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8 Bronze, Zinc, Aluminum, and Galvanized Steel: Corrosion Rates as a Function of Space and Time over the United States 1,3

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D. E . Patterson , R. B.Husaron ,and E. Escalante 1

Washington University, St. Louis, M O 63130

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National Bureau of Standards, Gaithersburg, M D 20899

The corrosion of metals exposed to the atmosphere is known to be caused by a mixture of natural and anthropogenic factors. To apportion the cause of metal corrosion, one may conduct controlled laboratory experiments or well designed field exposure experiments. A complicating factor may arise in the case that the progress of corrosion is not constant in time, but may change significantly after an initial period, e.g. due to erosion or formation of protective surface layers. Such erratic behavior has been documented by Guttman (1) for variability of zinc corrosion with season of first exposure. Therefore it is also desirable to examine the records of long term exposure studies to better reflect the actual fate exposed metals. Unfortunately, the environmental data associated with such exposures are generally insufficient to document the meteorological and chemical causes of the corrosion. The usefulness of existing long term exposure metals corrosion data thus depends upon reconstruction of the meteorological and chemical histories which are relevant to corrosion. To do so i n v o l v e s a n a l y s i s o f d a t a o n m e t e o r o l o g y and p o l l u t a n t e m i s s i o n s i n conjunction with data i n t e r p o l a t i o n t o o l s , i . e . p o l l u t a n t d i s p e r s i o n models. T h i s r e p o r t d i s c u s s e s t h e c u r r e n t s t a t u s o f s u c h an e f f o r t at Washington U n i v e r s i t y , and examines t h e e x i s t i n g exposure d a t a f o r e v i d e n c e o f key f e a t u r e s which may c l a r i f y t h e l i k e l y importance of manmade p o l l u t a n t s i n m e t a l s c o r r o s i o n . Metal

Corrosion

Data

The e x i s t i n g l o n g term e x p o s u r e m e t a l c o r r o s i o n d a t a has been r e p o r t e d i n a number o f papers i n j o u r n a l s and p r o c e e d i n g s o v e r t h e years. The c o n s i d e r a b l e t a s k o f l o c a t i n g , v e r i f y i n g , s t a n d a r d i z i n g and a s s e m b l i n g t h e numerous b i t s o f i n f o r m a t i o n i n t o a coherent whole was undertaken by E. E s c a l a n t e o f t h e N a t i o n a l Bureau o f S t a n d a r d s . Thus, t h e f i n a l d a t a s e t c o n t a i n s o n l y t h o s e e x p e r i m e n t s w h i c h were conducted a c c o r d i n g t o s t a n d a r d ASTM p r o c e d u r e s and had adequate d o c u m e n t a t i o n o f t h e s i t e c h a r a c t e r i s t i c s and p e r i o d o f e x p o s u r e . ^Current address: 3073 Andover, St. Louis, MO 63121 0097-6156/ 86/ 0318-0152$06.00/ 0 © 1986 American Chemical Society


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of Space and

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The c o r r o s i o n d a t a i n c l u d e s p e r i o d s from 1936 t o 1978, w i t h exposure d u r a t i o n s o f 1 t o 20 y e a r s . Most d a t a r e f e r t o t h e p e r i o d o f the 1960s, and r e f l e c t d u r a t i o n s o f 1 t o 7 y e a r s . The s i t e s a r e p r e d o m i n a n t l y i n the e a s t e r n U n i t e d S t a t e o r i n C a l i f o r n i a . S i x m e t a l s a r e i n c l u d e d i n the assembled r e c o r d s : carbon s t e e l , w e a t h e r i n g s t e e l , g a l v a n i z e d s t e e l , z i n c , aluminum and bronze. A t o t a l o f 63 s i t e s a r e found i n the d a t a base, w i t h many s i t e s r e p o r t i n g v a l u e s f o r numerous m e t a l s . S i t e s were c l a s s i f i e d as m a r i n e , i n d u s t r i a l , or u r b a n . The s i t e d i s t r i b u t i o n i s g i v e n below. Metal Aluminum Bronze Galvanized Steel Weathering S t e e l Zinc Carbon S t e e l

Rural

Industrial

5 5 9 2 8 10

7 3 5 2 17 22

Marine 6 2 1 10 16

The parameters f o r the c o r r o s i o n d a t a i n c l u d e s i t e l o c a t i o n and c l a s s i f i c a t i o n , d u r a t i o n and time p e r i o d o f e x p o s u r e , s o u r c e r e f e r e n c e , and a s e r i e s of c o r r o s i o n measures: c o r r o s i o n r a t e (mg/dm^/da, o r mdd), p e r c e n t a g e change i n s t r e n g t h and e l o n g a t i o n , p i t depth, y e a r s t o f i r s t r u s t , and o t h e r s which were r a r e l y used. The change i n s t r e n g t h and i n e l o n g a t i o n a r e more d i r e c t l y i m p o r t a n t as measures o f s t r u c t u r a l i n t e g r i t y t h a n i s the more commonly measured weight l o s s measure o f mdd. In the p r o c e s s o f e x a m i n i n g the metal c o r r o s i o n d a t a , c a r e was t a k e n t o a v e r a g e o n l y over narrow time r a n g e s . A g r a p h i c a l presen t a t i o n o f the averaged d a t a v a l u e i s c o n t a i n e d i n t h e f i n a l r e p o r t a s s o c i a t e d w i t h the metal c o r r o s i o n s t u d y ( 2 ) . The r e a d e r i s r e f e r r e d t o t h a t r e p o r t f o r the d e t a i l s o f the d a t a s e t . I n t h i s paper we p r e s e n t some key r e s u l t s . Paradigm The r e t r o s p e c t i v e s t u d y has been f o r m u l a t e d w i t h a c l e a r paradigm o f the parameters o f the c o r r o s i o n system. I t i s assumed t h a t the s i g n i f i c a n t c o n t r i b u t o r s t o the c o r r o s i o n o f m e t a l s exposed t o the atmosphere a r e water, s e a s a l t s , s u l f u r o x i d e s , n i t r o g e n o x i d e s , and the a c i d i t y o f p r e c i p i t a t e d w a t e r . I n the absence of w a t e r , i t i s presumed t h a t no s i g n i f i c a n t c o r r o s i o n a c t i v i t y w i l l t a k e p l a c e . S u r f a c e s may be m o i s t due t o p r e c i p i t a t i o n , dew, h y g r o s c o p i c a c t i o n , and o t h e r c a u s e s . Precipi t a t i o n i s presumed t o a c t i n t h r e e modes: by c l e a n s i n g the s u r f a c e o f accumulated d r y d e p o s i t e d m a t t e r , by m o i s t e n i n g t h e s u r f a c e , and (at low pH) by d i r e c t c h e m i c a l a t t a c k upon the m e t a l and c o r r o s i o n products. I n the presence o f m o i s t u r e , i t i s assumed t h a t the key c h e m i c a l r e a c t i o n s a r e i n i t i a t e d by S 0 and perhaps by Ν 0 gases. O t h e r mechanisms, s u c h as d r y d e p o s i t i o n o f a c i d i c a e r o s o l or d e p o s i t i o n o f HNO^ t o d r y s u r f a c e s , a r e n o t c o n s i d e r e d . Synergis t i c e f f e c t s are s i m i l a r l y disregarded. 2

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With t h i s o u t l o o k , the key f a c t o r s appear t o be the c o i n c i d e n t occurrence of S 0 and m o i s t u r e . The c o n t r o l l e d m e t a l exposures from the ASTM s t u d i e s were o f f l a t p a n e l s i n s t a n d a r d o r i e n t a t i o n . The s i m p l e geometry a v o i d s many c o m p l i c a t i o n s from m o i s t u r e r e t a i n e d i n s u r f a c e t e x t u r e f e a t u r e s . T h e r e f o r e , as a f i r s t approximation, we take the r e l e v a n t time o f wetness t o be s h o r t p e r i o d s f o l l o w i n g p r e c i p i t a t i o n and p e r i o d s o f dew f o r m a t i o n . The f o r m a t i o n o f a f i l m o f water i s most l i k e l y a t h i g h r e l a t i v e h u m i d i t i e s ; h i g h h u m i d i t i e s a r e most p r e v a l e n t d u r i n g t h e n i g h t h o u r s . I n the e a s t e r n U.S., summer predawn h u m i d i t y i s t y p i c a l l y 80-90$ compared t o a f t e r n o o n h u m i d i t i e s of 50-60$; w i n t e r p a t t e r n s a r e l e s s pronounced i n a r e a s n o r t h o f the Ohio r i v e r , but a l s o show a f t e r n o o n minima (3). The impact o f near s u r f a c e and e l e v a t e d e m i s s i o n s o u r c e s o f SOp on s u r f a c e c o n c e n t r a t i o n s has been shown t o d i f f e r v e r y s t r o n g l y E l e v a t e d e m i s s i o n s o u r c e s c o n t r i b u t e l i t t l e t o n i g h t time S 0 c o n c e n t r a t i o n s , w i t h the plume t y p i c a l l y t o u c h i n g down i n midmorning d u r i n g summer c o n d i t i o n s . Near s u r f a c e e m i s s i o n s , however, cause high concentrations of S 0 (and Ν 0 ) o v e r n i g h t . These s u r f a c e source emissions are trapped w i t h i n a shallow nocturnal mixing l a y e r w i t h r e l a t i v e l y l i t t l e d i s p e r s i o n , thus making S 0 a v a i l a b l e a t the same time t h a t m o i s t u r e i s most l i k e l y t o be p r e s e n t . The " c o r r o s i o n p o t e n t i a l " f o r nearby s u r f a c e s o u r c e s i s thus d i s p r o p o r t i o n a t e l y large f o r i t s emissions.


2

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2

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2

Retrospective

Reconstruction

of Environmental H i s t o r i e s

The r e c o n s t r u c t i o n o f the p o l l u t a n t c o n c e n t r a t i o n s t o which a m a t e r i a l i s l i k e l y t o have been exposed r e q u i r e s b o t h e m i s s i o n i n f o r m a t i o n and d i f f u s i o n models t o r e l a t e e m i s s i o n t r e n d s t o ambient c o n c e n t r a t i o n s downwind. U s i n g h i s t o r i c a l a r c h i v e s o f f u e l use, Husar (5) has d e v e l o p e d and a p p l i e d a methodology f o r e s t i m a t i n g the h i s t o r i c a l S 0 emis s i o n s on a s t a t e by s t a t e b a s i s back t o 1900. No s u c h c e r t r a l r e c o r d e x i s t s , u n f o r t u n a t e l y , f o r e m i s s i o n s on the urban s c a l e . The CAPITA Monte C a r l o r e g i o n a l model of p o l l u t a n t t r a n s m i s s i o n t h r o u g h the atmosphere was d e v e l o p e d as a d i a g n o s t i c t o o l t o s i m u l a t e the f o r m a t i o n and t r a n s p o r t o f s u l f a t e a e r o s o l . I t has s u b s e q u e n t l y been m o d i f i e d and a p p l i e d t o s i m u l a t i o n o f s u l f u r wet d e p o s i t i o n as w e l l as s u l f a t e c o n c e n t r a t i o n ( 6 , 7 ) . For the c u r r e n t p u r p o s e s , t h e model was s u b s t a n t i a l l y m o d i f i e d i n o r d e r t o a d e q u a t e l y model the d i u r n a l p a t t e r n s o f S 0 impact from near s u r f a c e and elevated sources. The s i m u l a t i o n i s a p p r o p r i a t e f o r t h e r e g i o n a l s c a l e , w i t h time s t e p o f 3 hours and s p a t i a l r e s o l u t i o n o f about 100 km i n a g r i d o f r o u g h l y 12000 km . T h i s s c a l e , however, i s i n a d e q u a t e f o r s i m u l a t i o n o f p r i m a r y p o l l u t a n t s s u c h as S 0 within t h e f i r s t 100-200 km o f t r a n s p o r t . The a v e r a g e o v e r the g r i d , even i f c o r r e c t , d o e s not r e v e a l the o r d e r s o f magnitude h i g h e r c o n c e n t r a t i o n s e x p e r i e n c e d i n s m a l l a r e a s n e a r e s t the s o u r c e s o f e m i s s i o n s . T h e r e f o r e a l o c a l model was d e v e l o p e d t o b e t t e r s i m u l a t e the n e a r f i e l d c o n c e n t r a t i o n and d e p o s i t i o n impacts of p r i m a r y e m i s s i o n s . T h i s s i m u l a t i o n u t i l i z e s much o f the same f o r m u l a t i o n i n terms o f m e t e o r o l o g y as the r e g i o n a l model ( w i t h improved i n i t i a l v e r t i c a l d i s p e r s i o n s i m u l a t i o n ) , and w i t h the i d e n t i c a l k i n e t i c s f o r t r a n s 2

2

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f o r m a t i o n o f S 0 t o SO^", d r y d e p o s i t i o n o f s u l f u r , and wet d e p o s i t i o n o f s u l f u r . T h i s model was used t o e s t i m a t e c o r r e c t i o n f a c t o r s t o be a p p l i e d t o t h e r e g i o n a l model r e s u l t s . The f u l l methodology o f r e t r o s p e c t i v e a n a l y s i s was a p p l i e d f o r the c a s e o f marble tombstone d e t e r i o r a t i o n i n an urban and a remote cemetery near New York C i t y (8,90. In t h i s e x e r c i s e i t was f o u n d t h a t our t e c h n i q u e may e s t i m a t e r u r a l S 0 c o n c e n t r a t i o n s acceptably w e l l , but i t does not r e p r o d u c e d e t a i l e d s t r u c t u r e o f urban concent r a t i o n s adequately. The main r e a s o n s a r e u n c e r t a i n t y about t h e e f f e c t i v e r e l e a s e h e i g h t s o f s o u r c e s and a l a c k o f d e t a i l e d l o c a l emission information. F o r t h e purposes o f metal c o r r o s i o n , t h e current s t a t e of the r e t r o s p e c t i v e r e c o n s t r u c t i o n of environmental h i s t o r i e s i s not s u f f i c i e n t l y q u a n t i t a t i v e t o warrant e x t r a c t i o n o f damage f u n c t i o n s . The d a t a s e t o f m e t a l c o r r o s i o n p r o v i d e s a v a l u a b l e i n s i g h t as t o which parameters dominate t h e c o r r o s i o n p r o c e s s . The g e n e r a l m e t e o r o l o g i c a l f e a t u r e s a r e w e l l e s t a b l i s h e d i n N o r t h America ( a l t h o u g h m i c r o m e t e o r o l o g i c a l f a c t o r s may dominate a t any s i n g l e exposure s i t e ) . The p o l l u t a n t r e l a t e d q u e s t i o n s may be s i m p l y s t a t e d : (1) what a r e t h e magnitudes o f marine and i n d u s t r i a l c o r r o s i o n r a t e s compared t o t h e r u r a l v a l u e s ; (2) what a r e t h e r e l a t i v e r a t e s i n t h e r u r a l e a s t e r n U n i t e d S t a t e s compared t o t h e w e s t e r n r u r a l v a l u e s ; (3) what a r e t h e broad t r e n d s i n c o r r o s i o n r a t e over the years? Roughly s p e a k i n g , t h e f i r s t q u e s t i o n a d d r e s s e s t h e importance o f S 0 , w h i l e t h e s e c o n d and t h i r d r e l a t e t o t h e r e g i o n a l s c a l e impacts o f S 0 , S 0 ^ , and a c i d i c d e p o s i t i o n . S 0 i n urban a r e a s i s a m a t t e r f o r l o c a l c o n t r o l e f f o r t s , whereas t h e r e g i o n a l impacts (which a r e due t o l o n g range t r a n s p o r t o f e m i t t e d m a t e r i a l ) r e q u i r e l a r g e r s c a l e c o n t r o l s and f a l l i n t o t h e c a t e g o r y o f " a c i d r a i n " a s s o c i a t e d damage. 2


2

2

=

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2

Observations T h i s s e c t i o n d i s c u s s e s a few o f t h e major p a t t e r n s e v i d e n t i n t h e metals c o r r o s i o n database. I n many r e s p e c t s , t h e r e s u l t s a r e r e s t a t e m e n t s o f the f i n d i n g s o f e a r l i e r r e s e a r c h by t h o s e a s s o c i a t e with the o r i g i n a l p r o j e c t s . F o r t h e purposes o f t h e r e t r o s p e c t i v e r e c o n s t r u c t i o n p r o j e c t , i t i s nevertheless u s e f u l t o determine whether t h e i n i t i a l assumptions appear r e a s o n a b l e and t o examine the r e l a t i v e importance o f the m e t e o r o l o g i c a l and c h e m i c a l f a c t o r s as r e v e a l e d i n t h e d a t a . T h i s w i l l t e l l where t o p l a c e a d d i t i o n a l e f f o r t f o r t h e b e s t improvement. More s p e c i f i c a l l y , i f t h e aim o f the study i s t o e x p l a i n t h e observed v a r i a t i o n s i n c o r r o s i o n r a t e , we f i r s t w i s h t o know what v a r i a t i o n s a r e s t r o n g enough t o w a r r a n t f u r t h e r study. F i r s t t h e d a t a were examined f o r v a r i a b i l i t y . The c o n v e n i e n t measure o f n o i s e t o s i g n a l ( s t a n d a r d d e v i a t i o n d i v i d e d by t h e mean) y i e l d e d v a l u e s which were q u i t e l a r g e f o r a l a r g e number o f c a r b o n s t e e l exposure t e s t s . Due t o t h i s v a r i a b i l i t y and t h e t y p i c a l l y s h o r t exposure t i m e s , c a r b o n s t e e l was n o t examined i n g r e a t e r detail. S i n g l e year e x p o s u r e s o f z i n c p l a t e s i n d i c a t e d t h a t t h e r e l a t i v e v a r i a b i l i t y was 30% f o r marine s i t e s , 20? f o r r u r a l s i t e s , and 10% f o r i n d u s t r i a l s i t e s . The m a r i n e s i t e s had t h e h i g h e s t


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RAIN

c o r r o s i o n r a t e s as w e l l as the h i g h e s t r e l a t i v e v a r i a b i l i t y , w h i l e t h e i n d u s t r i a l s i t e s i n d i c a t e d r a t h e r c o n s i s t e n t b e h a v i o r even o v e r t h i s s h o r t time p e r i o d . At the o t h e r extreme, bronze t r i p l i c a t e p a n e l s y i e l d e d r e l a t i v e v a r i a b i l i t y of below 10$ a f t e r 20 year e x p o s u r e s . The o v e r a l l r e s u l t of such c o n s i d e r a t i o n s was the d e c i s i o n t o average d a t a wherever m u l t i p l e v a l u e s were a v a i l a b l e with n e a r l y i d e n t i c a l exposure times. S i m i l a r l y , i t indicated that s m a l l d i f f e r e n c e s i n c o r r o s i o n r a t e s ( p a r t i c u l a r l y d u r i n g the f i r s t few y e a r s ) s h o u l d not be o v e r i n t e r p r e t e d . The near c o a s t a l e f f e c t of wetness and s e a s a l t i s e v i d e n t i n the d a t a , p a r t i c u l a r l y f o r 1 and 2 y e a r e x p o s u r e s . Zinc corrosion r a t e s a t Cape Kennedy, FL, dropped by a f a c t o r of M as d i s t a n c e from the c o a s t i n c r e a s e d from 60 t o 880 m. At Kure Beach,NC, Zn r a t e s dropped by a f a c t o r of 3 as d i s t a n c e i n c r e a s e d from 25m t o 250m from sea; c o r r e s p o n d i n g A1 c o r r o s i o n r a t e s f e l l by a f a c t o r of 2. C l e a r l y s i t e s w i t h i n a few hundred meters of t h e s e a must be s e g r e g a t e d from o t h e r s i t e s i n any f u r t h e r a n a l y s i s . The g e n e r a l m e t e o r o l o g i c a l environment i n the c o n t i n e n t a l U.S. i s w e l l known, r a n g i n g from the d e s e r t Southwest t o t h e r o u g h l y 1m a n n u a l r a i n f a l l o v e r much o f the c o u n t r y e a s t of the M i s s i s s i p p i river. The number o f days w i t h measurable p r e c i p i t a t i o n i s about 50$ h i g h e r i n t h e n o r t h e a s t e r n quadrant of t h e n a t i o n t h a n i n the G r e a t P l a i n s . The number o f days w i t h heavy f o g i s h i g h e r by a f a c t o r of 2 or more i n t h e mountainous p o r t i o n s o f t h e e a s t e r n U.S. and i n c o a s t a l a r e a s than i n the G r e a t P l a i n s or lower Ohio R i v e r v a l l e y (3). T h e r e f o r e some v a r i a t i o n i n c o r r o s i o n r a t e s w i t h i n t h e e a s t e r n U.S. may be e x p e c t e d j u s t from v a r i a t i o n i n time of w e t n e s s . Four s i t e s w i t h m u l t i p l e a l l o y s exposed over the y e a r s r e c u r i n most of t h e m e t a l c o r r o s i o n d a t a s e t s . These s i t e s i n c l u d e r u r a l marine exposures at P o i n t Reyes, CA and Kure Beach, NC; industrial e x p o s u r e s a t Newark, NJ; and an e a s t e r n U.S r u r a l i n l a n d l o c a t i o n a t S t a t e C o l l e g e , PA. Aluminum a l l o y s i n d i c a t e g e n e r a l l y low c o r r o s i o n r a t e s , except i n the C h i c a g o a r e a ; f o r s e v e r a l a l l o y s , c o r r o s i o n r a t e s t h e r e were n e a r l y an o r d e r o f magnitude h i g h e r t h a n a t o t h e r s i t e s . For AL 6061 ( T 6 ) , t h e i n d u s t r i a l environment caused s u b s t a n t i a l c o r r o s i o n and l o s s of s t r e n g t h . For t h r e e o t h e r a l l o y s (AL 201M ( T 4 ) , AL 202M (T81) and AL 7075 ( T 6 ) ) , the h i g h e r r a t e s were noted a t t h e marine s i t e s , with less i n d u s t r i a l e f f e c t . For a l l t h e s e a l l o y s , the r u r a l P e n n s y l v a n i a c o r r o s i o n r a t e s were lower t h a n the c o r r e s p o n d i n g r a t e s a t i n d u s t r i a l s i t e s , by a f a c t o r o f 2 t o 7 a f t e r 7 y e a r s e x p o s u r e . The r u r a l s i t e e x p o s u r e s i n d u c e d no s i g n i f i c a n t s t r e n g t h l o s s . T h e r e f o r e i t may be t e n t a t i v e l y c o n c l u d e d t h a t the r e g i o n a l s c a l e causes o f aluminum c o r r o s i o n a r e not s u f f i c i e n t l y pronounced t o warrant f u r t h e r r e s e a r c h a t t h i s t i m e . Weathering s t e e l data i n d i c a t e d r a t h e r constant c o r r o s i o n r a t e s a f t e r 8 year e x p o s u r e s (1967-1975) a t m a r i n e , urban and r u r a l s i t e s i n the e a s t e r n U.S. Although t h i s data i s l i m i t e d , i t s u g g e s t s t h a t the c o r r o s i o n h i s t o r y of t h i s w e a t h e r i n g s t e e l a l l o y must be r e l a t i v e l y i n s e n s i t i v e t o b o t h S 0 c o n c e n t r a t i o n and t o the pH o f r a i n f a l l . An a l t e r n a t i v e e x p l a n a t i o n i s t h a t p o l l u t a n t l e v e l s above some t h r e s h o l d v a l u e a r e s u f f i c i e n t t o induce the o b s e r v e d corrosion. Even s o , the d a t a do not o f f e r c l u e s as t o the p o l l u t a n t effect. 2


8.

P A T T E R S O N ET

AL.


157

G a l v a n i z e d s t e e l d a t a i s a v a i l a b l e from 20 year exposures (1936-1956) and from 2 year exposures (1971-1973). The twenty year exposures show t h a t c o r r o s i o n r a t e s i n the Great P l a i n s range from 0.2 t o 0.4 mdd, w h i l e r u r a l r a t e s i n the Ohio R i v e r v a l l e y and t h e n o r t h e a s t e r n U.S. range 0.5 t o 0.6 mdd. Such r u r a l d i f f e r e n c e s a r e l i k e l y a t t r i b u t a b l e t o v a r i a t i o n s i n time of wetness. I n d u s t r i a l exposures are a s s o c i a t e d w i t h r a t e s of 1-3 mdd. The more r e c e n t s h o r t term exposures a t the r u r a l S t a t e C o l l e g e s i t e i n d i c a t e lower c o r r o s i o n r a t e s than were r e g i s t e r e d d u r i n g the e a r l i e r 20 year e x p o s u r e s . A l t h o u g h i t i s d i f f i c u l t t o make such comparisons, the s u g g e s t i o n t h a t r u r a l e a s t e r n U.S. c o r r o s i o n r a t e s have i n c r e a s e d over the p a s t s e v e r a l decades i s not s u p p o r t e d by t h e s e measurements. Z i n c p r o v i d e s the most e x t e n s i v e s e t of measurements among the m e t a l s . Data are a v a i l a b l e from 7 year exposures e n d i n g i n 1956 and i n 1965, as w e l l as d u a l s e t s of 20 year exposures ending i n 1951 and 1978. The d a t a show s t r o n g dependence on presumed time of wetness. The two d i s t i n c t 20 year exposure p e r i o d s i n c l u d e use o f the same s i t e a t S t a t e C o l l e g e , PA. At t h i s r u r a l s i t e , t h e l o n g term c o r r o s i o n r a t e s over the 1931-1951 p e r i o d were i d e n t i c a l t o those of the 1958=1978 p e r i o d . Thus t h e i d e a of i n c r e a s i n g r u r a l c o r r o s i o n r a t e s over the p a s t 5 decades i s not s u p p o r t e d . Indust r i a l c o r r o s i o n r a t e s are s u b s t a n t i a l l y h i g h e r t h a n r u r a l r a t e s ; d u r i n g the e a r l i e r exposure s t u d y , n e i g h b o r i n g s i t e s i n d i c a t e d i n d u s t r i a l r a t e s 3 t o 6 t i m e s h i g h e r t h a n those r e c o r d e d a t r u r a l s i t e s (Figure 1). These d a t a do not o f f e r enough s p a t i a l coverage t o a s s e s s the d i f f e r e n c e s between e a s t e r n and n o n e a s t e r n r u r a l c o r r o s i o n r a t e s . The I n t e r s t a t e S u r v e i l l a n c e P r o j e c t (10) p r o v i d e d measures of z i n c c o r r o s i o n r a t e s a t a number o f s i t e s i n t h e Great P l a i n s s o u t h t o Texas as w e l l as i n the e a s t e r n U.S. T h i s d a t a r e p r e s e n t s the e f f e c t s a f t e r s i n g l e year e x p o s u r e s , so t h a t l a r g e v a r i a b i l i t y i s e x p e c t e d . A c l e a r o v e r a l l p a t t e r n i s e v i d e n t , however, w h i c h s u g g e s t s t h a t e a s t e r n U.S. r u r a l c o r r o s i o n r a t e s f o r z i n c are h i g h e r than the c o r r e s p o n d i n g r a t e s i n the Great P l a i n s by a f a c t o r of 2 t o 3· T h i s i s more t h a n can be accounted f o r by m e t e o r o l o g i c a l f a c t o r s a l o n e ; i t i s presumed, t h e r e f o r e , t h a t r u r a l z i n c c o r r o s i o n r a t e s i n the e a s t e r n U.S. are a f f e c t e d t o a s i g n i f i c a n t degree by r e g i o n a l p o l l u t i o n . The r e g i o n a l e f f e c t i s not l i k e l y t o be a new phenomena. Bronze d a t a i s s p a r s e . An a l l o y o f 92% Cu exposed from 19311951 showed no c o r r o s i o n i n the d e s e r t environment and very l i t t l e a t the r u r a l s i t e s . C o i n c i d e n t z i n c d a t a i n d i c a t e s t h a t over the 20 year p e r i o d the average c o r r o s i o n r a t e s were h i g h e r f o r bronze a t marine s i t e s but much lower f o r bronze than f o r z i n c a t r u r a l s i t e s (Figure 2). At the r u r a l S t a t e C o l l e g e , PA, s i t e the c o r r o s i o n r a t e f o r two d i f f e r e n t bronze a l l o y s d u r i n g the 1958-1978 p e r i o d was n e a r l y t w i c e t h a t of the e a r l i e r bronze e x p o s u r e , w h i l e the i n d u s t r i a l Newark a r e a c o r r o s i o n r a t e s appeared t o have d e c l i n e d . A bronze w i t h 99% Cu i n a l l o y w i t h 1.25% Sn and Ρ was exposed d u r i n g the 1958-1978 p e r i o d a t the f o u r s t a n d a r d ASTM s i t e s . The c o r r o s i o n r a t e s at the r u r a l s i t e and the w e s t e r n marine s i t e were l e s s than h a l f those a t the e a s t e r n marine and i n d u s t r i a l s i t e s .


158

M A T E R I A L S D E G R A D A T I O N C A U S E D BY A C I D R A I N

Zinc 7-year exposures 1956


ψ \

68


20-year

Zinc exposures 1951

Zinc

1-year exposures 1965


jïio I 33

/ J

y\ /

\

22 132


Figure

1.

Corrosion rates

20-year

2

Zinc exposures 1978

(mg/dm ,da) f o r z i n c .


8.

PATTERSON ET AL.

Corrosion

Rates as a Function

Bronze C o r r o s i o n 1.25% Sn, P: 1958-1978 20 y r

Bronze C o r r o s i o n 1.25% Sn, P: 1958-1978 % S t r e n g t h (7)

Figure

2.

Corrosion

rates

of Space and Time

2

(mg/dm ,da) o f bronze

(92% C u ) .

159

M A T E R I A L S D E G R A D A T I O N C A U S E D BY A C I D


160

RAIN

Chemical a n a l y s e s of the c o r r o s i o n product i n d i c a t e d t h a t no s u l f u r compounds were p r e s e n t a t e i t h e r of t h e marine s i t e s ; a p p a r e n t l y p o l l u t a n t s d i d not cause the marine bronze d e t e r i o r a t i o n . Over 90% of the c o r r o s i o n p r o d u c t was s t i l l a d h e r i n g a t t h e s e m a r i n e s i t e s a f t e r 20 y e a r s . At the r u r a l s i t e , 67% remained, w h i l e o n l y 27% of the c o r r o s i o n p r o d u c t remained at the i n d u s t r i a l s i t e . A bronze a l l o y more r e p r e s e n t a t i v e o f s t a t u a r y bronzes (5% Sn and P) was a l s o exposed a t t h e f o u r s t a n d a r d s i t e s . The r u r a l and Western marine s i t e s a g a i n show c o r r o s i o n r a t e s l e s s than h a l f t h o s e of the e a s t e r n marine and the i n d u s t r i a l s i t e . The c o r r o s i o n r a t e s at the i n d u s t r i a l and the marine s i t e i n c r e a s e w i t h e x p o s u r e time f o r t h i s a l l o y , i n marked c o n t r a s t t o t h e u s u a l p a t t e r n of d e c l i n i n g c o r r o s i o n r a t e with time. I t i s a l s o c u r i o u s t h a t the measured l o s s of s t r e n g t h a t t h e r u r a l PA s i t e i s d i s p r o p o r t i o n a t e l y l a r g e f o r i t s c o r r o s i o n r a t e (1.4% vs 2.0% a t the i n d u s t r i a l s i t e ) . The bronze a l l o y s were exposed at t h e same time and p l a c e as the z i n c d u r i n g t h e p e r i o d 1958-1978, w i t h measurements a t 2, 7 and 20 year e x p o s u r e s . E x a m i n a t i o n of the c o r r o s i o n r a t e s r e v e a l e d two s e p a r a t e p a t t e r n s : a m a r i n e t r e n d o f h i g h i n i t i a l c o r r o s i o n r a t e w i t h s h a r p r e d u c t i o n i n r a t e a f t e r t h e f i r s t few y e a r s , and an i n d u s t r i a l - r u r a l t r e n d c o r r e s p o n d t o d i f f e r e n t c h e m i c a l mechanisms at work, w i t h o n l y t h e i n d u s t r i a l - r u r a l c o r r o s i o n b e i n g r e l a t e d t o s u l f u r compounds. The c o r r o s i o n r a t e s over d i s t i n c t time p e r i o d s a r e g i v e n below. The t h r e e s e t s o f r a t e s c o r r e s p o n d t o t h e i n i t i a l two y e a r s o f e x p o s u r e , the 5 y e a r s c o v e r i n g the p e r i o d from 2 t o 7 y e a r s a f t e r e x p o s u r e , and t h e f i n a l 13 y e a r s f o r the p e r i o d of 7 t o 20 y e a r s a f t e r exposure. Site

Kure Beach, NC P o i n t Reyes, CA Newark, NJ S t a t e C o l l e g e , PA

Zinc

1% Bronze

5%

Bronze

2

5

13

2

5

13

2

5

13

.77 .26 .68 .18

.81 .05 1.11 .24

.30

.51 .26 .42 .20

.40 .04 .38 .14

.18 .09 .33 .15

.92 .55 .34 .1.4

.49

.30

—

.13 .51 .20

.13 .56 .23

.51 .14

A l t h o u g h r a t i o s of the a c c u m u l a t e d w e i g h t l o s s e s do not show a c o n s i s t e n t p a t t e r n , the d i s c r e t e time p e r i o d s do show a sudden agreement a f t e r t h e i n i t i a l 7 year exposure p e r i o d . D u r i n g the l a s t 13 y e a r s , the r a t i o o f bronze t o z i n c c o r r o s i o n r a t e s ( i n mdd) f a l l s i n t o t h e range 0.65 ± .05 f o r b o t h t h e marine and nonmarine s i t e s f o r the 1% a l l o y , w i t h marine v a l u e s of 1.00 and nonmarine v a l u e s o f 0.90 ± .03 f o r the 5% a l l o y . These r a t i o s c o r r e s p o n d t o f a c t o r s of the c o e f f i c i e n t s f o r C1 and S 0 , r e s p e c t i v e l y , i n damage f u n c t i o n s such as proposed by B e n a r i e (J_2). An a d d i t i o n a l bronze a l l o y c o n t a i n i n g 7% A1 was examined, and no s u c h r e l a t i o n s h i p was f o u n d . C l e a r l y the b e h a v i o r o f a bronze i s s t r o n g l y dependent upon the a l l o y ; i t appears t h a t the phosphor bronzes e x h i b i t c o r r o s i o n b e h a v i o r q u a n t i t a t i v e l y s i m i l a r t o t h a t of z i n c a f t e r the f i r s t few years of exposure. 2

8.

PATTERSON ET AL.


161


Summary A p r o j e c t was undertaken t o perform r e t r o s p e c t i v e r e c o n s t r u c t i o n of e n v i r o n m e n t a l h i s t o r i e s a t t h e s i t e s o f p r e v i o u s l o n g term a t m o s p h e r i c m e t a l e x p o s u r e s . The e f f o r t r e q u i r e d development o f a p p r o p r i a t e e m i s s i o n i n f o r m a t i o n and d i s p e r s i o n m o d e l i n g c a p a b i l i t i e s on both t h e r e g i o n a l and urban s p a t i a l s c a l e s . The development of u s e f u l urban s c a l e e m i s s i o n i n v e n t o r i e s d a t i n g back s e v e r a l decades proved t o be a l i m i t i n g f a c t o r . A t p r e s e n t , t h e r e f o r e , t h e r e t r o s p e c t i v e r e c o n s t r u c t i o n o f environmental h i s t o r i e s i s not p o s s i b l e f o r t h e l a r g e number o f s i t e s i n t h e m e t a l s c o r r o s i o n d a t a base. T h i s p r e c l u d e s d e r i v a t i o n o f damage f u n c t i o n s a t t h i s t i m e . E x a m i n a t i o n o f t h e m e t a l c o r r o s i o n d a t a base i t s e l f has i n d i c a t e d t h a t t h e r e p o r t e d c o r r o s i o n v a l u e s f o r z i n c and b r o n z e , at l e a s t , may be a s s o c i a t e d w i t h r e g i o n a l p o l l u t i o n as w e l l as l o c a l S 0 sources. I t a l s o was found t h a t z i n c and t h e two t i n phosphor bronzes behave s i m i l a r l y a f t e r an i n i t i a l p e r i o d o f exposure t o t h e atmosphere. 2

Literature Cited 1. Guttman, H. (1968) Effects of atmospheric factors on rolled zinc. Atmospheric Corrosion of Metals, ASTM STP 767, American Society for Testing and Materials, 286-308. 2. Patterson, D.E. and Husar, R.B. (1985) Final Report on Metals Damage. Submitted to National Park Service, Preservation Assistance Division, Washington, DC. 3. Baldwin, J.L.(1973) Climate of the United States. Environmental Data Service, NOAA, U.S. Dept. of Commerce, UDC 551.582(73). 4. Husar, R.B., Patterson, D.E., Husar, J.D., Gillani, N.V. and Wilson, W.E. (1978). Sulfur budget of a power plant plume. Atmos. Environ. 12, 549-568. 5. Husar, R.B. (1985). Manmade SO andNO Emission Patterns and Trends. In National Academy of Sciences Trend Report. 6. Husar, R.B., Patterson, D.E. and Wilson, W.E. (1985a) Monte Carlo simulation of regional air pollution: transport dynamics. Submitted to Journal of Climate and Applied Meteorology, June 1985. 7. Husar, R.B., Patterson, D.E. and Wilson, W.E. (1985b) Monte Carlo simulation of regional air pollution: semiempirical regional scale source-receptor relationship. In internal EPA review. 8. Husar, R.D., Patterson, D.E.(1985c) SO concentration estimates for New York City,1880-1980. Report to EPA Atmospheric Sciences Research Laboratory, Research Triangle Park, NC. 9. Husar, R.B., Patterson, D.E. (1985d). Marble tombstone deterioration in New York City area.Report to EPA Atmospheric Research Laboratory, Research Triangle Park, NC. 10. Cavendar, J., Cox, W., Georgevich, M., Huey, N., Yutze, G. and Zimmer C. (1971). Interstate Surveillance Project. US EPA, GPO 5503-0006, Washington, DC. 11. Costas, L.(1982) Atmospheric corrosion of copper alloys exposed for 15 to 20 years. Atmospheric Corrosion of Metals, ASTM STP 767, American Society for Testing and Materials, 106-115. x

x

2

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MATERIALS DEGRADATION CAUSED BY ACID RAIN

12. Benarie, M. (1984) Metallic corrosion functions of atmospheric pollutant concentrations and rain pH. Report under contract DE-ACO2-76CH0016 to Department of Energy.


RECEIVED

March 19, 1986

Materials Degradation Caused by Acid Rain - American Chemical

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