and 5.0 Inch Thick P l a t e of Aluminum Alloy 7075-T6...... ......................................... 11 3 E f f e c t of E l e c t r o p o l i s h i n g Specimens Taken From 5.0 Inch Thick P l a t e Aluminum of Alloy 7075-T6.. .......... 12 4 E f f e c t s of High Humidity on Electropolished and Mechanically Polished Brass (34 Days Exposure) ........ 13 5 E f f e c t s of High Humidity on Electropolished and Mechanically Polished Brass (190 Days Exposure) ....... 14

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TECHNICAL MEMORANDUM X-64516 INVESTIGATION OF THE TECHNICAL M I CRONI CS CONTROL (TMC) PROCESS FOR ELECTROPOLISHING VARIOUS METALS SUMMARY An e v a l u a t i o n was made of t h e T e c h n i c a l M i c r o n i c s C o n t r o l (TMC) p r o c e s s f o r e l e c t r o p o l i s h i n g a v a r i e t y of m e t a l s and metal a l l o y s . S p e c i f i c o p e r a t i n g d e t a i l s f o r e a c h of t h e s e m e t a l s were d e t e r m i n e d w i t h r e g a r d t o optimum v o l t a g e , t i m e a n d t e m p e r a t u r e of t h e b a t h . P r i m a r i l y , t h e e v a l u a t i o n was based on g e n e r a l a p p e a r a n c e of t r e a t e d specimens. However, o t h e r p h a s e s of t h e program i n c l u d e d d e t e r m i n i n g t h e p a s s i - v a t i n g e f f e c t s of t h e s o l u t i o n on s t a i n l e s s s t e e l , t h e t a r n i s h a n d c o r r o s i o n r e s i s t a n c e of t r e a t e d specimens of b r a s s and m i l d s t e e l , a n d t h e manner i n which a p o l i s h e d s u r f a c e of b r a s s a n d s t a i n l e s s s t e e l a c c e p t e d a n e l e c t r o p l a t e w i t h r e g a r d t o a d h e r e n c e . The s t u d y showed t h a t t h i s e l e c t r o p o l i s h i n g method produced smooth and l u s t r o u s s u r f a c e s on s t a i n l e s s steel, b r a s s , c o p p e r , n i c k e l , aluminum, and s t e e l . Smooth b u t o n l y s e m i - b r i g h t s u r f a c e s were produced o n I n v a r a n d Rene' 41. s u b s e q u e n t s a l t s p r a y tests i n d i c a t e d t h a t s t a i n l e s s The r e s u l t s of steel was made c o n s i d e r a b l y more p a s s i v e by t h e TMC e l e c t r o p o l i s h i n g s o l u t i o n t h a n by c o n v e n t i o n a l methods of p a s s i v a t i o n such as p i c k l i n g i n n i t r i c a c i d . R e s u l t s of c o n t r o l l e d h i g h h u m i d i t y a n d l a b o r a t o r y e x p o s u r e tests showed t h a t TMC e l e c t r o p o l i s h e d s u r f a c e s of b r a s s a n d s t e e l were more r e s i s t a n t t o t a r n i s h o r c o r r o s i o n t h a n m e c h a n i c a l l y p o l i s h e d s u r f a c e s , b u t t h a t "as p o l i s h e d " s u r f a c e s of t h e s e m e t a l s , as w e l l as s t a i n l e s s s t e e l , d i d n o t a c c e p t e l e c t r o p l a t e s w i t h good a d h e r e n c e . INTRODUCrI O N E l e c t r o p o l i s h i n g i s a n e l e c t r o c h e m i c a l p r o c e s s t h a t , under p r o p e r c o n d i t i o n s , makes a m e t a l smooth a n d b r i g h t by d i s s o l u t i o n of t h e metal s u r f a c e . The p r o c e s s i s t h e r e v e r s e of e l e c t r o p l a t i n g i n t h a t t h e e l e c t r o p o l i s h e d i s made t h e anode and m e t a l i s r'emoved m e t a l b e i n g r a t h e r t h a n d e p o s i t e d . On t h e o t h e r hand, it i s similar t o e l e c t r o - f a c t o r of t h e s o l u t i o n i s t h e t o t a l p l a t i n g i n t h a t t h e c o n t r o l l i n g

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amount of c u r r e n t t h a t p a s s e s through it. T h i s being t h e c a s e , s e v e r a l o t h e r i n t e r - r e l a t e d f a c t o r s such a s c u r r e n t d e n s i t y , temperature ( t h e e l e c t r o l y t e d e c r e a s e s as t h e temperature i n c r e a s e s ) , r e s i s t i v i t y of t h e and t i m e of treatment have a d i r e c t i n f l u e n c e on t h e end r e s u l t s i n any e l e c t r o p o l i s h i n g o p e r a t i o n . These v a r i a b l e s a r e a d j u s t e d t o h e l p produce a p a r t i c u l a r type of s u r f a c e f i n i s h . Also, a s w i t h e l e c t r o - p l a t i n g , t h e type and c o n d i t i o n of t h e metal being p o l i s h e d i n f l u e n c e t h e r e s u l t a n t s u r f a c e f i n i s h . E l e c t r o p o l i s h i n g techniques were i n i t i a l l y developed f o r use i n t h e a r e a of i n d u s t r i a l d e c o r a t i v e f i n i s h i n g . These p o l i s h i n g techniques were r e p o r t e d t o provide a n improvement i n t h e q u a l i t y and r e d u c t i o n i n t h e c o s t of f i n i s h i n g commercial items over t h a t of mechanical f i n i s h i n g . Since t h e e a r l y days of e l e c t r o p o l i s h i n g , non-decorative uses have evolved and p r e s e n t l y many e n g i n e e r i n g m e t a l s a r e e l e c t r o p o l i s h e d f o r v a r i o u s o t h e r reasons. Some of t h e u s e s and d i s t i n c t advantages of e l e c t r o p o l i s h i n g over r e l a t e d mechanical processing methods a r e a s follows ( r e f . 1, 2 ) : Deburring and p o l i s h i n g i n l o c a t i o n s i n a c c e s s i b l e t o mechanicalmeans,(2) removing "smeared" and l o o s e l y adhered m e t a l on s u r f a c e s t h a t were mechanically f i n i s h e d , (3) reducing o r l e f t e l i m i n a t i n g s u r f a c e s t r e s s e s caused by mechanical processing ( 4 ) i m - proving t h e c o r r o s i o n r e s i s t a n c e of c e r t a i n m e t a l s (5) reducing t h e f r i c t i o n on bearing s u r f a c e s , (6) producing a s u r g i c a l l y c l e a n s u r f a c e by removing contaminants, and reducing t h e e f f e c t i v e molecular t r a p p i n g s u r f a c e of vacuum components. Because e l e c t r o p o l i s h i n g does enhance t h e appearance of m e t a l s and l i s t e d advantages, it has gained e x t e n s i v e because of some of t h e above i n d u s t r i a l use f o r a number a p p l i c a t i o n s . Many types of b a t h s and of procedures have been developed, some of which have been t a i l o r e d j u s t f o r s p e c i a l m e t a l s and a l l o y s , w h i l e o t h e r p o l i s h i n g baths a r e claimed t o be e f f e c t i v e on a v a r i e t y of m e t a l s and metal a l l o y s . An e l e c t r o - p o l i s h i n g b a t h , developed by "Technical Micronics Control'' was r e c e n t l y r e p o r t e d t o p o l i s h p r a c t i c a l l y any metal a l l o y t o a f a i r l y high degree of l u s t e r , and by changing t h e o p e r a t i n g c o n d i t i o n s of the bath, t h e same bath could be used f o r producing a matte f i n i s h on metal a l l o y s . I n view of t h i s broad claim f o r t h e TMC bath and t h e d e s i r a b l e f e a t u r e s g e n e r a l l y produced by e l e c t r o p o l i s h i n g , some of which have d i r e c t aerospace a p p l i c a t i o n , a n i n v e s t i g a t i o n of t h i s process was made.

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EXPERIMENTAL PROCXDURE Two g e n e r a l types of e l e c t r o p o l i s h i n g s o l u t i o n s a r e produced by "Technical Micronics Control (TMC)", type I and 11. The e x a c t formul a t i o n of t h e s e b a t h s was h e l d p r o p r i e t a r y by TMC, but f o r e v a l u a t i o n purposes, they r e p o r t e d t h a t t h e type I s o l u t i o n w a s b a s i c a l l y a p h o s p h o r i c / s u l f u r i c a c i d mixture and t h a t type I1 was b a s i c a l l y a chromic/phosphoric a c i d mixture. The type I s o l u t i o n was recommended f o r a l l t h e common metals where matte f i n i s h e s were d e s i r e d and e i t h e r type I o r I1 was recommended f o r use on a l l common m e t a l s (except aluminum and brass) where a h i g h l u s t e r f i n i s h was d e s i r e d . Type I1 s o l u t i o n o n l y was recommended f o r aluminum and b r a s s . Because of t h e i n c r e a s i n g d e s i r e f o r s u r g i c a l l y c l e a n and p o l i s h e d s u r f a c e s i n c e r t a i n c u r r e n t aerospace a p p l i c a t i o n s , t h e s o l u t i o n s were e v a l u a t e d , p r i m a r i l y f o r t h e i r a b i l i t y t o produce a h i g h l u s t e r f i n i s h based on v i s u a l appearance of t r e a t e d specimens. L a t e r i n t h e program, t h e s o l u t i o n s were a l s o e v a l u a t e d f o r t h e i r a b i l i t y t o p a s s i v a t e s t a i n l e s s s t e e l and prot e c t b r a s s and s t e e l from t a r n i s h and c o r r o s i o n i n a m i l d l y c o r r o s i v e atmosphere. Also, t o a more l i m i t e d e x t e n t , t e s t s were made t o d e t e r - mine i f s u r f a c e s produced by t h e e l e c t r o p o l i s h i n g s o l u t i o n s were r e c e p t i v e t o a n e l e c t r o p l a t e . The m e t a l s used i n t h e o v e r a l l t e s t program included s t e e l (Invar, 4130, 1010), and s t a i n l e s s steel (300 and 400 series), aluminum a l l o y s (2014-T6, 7075-T6, 6061-T6) and t o a more l i m i t e d e x t e n t , pure n i c k e l and a n i c k e l a l l o y (Rene' 41). Generally, t h e e l e c t r o p o l i s h i n g of t h e s e m e t a l s was c a r r i e d o u t according t o t h e recommendation of TMC. However, due t o t h e many v a r i a b l e s r e l a t e d t o a n e l e c t r o p o l i s h i n g p r o c e s s , t h e TMC recommendat i o n s were v e r y g e n e r a l , and t h u s , s p e c i f i c o p e r a t i n g d e t a i l s had t o be determined f o r t h e d i f f e r e n t m e t a l s and d i f f e r e n t types of f i n i s h e s d e s i r e d . For example, where a h i g h l u s t e r f i n i s h was d e s i r e d on b r a s s , t h e manufacturer recommended t h a t t h e p o l i s h i n g o p e r a t i o n be c a r r i e d o u t a t a v o l t a g e range of 6 t o 15 v o l t s and a temperature range of 130" - 160°F (54-71°C). A f t e r v a r y i n g t h e v o l t a g e w i t h i n t h i s range, it was found t h a t 1 2 v o l t s produced t h e h i g h e s t degree of l u s t e r . The temperature was not c r i t i c a l , and h i g h l u s t e r f i n i s h e s were produced a t temperatures i n t h e range of 140'-180°F (60-82OC). P r o c e s s i n g det a i l s f o r o t h e r m e t a l s used i n t h e e v a l u a t i o n program were s i m i l a r l y determined. T e s t specimens of t h e v a r i o u s m e t a l s and metal a l l o y s were e l e c t r o - p o l i s h e d under t h e optimum c o n d i t i o n s determined f o r each of t h e d i f - f e r e n t m e t a l s and t h e r e s u l t a n t s u r f a c e s v i s u a l l y observed f o r t h e i r

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l u s t e r . These specimens were compared w i t h specimens cleaned and e l e c t r o p o l i s h e d w i t h o t h e r b a t h s and a l s o w i t h specimens t h a t were mechanically p o l i s h e d . A f t e r e l e c t r o p o l i s h i n g under optimum c o n d i t i o n s determined f o r each of t h e a l l o y s , one group of b r a s s and s t e e l p a n e l s was exposed t o a p l a t i n g l a b o r a t o r y environment and a n o t h e r group w a s exposed t o a c o n t r o l l e d high humidity environment ( r e l a t i v e humidity of 95-98 p e r - c e n t and a temperature of 95-100°F) and compared w i t h mechanically f i n i s h e d panels which were t e s t e d simultaneously i n t h e same environments. S t a i n l e s s steel p a n e l s were contaminated w i t h s t e e l p a r t i c l e s by brushing w i t h a w i r e brush o r machining w i t h a steel t o o l . One group of t h e p a n e l s was e l e c t r o p o l i s h e d and a n o t h e r group w a s p a s s i v a t e d i n a s t a n d a r d n i t r i c a c i d bath. The two groups of p a n e l s were exposed t o a 5 p e r c e n t s a l t s p r a y environment and observed f o r t h e occurrence of r u s t . T e s t p a n e l s of b r a s s and s t a i n l e s s s t e e l were e l e c t r o p o l i s h e d under t h e c o n d i t i o n s determined t o be optimum f o r t h e s e metals. An a t t e m p t was t h e n made t o e i t h e r e l e c t r o p l a t e d i r e c t l y on t h e p o l i s h e d s u r f a c e , o r t o p l a t e on t h e p o l i s h e d s u r f a c e which had been f u r t h e r t r e a t e d t o remove any f i l m t h a t might have been formed by t h e e l e c t r o - p o l i s h i n g . S p e c i a l emphasis was placed on t h e s t a i n l e s s steel i n t h i s p a r t i c u l a r t e s t because e l e c t r o p o l i s h i n g has r e p o r t e d l y beenf'used i n p r e p a r i n g s t a i n l e s s s t e e l s f o r e l e c t r o p l a t i n g (gold p l a t i n g d i r e c t l y on t h e s t a i n l e s s s t e e l ) . Most s t a i n l e s s s t e e l s g e n e r a l l y r e q u i r e a more s e v e r e a c i d t r e a t m e n t t h a n do o t h e r m e t a l s , and o c c a s i o n a l l y need a s p e c i a l " s t r i k e " b e f o r e t h e y can be s u c c e s s f u l l y e l e c t r o p l a t e d . P r e p a r i n g s t a i n l e s s s t e e l s f o r e l e c t r o p l a t i n g by e l e c t r o p o l i s h i n g would n o t only be less d i f f i c u l t , but would f u r n i s h a b r i g h t e r and smoother s u r f a c e upon which t o p l a t e , i f proven f e a s i b l e . DISCUSSION OF RESULTS Appearance. - The e l e c t r o p o l i s h i n g r e s u l t e d , t o some degree, i n s u r f a c e on a l l metals t e s t e d . With t h e exa b r i g h t e r and smoother c e p t i o n of a few c a s e s which w i l l be d i s c u s s e d l a t e r , t h e s u r f a c e s of a l l t h e common m e t a l s t e s t e d (which included b r a s s , copper, mild s t e e l , s t a i n l e s s s t e e l , aluminum a l l o y s , and n i c k e l ) were s u b s t a n t i a l l y and smoothed by t h e e l e c t r o p o l i s h i n g a c t i o n . A v o l t a g e brightened range of 1 2 t o 14 v o l t s a t a temperature of 140-160°F (60-61°C) appeared t o be optimum f o r s t e e l , s t a i n l e s s s t e e l , copper, and b r a s s . 10 t o 12 v o l t s a t a temperature of The optimum range f o r n i c k e l was 140-160°F (60-71°C) while a range of 1 2 t o 20 v o l t s a t a temperature of

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140-160°F (60-71°C) produced t h e most d e s i r a b l e f i n i s h on t h e aluminum a l l o y s . The s u r f a c e s of t h e l e s s common a l l o y s t e s t e d which included I n v a r (an i r o n - n i c k e l a l l o y ) and Rene' 4 1 ( a nickel-base a l l o y ) were smoothed t o about t h e same degree a s t h e common a l l o y s b u t were much l e s s brightened. A s l i g h t l y b r i g h t e r s u r f a c e was o b t a i n e d a t a considerably wider v o l t a g e - temperature range on Rene' 41 t h a n on I n v a r . I n t h e case of Rene' 41, a f a i r l y b r i g h t s u r f a c e was o b t a i n e d a t v o l t a g e s ranging from 6 t o 15 and a t temperatures of 130°F (54°C) t o 180°F (82°C). On t h e o t h e r hand, a s a t i s f a c t o r y p o l i s h could n o t be o b t a i n e d on t h e I n v a r a t v o l t a g e s a p p r e c i a b l y l e s s t h a n 12. The optimum range f o r t h i s a l l o y appeared t o be from 12 t o 15 v o l t s detemperature which w a s more i n f l u e n t i a l t h a n pending somewhat upon t h e had been observed w i t h t h e p o l i s h i n g of o t h e r metals. The b e s t p o l i s h i n g a c t i o n occurred on I n v a r a t a b a t h temperature of between 150" - 170°F( 66-77°C). I n t h e i n i t i a l phase of t h e program, a l l of t h e aluminum t e s t p a n e l s were prepared from s h e e t m a t e r i a l ranging from about 0.050 t o 0.125 i n c h t h i c k . Generally, a l l of t h i s s h e e t m a t e r i a l was p o l i s h e d t o a f a i r degree of l u s t e r w i t h no p a r t i c u l a r d i f f e r e n c e observed between t h e d i f f e r e n t a l l o y s . During t h e l a t t e r p a r t of t h e program i n connection w i t h a d i f f e r e n t program, t o e l e c t r o - t h e r e a r o s e a need, p o l i s h t e n s i l e specimens of a l l o y 7075-T6 (0.180 i n c h t h i c k ) t h a t had from a f i v e i n c h t h i c k p l a t e m a t e r i a l . When t h e s e specimens been c u t were polished, a s i g n i f i c a n t d i f f e r e n c e i n s u r f a c e f i n i s h was observed between t h o s e of t h e t h i n s h e e t m a t e r i a l of t h e same a l l o y . The s u r - f a c e s of t h e p l a t e specimens were etched by t h e electro-chemical a c t i o n and were not p o l i s h e d t o a s g r e a t a degree a s t h e s h e e t s p e c i - mens. A f t e r c o n s i d e r a b l e e f f o r t (varying t h e o p e r a t i n g conditions) f a i l e d t o produce a comparable s u r f a c e t o t h a t of s h e e t specimens, a conducted t o determine t h e cause of t h e d i f - m e t a l l o g r a p h i c s t u d y was f e r e n c e i n behavior of t h e two c o n d i t i o n s of t h e same m a t e r i a l . The r e s u l t s of t h i s s t u d y showed s i g n i f i c a n t d i f f e r e n c e s i n t h e micros t r u c t u r e of t h e s e m a t e r i a l s . The s h e e t m a t e r i a l s had much s m a l l e r g r a i n s i z e and l e s s i n t e r m e t a l l i c phases t h a n d i d t h e 5 inch p l a t e . (See F i g u r e s 1 and 2). The maximum depth of p i t t i n g on t h e 5.0 i n c h (Figure 3 ) . These d i f f e r e n c e s i n s u r f a c e p l a t e m a t e r i a l was 0.000625 appearance r e s u l t i n g from t h e e l e c t r o p o l i s h i n g were a t t r i b u t e d t o t h e v a r i a t i o n s i n t h e m i c r o s t r u c t u r e which occurred from v a r i a t i o n s i n f a b r i c a t i o n technique and h e a t t r e a t i n g procedures. S i m i l a r r e s u l t s s t u d y of a n o t h e r a l l o y , 2014-T6 aluminum, were o b t a i n e d from a f u r t h e r which f u r t h e r confirmed t h a t g r a i n s i z e s i g n i f i c a n t l y a f f e c t s t h e e l e c t r o p o l i s h ( t h e f i n e r t h e g r a i n s i z e , t h e b e t t e r degree degree of of p o l i s h i n g ) .

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I n conducting p o l i s h i n g experiments w i t h b r a s s , it was found t h a t t e s t specimens r e a c t e d d i f f e r e n t l y from t h e aluminum specimens i n t h e p o l i s h i n g s o l u t i o n . Very s i m i l a r r e s u l t s were obtained i n t h a t some specimens were d u l l and p i t t e d while o t h e r s were extremely b r i g h t . The r e s u l t s of a metallographic s t u d y on specimens of t h i s m a t e r i a l i n d i c a t e d t h a t t h e m i c r o s t r u c t u r e s d i d v a r y somewhat randomly and c o r r e l a t i o n between t h i s v a r i a n c e and s u r f a c e appearances could not be e s t a b l i s h e d . However, t h e r e s u l t s of chemical a n a l y s i s showed t h a t a l l o y type l a r g e l y determined t h e degree of p o l i s h o r t h e type of s u r - f a c e f i n i s h . For example, yellow b r a s s which contained no l e a d could be e l e c t r o p o l i s h e d t o a much h i g h e r degree of l u s t e r t h a n h i g h l y leaded b r a s s which contained approximately 1.75 p e r c e n t l e a d . R e s u l t s of f u r t h e r experiments i n d i c a t e d t h a t t h e presence of any s i g n i f i c a n t amount of l e a d i n b r a s s would, t o a degree, a f f e c t t h e e l e c t r o c h e m i c a l a c t i o n and t h e r e s u l t a n t s u r f a c e f i n i s h . P a s s i v a t i n g E f f e c t s . r T e s t s conducted t o determine t h e p a s s i v a t i n g e f f e c t s of t h e TMC e l e c t r o p o l i s h i n g s o l u t i o n on s t a i n l e s s s t e e l showed, i n g e n e r a l , t h a t t h i s treatment i n c r e a s e d t h e p a s s i v i t y of t h i s metal over t h a t of conventional methods of p a s s i v a t i o n . Specimens of 321 s t a i n l e s s s t e e l , contaminated and t h e n t r e a t e d by v a r i o u s methods, were exposed f o r approximately 560 hours t o a s a l t s p r a y environment. A l l of t h e specimens t r e a t e d conventionally ( n i t r i c a c i d a t varying temp e r a t u r e s and concentration) showed a c o n s i d e r a b l e amount of r u s t a f t e r 8 hours and were removed from t h e t e s t w h i l e t h e electropol$shed s p e c i - mens d i d n o t show any change u n t i l a f t e r about 40 hours. A t t h i s p o i n t , l i g h t s t a i n was observed i n t h e c e n t e r of t h e specimens, and a f t e r 146 hours, l i g h t r u s t was formed. A t t h e end of t h e t e s t , a moderate and a few small p i t s had formed on t h e s p e c i - amount of r u s t was p r e s e n t mens. During t h e course of t h i s study, t h i s t e s t was e s s e n t i a l l y r e p e a t e d w i t h t h e exception of some v a r i a t i o n s i n t h e conventional a c i d treatments. I n t h e r e p e a t e d t e s t , a l l specimens were exposed f o r approximately 750 hours, and a g a i n , t h e e l e c t r o p o l i s h e d specimens t h a n d i d t h e specimens t h a t were p a s s i - showed c o n s i d e r a b l y l e s s r u s t v a t e d by conventional means. T a r n i s h R e s i s t a n c e . - R e s u l t s of t e s t s showed t h a t t h e t a r n i s h r e s i s t a n c e of b r a s s was enhanced s u b s t a n t i a l l y by e l e c t r o p o l i s h i n g . On t h e o t h e r hand, r e s u l t s of s i m i l a r t e s t s showed t h a t t h e r u s t r e - s i s t a n c e of s t e e l w a s only s l i g h t l y improved by t h e e l e c t r o p o l i s h i n g . Specimens of b r a s s (one group mechanically p o l i s h e d and a n o t h e r group e l e c t r o p o l i s h e d ) were exposed t o a p l a t i n g l a b o r a t o r y environment f o r 25 days. The e l e c t r o p o l i s h e d p a n e l s showed some s t a i n while t h e mechanically p o l i s h e d panels had changed t o a dark c o l o r t y p i c a l of severe t a r n i s h on b r a s s . The only d i f f e r e n c e between s i m i l a r l y t r e a t e d specimens of s t e e l exposed t o t h e same environment was t h a t t h e mechanically p o l i s h e d panels began t o r u s t a few hours sooner than

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t h e e l e c t r o p o l i s h e d panels. A t t h e end of t h e t e s t , no d i f f e r e n c e could be observed between t h e two groups. Mechanically and e l e c t r o p o l i s h e d specimens of b r a s s were exposed t o a c o n t r o l l e d high humidity environment f o r approximately 190 days and s i m i l a r l y t r e a t e d panels of s t e e l were exposed t o t h i s environment f o r a few hours. The mechanically p o l i s h e d panels of b r a s s began t o t a r n i s h ( t u r n brown) a f t e r seven days and g r a d u a l l y became d a r k e r a s exposure time i n c r e a s e d w h i l e t h e e l e c t r o p o l i s h e d panels r e t a i n e d a l l of t h e i r o r i g i n a l l u s t e r f o r 34 days and was s t i l l f a i r l y b r i g h t a f t e r (Figure 4). A t t h e end of t h e t e s t p e r i o d , t h e approximately 100 days e l e c t r o p o l i s h e d p a n e l s were semi-bright w i t h a few dark s p o t s but t h e mechanically p o l i s h e d p a n e l s were s e v e r e l y t a r n i s h e d (Figure 5). The two groups of s t e e l panels exposed t o t h i s environment showed e s s e n t i - a t t h e end of a 30-hour t e s t p e r i o d . a l l y t h e same degree of r u s t The only d i f f e r e n c e was t h a t t h e mechanically p o l i s h e d p a n e l s began t o r u s t s e v e r a l hours sooner t h a n t h e e l e c t r o p o l i s h e d panels. E l e c t r o p l a t i n g on E l e c t r o p o l i s h e d S u r f a c e s . - T e s t r e s u l t s i n d i - s u r f a c e s produced by t h e TMC e l e c t r o p o l i s h i n g s o l u t i o n s c a t e d t h a t were t o t a l l y non-receptive t o a n e l e c t r o p l a t e i n t h e "as polished" c o n d i t i o n and were considerably more d i f f i c u l t t o a c t i v a t e f o r p l a t i n g t h a n t h e same unpolished metals. T h i s i n c r e a s e d d i f f i c u l t y i n a c t i - v a t i o n i s understandable from t h e f a c t t h a t e l e c t r o p o l i s h e d s u r f a c e s e x h i b i t a f a i r amount of improvement i n r e s i s t a n c e t o t a r n i s h i n g and c o r r o s i o n over non-polished s u r f a c e s , a s d i s c u s s e d e a r l i e r . Generally, e l e c t r o p o l i s h e d copper and b r a s s must be a c t i v a t e d by i n s o l u t i o n s such as a l k a l i n e c l e a n e r s o r cyanides c a t h o d i c treatment and followed by dipping i n a c i d s o l u t i o n s such as h y d r o c h l o r i c b e f o r e p l a t i n g . Although some degree of adhesion was obtained without t h e use of c a t h o d i c t r e a t m e n t , t h e acceptance of e l e c t r o p l a t e s w i t h p o s i - t i v e and uniform adherence depended upon a more thorough d e - p a s s i - v a t i o n treatment. E l e c t r o p o l i s h e d s t a i n l e s s s t e e l behaved s i m i l a r l y t o b r a s s and copper i n t h a t e l e c t r o p l a t e s were not a c c e p t e d w i t h a s a t i s f a c t o r y l e v e l of adhesion. Uulike b r a s s and copper, s t a i n l e s s s t e e l i n i t s o r d i n a r y form (unpolished) possesses a v e r y t e n a c i o u s oxide f i l m which makes it much more d i f f i c u l t t o p l a t e . I n e l e c t r o p o l i s h i n g , i t i s b e l i e v e d t h a t t h i s n a t u r a l f i l m i s removed and t h e metal i s m i l d l y "anodized" a g a i n (another oxide f i l m produced by t h e e l e c t r o c h e m i c a l making it necessary t o r e a c t i v a t e before p l a t i n g . Whether o x i d a t i o n ) , r e s u l t s of t h i s test showed t h a t o r n o t t h i s was t h e case, t h e TMC e l e c t r o p o l i s h e d s t a i n l e s s s t e e l was l e f t i n a p a s s i v e c o n d i t i o n and t h i s p a s s i v e f i l m had t o be removed b e f o r e e l e c t r o p l a t i n g could be c a r r i e d o u t s u c c e s s f u l l y .

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CONCLUSIONS AND RECOMMENDATIONS The TMC method of e l e c t r o p o l i s h i n g produced b r i g h t s u r f a c e s on a l l t h e common m e t a l s and a l l o y s t e s t e d which included, copper, b r a s s , s t e e l , s t a i n l e s s s t e e l , n i c k e l , and aluminum a l l o y s . Of t h i s group, copper, b r a s s , and n i c k e l could be e l e c t r o p o l i s h e d w i t h t h e g r e a t e s t degree of l u s t e r . It a p p e a r s t h a t t h e composition of b r a s s and t h e m i c r o s t r u c t u r e ( g r a i n s i z e ) of c e r t a i n aluminum a l l o y s a f f e c t t h e type of s u r f a c e r e s u l t i n g from e l e c t r o p o l i s h i n g t h e s e metals. T e s t r e s u l t s show t h a t specimens of b r a s s t h a t c o n t a i n l e a d do n o t p o l i s h a s w e l l a s specimens t h a t do n o t c o n t a i n lead. T e s t s of t h e aluminum a l l o y s (7075-T6 and 2014-T6) show t h a t t h e s m a l l e r t h e g r a i n s i z e , t h e b e t t e r t h e degree of p o l i s h . l e s s common metals tested,which included I n v a r and Surfaces of t h e Rene' 41,can be made smooth by t h e TMC e l e c t r o p o l i s h i n g s o l u t i o n , but cannot be produced w i t h a + h i g hdegree of l u s t e r . . S t a i n l e s s s t e e l can be made considerably more p a s s i v e by e l e c t r o - p o l i s h i n g w i t h t h e TMC s o l u t i o n t h a n by t r e a t i n g w i t h conventional p a s s i v a t i n g a c i d s . E l e c t r o p o l i s h e d s u r f a c e s of s t a i n l e s s s t e e l , s t e e l , treatment w i l l n o t a c c e p t a n e l e c t r o - and b r a s s without subsequent p l a t e w i t h good adherence, b u t such s u r f a c e s a r e more r e s i s t a n t t o t a r n i s h i n g o r c o r r o s i o n t h a n mechanically p o l i s h e d s u r f a c e s . A The r e s u l t s of t h i s s t u d y i n d i c a t e t h a t t h e TMC e l e c t r ~ p o l i s h i n ~ p r o c e s s , when c a r r i e d o u t according t o t h e recommendations of TMC, i s a good g e n e r a l e l e c t r o p o l i s h i n g process f o r a f a i r l y l a r g e v a r i e t y of metals and metal a l l o y s and is, t h e r e f o r e , recommended i n a p p l i - c a t i o n s t h a t r e q u i r e e l e c t r o p o l i s h i n g .

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REFERENCES 1. Gurklis, J. A., McGraw, L. D., and Faust, C. L., "Electropolishing and Chemical Polishing of High-Strength, High-Temperature Metals and Alloys," Battelle Memorial Institute, DMIC Memorandum 98, April 12, 1961 2. Cole, J. K., "Electropolish ProcessingYt'Technical Micronics Control, Nr: 4001, September 16, 1966

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5.0 Inch Thick P l a t e lOOX 0.125 Inch Thick P l a t e lOOX Figure 1 - D i f f e r e n c e s i n t h e Grain S i z e of 0.125 and 5.0 Inch t h i c k Specimens of 7075-T6 Aluminum Alloy

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0.125 Inch Sheet 375X Figure 2 - Difference in the Intermetallic Phases of 0.125 and 5.00 Inch Thick Specimens of 7075-T6 Aluminum Alloy

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Figure 3 - E f f e c t of E l e c t r o p o l i s h i n g on 5.0 Inch Thick Specimen of 7075-T6 Aluminum Alloy 300X

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Mechanically Polished Electropolished Figure 4 - Effects of High Humidity on Electropolished and Mechanically Polished Brass ( 34 Days Exposure)

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APPROVAL INVESTIGATION OF THE TECHNICAL M ICRONICS CONTROL (TMC) PROCESS FOR ELECIROPOLISHING VARIOUS METALS James R. Lowery BY The information i n t h i s r e p o r t has been reviewed f o r s e c u r i t y c l a s s i f i c a t i o n . Review of any information concerning Department of Defense o r Atomic Energy Commission programs has been made by the MSFC S e c u r i t y C l a s s i f i c a t i o n O f f i c e r . T h i s r e p o r t , i n its e n t i r e t y , has been determined t o be u n c l a s s i f i e d . T h i s document has a l s o been reviewed and approved f o r t e c h n i c a l accuracy. E . C. McKannan Chief, M e t a l l i c Materials m n c h - Chief, M a t e r i a l s Division Astronautics Laboratory I Director, MSFC-RSA, Ala 15

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DISTRIBUTION S&E-ASTN-DIR S&E-ME -DIR S&E-ASTR-DIR S&E-QUAL-DIR S&E-CSE -DIR S&E-ASTN -M S&E-ASTN -MX S&E-ASTN -P S&E-ASTN -E S&E-ASTN -MM S&E-ASTN -MN S&E-ASTN -ME S&E-ASTN-MMC S&E-ASTN -RM S&E-ASTN -S S&E-ASTN -ESP S&E-ASTN -EA S&E-ASTN -EAA A&TS-MS -H A&TS-PAT A&TS-TU A&TS-MS -1P A&TS-MS-IL S&E ME-M SGcE-ME -M S&E-ASTN -ME S&E-ASTN-ME S&E-A STN -ME SGcE-ASTR-P S&E-ME -DPP M r . Heimburg D r . S i e b e l M r . Moore M r . Grau D r . Hauessermann M r . Schwinghamer M r . C a t a l d o M r . P a u l M r . K r o l l M r . McKannan M r . C u r r y D r . P s c h e r a M r . Lowery (20) Miss S c o t t M r . V e r b l e M r . G a i n e s M r . P r a s t h o f e r M r . G e n t e r M r . Akens M r . Wofford M r . Wiggins ( 5 ) M r . Ziak M i s s R o b e r t s o n (8) M r . Wilson M r . Sharp M r . Smith M r . Horton M r . Demorest M r . J a c k s o n M r . B e r r y S c i e n t i f i c a n d T e c h n i c a l I n f o r m a t i o n F a c i l i t y (25) A t t n : NASA R e p r e s e n t a t i v e (S -AK/RKT) P . 0 . Box 33 C o l l e g e P a r k , Maryland 20740