A paper to be presented at the 29th Annual Con­vention of the American Institute of Electrical Engineers, Boston, Mass., June 25 , 1912.

Copyright, 1912. B y Α. I. Ε . E . (Subject to final revision for the Transactions.)

E L E C T R O L Y T I C C O R R O S I O N O F I R O N B Y D I R E C T C U R R E N T I N S T R E E T S O I L

B Y A L B E R T F . G A N Z

T h e e l e c t r o l y t i c corros ion of i ron w h e n e x p o s e d t o a n e lec tr ic current l e a v i n g t h e i ron i n d a m p soi l h a s a l r e a d y r e c e i v e d a grea t dea l of a t t e n t i o n a n d i s a m a t t e r of v e r y grea t prac t i ca l i m p o r t a n c e . S t r a y currents f r o m d irec t - current e l ec tr i c rai l ­w a y s y s t e m s e m p l o y i n g t h e r u n n i n g t r a c k s as r e t u r n c o n d u c t o r s f r e q u e n t l y reach u n d e r g r o u n d p i p i n g s y s t e m s a n d c a u s e cor­ros ion b y e l ec tro lys i s . T h e r e h a s b e e n s o m e q u e s t i o n a s t o w h e t h e r or n o t t h e w e i g h t of i ron o x i d i z e d or d e s t r o y e d u n d e r t h e c o n d i t i o n s e x i s t i n g o n u n d e r g r o u n d s t r u c t u r e s i s e q u a l t o t h a t c a l c u l a t e d f r o m F a r a d a y ' s l a w , o n t h e bas i s of 1 .044 g r a m s of i r o n d e s t r o y e d b y o n e a m p e r e - h o u r , a n d w h e t h e r v a r i o u s k i n d s of i ron are corroded t o t h e s a m e e x t e n t w i t h t h e s a m e a m o u n t of current l e a v i n g t h e i ron t o p a s s t o d a m p soi l ; a n d c e r t a i n c las ses of i ron h a v e b e e n c l a i m e d t o re s i s t e l e c ­t r o l y t i c corros ion . S o m e h a v e a l so b e l i e v e d t h a t w i t h v e r y l o w current dens i t i e s t h e a m o u n t of corros ion p r o d u c e d i s l e s s t h a n t h e t h e o r e t i c a l a m o u n t .

I t h a s b e e n s h o w n t h a t u n d e r s o m e l a b o r a t o r y c o n d i t i o n s i ron a s s u m e s a p a s s i v e s t a t e w h e r e t h e a c t u a l a m o u n t of cor­ros ion p r o d u c e d is l e ss t h a n t h a t c a l c u l a t e d b y F a r a d a y ' s l a w , b u t i t h a s n o t b e e n s h o w n t h a t s u c h c o n d i t i o n s e x i s t i n t h e case of u n d e r g r o u n d s t ruc tures bur ied i n s tree t soi ls . T h e s u b j e c t of e l e c t r o l y t i c corros ion of i ron i s h o w e v e r so large a n d t h e r e are so m a n y poss ib le v a r i a t i o n s t h a t a c o m p l e t e i n v e s t i g a t i o n c o v e r i n g e v e r y poss ib le p h a s e of t h e s u b j e c t i s i m p r a c t i c a b l e i n a n y o n e series of t e s t s . I n a recen t paper* , M r . J . L . R .

*" Electrolytic Corrosion of Iron by Direct Current by J. L. R. Hayden, Journal of the Franklin Institute, October, 1911.

1001

1002 GANZ: ELECTROLYTIC CORROSION Qune 25

H a y d e n descr ibes s o m e l a b o r a t o r y e x p e r i m e n t s d e a l i n g a d m i r ­a b l y w i t h t h e p r o b l e m of t h e e l ec t ro ly t i c corros ion of i ron i n v a r i o u s l a b o r a t o r y s o l u t i o n s w h e n s u b j e c t e d t o large current dens i t i e s . T h e current dens i t i e s u s e d b y M r . H a y d e n r a n g e d f rom 0 .21 a m p e r e per sq . ft . (2 .25 a m p e r e s per sq . m . ) t o 3 .35 a m p e r e s per sq . ft . (36 a m p e r e s per sq . m . ) . M r . H a y d e n f o u n d t h a t w i t h a n u m b e r of e l e c t r o l y t e s , par t i cu lar ly w i t h c e r t a i n n i t ra te s a n d w i t h b i c h r o m a t e s , m u c h smal l er a m o u n t s of cor­ros ion w e r e p r o d u c e d t h a n t h e theore t i ca l a m o u n t . I n all of these cases , h o w e v e r , t h e g r e a t e s t d e v i a t i o n f r o m F a r a d a y ' s l a w w a s f o u n d w i t h t h e larges t current dens i t i e s . W i t h ch lor ides a n d s u l p h a t e s M r . H a y d e n a l w a y s f o u n d a n a m o u n t of corros ion corresponding t o t h e t h e o r e t i c a l v a l u e . A s M r . H a y d e n h a s p o i n t e d o u t i n t h e p a p e r , h i s t e s t s w e r e m a d e w i t h l a b o r a t o r y so lu t ions a n d n o t w i t h s tree t soi ls . T h e l o w e s t current d e n s i t i e s u s e d b y M r . H a y d e n are a l so f r o m 10 t o 100 t i m e s greater t h a n t h e dens i t i e s w i t h w h i c h current s are f o u n d i n prac t i s e l e a v i n g u n d e r g r o u n d s t ruc tures . T h e t e s t s descr ibed i n M r . H a y d e n ' s paper w h i l e e x c e e d i n g l y v a l u a b l e d o therefore n o t nece s sar i l y w a r r a n t a n y c o n c l u s i o n a s t o w h a t wil l h a p p e n t o p i p e s bur i ed i n s tree t soi ls w h e n s u b j e c t e d t o s t r a y e lectr ic current s .

T h e e x p e r i m e n t s descr ibed i n t h e p r e s e n t p a p e r w e r e p a r t i c u ­lar ly d e s i g n e d t o d e t e r m i n e t h e re la t ive r a t e s of corros ion of v a r i o u s k i n d s of i ron in t w o t y p i c a l k i n d s of s t ree t soi l , w h e n s u b j e c t e d t o s u c h l o w current d e n s i t i e s a s are ord inar i ly f o u n d in pract i ce o n u n d e r g r o u n d s t ruc tures . F o u r s e t s of t e s t s w e r e m a d e , e a c h e x t e n d i n g t h r o u g h o u t 47 d a y s t o d e t e r m i n e t h e rate of corros ion of c o m m e r c i a l s tee l , c o m m e r c i a l w r o u g h t i r o n , i n g o t i ron , a n d c a s t i ron , a n d t o c o m p a r e t h e a c t u a l a m o u n t of corros ion w i t h t h a t c a l c u l a t e d b y F a r a d a y ' s l a w . I t w a s a t ­t e m p t e d i n t h e s e t e s t s t o a p p r o a c h pract ica l c o n d i t i o n s as n e a r l y as poss ib le . T h e t e s t s were c o n d u c t e d in t h e e lectr ica l l abora­t o r y of S t e v e n s I n s t i t u t e of T e c h n o l o g y , a n d t h e d e t a i l e d r e s u l t s are g i v e n in t h e f o l l o w i n g :

G E N E R A L D A T A

T w o k i n d s of soil w e r e u s e d in t h e t e s t s , w h i c h w e r e o b t a i n e d f rom s tree t e x c a v a t i o n s near gas m a i n s in L o n g I s l a n d C i t y . O n e of t h e s e e x c a v a t i o n s w a s c h o s e n w h e r e t h e soil a p p e a r e d t o b e l arge ly l i g h t s a n d m i x e d w i t h s o m e c l a y , a n d t h e s e c o n d w a s c h o s e n w h e r e t h e soil w a s h e a v y a n d dark a n d a p p e a r e d t o b e a m i x t u r e of c l a y a n d l o a m . Severa l barre ls of soi l f r o m

1912] G AN Ζ: ELECTROLYTIC CORROSION 1003

e a c h e x c a v a t i o n w e r e o b t a i n e d for t h e s e t e s t s . T o as sure un i ­f o r m i t y of soi l c o n d i t i o n s all of t h e soil of each k ind w a s p a s s e d t h r o u g h a fine m e s h s i e v e a n d t h e n t h o r o u g h l y m i x e d i n a large w o o d e n b o x before b e i n g used i n t h e t e s t s .

F o r t h e s a m p l e s of i ron , s t a n d a r d l | - i n . ( 3 . 1 7 5 - c m . ) p i p e s w e r e c h o s e n s u c h as are u s e d ordinar i ly for s erv i ce p i p e s , a n d t h e s e p i p e s w e r e of c o m m e r c i a l s t ee l , c o m m e r c i a l w r o u g h t i ron a n d i n g o t i ron , e a c h t e s t p i p e b e i n g a b o u t 18 in . ( 4 5 . 7 2 c m . ) l o n g . T h e first t w o t e s t s were c o n d u c t e d w i t h t h e s e t h r e e k i n d s of i ron p ipe . F o r t h e th i rd a n d f o u r t h t e s t s c a s t i ron p i p e s of t h e s a m e s ize w e r e a d d e d .

T h e four s a m p l e p i p e s of e a c h k i n d of i ron u s e d for t h e t e s t s w e r e p l a c e d i n s u i t a b l e t e s t b o x e s . E a c h p i p e w a s m a r k e d i n t h e m e t a l a t o n e e n d w i t h i d e n t i f y i n g n u m b e r s , a n d a h o l e w a s dri l led a n d t a p p e d a t t h e s e e n d s for c o n v e n i e n t e lectr ica l c o n ­n e c t i o n . T w o p i p e s of e a c h k i n d were p l a c e d i n c l a y a n d l o a m soil , a n d t w o p ipes of e a c h k i n d i n c l a y a n d s a n d so i l ; o n e s e t of p i p e s w a s s u b j e c t e d t o d a m p soil o n l y , a n d t h e o t h e r s e t of p i p e s w a s s u b j e c t e d t o d a m p soil a n d t o a n e x t e r n a l l y a p p l i e d e lec tr ic current .

B e f o r e t h e t e s t s w e r e b e g u n e a c h p ipe w a s t h o r o u g h l y c l e a n e d i n a m a n n e r t o b e descr ibed i n greater de ta i l la ter , a n d w a s t h e n w e i g h e d b y t w o i n d e p e n d e n t o b s e r v e r s o n a p h y s i c a l b a l a n c e w h i c h r e s p o n d s t o 10 m i l l i g r a m s . A t t h e e n d of e a c h t e s t e a c h p ipe w a s s imi lar ly c l e a n e d and a g a i n w e i g h e d o n t h e s a m e b a l a n c e b y t w o observers . F r o m t h e difference in t h e w e i g h i n g s before a n d after t h e t e s t t h e los s of w e i g h t w a s d e t e r m i n e d . T h e dif­ference b e t w e e n t h e los s of w e i g h t of a n y o n e p i p e s u b j e c t e d t o d a m p soi l w i t h ex terna l e lec tr ic current and t h e l o s s of w e i g h t of t h e c o r r e s p o n d i n g p i p e s u b j e c t e d t o t h e a c t i o n of t h e s a m e k i n d of soi l b u t w i t h o u t ex terna l e lec tr ic current w a s t a k e n as t h e l o s s of w e i g h t d u e t o e l ec tro lys i s .

I n t h e case of t h e p i p e s w h i c h w e r e s u b j e c t e d t o e x t e r n a l e lec tr ic current , t h e s t r e n g t h of t h i s current w a s d e t e r m i n e d i n e v e r y case b o t h b y m e a n s of c o p p e r v o l t a m e t e r s p l a c e d i n ser ies w i t h t h e p i p e s , a n d b y m e a n s of a c u r r e n t - t i m e c u r v e p l o t t e d f r o m i n d i c a t i n g m e t e r r e a d i n g s t a k e n e v e r y f e w d a y s t h r o u g h o u t t h e t e s t . I n genera l t h e resu l t s o b t a i n e d f r o m t h e v o l t a m e t e r s a n d f r o m t h e c u r r e n t - t i m e c u r v e s a g r e e d q u i t e we l l , a n d t h e a v e r a g e of t h e t w o v a l u e s w a s t a k e n as t h e t o t a l q u a n t i t y of e l ec t r i c i ty p a s s e d .

F o u r s e p a r a t e t e s t s e a c h of 47 d a y s ' d u r a t i o n w e r e m a d e .

1004 G AN Ζ: ELECTROLYTIC CORROSION [June 25

T h e genera l resu l t s of t h e s e t e s t s are g i v e n i n T a b l e I . I n t h e first a n d s e c o n d t e s t s all of t h e p i p e s w h i c h w e r e t o b e s u b j e c t e d t o e x t e r n a l e lec tr ic current w e r e c o n n e c t e d i n paral le l t o a c o n ­s t a n t - v o l t a g e s u p p l y , w h i l e i n t h e t h i r d a n d f o u r t h t e s t s t h e p i p e s w e r e c o n n e c t e d i n ser ies , t h u s p r o d u c i n g t h e s a m e current f low f r o m e a c h p i p e t o t h e surround ing soi l . D u r i n g all t e s t s t h e t e s t b o x e s w e r e k e p t i n o n e of t h e l a b o r a t o r y r o o m s . T h e d e t a i l e d c o n d i t i o n s a n d a r r a n g e m e n t s of e a c h t e s t are g i v e n b e l o w .

Test No. 1. T h e p i p e s as r e c e i v e d f r o m t h e m a n u f a c t u r e r w e r e c l e a n e d w i t h k e r o s e n e a n d b e n z o l a n d wire b r u s h e d t o r e m o v e l o o s e sca le a n d rust . Af t er h a v i n g b e e n t h o r o u g h l y c l e a n e d a n d dr ied t h e y w e r e p l a c e d i n w o o d e n t e s t b o x e s . F o u r b o x e s w e r e p r o v i d e d , e a c h b e i n g a p p r o x i m a t e l y 12 in . b y 12 i n . b y 12 in . (30 . 4 8 c m . b y 3 0 . 4 8 c m . b y 3 0 . 4 8 c m . ) in s ide d i m e n s i o n s , w i t h three h o l e s b o r e d i n e a c h e n d t h r o u g h w h i c h t h e p i p e s p r o ­j e c t e d . T h r e e p i p e s , o n e of e a c h k i n d of i ron , w e r e p l a c e d i n e a c h t e s t b o x . A s h e e t of c o p p e r p l a c e d a t t h e b o t t o m a n d a p iece of c o p p e r wire g a u z e p l a c e d o n t o p served a s g r o u n d p l a t e s . T w o b o x e s , c o n t a i n i n g c l a y a n d s a n d soil w i t h t h r e e t e s t p i p e s

. i n e a c h b o x , a n d t w o b o x e s c o n t a i n i n g c l a y a n d l o a m soil w i t h t h r e e t e s t p i p e s i n e a c h b o x , w e r e se t u p w i t h g r o u n d p l a t e s a s a b o v e descr ibed . A s torage b a t t e r y , h a v i n g a v o l t a g e of a p ­p r o x i m a t e l y 14 v o l t s , w a s c o n n e c t e d w i t h i t s p o s i t i v e ter ­m i n a l t o t h e p i p e s i n t w o b o x e s o n e w i t h e a c h k i n d of so i l , a n d i t s n e g a t i v e t ermina l t o t h e g r o u n d p l a t e s . I n c ircui t w i t h e a c h of t h e s e p i p e s w a s p l a c e d a s u i t a b l e c o p p e r v o l t ­a m e t e r . N o e x t e r n a l e l e c t r o m o t i v e force w a s a p p l i e d b e ­t w e e n p i p e s a n d g r o u n d p l a t e s i n t h e r e m a i n i n g t w o b o x e s . D u r i n g t h e t e s t , 2 5 0 cu . c m . of h y d r a n t w a t e r w a s a d d e d t o e a c h t e s t b o x a p p r o x i m a t e l y e v e r y t h i r d d a y i n order t o k e e p t h e soi l d a m p . I n t h e case of t h e p i p e s s u b j e c t e d t o e x t e r n a l e l e c t r i c i t y , t h e current d e l i v e r e d t o e a c h p i p e w a s m e a s u r e d prac ­t i c a l l y e v e r y d a y b y m e a n s of a n a m m e t e r i n s e r t e d t e m p o r a r i l y , a n d t h e v o l t a g e b e t w e e n p i p e s a n d g r o u n d p l a t e s w a s a l so m e a s ­ured . Af t er t h e t e s t h a d b e e n c o n t i n u e d i n t h i s w a y for 47 d a y s t h e t e s t p i p e s w e r e r e m o v e d f r o m the i r b o x e s , w a s h e d i n h o t w a t e r t o r e m o v e a d h e r i n g soi l , s o a k e d in k e r o s e n e t o s o f t e n t h e rus t , w ire b r u s h e d u n t i l c l ean , t h e n w a s h e d i n b e n z o l , dr ied a n d w e i g h e d as before .

Test No. 2 . F o r t e s t N o . 2 t h e p i p e s w e r e t a k e n d i rec t ly a t t h e e n d of T e s t N o . 1 a n d r e p l a c e d in the ir t e s t b o x e s . T h e s a m e b o x e s a n d t h e s a m e soil a n d g r o u n d p l a t e s w e r e u s e d a s

1912] G AN Ζ: ELECTROLYTIC CORROSION 1005

in t e s t N o . 1. Al l c o n d i t i o n s of t e s t N o . 2 w e r e i d e n t i c a l w i t h t h o s e of t e s t N o . 1 e x c e p t t h a t a c a u s t i c p o t a s h b a t t e r y w a s u s e d t o m a i n t a i n t h e p i p e s 0 .7 v o l t p o s i t i v e t o t h e g r o u n d p l a t e s a n d 2 0 0 cu . c m . of h y d r a n t w a t e r w a s a d d e d t o e a c h b o x prac t i ­c a l l y e v e r y d a y , t h u s m a i n t a i n i n g t h e soil i n a d a m p e r c o n d i t i o n t h a n i n t e s t N o . 1. A t t h e e n d of 4 7 d a y s t h e p i p e s were a g a i n r e m o v e d , a n d c l e a n e d a n d w e i g h e d i n t h e s a m e m a n n e r a s af ter t e s t . N o . 1.

Test No. 3 . F o r t e s t N o . 3 t h e s a m e p i p e s w e r e u s e d as r e c e i v e d f r o m t e s t N o . 2 , w i t h t h e a d d i t i o n of four c a s t i ron p i p e s , w h i c h before b e i n g s u b j e c t e d t o t e s t w e r e a l so c l e a n e d w i t h k e r o s e n e a n d b e n z o l a n d t h e n dr ied a n d w e i g h e d . I n t h i s t e s t i t w a s des ired t o s u b j e c t e a c h p i p e t o t h e s a m e current i n s t e a d of t o t h e s a m e v o l t a g e as in t e s t s N o . 1 a n d N o . 2 . F o r th i s r e a s o n 16 t e s t b o x e s w e r e c o n s t r u c t e d e a c h a p p r o x i m a t e l y 16 in . b y 6 in . b y 6 in . (40 .6 c m . b y 15 c m . b y 15 c m . ) in terna l d i m e n s i o n s , w i t h o n e h o l e i n e a c h e n d t h r o u g h w h i c h t h e p i p e p r o j e c t e d , t h u s e x p o s i n g 16 in . (40 .6 c m . ) of e a c h t e s t p i p e t o soi l . T h e g r o u n d p l a t e i n e a c h t e s t b o x w a s p l a c e d a t t h e t w o s ides a n d t h e b o t t o m a n d t o p of e a c h b o x , t h u s c o m p l e t e l y sur­r o u n d i n g t h e t e s t p i p e , t h e l o w e r p o r t i o n of t h e g r o u n d p l a t e c o n s i s t i n g of c o p p e r s h e e t i n g a n d t h e u p p e r p o r t i o n of c o p p e r wire g a u z e . T h e soil w a s o b t a i n e d f r o m t h e t e s t b o x e s u s e d i n t e s t s N o . 1 a n d N o . 2 , e a c h k i n d of soi l b e i n g m i x e d t h o r o u g h l y t o g e t h e r w i t h s o m e a d d i t i o n a l soi l of t h e s a m e k i n d lef t o v e r f r o m t h e or ig inal s h i p m e n t . E i g h t of t h e n e w t e s t b o x e s w e r e filled w i t h c l a y a n d s a n d soil a n d e i g h t w i t h c l a y a n d l o a m soi l , a n d o n e p i p e w a s p l a c e d in e a c h t e s t b o x . F o u r b o x e s c o n t a i n i n g c l a y a n d s a n d soi l , e a c h c o n t a i n i n g a different k i n d of i r o n p i p e , a n d four b o x e s c o n t a i n i n g c l a y a n d l o a m soi l , e a c h c o n t a i n i n g a different k i n d of i r o n p i p e w e r e c o n n e c t e d e l ec t r i ca l l y i n s e r i e s — t h a t i s , t h e p i p e i n o n e b o x w a s c o n n e c t e d t o t h e g r o u n d p l a t e of t h e n e x t b o x . A p o t e n t i a l difference of a b o u t 125 v o l t s w a s m a i n t a i n e d across t h e e i g h t b o x e s b y m e a n s of a s t o r a g e b a t t e r y . I n t h i s w a y t h e s a m e current w a s m a d e t o f low f rom e a c h p i p e t o t h e s u r r o u n d i n g soi l . I n series w i t h t h e c ircui t w e r e p l a c e d t w o c o p p e r v o l t a m e t e r s . T h e re­m a i n i n g e i g h t b o x e s , four w i t h e a c h k i n d of soi l , w e r e lef t w i t h o u t e lec tr ica l c o n n e c t i o n s so t h a t t h e p i p e s i n t h e m w e r e s u b j e c t e d t o d a m p soi l o n l y . D u r i n g t h e t e s t a p p r o x i m a t e l y 100 c u . c m . of h y d r a n t w a t e r w a s a d d e d p r a c t i c a l l y e v e r y d a y t o e a c h b o x , a n d t h e current in t h e c ircui t a n d t h e p o t e n t i a l

1006 GANZ: ELECTROLYTIC CORROSION [June 25

difference b e t w e e n p ipe a n d g r o u n d p l a t e i n e a c h b o x w a s m e a s ­ured e v e r y f e w d a y s . A t t h e e n d of 4 7 d a y s e a c h p i p e w a s r e m o v e d f r o m i t s b o x , w a s h e d w i t h h o t w a t e r t o r e m o v e t h e adher ing soi l , soaked i n a m m o n i u m c i tra te t o l o o s e n a n d r e m o v e t h e r u s t , w ire b r u s h e d , t h e n w a s h e d i n n a p t h a , dr i ed a n d w e i g h e d . A m m o n i u m c i t ra te w a s u s e d as i t h a d b e e n f o u n d b y tr ial t h a t t h i s w o u l d m o r e q u i c k l y a n d m o r e e f f ec t ive ly l o o s e n rus t t h a n w o u l d k e r o s e n e or benzo l .

Test No. 4 . I n order t o e l i m i n a t e all poss ib le ef fects of pre­v i o u s corros ion , sca le , e t c . , e a c h t e s t p ipe w a s t u r n e d d o w n i n a l a t h e t o b r i g h t m e t a l , t h e n c l e a n e d w i t h n a p t h a , s o a k e d i n a m m o n i u m c i t ra te , rinsed in w a t e r a n d n a p t h a , t h e n dr ied a n d w e i g h e d . T h e p i p e s w e r e t h e n rep laced i n t h e i r b o x e s a n d t h e t e s t run prac t i ca l l y in the s a m e m a n n e r as t e s t N o . 3 , e x c e p t t h a t 1 5 0 c u . c m . of h y d r a n t w a t e r w a s a d d e d t o e a c h t e s t b o x e a c h d a y , t h u s m a i n t a i n i n g t h e soi l d a m p e r t h a n i n t e s t N o . 3 , a n d t h a t a record ing m i l l i a m m e t e r w a s k e p t i n c ircui t . A t t h e e n d of 4 7 d a y s e a c h p i p e w a s r e m o v e d a n d s u b j e c t e d t o e x a c t l y t h e s a m e c l e a n i n g proces se s a s a t t h e e n d of t e s t N o . 3 . T h e y were t h e n dr ied a n d w e i g h e d as before .

R E S U L T S OF PHYSICAL EXAMINATION OF P I P E S AFTER T E S T S

T h e p i p e s w h i c h h a d b e e n s u b j e c t e d t o d a m p soi l o n l y s h o w e d s l i gh t surface corros ion b u t n o p i t t i n g af ter t e s t N o . 1 a n d t e s t N o . 2 , a n d s o m e s l ight p i t t i n g after t e s t N o . 3 , b u t i n n o case w a s t h i s p i t t i n g c o m p a r a b l e i n s e v e r i t y w i t h t h a t p r o d u c e d w h e r e a n e x t e r n a l e lec tr ic current h a d b e e n a p p l i e d . A f t e r t e s t N o . 1 t h e c o a t i n g of sca le or ig ina l ly o n t h e p i p e s w a s f o u n d p r a c t i c a l l y i n t a c t , b u t i t w a s f o u n d p a r t l y r e m o v e d after t e s t N o . 2 a n d h a d a l m o s t c o m p l e t e l y d i s a p p e a r e d af ter t e s t N o . 3 , t h e sca le t h e n r e m a i n i n g b e i n g i n we l l de f ined p a t c h e s w h i c h s h o w e d l i t t l e corros ion . Af ter t e s t N o . 4 w h e r e all s ca le h a d p r e v i o u s l y b e e n r e m o v e d t h e corros ion w a s e x t r e m e l y u n i f o r m w i t h o u t t h e s l i g h t e s t e v i d e n c e of p i t t i n g . I t wi l l b e n o t e d f r o m T a b l e I , i n t h e c o l u m n m a r k e d B, t h a t t h e s e p i p e s i n genera l s h o w e d a n increas ing loss i n w e i g h t i n t h e s u c c e s s i v e t e s t s , w i t h t h e e x c e p t i o n of t e s t N o . 4 , w h e r e t h e los s i n w e i g h t w a s i n genera l s l i g h t l y l e s s t h a n t h a t i n t e s t N o . 3 .

P i p e s s u b j e c t e d t o d a m p soil p lus a p p l i e d ex terna l e l ec tr ic cur­ren t s h o w e d s o m e e v i d e n c e of p i t t i n g after t h e first t e s t , t h e p i t s b e c o m i n g m o r e p r o n o u n c e d after t e s t s N o . 2 a n d N o . 3 . A f t e r t e s t N o . 4 t h e p i t t i n g w a s f o u n d q u i t e a s s evere a s af ter t e s t N o . 3 . I t

TABLE I.

C O M P A R A T I V E R E S U L T S OF T E S T S ON C O R R O S I O N OF C O M M E R C I A L S T E E L , C O M M E R C I A L W R O U G H T I R O N , A N D C A S T I R O N P I P E S , W H E N S U B J E C T E D T O D A M P SOIL W I T H A N D W I T H O U T

A P P L I E D E X T E R N A L E L E C T R I C C U R R E N T .

General Data Test N o . 1 Test No. 2 Test No . 3 Duration of test , days 47 47 47 Size of test pipes, inches 1 .25 b y 18 1.25 by 18 1 .25 b y 18

cm. 3 . 1 7 5 b y 4 5 . 7 2 3 . 1 7 5 b y 4 5 . 7 2 3 . 1 7 5 b y 4 5 . 7 2 Length of pipe exposed to soil, inches 12 12 16

cm. 3 0 . 4 8 3 0 . 4 8 4 0 . 6 4 Quantity of soil in each test box, cu. ft 1 1 0 . 3 3

cu. m. 0 . 0 2 8 3 0 . 0 2 8 3 0 . 0 0 9 4 3 Number of test pipes in each test box 3 3 1 Quantity of water added to each test box 250 çu. cm. 200 cu. cm. 100 cu. cm.

every 3rd day every day every day

I N G O T I R O N ,

Test No. 4

47 1 .25 by 18

1.175 by 4 5 . 7 2 16

4 0 . 6 4 0 . 3 3

0 . 0 0 9 4 3 1

150 cu. cm. every day

Kind of

soil

Clay and sand soil from excavation in Long Island City

Clay and loam soil from street excavat ion in Long Island City

Kind of

iron.

Steel

Wrought iron

Ingot iron

Cast iron

Steel

Wrought iron

Ingot iron

Cast iron

Test No.

Average positive

potential of test pipe to ground

plate

Volts

1 4 . 0 0 . 6 9

2 4 . 5 2 6 . 3

1 4 . 0 0 . 6 9

3 0 . 0 1 5 . 4

1 4 . 0 0 . 6 9

3 1 . 7 2 2 . 0

3 7 . 3 1 6 . 7

1 4 . 0 0 . 6 9 0 . 8 0 9 . 3 5

1 4 . 0 0 . 6 9 0 . 7

1 3 . 1

1 4 . 0 0 . 6 9 0 . 6 8 . 5 3

0 . 9 1 7 . 0

Average density of current

leaving surface of pipe

Amperes per

sq. ft.

0 . 0 0 5 9 0 . 0 0 1 7 0 . 0 1 5 6 0 . 0 5 3 7

0 . 0 0 4 5 0 . 0 0 0 9 0 . 0 1 5 6 0 . 0 5 3 7

0 . 0 0 4 7 0 . 0 0 1 7 0 . 0 1 5 6 0 . 0 5 3 7

0 . 0 1 5 6 0 . 0 5 3 7

0 . 0 1 5 3 0 . 0 0 7 0 0 . 0 1 5 6 0 .0537

0 . 0 1 1 1 0 . 0 0 2 4 0 . 0 1 5 6 0 .0537

0 . 0 1 3 2 0 . 0 0 6 2 0 . 0 1 5 6 0 . 0 5 3 7

0 . 0 1 5 6 0 . 0 5 3 7

Amperes per

sq. m.

0 . 0 6 3 5 0 . 0 1 8 3 0 . 1 6 8 0 0 . 5 7 8

0 . 0 4 8 4 0 . 0 0 9 7 0 . 1 6 8 0 . 5 7 8

0 . 0 5 0 6 0 . 0 1 8 3 0 . 1 6 8 0 . 5 7 8

0 . 1 6 8 0 . 5 7 8

0 . 1 6 5 0 . 0 7 5 0 . 1 6 8 0 . 5 7 8

0 . 1 1 9 0 . 0 2 5 8 0.1.68 0 . 5 7 8

0 . 1 4 2 0 . 0 6 6 7 0 . 1 6 8 0 . 5 7 8

0 . 1 6 8 0 . 5 7 8

Loss of weight of pipes

i£.axios oi actual Approxi­ t o

Actual mate Theoretical Theoretical Loss

A Β C = 4 - B D _ A C D D

Pipe buried Pipe buried D u e to D u e to elec­ Actual Actual loss in soil and in soil action trolysis only total from elec­subjected only. of external (Calculated loss trolysis

to external No external electric from electro­ divided by divided b y electric electric current chemical theoretical theoretical current. current. only . equivalent) loss from loss from Grams Grams Grams Grams electrolysis electrolysis

18 13 2 36 15 77 2 97 6 11 5 . 3 4 72 1 90 2 82 0 83 5 71 3 . 4 1

21 09 4 88 16 21 10 5 2 01 1 .54 39 50 4 24 35 26 34 2 1 15 1 .03

3 69 0 21 3 48 2 07 1 .78 1 .68 1 63 0 85 0 78 0 45 3 . 62 1 . 7 3

20 48 2 56 17 90 10 5 1 .95 1 .71 41 03 3 12 37 91 34 2 1 20 1 .11

8 54 0 50 8 04 2 34 3 .65 3 . 4 3 2 46 0 21 2 25 0 84 2 . 92 2 . 6 7

21 24 2 85 18 39 10 5 2 02 1 .75 41 71 1 83 39 88 34 2 1 22 1 .17

13 51 2 . 0 5 11 46 10 5 1 29 1 .09 41 39 3 . 6 4 3 7 . 7 5 3 4 . 2 1 21 1 .10

17 13 3 29 13 84 7 73 2 .22 1 .79 6 82 3 63 3 19 3 37 2 03 0 . 9 5

20 48 6 16 14 32 10 5 1 95 1 .30 41 19 2 44 38 75 34 2 1 20 1 .13

6 74 1 01 5 73 5 53 1 22 1 .04 2 57 1 44 1 13 1 17 2 19 0 . 9 6

17 60 4 27 13 33 10 5 1 68 1 .27 45 24 3 07 42 17 34 2 1 32 1 .23

14 01 1 00 13 01 6 58 2 13 1 . 9 8 5 75 1 45 4 30 3 09 1 86 1 .39

16 44 4 64 11 80 10 5 1 57 1 . 1 2 38 90 3 72 35 08 34 2 1 14 1 .03

13 19 3 . 9 9 9 . 2 0 10 5 1 26 0 . 8 8 4 5 78 4 26 41 52 34 2 1 34 1 .21

[GANZ]

1912] GANZ: ELECTROLYTIC CORROSION 1007

wi l l b e n o t i c e d a l so f r o m t h e las t c o l u m n i n T a b l e I t h a t t h e v a l u e of t h e ra t io of t h e a c t u a l lo s s f r o m e l ec t ro ly s i s t o t h e t h e o ­ret ical loss decreases i n genera l i n t h e s u c c e s s i v e t e s t s , t h a t is* as t h e sca le i s r e m o v e d . H e n c e i t w o u l d a p p e a r t h a t w i t h t h e e x c e p t i o n of c a s t i ron t h e sca le d o e s n o t p r o t e c t p i p e s f r o m e l ec ­tro lys i s a n d m a y e v e n a g g r a v a t e i t . I t is a l so e v i d e n t t h a t l oca l ­i zed corros ion re su l t ing i n p i t t i n g occurs e v e n w h e n all s ca le h a s b e e n r e m o v e d a n d a u n i f o r m m e t a l l i c surface is e x p o s e d t o t h e soil . I n t h e case of c a s t i ron t h e iron is o x i d i z e d b y e lec tro l ­y s i s b u t r e m a i n s in p lace as a g r a p h i t i c m a s s h a v i n g l i t t l e m e c h a n ­ical s t r e n g t h b u t r e t a i n i n g t h e surface i n i t s or ig inal c o n d i t i o n w i t h o u t e x p o s i n g p i t s . T h i s g r a p h i t i c m a t e r i a l i s q u i t e soft a n d c a n b e eas i ly d u g o u t w i t h a kn i fe or o t h e r su i tab l e t o o l .

DISCUSSION OF R E S U L T S

T h e c u r r e n t - t i m e c u r v e s for e a c h t e s t s h o w a rap id fa l l ing off of t h e current d u r i n g the first f e w d a y s . A s a n e x a m p l e of t h i s t h e fo l l owing v a l u e s of current are g i v e n f r o m t e s t N o . 3 :

At beginning of test 0 .100 ampere. At end of 1st day 0.050 " " " "· 2nd day 0.0298 " " " 5th day 0.0125 " " " 12th day 0.0072 " " " 19th day 0.0062 " " " 29th day 0.0050 u " " 47th day 0.0033

T h i s i n d i c a t e s t h a t t h e re s i s tance increases v e r y r a p i d l y a t first a n d t h e n increases o n l y v e r y s l o w l y . T h i s increase i n res i s tance w a s f o u n d t o t a k e p l a c e a t t h e surface of t h e p i p e or in t h e l a y e r of soi l i m m e d i a t e l y surround ing t h e p ipe . T h i s w a s d e t e r m i n e d b y m e a n s of a s e p a r a t e t e s t , i n w h i c h , af ter t h e res i s tance h a d i n c r e a s e d , t h e p i p e w a s r e m o v e d , w i p e d off w i t h w a s t e , a n d i m m e d i a t e l y r e p l a c e d , w h e n i t w a s f o u n d t h a t t h e res i s tance w a s a g a i n as l o w as a t t h e b e g i n n i n g of t h e t e s t .

T h e d e t a i l e d resu l t s of t h e t e s t s a n d of t h e c a l c u l a t i o n s m a d e from t h e s e re su l t s are g i v e n i n T a b l e I . I n t h e f o u r t h c o l u m n of t h i s t a b l e t h e a v e r a g e v o l t a g e b e t w e e n e a c h t e s t p i p e a n d t h e g r o u n d p l a t e i s g i v e n i n v o l t s . I n t h e first t e s t e a c h p i p e w a s m a i n t a i n e d a t a c o n s t a n t p o s i t i v e p o t e n t i a l of 14 v o l t s , a n d t h e current p r o d u c e d b y t h i s v o l t a g e v a r i e d w i t h e a c h p i p e , b e i n g d e t e r m i n e d b y t h e a c c i d e n t a l r e s i s t a n c e of soi l a n d of t h e surface c o n t a c t s i n e a c h case . I n t h e s e c o n d t e s t t h e p i p e s w e r e m a i n ­t a i n e d a t a c o n s t a n t v o l t a g e of 0 .69 v o l t , a n d t h e current f r o m

1008 GANZ: ELECTROLYTIC CORROSION [June 25

e a c h p ipe v a r i e d as before . I n t e s t N o . 3 t h e p i p e s w e r e all c o n ­n e c t e d i n series so t h a t t h e current l e a v i n g e a c h p i p e w a s t h e s a m e , b u t t h e v o l t a g e b e t w e e n e a c h p i p e a n d t h e g r o u n d p l a t e n o w v a r i e d . I t wi l l be n o t e d t h a t t h i s v o l t a g e r a n g e d f r o m 2 4 . 5 t o 3 7 . 3 v o l t s i n t h e c l a y a n d s a n d soi l , w h i l e i n t h e c l a y a n d l o a m soi l i t w a s l e s s t h a n o n e v o l t . I n t e s t N o . 4 t h e p i p e s w e r e a g a i n c o n n e c t e d i n series , a n d t h e v o l t a g e i n t h e c l a y a n d s a n d soi l r a n g e d f r o m 15 .4 t o 26 .3 v o l t s , w h i l e i n t h e c l a y a n d l o a m soi l i t r a n g e d f r o m 8 .53 t o 17 .0 v o l t s . W h i l e t h e a p p l i e d v o l t a g e i n t e s t s N o . 3 a n d N o . 4 w a s t h e s a m e , i t wi l l b e n o t e d t h a t t h e a v e r a g e current i n t e s t N o . 4 w a s o v e r t h r e e t i m e s as large a s i n t e s t N o . 3 .

S i n c e t h e s a m e soi ls w e r e u s e d i n t e s t s N o . 3 a n d N o . 4 , t h i s dif ference i n current a n d i n r e l a t i v e v o l t a g e s , i n d i c a t i n g c h a n g e s i n re s i s tance , m a y b e d u e t o t h e f a c t t h a t for t e s t N o . 4 t h e p i p e s w e r e t u r n e d d o w n t o a s m o o t h surface w h e r e b y b e t t e r a n d m o r e u n i f o r m c o n t a c t w a s s e c u r e d b e t w e e n p i p e s a n d soi l .

T h e a c t u a l lo s s of w e i g h t of t h e p ipes i n t h e t e s t s a s d e t e r m i n e d f r o m t h e w e i g h i n g s m a d e before a n d after t h e t e s t s , i s g i v e n i n c o l u m n s A a n d Β of T a b l e 1. T h e difference b e t w e e n t h e l o s s of w e i g h t of a n y p ipe s u b j e c t e d t o d a m p soi l w i t h a p p l i e d e lec tr ic current a n d t h e l o s s of w e i g h t of t h e c o r r e s p o n d i n g p i p e s u b j e c t e d o n l y t o d a m p s o i l i s t a k e n as t h e los s of w e i g h t d u e t o e l ec tro lys i s a n d is g i v e n i n c o l u m n C. T h e t h e o r e t i c a l l o s s f r o m e l e c t r o l y s i s as c a l c u l a t e d f r o m t h e a m p e r e - h o u r s w h i c h p a s s e d f r o m e a c h p ipe t o t h e s u r r o u n d i n g soi l , a n d f r o m t h e e l e c t r o c h e m i c a l e q u i v a l e n t of i ron , n a m e l y 1 . 0 4 4 e r a m s per a m p e r e - h o u r , i s g i v e n in c o l u m n D. T h e a c t u a l t o t a l l o s s ( c o l u m n A) d i v i d e d b y t h e t h e o r e t i c a l l o s s f r o m e l ec ­t ro ly s i s ( c o l u m n D) i s g i v e n i n t h e n e x t t o t h e l a s t c o l u m n

m a r k e d -^-, a n d t h e a p p r o x i m a t e a c t u a l l o s s f r o m e l e c t r o l y s i s

( c o l u m n C) d i v i d e d b y t h e t h e o r e t i c a l lo s s f r o m e l e c t r o l y s i s C

( c o l u m n D) i s g i v e n i n t h e l a s t c o l u m n m a r k e d ^ j - .

I t wi l l b e n o t e d t h a t i n e v e r y case t h e a c t u a l t o t a l l o s s d i v i d e d b y t h e t h e o r e t i c a l lo s s f r o m e l ec t ro ly s i s i s greater t h a n u n i t y , a n d t h a t t h e a p p r o x i m a t e a c t u a l lo s s f r o m e l ec t ro ly s i s d i v i d e d b y t h e t h e o r e t i c a l lo s s f r o m e l ec tro lys i s , a s g i v e n i n t h e l a s t c o l u m n of T a b l e I , i s a l so greater t h a n u n i t y i n e v e r y case b u t t h r e e , i n w h i c h c a s e s these v a l u e s are 0 .96 , 0 .95 , a n d 0 .88 . A s a l r e a d y

1912] GANZ: ELECTROLYTIC CORROSION 1009

s t a t e d , i n t h e case of the c a s t i ron p ipes there is left a s t h e resu l t of e l ec tro lys i s a soft graph i t i c m a t e r i a l w h i c h is difficult t o r e m o v e . T h e o n e l o w v a l u e of 0 .88 , o b t a i n e d for cas t i ron in t e s t N o . 3 , is therefore p r o b a b l y a c c o u n t e d for b y t h e fac t t h a t t h i s graph i t i c m a t e r i a l h a d n o t all b e e n r e m o v e d .

A n e x a m i n a t i o n of t h e las t c o l u m n of t h e t a b l e s h o w s t h a t t h e a c t u a l loss f r o m e l ec tro lys i s d i v i d e d b y t h e t h e o r e t i c a l lo s s w a s m u c h larger i n t h e first t e s t t h a n i n t h e l a s t t e s t , par t i cu lar ly in t h e case of c l a y a n d s a n d soil . I n o n e of t h e s e case s i n t h e first t e s t t h e a c t u a l lo s s of w e i g h t f r o m e l ec tro lys i s d i v i d e d b y t h e t h e o r e t i c a l l o s s w a s as h i g h as 5 .3 . T h i s m a y b e p a r t l y d u e t o s o m e sca le h a v i n g b e e n l o o s e n e d b y corros ion of t h e m ι t a l a n d a f t erward b r u s h e d off i n t h e c l e a n i n g of t h e p ipes . I t m a y b e r e m a r k e d , h o w e v e r , t h a t after t e s t N o . 3 a p h y s i c a l e x a m i n a t i o n of t h e p i p e s s h o w e d t h a t t h o s e w h i c h h a d b e e n s u b j e c t e d t o t h e a c t i o n of d a m p soil o n l y a p p e a r e d t o h a v e lo s t n e a r l y a s m u c h of t h e or ig inal sca le a s t h o s e a c t e d u p o n b y e lec tr ic current . I t , therefore , d o e s n o t s e e m l ike ly t h a t t h e large v a l u e s of t h e ra t ios a b o v e m e n t i o n e d are en t i re ly d u e t o los s of sca le , b u t ra ther i n d i c a t e s t h a t a great dea l m o r e t h a n t h e t h e o r e t i c a l a m o u n t of i ron w a s l o s t b y e l ec tro lys i s .

I t wi l l b e n o t e d t h a t t h e loss of w e i g h t b y e l e c t r o l y s i s a p p e a r s t o b e a b s o l u t e l y i n d e p e n d e n t of t h e a p p l i e d v o l t a g e , e x c e p t in s o far a s t h e v o l t a g e d e t e r m i n e s t h e a m o u n t of current p r o d u c e d , 0.7 v o l t b e i n g q u i t e a s e f fect ive i n p r o d u c i n g corros ion a s 3 0 v o l t s .

N o c h e m i c a l a n a l y s e s of t h e soi ls u s e d w e r e m a d e , e x c e p t t h a t a s a m p l e f r o m e a c h k i n d of soi l w a s t e s t e d for ch lor ides a n d s h o w e d t h e presence of t h e s e sa l t s .

CONCLUSIONS

T h e d u r a t i o n of t h e s e t e s t s w e r e n o t suff ic ient ly l o n g t o w a r r a n t p o s i t i v e c o n c l u s i o n s t o b e d r a w n regard ing t h e r e l a t i v e corros ion of t h e four k i n d s of i r o n t e s t e d , w h e n s u b j e c t e d o n l y t o t h e a c t i o n of d a m p soi l . T h e f o l l o w i n g c o n c l u s i o n s a p p e a r h o w e v e r t o b e w a r r a n t e d :

T h e corros ion of i ron b y e l ec tro lys i s i n t h e t w o k i n d s of s tree t soi l t e s t e d is i n d e p e n d e n t of t h e v a l u e of t h e a p p l i e d v o l t a g e , e x c e p t i n so far as t h i s d e t e r m i n e s t h e a m o u n t of current pro­d u c e d , and less t h a n o n e v o l t c a n p r o d u c e corros ion b y e l ec ­t ro ly s i s .

F o r t h e t w o k i n d s of s t ree t so i l s t e s t e d , a n d w i t h current

1010 GANZ: ELECTROLYTIC CORROSION [June 25

dens i t i e s r a n g i n g f r o m 1.7 m i l l i a m p e r e s per sq . ft . ( 18 .3 mi l l i -a m p e r e s per sq . m . ) t o 5 4 m i l l i a m p e r e s per sq . . f t . (581 mi l l i ­a m p e r e s per sq . m . ) , t h e loss of w e i g h t of i ron b y e l ec tro lys i s i s ' a t l e a s t e q u a l t o t h a t c a l c u l a t e d b y F a r a d a y ' s l a w , a n d i s i n general greater t h a n t h e t h e o r e t i c a l loss . I n all case s e l ec t ro ly s i s t e n d s t o c a u s e loca l i zed corros ion a n d d e c i d e d p i t t i n g . Surface scale a p p e a r s t o acce lera te corros ion f rom e l ec tro lys i s w i t h all i rons e x c e p t cas t i r o n ; t h i s w a s e spec ia l l y p r o n o u n c e d i n t h e case of t h e s tee l p i p e s t e s t e d . W h e n t h e surface sca le w a s re­m o v e d there w a s prac t i ca l l y n o difference i n t h e a m o u n t of corros ion p r o d u c e d b y a g i v e n current l e a v i n g i ron for d a m p soil b e t w e e n c o m m e r c i a l s tee l , c o m m e r c i a l w r o u g h t i ron , i n g o t i ron a n d c a s t i ron .

I t s h o u l d b e p o i n t e d o u t t h a t t h e e lectr ica l r e s i s tance of c a s t i ron i s a b o u t t e n t i m e s as great a s t h a t of w r o u g h t i ron , s tee l , or i n g o t i ron , a n d t h e u s u a l l e a d j o i n t s in c a s t i ron p i p e s a l so h a v e a re s i s tance w h i c h is m a n y t i m e s greater t h a n t h e screw c o u p l i n g j o i n t s u s u a l w i t h w r o u g h t iron a n d s tee l p ipes . F o r t h e s e r e a s o n s a g i v e n v o l t a g e d r o p t h r o u g h g r o u n d wil l c a u s e a m u c h s m a l l e r current t o flow o n a cas t iron p ipe t h a n o n a w r o u g h t i ron or a s t ee l p ipe , t h u s p r a c t i c a l l y m a k i n g cas t i ron p ipes m u c h Jess s u b j e c t t o e l ec tro lys i s t h a n w r o u g h t i ron or s tee l p ipes . I t m u s t a l so b e n o t e d t h a t w h e n a c a s t i ron p ipe i s corroded b y e l ec tro lys i s , t h e i ron i s o x i d i z e d b u t r e m a i n s in p lace as a graph i t i c m a s s h a v i n g l i t t l e m e c h a n i c a l s t r e n g t h , b u t pos se s s ing t h e a b i l i t y t o m a i n t a i n t h e p ipe g a s - t i g h t a n d s o m e t i m e s e v e n w a t e r - t i g h t for c o n s i d e r a b l e per iods , w h i l e w i t h w r o u g h t i ron or s tee l p i p e s t h i s d o e s n o t occur so t h a t h o l e s a n d c o n s e q u e n t l e a k s are m o r e q u i c k l y p r o d u c e d . F r e q u e n t l y w h e r e c a s t iron p ipes a p p e a r t o b e i m m u n e from elec­t ro lys i s b e c a u s e n o e v i d e n c e s of l e a k a g e h a v e d e v e l o p e d , an e x a m i n a t i o n of t h e p ipes w o u l d revea l t h a t a grea t dea l of cor­ros ion h a s a c t u a l l y t a k e n p lace a n d t h a t t h e p i p e s h a v e b e e n v e r y g r e a t l y w e a k e n e d .

T h e t e s t s descr ibed in t h i s paper are b y n o m e a n s c o n s i d e r e d c o m p l e t e . T h e r e are i n fac t so m a n y poss ib le v a r i a b l e s , s u c h as different k i n d s of soi l , different degrees of w e t t i n g t h e soi l , dif ferent k i n d s of i ron , different v o l t a g e s , different current d e n s i t i e s , e t c . , t h a t i t w o u l d b e e x t r e m e l y l abor ious t o m a k e a c o m p l e t e se t of t e s t s . T h e wr i ter e x p e c t s h o w e v e r t o c o n t i n u e t h e e x p e r i m e n t s a l o n g t h e l ine o u t l i n e d i n t h i s paper a n d h o p e s t h a t t h e d i s c u s ­s ion wi l l br ing o u t s u g g e s t i o n s w h i c h wi l l s erve t o m a k e t h e n e x t series of t e s t s m o r e v a l u a b l e .