Materials Degradation Caused by Acid Rain - American Chemical

7 Environmental Effects on Metallic Corrosion Products Formed in Short-Term Atmospheric Exposures D. R. Flinn, S. D. Cramer, J. P. Carter, D. M . Hurwitz, and P. J. Linstrom Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 8, 2015 | http://pubs.acs.org Publication Date: September 25, 1986 | doi: 10.1021/bk-1986-0318.ch007

Bureau of Mines, U.S. Department of the Interior, Avondale, M D 20782-3393

The Bureau of Mines has measured short- and long-term atmospheric corrosion damage on five metals and two metal-coated steel products at four sites in the east and northeast United States as part of the National Acid Precipitation Assessment Program to evaluate the effects of acid deposition on materials. The composition of the corrosion product on carbon steel, weathering steel, copper, zinc, and galvanized steel is relatively unchanged in 1- and 3-month exposures over a wide variety of environmental conditions. Spalling and runoff losses are observed on all metals. Massive reorganization of the corrosion film by a mechanism of cyclic dissolution and precipitation was observed on carbon steel, Cor-Ten A, zinc, and galvanized steel. Loss of corrosion product from zinc in runoff was a function of both dissolution in rain water and neutralization by hydrogen ion loading, with dissolution contributing the greater portion of the loss. The c o r r o s i o n o f m e t a l l i c m a t e r i a l s i n the atmosphere has been s t u d i e d e x t e n s i v e l y (1). The m a j o r i t y o f the work i n t h i s a r e a has been t o determine the performance o f m a t e r i a l s and t o e v a l u a t e m i t i g a t i o n t e c h n i q u e s i n environments o f i n t e r e s t . With o n l y a few e x c e p t i o n s ( s e e f o r example r e f e r e n c e s 2, 3), attempts have not been made i n s t u d i e s c o n d u c t e d i n t h e U n i t e d S t a t e s t o f u l l y c h a r a c t e r i z e the environment and t o d e t e r m i n e the r e l a t i o n s h i p s between components o f the environment and t h e performance o f the m a t e r i a l o f i n t e r e s t ( s e e r e f e r e n c e 4 f o r a r e c e n t assessment o f t h i s a r e a ) . Adherent c o r r o s i o n p r o d u c t s a r e o f t e n c h a r a c t e r i z e d , but no attempts have been made, except i n l a b o r a t o r y s t u d i e s (5), to quantitatively r e l a t e the c o r r o s i o n f i l m chemistry t o environmental parameters. In 1981, a f i e l d s t u d y was i n i t i a t e d by the Bureau o f Mines t o d e t e r m i n e t h e e f f e c t s o f t h e environment, i n c l u d i n g a c i d d e p o s i t i o n , on the c o r r o s i o n o f a number o f commonly used m e t a l l i c m a t e r i a l s o f construction. T h i s s t u d y , which i s f u l l y d e s c r i b e d i n a r e c e n t paper (6_), i s b e i n g conducted a t f i e l d s i t e s where c o n t i n u o u s a i r

This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

In Materials Degradation Caused by Acid Rain; Baboian, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

120

MATERA ILS DEGRADATO IN CAUSED BY ACID RAN I

q u a l i t y , r a i n c h e m i s t r y , and m e t e o r o l o g y measurements are b e i n g made. Of e q u a l i m p o r t a n c e , the c o r r o s i o n p r o d u c t s are c h a r a c t e r i z e d by both wet c h e m i c a l and i n s t r u m e n t a l methods i n o r d e r t o p r o v i d e a d d i t i o n a l i n f o r m a t i o n r e g a r d i n g e n v i r o n m e n t a l e f f e c t s on the materials. I n t h i s paper, the r e s u l t s of the c o r r o s i o n p r o d u c t c h a r a c t e r i z a t i o n s f o r s h o r t - t e r m ( g e n e r a l l y 1 y e a r or l e s s ) exposures are described.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 8, 2015 | http://pubs.acs.org Publication Date: September 25, 1986 | doi: 10.1021/bk-1986-0318.ch007

Experimental The f i e l d study i s b e i n g conducted at f i v e s i t e s : Research T r i a n g l e Park, NC; Washington, DC; C h e s t e r , NJ; Newcomb, NY; and a r e c e n t l y added s i t e at S t e u b e n v i l i e , OH. These s i t e s encompass the range o f gaseous p o l l u t a n t l e v e l s and m e t e o r o l o g y v a r i a b l e s t y p i c a l l y o b s e r v e d i n most r u r a l , urban, and suburban l o c a t i o n s i n the U n i t e d States. Hydrogen i o n l o a d i n g i n the r a i n f a l l v a r i e s by a f a c t o r of 2 between the f o u r o r i g i n a l s i t e s , and annual average r a i n f a l l pH ranges from 4.18 t o 4.41. The m e t a l s exposed i n the program are 1010 c a r b o n s t e e l , C o r Ten A w e a t h e r i n g s t e e l , 3003-H14 aluminum, 110 c o p p e r , r o l l e d z i n c ( a l l o y 191), G-90 g a l v a n i z e d s t e e l , and Galvalume (a 55Al-45Zn c o a t i n g on s t e e l ) . W e i g h t - l o s s samples, 4 by 6 i n c h e s , a r e i n s t a l l e d ( i n t r i p l i c a t e ) and removed f o l l o w i n g p r e s e l e c t e d exposure p e r i o d s o f 1 month, 3 months, 1 y e a r , and 3 y e a r s ; the 3-raonth and 1-year exposures are i n s t a l l e d e v e r y 3 months, w i t h the w i n t e r season d e f i n e d as December through F e b r u a r y . The sample exposure r a c k s are i n c l i n e d at 30° t o the h o r i z o n and face s o u t h . A f t e r the exposed samples a r e weighed, the c o r r o s i o n p r o d u c t i s c h e m i c a l l y removed from the w e i g h t - l o s s samples, and t h e s e s t r i p p i n g s o l u t i o n s are a n a l y z e d by s t a n d a r d c h e m i c a l t e c h n i q u e s ( 7 ) . The c l e a n e d samples are weighed a g a i n t o determine m e t a l l o s s d u r i n g e x p o s u r e . Smaller, 1 by 1.75 i n c h , samples ( " m i c r o a n a l y s i s samples") are exposed c o n c u r r e n t l y w i t h the l a r g e r samples. The m i c r o a n a l y s i s samples a r e used i n s t u d i e s o f the c o r r o s i o n f i l m by a number o f i n s t r u m e n t a l t e c h n i q u e s . X-ray d i f f r a c t i o n (XRD) was performed on a P h i l l i p s X-ray d i f f r a c t i o n u n i t at 40 KV u s i n g CuKa r a d i a t i o n . Samples were examined on an I S I s c a n n i n g e l e c t r o n m i c r o s c o p e (SEM) u s i n g b a c k s c a t t e r e d e l e c t r o n s . Elemental depth p r o f i l e s f o r the c o r r o s i o n f i l m s were measured by a combinat i o n o f i o n s c a t t e r i n g s p e c t r o s c o p y (ISS) and i o n e t c h i n g w i t h % e at a r a t e o f 0.3 t o 0.5 nm per minute. C h e m i c a l i n f o r m a t i o n from the c o r r o s i o n f i l m s u r f a c e s was o b t a i n e d w i t h a S u r f a c e S c i e n c e L a b o r a t o r y SSX-100 X-ray p h o t o e l e c t r o n s p e c t r o m e t e r (XPS) u s i n g an A l K a monochromatic X - r a y s o u r c e . T h e r r a o g r a v i m e t r i c a n a l y s i s (TGA) was conducted u s i n g a Cahn RH vacuum e l e c t r o b a l a n c e system and a Cahn Mark I I time d e r i v a t i v e computer t o determine the amount of water i n the c o r r o s i o n f i l m s and the p r e s e n c e o f compounds t h a t may decompose at temperatures up t o about 600° C. TGA was done i n an argon atmosphere. F o r complete d e s c r i p t i o n o f the s i t e environments, m a t e r i a l s s t u d i e d , and e x p e r i m e n t a l p r o c e d u r e s , see r e f e r e n c e 6^. Complete 1

+


7. F L I N N E T A L .

Environmental

Effects on Metallic

Corrosion

121

Products

computer f i l e s o f the a i r q u a l i t y and meteorology d a t a a r e b e i n g p r e p a r e d so that i t w i l l soon be p o s s i b l e t o e v a l u a t e c o r r e l a t i o n s between t h e s e d a t a and t h e m e t a l l i c c o r r o s i o n r a t e s . With t h e e x c e p t i o n o f S t e u b e n v i l l e , OH, more than 2 y e a r s o f c o r r o s i o n r a t e d a t a (25 one-month exposure p e r i o d s , 11 three-month p e r i o d s , and 7 y e a r - l o n g e x p o s u r e s ) have been measured at each s i t e .


Results

and D i s c u s s i o n

T a b l e I shows the average c o r r o s i o n r a t e s observed f o r Cor-Ten A, z i n c , and copper i n 1-month, 3-month, and 1-year e x p o s u r e s . The c o r r o s i o n r a t e s f o r copper and z i n c were lowest at the Washington, DC, s i t e , w h i l e t h e Cor-Ten A c o r r o s i o n r a t e was lowest at the New York s i t e . The c o r r o s i o n r a t e s f o r g a l v a n i z e d s t e e l and f o r Galvalume were l e s s than 1 μπι/y i n exposures up t o 1 y e a r . The c o r r o s i o n r a t e f o r aluminum, as e x p e c t e d , was low ( l e s s than 0.1 Mm/y).

Table I.

Metal

Average c o r r o s i o n r a t e s Exposure Months

1

NC

at four

Corrosion DC 50.7 36.3 15.1

33.8 27.1 16.1

field

sites

R a t e . Mm/v NJ

NY

43.4 34.5 20.1

18.6 14.7 10.6

Cor-Ten A

1 3 12

191

Zinc

1 3 12

6.5 2.6 1.4

1.0 0.9 1.1

3.5 2.0 1.4

4.6 2.4 1.0

110

Copper

1 3 12

6.2 4.3 2.4

1.9 1.4 1.1

5.4 3.6 2.2

5.8 3.8 2.1

1

0ne-month exposures f o r p e r i o d May 1982 through May 1984; 3- and 12-month exposures begun i n p e r i o d March 1982-December 1983.

In a g e n e r a l way, the growth o f a c o r r o s i o n f i l m on a metal c a n be viewed s c h e m a t i c a l l y as a p r o c e s s w i t h t h r e e d i s t i n c t s t e p s . The f i r s t i s the r e a c t i o n o f metal A w i t h e n v i r o n m e n t a l s p e c i e s Β A + Β

AB

(1)

The second step i s the i n t e r a c t i o n o f t h i s c o r r o s i o n product w i t h e n v i r o n m e n t a l f a c t o r s such as the weather, gaseous p o l l u t a n t s , and wet and d r y d e p o s i t i o n t o produce, w h i l e c o n s e r v i n g m e t a l i n the f i l m , a m o d i f i e d o r weathered c o r r o s i o n product AB* AB + environment + AB*


(2)

122

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

The t h i r d i n v o l v e s the l o s s o f mass from the c o r r o s i o n through m e c h a n i c a l and c h e m i c a l p r o c e s s e s


AB*

+ environment

= AB** + R + L

S

product

L

(3)

where r e p r e s e n t s the r u n o f f l o s s e s produced by the d i s s o l u t i o n and removal o f c o r r o s i o n product i n p r e c i p i t a t i o n , and represents the s p a l l i n g l o s s e s caused by thermal and volume d i s t o r t i o n stresses. In a w e l l - d e v e l o p e d c o r r o s i o n f i l m , a l l t h r e e s t e p s may be o c c u r r i n g at the same time. I f m^ i s the mass of A l o s t due t o c o r r o s i o n , and R^ are the mass of A l o s t from m^ i n s p a l l i n g and r u n o f f , and m ^ Ç * * i s the mass of weathered c o r r o s i o n product r e t a i n e d on the metal i n E q u a t i o n 3 , then a mass b a l a n c e f o r the c o r r o s i o n f i l m y i e l d s :

m

m

R

S

AB** = ( A- L- J[f

X =1

t(î!j-)]

( 4 )

where i s the f r a c t i o n o f A i n the i - t h c o n s t i t u e n t o f n^yj**, compared t o the t o t a l mass o f A r e t a i n e d i n m^g**. M and M are the m o l e c u l a r weight o f A and the i - t h c o n s t i t u e n t , r e s p e c t i v e l y , i s the number of c o n s t i t u e n t s p r e s e n t i n AB**, and

Ν

Ν

Σ

1

Χ- = i=l

(5)

L

For some m a t e r i a l s , r u n o f f and s p a l l i n g l o s s e s are n e g l i g i b l e i n s h o r t - t e r m e x p o s u r e s , e.g., ^ L S ^ = 0 , so that m^ r e p l a c e s ( ^ ^ T ' ^ L ) i n E q u a t i o n 4. I f , f u r t h e r m o r e , the system o f o x i d e s , h y d r o x i d e s , e t c . , which comprise the c o r r o s i o n product are t h e r m o d y n a m i c a l l y s t a b l e , i . e . , r e l a t i v e l y i n s e n s i t i v e t o changes i n the environment so t h a t the c o m p o s i t i o n does not v a r y g r e a t l y , a p l o t o f m^** as a f u n c t i o n o f m^, u s i n g the g r a v i m e t r i c a l l y determined f i l m weight and weight-loss data, should y i e l d a s t r a i g h t l i n e with slope =

b

=

m

/ m

AB** A

=

i

X = 1

i^V

sjjch as t h a t shown i n F i g u r e 1 f o r Cor-Ten A. The c o n c e n t r a t i o n , X^, of the i - t h c o n s t i t u e n t i n the c o r r o s i o n product e x p r e s s e d as a f r a c t i o n o f n i ^ g ^ i s then

* - ^j™A

y

X

M

_ f iir il

() 7

Data p r e s e n t e d as i n F i g u r e 1 has the advantage of a c c e n t u a t i n g d i f f e r e n c e s due t o s p a l l i n g and r u n o f f l o s s e s . These d i f f e r e n c e s r e s u l t i n p o i n t s which l i e below the l i n e d e s c r i b i n g the s h o r t - t e r m data. In cases where r u n o f f or s p a l l i n g l o s s e s are s i g n i f i c a n t , (m^R^-S^) must be e s t i m a t e d from an a n a l y s i s o f the c o r r o s i o n p r o d u c t r e t a i n e d on the c o r r o d i n g m e t a l . T h i s was done by wet c h e m i c a l a n a l y s i s o f the s t r i p p i n g s o l u t i o n s that c o n t a i n the c o r r o s i o n p r o d uct removed from exposed samples. A g a i n , i f the c o r r o s i o n p r o d u c t i s t h e r m o d y n a m i c a l l y s t a b l e f o r a wide range of e n v i r o n m e n t a l


FLINN ET AL.

7.

Environmental Effects on Metallic Corrosion Products

123

2,800


2,400

Ο Ο • Δ

KEY North Carolina District of Columbia New Jersey N e w York

2,000

ε " O

\ O )

E_ I

S l o p e = 1.686 1,600

LU

5

1,200

Q ω Ο

DC Œ

Ο ϋ

800

400

0

400

800

1,600

1,200

WEIGHT L O S S . m g / d m

2,000

2

F i g u r e 1. Mass of c o r r o s i o n p r o d u c t r e t a i n e d on Cor-Ten A w e i g h t - l o s s p a n e l s i n exposures of 1, 3, and 12 months a t 4 sites. Exposures were i n i t i a t e d i n p e r i o d May 1982 - May 1983.


124

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 ACID RAIN

c o n d i t i o n s , E q u a t i o n 4 shows t h a t a p l o t o f m^ ** as f u n c t i o n o f ^ A~ L~ L^ l d y i e l d a s t r a i g h t l i n e with a slope B

m

R

S

s

n

o

u

m

b =

:

AB**

m

R

(8) S


l A- L" LJ With t h i s approach, s p a l l i n g and r u n o f f l o s s e s do not a f f e c t t h e computation o f the s l o p e . On the o t h e r hand, the p r e c i s i o n o f the wet c h e m i c a l measurements i s lower than f o r the g r a v i m e t r i c measure ments and s l o p e s computed by E q u a t i o n 8 are not as w e l l d e f i n e d as those from E q u a t i o n 6. I n the l i m i t when S^=R^=0, these two approaches t o computing t h e s l o p e s g i v e s i m i l a r r e s u l t s . E x p e r i m e n t a l e s t i m a t e s o f b a r e g i v e n i n T a b l e I I . Except f o r g a l v a n i z e d s t e e l , these v a l u e s were c a l c u l a t e d u s i n g g r a v i m e t r i c r e s u l t s and E q u a t i o n 6. F o r g a l v a n i z e d s t e e l , b was c a l c u l a t e d from wet c h e m i c a l r e s u l t s f o r the amount o f z i n c i n the c o r r o s i o n f i l m , i . e . , (n^-R^-S^) E q u a t i o n 8. The number o f d a t a p o i n t s , N, used i n the computation a r e a l s o g i v e n t o show t h a t the s l o p e s are based on a l a r g e d a t a s e t . E x p e r i m e n t a l v a l u e s o f the r e l a t i v e u n c e r t a i n t y i n b, i . e . , Ab/b, are g i v e n i n T a b l e I I , column 5, as 2tS/b, where S i s the s t a n d a r d e r r o r i n the e s t i m a t e o f b, and t i s the measure o f the d i s p e r s i o n about b, at a g i v e n c o n f i d e n c e l e v e l . One- and 3-month exposure g r a v i m e t r i c d a t a were used t o compute b f o r Cor-Ten A and carbon s t e e l s i n c e 1-year data showed e v i d e n c e o f s p a l l i n g l o s s e s ( c o n f i r m e d by SEM e x a m i n a t i o n ) and t o a l e s s e r extent r u n o f f l o s s e s . One- and 3-month g r a v i m e t r i c d a t a were a l s o used f o r copper. The z i n c and g a l v a n i z e d d a t a showed c o n s i d e r a b l e s c a t t e r due t o r u n o f f l o s s e s . No s p a l l i n g o f these m a t e r i a l s was observed u s i n g SEM e x a m i n a t i o n o f t h e i r s u r f a c e s . The 1- and 3month exposure z i n c g r a v i m e t r i c d a t a d e f i n e an upper l i m i t t o t h e amount o f c o r r o s i o n f i l m r e t a i n e d on the z i n c and were used t o com pute the s l o p e b. On the o t h e r hand, t h e g a l v a n i z e d s t e e l d a t a had much more s c a t t e r than the z i n c d a t a and d e f i n i n g the l i m i t i n g s l o p e was more d i f f i c u l t . As noted above, t h i s was overcome by u s i n g the wet c h e m i c a l e s t i m a t e f o r (m^-R^-S^j i n E q u a t i o n 8. a

Table I I .

(l/b)3b/3x

Cor-Ten A Steel

191 Z i n c Galvanized 110 Copper

d

Relative sensitivity

Metal

1010 Carbon

n

Steel

f a c t o r and e x p e r i m e n t a l b

Constituents

slopes

2tS/b

x

Ν

1

FeOOH or F e 0 ,

2°

1.686 0.005

420

1

FeOOH o r F e 0 , H 0

1.677 0.009

425

0.2

Zn(0H) ,

1.645 0.014

280

0.4

ZnO,

ZnC0

1.512 0.030

136

0.1

CuO,

Cu 0

1.115 0.007

429

2

2

2

2

ZnC0

3

3

3

3

H

2

*S e v a l u a t e d at 99 pet c o n f i d e n c e l e v e l f o r a l l metals except g a l v a n i z e d s t e e l ; g a l v a n i z e d s t e e l e v a l u a t e d at 90 pet confidence l e v e l .



7.

FLINNETAL.


125

The s e n s i t i v i t y o f t h e s l o p e b t o changes i n t h e c o m p o s i t i o n o f the c o r r o s i o n f i l m i s c h a r a c t e r i z e d by t h e r e l a t i v e s e n s i t i v i t y factor, [l/bj8b/3x. Values of t h i s f a c t o r are given i n Table I I f o r the case where the c o r r o s i o n f i l m c o n s i s t s o f the 2 major c o n s t i t u ents shown i n column 3. Cor-Ten A and 1010 carbon s t e e l had t h e h i g h e s t s e n s i t i v i t y f a c t o r ; a 1 pet change i n c o m p o s i t i o n would r e s u l t i n a 1 pet change i n s l o p e . Copper had t h e lowest s e n s i t i v i t y f a c t o r and a 1 pet change i n c o m p o s i t i o n would r e s u l t i n o n l y a 0.1 pet change i n s l o p e . Comparing v a l u e s o f the r e l a t i v e u n c e r t a i n t y i n b, column 5, w i t h the s e n s i t i v i t y f a c t o r s s u g g e s t s t h a t the c o m p o s i t i o n o f most o f the c o r r o s i o n f i l m s i n s h o r t exposures i s independent o f e n v i r o n m e n t a l i n f l u e n c e s . I n f a c t , u s i n g the 2c o n s t i t u e n t c o r r o s i o n f i l m model, t h e e x p e r i m e n t a l s l o p e s d e f i n e t h e amount o f each c o n s t i t u e n t i n the c o r r o s i o n product t o w i t h i n s e v e r a l weight p e r c e n t . Of c o u r s e , t h e c o r r o s i o n f i l m s on t h e s e m a t e r i a l s a r e more c o m p l i c a t e d than the 2 - c o n s t i t u e n t model. Never t h e l e s s , these r e s u l t s i n d i c a t e t h a t the c o m p o s i t i o n o f t h e c o r r o s i o n f i l m s i s q u i t e s t a b l e and t h a t , r e g a r d l e s s o f s i t e - t o - s i t e d i f f e r e n c e s i n t h e important e n v i r o n m e n t a l f a c t o r s , the r e l a t i v e amounts o f each c o n s t i t u e n t a r e f a i r l y c o n s t a n t . The major i n f l u ence o f the e n v i r o n m e n t a l f a c t o r s then i s not t o a l t e r the thermo dynamic s t a b i l i t y o f the c o r r o s i o n p r o d u c t s but t o i n f l u e n c e t h e k i n e t i c s of the rate l i m i t i n g c o r r o s i o n r e a c t i o n . C o n s i d e r now E q u a t i o n 3 d e s c r i b i n g l o s s e s o f c o r r o s i o n p r o d u c t . Assuming t h a t s p a l l i n g does not o c c u r , e v i d e n t l y t r u e f o r most mate r i a l s i n t h i s study except Cor-Ten A and carbon s t e e l , and that d r y d e p o s i t i o n e f f e c t s can be n e g l e c t e d , t h e p r i n c i p a l e f f e c t s l e a d i n g t o l o s s o f c o r r o s i o n product i n r u n o f f a r e : (1) d i s s o l u t i o n o f the c o r r o s i o n p r o d u c t i n r a i n w a t e r ; and (2) r e a c t i o n o f c o r r o s i o n p r o d uct w i t h hydrogen i o n i n the r a i n t o form a more s o l u b l e s p e c i e s . L e t t h e f l u x o f hydrogen i o n , i . e . , t h e hydrogen i o n l o a d , t o the skyward s i d e o f the weight l o s s p a n e l s be χ ( i n mg H /m ) and t h e f l u x o f p r e c i p i t a t i o n be y ( i n L H 0 / m ) . The l o s s o f m e t a l A from the c o r r o s i o n p r o d u c t AB* t o r u n o f f i s ( i n mg A/dm ). For sim p l i c i t y , assume t h a t the AB* i s a s i n g l e compound. L e t H react with t h i s product according to +

2

2

2

2

+

AB*

+ nH

+

·*· A

n +

+ H B*

(9)

n

The amount o f AB consumed i n r u n o f f due t o t h i s r e a c t i o n i s (xM^/nMy), where and a r e t h e m o l e c u l a r weights o f H and A, respectively. I n a d d i t i o n , t h e amount o f A d i s s o l v e d as AB* i n H^O i s f S g * y ( M ^ / M ^ * J , where S^g* i s the s o l u b i l i t y o f AB* i n water ( i n mg AB*/L), M^ * i s the m o l e c u l a r weight o f AB*, and f i s t h e f r a c t i o n of s a t u r a t i o n that i s achieved. T h i s f r a c t i o n i s l i k e l y t o be below 1.0 s i n c e the r e s i d e n c e time f o r p r e c i p i t a t i o n on the i n c l i n e d s u r f a c e o f the p a n e l s i s s h o r t . Combining terms, r a t i o n a l i z i n g u n i t s , and c o r r e c t i n g areas f o r the i n c l i n e o f the p a n e l leads t o : A

B

B

*L =

[fs * AB

y

ivfc;

+

) (

) f-1

C

r

3

+

15

FeOOH (0/Fe~2.2) 0 a/o C l / S

0-1

10-55 ZnO

1-15

0.5-5 a/o C l / S ZnO, w i t h 2-4 a/o excess Zn

>15

0>Zn, c o m p o s i t i o n towards c a r b o n a t e

0-1

3-13 a/o Cr .2-2 a/o A l 16-29 a/o Zn 55-80 a/o 0

>1

ZnO 9-13 a/o Cr 2-7 a/o A l

0-0.3

3-8 a/o C l / S 30-85 a/o Cu 10-62 a/o 0

0.3-2

>2 Data normalized

Descript ion

to t h i s value

CuO 1-3 Cu

~

1)

1

a/o

Cl/S

shifting or h y d r o x i d e

1

a/o

Cl/S

r i c h phase

(Cu/0~1.6)

f o r computational

purposes.



7.

F L I N N ET A L .


133

g i v e the smooth f l a t s u r f a c e c h a r a c t e r i s t i c o f the c o r r o s i o n f i l m formed i n l o n g e x p o s u r e s . F o r such a r e e f - l i k e f e a t u r e t o grow, the " v a l l e y " or d e p r e s s e d a r e a between mounds must be f i l l e d w i t h a c o n c e n t r a t e d s o l u t i o n o f d i s s o l v e d r u s t . T h i s o f c o u r s e c o u l d not have o c c u r r e d d u r i n g a r a i n because the s o l u t i o n would have washed away and the f e a t u r e s seen i n F i g u r e 2 o b l i t e r a t e d . I n s t e a d , i t i s proposed t h a t the t h i n l a y e r o f water which remains a f t e r a r a i n f a l l p r e c i p i t a t i o n event a b s o r b s a c i d i c gases from the a i r b e f o r e the s u r f a c e d r i e s and r e s u l t s i n an a c i d e l e c t r o l y t e t h a t d i s s o l v e s rust. Then, on d r y i n g the d i s s o l v e d m a t e r i a l p r e c i p i t a t e s i n the " v a l l e y s " and at the edge o f the " r e e f - l i k e " s t r u c t u r e s t o g r a d u a l l y f i l l i n the " v a l l e y " areas and l e v e l the c o r r o s i o n f i l m . The numerous s m a l l mounds i n the v a l l e y s appear t o be p r e c i p i t a t e n u c l e a t i o n sites. The s i t e s are fewer and l a r g e r at the r i m of the s o l u t i o n f i l l e d a r e a s , i n d i c a t i n g a lower r a t e of p r e c i p i t a t i o n when t h e s o l u t i o n volume i s l a r g e r . But, as e l e c t r o l y t e e v a p o r a t e s and the s o l u t i o n draws down i n t o the v a l l e y , the r a t e o f p r e c i p i t a t e n u c l e a t i o n i n c r e a s e s and numerous s m a l l mounds c o n s i s t e n t w i t h a c c e l e r a t e d d r y i n g are formed. C r a c k s from an e a r l i e r d r y i n g c y c l e would not s u r v i v e such e x t e n s i v e a l t e r a t i o n o f the s u r f a c e . This e v i d e n t l y accounts f o r the absence of h e a l e d or p a r t i a l l y h e a l e d c r a c k s i n the s u r f a c e . The c r a c k s a p p a r e n t l y form as the l a s t s t e p i n the d r y i n g c y c l e . New c r a c k s , of c o u r s e , can form a l o n g the path of e a r l i e r c r a c k s . The o v e r a l l l e v e l i n g t h a t o c c u r s i n l o n g e r exposures s u g g e s t s t h a t the c r e s t s of the mounds are the most a c t i v e s i t e s f o r d i s s o l u t i o n when wet. The water l a y e r i s t h i n n e s t h e r e , and exchange w i t h a c i d i c gases from the atmosphere t o produce a s o l u t i o n which w i l l d i s s o l v e the r u s t s h o u l d o c c u r most r a p i d l y at such s i t e s . I f the c r e s t s are s i t e s of most a c t i v e d i s s o l u t i o n , then the " v a l l e y s " are the s i t e s o f most a c t i v e p r e c i p i t a t i o n and one f o l l o w s the o t h e r as the volume o f water r e t a i n e d on the s u r f a c e d e c r e a s e s d u r i n g d r y i n g . H o r t o n (25) has o b s e r v e d t h a t dust p a r t i c l e s are found e x c l u s i v e l y i n the o u t e r l a y e r o f the c o r r o s i o n f i l m . This i s consistent with the m a s s i v e r e o r g a n i z a t i o n o f the f i l m proposed h e r e by a c y c l e o f d i s s o l u t i o n and p r e c i p i t a t i o n d u r i n g the l a t t e r stages of d r y i n g . The o u t e r l a y e r then c o r r e s p o n d s t o a s o l u t i o n a l t e r e d s t r u c t u r e m o d i f i e d i n t i m a t e l y by c o n t a c t w i t h the environment, w h i l e the i n n e r l a y e r i s m o d i f i e d b a s i c a l l y by d i f f u s i o n and c o n d u c t i o n processes and i s r e l a t i v e l y i s o l a t e d , except a l o n g c r a c k l i n e s , t o m a s s i v e i n t r u s i o n s o f water. In a few cases p r e c i p i t a t i o n of m a t e r i a l a l o n g c r a c k s was o b s e r v e d , F i g u r e 3. T h i s was more l i k e l y t o be seen on the g r o u n d ward s i d e than the skyward s i d e , more o f t e n f o r exposures at the N o r t h C a r o l i n a s i t e than New J e r s e y , and i n l o n g e r exposures than i n s h o r t e x p o s u r e s . The p r e c i p i t a t e d m a t e r i a l i s seen i n F i g u r e 3B t o be densest a l o n g t h a t p o r t i o n of the c r a c k l y i n g i n the d e p r e s s i o n s above and below the r i m i n the c e n t e r of t h i s f i g u r e . T h i s i s cons i s t e n t w i t h the o b s e r v a t i o n t h a t the h i g h e r areas o f the f i l m are s i t e s of d i s s o l u t i o n and the lower areas are s i t e s f o r p r e c i p t a t i o n . E v i d e n t l y , i n the case shown i n F i g u r e 3, d i s s o l v e d m a t e r i a l from w i t h i n the c r a c k has moved t o the s u r f a c e where i t p r e c i p i t a t e d as the c o r r o s i o n f i l m d r i e d . F u r t h e r d r y i n g then reformed the c r a c k .


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

134

RAIN


The t h r e e f e a t u r e s d e s c r i b e d h e r e suggest t h a t the o u t e r p a r t of the c o r r o s i o n f i l m s on c a r b o n s t e e l and Cor-Ten A undergo m a s s i v e r e o r g a n i z a t i o n by a mechanism of s o l u t i o n and p r e c i p i t a t i o n d u r i n g the l a t t e r s t a g e s o f d r y i n g . In many ways t h i s p r o c e s s i s s i m i l a r f o r both m a t e r i a l s and o n l y d i f f e r s i n s m a l l but s i g n i f i c a n t d e t a i l s r e l a t e d t o the p r e s e n c e of a l l o y i n g elements i n the w e a t h e r i n g steel. The a b i l i t y t o h e a l and s e l f - r e p a i r the c o r r o s i o n f i l m i s an e s s e n t i a l aspect of f o r m i n g a more p r o t e c t i v e c o r r o s i o n f i l m and would appear l i n k e d not o n l y t o the c h e m i s t r y of the a l l o y but a l s o t o the c h e m i s t r y o f the i n t e r a c t i n g environment. 191 Z i n c . In c o n t r a s t t o the s t e e l s , z i n c does not y i e l d a s i m p l e l i n e a r r e l a t i o n s h i p between the amount o f c o r r o s i o n f i l m r e t a i n e d on the s u r f a c e and the z i n c l o s s due t o c o r r o s i o n , F i g u r e 4. The d a t a fans out c o n s i d e r a b l y w i t h i n c r e a s i n g weight l o s s . A mass b a l a n c e on the 1-and 3-month d a t a which d e f i n e an upper l i m i t t o the amount of c o r r o s i o n f i l m r e t a i n e d on the s u r f a c e , u s i n g E q u a t i o n 6, y i e l d s a l i n e w i t h a s l o p e b = 1.645. The s m a l l c o n f i d e n c e i n t e r v a l f o r b, T a b l e I I I , and the l a r g e number of p o i n t s (N = 280) used i n computi n g b would suggest t h a t r u n o f f and s p a l l i n g of the z i n c c o r r o s i o n f i l m are not a s i g n i f i c a n t f a c t o r i n the s h o r t - t e r m exposures and t h a t b i s a consequence o n l y of E q u a t i o n s 1 and 2. V a l u e s of b computed f o r the i n d i v i d u a l s i t e s agree w e l l w i t h t h a t shown i n F i g u r e 4 except f o r the D i s t r i c t o f Columbia s i t e , where b = 1.41. Comparison of b w i t h the t h e o r e t i c a l v a l u e s f o r v a r i o u s compounds, T a b l e I I I , s u g g e s t s t h a t the c o r r o s i o n f i l m at the N o r t h C a r o l i n a , New J e r s e y , and New York s i t e s i s l a r g e l y a c o m b i n a t i o n o f Z n ( 0 H ) and Z n C 0 . A p p l y i n g E q u a t i o n 7 f o r a f i l m composed of two main c o n s t i t u e n t s , the d i f f e r e n c e between the e x p e r i m e n t a l s l o p e 1.645 and the t h e o r e t i c a l s l o p e s , 1.52 f o r Z n ( 0 H ) and 1.92 f o r Z n C 0 , i n d i c a t e s a f i l m c o n s i s t i n g o f about 30 wt pet Z n C 0 and 70 wt pet Zn(0H) i n short-terra exposures. The lower s l o p e f o r the D i s t r i c t of Columbia s i t e s t r o n g l y i n d i c a t e s the p r e s e n c e of ZnO. Slopes were a l s o computed from E q u a t i o n 8 u s i n g the r e s u l t s from the wet c h e m i c a l a n a l y s e s o f the s t r i p p i n g s o l u t i o n s . The r e s u l t s agree w i t h those from g r a v i m e t r i c d e t e r m i n a t i o n s and l e a d t o the same s e t of major c o n s t i t u e n t s f o r the c o r r o s i o n f i l m , T a b l e I I I . The computed s l o p e s were, however, somewhat h i g h e r , as would be e x p e c t e d i f t h e r e were some r u n o f f l o s s e s , and recoraputation o f the c o r r o s i o n f i l m c o m p o s i t i o n based on t h e s e s l o p e s gave v a l u e s n e a r e r t o 65 wt pet Z n C 0 and 35 wt pet Z n ( 0 H ) f o r s h o r t - t e r m exposures at the N o r t h C a r o l i n a , New J e r s e y , and New York s i t e s . 2

3

2

3

3

2

3

2

C o r r o s i o n f i l m c o n s t i t u e n t s i d e n t i f i e d by TGA i n an i n e r t atmosphere, T a b l e IV, agree w e l l w i t h those determined by the mass b a l a n c e on the c o r r o s i o n f i l m . C o r r o s i o n f i l m s on samples exposed at the D i s t r i c t o f Columbia and N o r t h C a r o l i n a s i t e s f o r 1 t o 12 months gave s i m i l a r r e s u l t s . Pure Z n C 0 was run as a s t a n d a r d . The l a r g e weight l o s s at 2 0 0 - 2 5 0 ° C c o i n c i d e d e x a c t l y w i t h the decompos i t i o n of Z n C 0 and r e s u l t s r e p o r t e d by Anderson (26). Mass b a l a n c e c a l c u l a t i o n s i n d i c a t e t h a t the weight l o s s c o r r e s p o n d s t o between 40 and 100 wt pet Z n C 0 . No weight l o s s of the Z n C 0 s t a n d a r d was o b s e r v e d below 200° C. T h e r e f o r e , a s m a l l l o s s of weight from the c o r r o s i o n f i l m t h a t o c c u r s i n the range 110°-200° C was i n t e r p r e t e d as Z n ( 0 H ) d e c o m p o s i t i o n . X-ray d i f f r a c t i o n s p e c t r a from the 3

e

3

3

3

2



7.

FLINN ET AL.


135

F i g u r e 3. Groundward s i d e of 1010 c a r b o n s t e e l microsample M 6 exposed 3 y e a r s a t NC s i t e showing d i s s o l u t i o n and p r e c i p i t a t i o n features. Magnifications: (a) 80X; (b) 240X.

τ

2

Γ

Slope= 1.645 Ο •

%

ο ο •

Ο Ο • Δ

KEY North Carolina District of Columbia New Jersey New York

60 80 WEIGHT LOSS, mg/dm

2

F i g u r e 4. Mass o f c o r r o s i o n p r o d u c t r e t a i n e d on 191 z i n c weightl o s s p a n e l s i n exposures o f 1, 3, and 12 months a t 4 s i t e s . Exposures were i n i t i a t e d i n p e r i o d May 1982 - May 1983.


136


RAIN

c o r r o s i o n f i l m on samples exposed f o r 3 y e a r s , T a b l e V, c o n t a i n e d s e v e r a l peaks which c o u l d not be matched w i t h known phases. X~ray d i f f r a c t i o n of the samples f o l l o w i n g TGA showed the c o r r o s i o n f i l m to be ZnO, the normal thermal d e c o m p o s i t i o n product of Z n ( 0 H ) and ZnC0 . 2

3

Ion s c a t t e r i n g s p e c t r o s c o p y o f the c o r r o s i o n f i l m formed i n 1raonth exposures at the New York s i t e gave an O/Zn atomic r a t i o con s i s t e n t w i t h ZnO i n the o u t e r 15 nm of the s u r f a c e and which trended towards Z n C 0 or Z n ( O H ) deeper i n t o the f i l m , T a b l e V I I . The v e r y o u t e r 1 nm was r i c h i n the s u r f a c e contaminants CI or S. Some Zn enrichment of the o u t e r 15 nm of the f i l m i s observed which c o u l d be a s s o c i a t e d w i t h the s u r f a c e contaminants or r e l a t e d t o d r y i n g effects.


3

2

A l t h o u g h many of the d a t a p o i n t s i n F i g u r e 4 l i e on the mass b a l a n c e l i n e d e f i n e d by the s h o r t - t e r m d a t a , a s i g n i f i c a n t number of 3- and 12-month p o i n t s l i e i n the fan-shaped area t o the r i g h t of the l i n e . G i v e n the c h e m i s t r y of long-terra z i n c c o r r o s i o n p r o d u c t s i n the atmosphere (27), t h i s s h i f t cannot be due t o the t r a n s f o r m a t i o n of s h o r t - t e r r a c o r r o s i o n p r o d u c t s by w e a t h e r i n g i n t o some lower m o l e c u l a r weight compound. Instead, i t represents a s u b s t a n t i a l l o s s of z i n c c o r r o s i o n product from the s u r f a c e i n r u n o f f or pos s i b l y by s p a l l i n g . SEM e x a m i n a t i o n of the c o r r o s i o n f i l m on z i n c samples exposed up t o 36 months showed no e v i d e n c e o f s p a l l i n g and, hence, i t i s assumed t h a t t h i s m a t e r i a l l o s s i s due e n t i r e l y t o runoff. The v e r t i c a l d i f f e r e n c e between the d a t a p o i n t s i n F i g u r e 4 and the mass b a l a n c e l i n e r o u g h l y approximates the amount of t h i s loss. However, a b e t t e r e s t i m a t e of the z i n c l o s s t o the e n v i r o n ment i n r u n o f f i s g i v e n by the d i f f e r e n c e i n the g r a v i m e t r i c z i n c weight l o s s and the z i n c r e t a i n e d i n the c o r r o s i o n f i l m as d e t e r mined from the wet c h e m i c a l a n a l y s e s , i . e . , (m^-R^-S^). T h i s quan t i t y i s p l o t t e d i n F i g u r e 5 f o r 1-raonth exposures and i n F i g u r e 6 f o r 3- and 12-month exposures as a f u n c t i o n of the t o t a l hydrogen i o n load d u r i n g the exposure p e r i o d . The 1-raonth d a t a i n F i g u r e 5 show that l i t t l e , i f any, of the c o r r o s i o n f i l m i s l o s t i n r u n o f f due t o hydrogen i o n l o a d or through d i s s o l u t i o n i n water. The z i n c l o s s e s e x h i b i t a r o u g h l y normal d i s t r i b u t i o n about z e r o , and the wide d i s t r i b u t i o n i n v a l u e s , p a r t i c u l a r l y the n e g a t i v e v a l u e s , are due t o low p r e c i s i o n i n the wet c h e m i c a l a n a l y s i s of the c o r r o s i o n f i l m chemistry. T h i s o b s e r v a t i o n , that e s s e n t i a l l y no r u n o f f l o s s e s occur i n s h o r t - t e r m e x p o s u r e s , i s c o n s i s t e n t w i t h the e a r l i e r i n t e r p r e t a t i o n of the mass b a l a n c e r e s u l t s f o r c o r r o s i o n f i l m s formed i n 1- and, i n some c a s e s , 3-raonth e x p o s u r e s . The z i n c l o s s e s f o r l o n g e r e x p o s u r e s , F i g u r e 6, show a d e c i d edly d i f f e r e n t r e s u l t . Here t h e r e i s an i n c r e a s i n g t r e n d of z i n c l o s s from the c o r r o s i o n f i l m as the hydrogen i o n l o a d i n c r e a s e s . A l e a s t squares f i t of the d a t a y i e l d s the f o l l o w i n g r e l a t i o n s h i p between the z i n c r u n o f f l o s s , R ( i n mg Zn/dm ), and the hydrogen i o n l o a d , χ (mg H / m ) : 2

L

+

2

R

L

= 0.746 χ

(11)

Assume, f o r s i m p l i c i t y , t h a t the c o r r o s i o n f i l m i s Z n C 0 and t h a t hydrogen i o n r e a c t s w i t h the c o r r o s i o n f i l m t o form b i c a r b o n a t e . E q u a t i o n 9 then becomes 3


F U N Ν ET AL.

Environmental

Effects on Metallic Corrosion

Products

137

Ε


Ε 20h-

Ο

ο

φ

0 < 0

ο

ο ο_

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0 0

ο

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ο°°ο

0

ο ο

ο

ο ο

2

4

6

ο

ο

ο

8

ο ο

10

12

14

16

2

HYDROGEN ΙΟΝ LOAD, mg/m

F i g u r e 5. I n f l u e n c e of hydrogen i o n l o a d i n g on t h e d i s s o l u t i o n of c o r r o s i o n p r o d u c t d u r i n g one-month exposures of 191 z i n c a t 4 sites.


138


Z11CO3 + H+ U s i n g the annual average pH tr-af liln- nf- a F all l1 Ιvolume ;Λ1 i t m a

and


R

L

rewritten

= 0.0043 [ f S

Z

n

C

_

3

+

P

H

)

estimate

X

ajj—i

y= then be

(12)

the hydrogen i o n l o a d to

( I O

E q u a t i o n 10 can

+ HCO3

2 +

> Zn

0

3

(13)

i n form o f E q u a t i o n 11

10

RAIN

-3+pH Μ(s^)

,

_ +

MJ Z

to

give

Y

^

(14)

The s o l u b i l i t y o f Z n C 0 i n n e u t r a l water i s g i v e n as 10 mg/L (28). (The o f t e n quoted v a l u e g i v e n by Ageno and V a l l a ( 2 9 ) , 206 mg/L"7 i s e v i d e n t l y i n e r r o r by a f a c t o r o f 10 s i n c e they a l s o r e p o r t t h e i r v a l u e as 1.64X10"** mole/L.) S u b s t i t u t i n g i n t o E q u a t i o n 14, u s i n g 4.2 as a r e a s o n a b l e annual average pH f o r the s i t e s ( 6 ) , and assuming t h a t the r u n o f f from the sample i s s a t u r a t e d w i t h ZnCO^, i . e . , f=l g i v e s 3

R

L

=

0.28DX

(0.356 +

- 0.637X

(15)

T h i s i s i n good agreement w i t h E q u a t i o n 11 and i n d i c a t e s that r u n o f f l o s s e s from z i n c are due t o a c o m b i n a t i o n of d i s s o l u t i o n of c o r r o s i o n product i n water, the p r i n c i p a l e f f e c t , and hydrogen i o n l o a d ing. E q u i l i b r i a i n v o l v i n g C 0 , H C 0 , c a r b o n a t e , and bicarbonate are a l s o i n v o l v e d i n the d i s s o l u t i o n r e a c t i o n and rough c a l c u l a t i o n s have i n d i c a t e d t h a t the r u n o f f w i l l be d e c i d e d l y more b a s i c than the incident rain. However, t h i s does not a l t e r the f a c t t h a t hydrogen i o n l o a d i s a s i g n i f i c a n t f a c t o r i n the d i s s o l u t i o n of the z i n c c o r r o s i o n product. Z i n c c o r r o s i o n r a t e s d e c r e a s e s u b s t a n t i a l l y w i t h exposure time, T a b l e I, i n d i c a t i n g t h a t the c o r r o s i o n f i l m i s becoming i n c r e a s i n g l y more p r o t e c t i v e and r e t a r d i n g f u r t h e r c o r r o s i o n . Moreover, c o r r o s i o n r a t e s at the f o u r s i t e s are c o n v e r g i n g t o s i m i l a r v a l u e s even though t h e r e was a 6 - f o l d d i f f e r e n c e i n t h e i r i n i t i a l r a t e s . Thus, i t would appear that a c e r t a i n mass of c o r r o s i o n product must form on the metal s u r f a c e t o p r o v i d e the degree o f p r o t e c t i o n observed i n longer exposures. I f r u n o f f l o s s e s are s i g n i f i c a n t , as s u g g e s t e d by F i g u r e 6 and E q u a t i o n 11, then z i n c must c o r r o d e at a r a t e s u f f i c i e n t t o r e p l a c e the c o r r o s i o n product l o s t by d i s s o l u t i o n i n long exposure times when r o u g h l y s t e a d y s t a t e c o n d i t i o n s e x i s t . I n t h i s way hydrogen i o n l o a d and a c i d d e p o s i t i o n have a very s p e c i f i c and d e f i n a b l e r o l e i n a c c e l e r a t i n g the c o r r o s i o n o f z i n c . 2

2

3

S c a n n i n g e l e c t r o n m i c r o s c o p i c e x a m i n a t i o n s of z i n c samples exposed 1, 12, and 36 months showed e x t e n s i v e r e o r g a n i z a t i o n of the c o r r o s i o n f i l m through r e p e a t e d c y c l e s of d i s s o l u t i o n and p r e c i p i t a t i o n not u n l i k e that which o c c u r s on c a r b o n s t e e l and Cor-Ten A. The c o r r o s i o n product on z i n c exposed 3 y e a r s at the New Jersey s i t e , where s h o r t - t e r m z i n c c o r r o s i o n r a t e s are low, i s u n i f o r m , somewhat n o d u l a r , and f i n e g r a i n e d on both the skyward and groundward s i t e s , F i g u r e 7. The same was t r u e f o r samples from the D i s t r i c t of Columbia s i t e . On the o t h e r hand, the c o r r o s i o n f i l m on




FLINN E T A L .

F i g u r e 7. M i c r o s a m p l e s of z i n c 191 exposed 3 y e a r s , (a) s k y ward s i d e and (b) groundward s i d e , sample C43, from NJ s i t e ; (c) skyward s i d e and (d) groundward s i d e , sample C31, from NC site. Magnification: 136X.



140


RAIN

z i n c exposed 3 y e a r s at the North C a r o l i n a s i t e , where s h o r t - t e r m c o r r o s i o n r a t e s were h i g h e s t , was v e r y d i f f e r e n t from those f o r New J e r s e y and skyward and groundward s i t e s were d i s s i m i l a r . M a s s i v e d i s s o l u t i o n and p r e c i p i t a t i o n f e a t u r e s are superimposed on the skyward s i d e over a u n i f o r m , n o d u l a r c o r r o s i o n product s i m i l a r t o that o b s e r v e d at New J e r s e y . These f e a t u r e s , the e n l a r g e d and filled o v a l shapes and the c u r v i n g s o l i d b a r r i e r c r o s s i n g dozens of the s m a l l e r n o d u l a r shapes, are e v i d e n c e f o r a p r o c e s s of d i s s o l u t i o n , c o n c e n t r a t i o n , r e d i s t r i b u t i o n , and p r e c i p i t a t i o n t h a t b u i l d s l a r g e r s t r u c t u r e s on the m e t a l s u r f a c e . Were t h i s t o happen d u r i n g r a i n f a l l the d i s s o l v e d m a t e r i a l would wash away. I n s t e a d , as f o r the s t e e l s , i t would appear t o happen when the s u r f a c e i s d r y i n g and i n v o l v e s a b s o r p t i o n of a c i d i c gases from the atmosphere. The groundward s i d e , F i g u r e 7D, shows broad open areas covered by o n l y a t h i n , porous c o r r o s i o n p r o d u c t and bound by l o o p i n g b a r r i e r s o f precipitated material. I t would appear t h a t c o r r o s i o n on the groundward s i d e of z i n c exposed at the North C a r o l i n a s i t e i s subs t a n t i a l l y l e s s than on the skyward s i d e . S i n g l e - s i d e d experiments are now i n p r o g r e s s t o c o n f i r m t h i s p o s s i b i l i t y ( 6 ) . F i g u r e 8 shows i n g r e a t e r d e t a i l the c o r r o s i o n product t h a t forms on the skyward s i d e of z i n c exposed at the N o r t h C a r o l i n a and New J e r s e y s i t e s f o r 1 and 3 y e a r s . The massive f e a t u r e s that d e v e l o p at N o r t h C a r o l i n a i n 3 y e a r s , F i g u r e 8B, are not yet apparent a f t e r 1 y e a r and the c o r r o s i o n p r o d u c t t h a t u n i f o r m l y c o v e r s the s u r f a c e , and on which i s superimposed the n o d u l a r shapes, appears porous, F i g u r e 8A. In c o n t r a s t , the u n i f o r m l a y e r c o v e r i n g the s u r f a c e at New J e r s e y a f t e r 1 y e a r , w h i l e c r a c k e d , i s dense and shows e v i d e n c e of d i s s o l u t i o n and r e d i s t r i b u t i o n o f m a t e r i a l t o g i v e a c o r r o s i o n f i l m of more u n i f o r m t h i c k n e s s , F i g u r e 8C. The c o r r o s i o n p r o d u c t formed i n 3 y e a r s e x h i b i t s s e v e r a l types of s t r u c t u r e s due t o s t i l l u n i d e n t i f i e d phases, F i g u r e 8D. The SEM p h o t o m i c r o g r a p h s i n F i g u r e s 7 and 8 show that r e o r g a n i z a t i o n o f the z i n c c o r r o s i o n product on w e a t h e r i n g i s a complex phenomena i n v o l v i n g competing p r o c e s s e s . F o r the h i g h l y s t r u c t u r e d f e a t u r e s t o d e v e l o p and be p r e s e r v e d over l o n g p e r i o d s , i t i s c l e a r t h a t , w h i l e d i s s o l u t i o n does o c c u r , the c o r r o s i o n p r o d u c t s are not r e a d i l y s o l u b l e and accumulate r a t h e r than wash from the s u r f a c e . G a l v a n i z e d S t e e l . The r e l a t i o n s h i p between c o r r o s i o n f i l m weight and z i n c weight l o s s f o r g a l v a n i z e d s t e e l was s i m i l a r t o t h a t shown i n F i g u r e 4 f o r z i n c . There were, however, fewer p o i n t s d e f i n i n g the l i n e f o r the upper l i m i t t o the amount of c o r r o s i o n p r o d u c t r e t a i n e d on the s u r f a c e . T h e r e was a l s o much more s c a t t e r i n the d a t a below t h i s l i n e i n d i c a t i n g that s u b s t a n t i a l c o r r o s i o n p r o d u c t was l o s t i n r u n o f f . The s l o p e f o r the l i m i t i n g l i n e was 1.178, which would i n d i c a t e t h a t ZnO was a major c o n s t i t u e n t o f the c o r r o sion film. However, the s l o p e was recomputed u s i n g the z i n c p r e s e n t i n the c o r r o s i o n f i l m as determined by wet c h e m i c a l a n a l y s i s o f the f i l m s t r i p p i n g s o l u t i o n , E q u a t i o n 8. T h i s produced a l i n e a r r e l a t i o n s h i p w i t h v e r y l i t t l e s c a t t e r between f i l m weight and z i n c i n the c o r r o s i o n f i l m . S l o p e s computed from t h i s d a t a f o r each o f the s i t e s , i n c l u d i n g the Washington, DC, s i t e , were i n good agreement and ranged from 1.45 t o 1.55. These v a l u e s are c o n s i s t e n t w i t h a c o r r o s i o n f i l m c o n t a i n i n g ZnCOg, ZnO, and Z n ( O H ) , as r e p o r t e d by 2




FLINN ET AL.

F i g u r e 8. Skyward s i d e of 191 z i n c m i c r o s a m p l e s . (a) 1-year exposure and (b) 3-year exposure a t NJ s i t e ; (c) 1-year and (d) 3-year exposure a t NC s i t e . Samples a r e (a) C229; (b) C31 (c) C231; (d) C43. Magnification: 720X.


142


RAIN

B i e s t e k (27) and B a r t o n ( 3 0 ) . Assuming a 2 - c o n s t i t u e n t corrosion f i l m of ZnC0 and ZnO, a sTope of 1.5 c o r r e s p o n d s r o u g h l y t o a c o r r o s i o n f i l m c o n s i s t i n g of 60 wt pet Z n C 0 and 40 wt pet ZnO. The s l o p e s and the c o n s t i t u e n t s a s s o c i a t e d w i t h those s l o p e s are d i f f e r e n t f o r z i n c and g a l v a n i z e d s t e e l . T h i s was unexpected s i n c e , t o the atmosphere, they s h o u l d appear the same. However, t h e r e was one s i g n i f i c a n t d i f f e r e n c e between the two m a t e r i a l s . The s u r f a c e o f the g a l v a n i z e d s t e e l r e c e i v e d a chromate treatment t o improve i t s r e s i s t a n c e to "white r u s t i n g " d u r i n g s t o r a g e (31, 32). X-ray p h o t o e l e c t r o n s p e c t r o s c o p y , and ISS a n a l y s i s of the c o r r o s i o n f i l m on g a l v a n i z e d s t e e l has shown that the s u r f a c e i s r i c h i n chromium from t h i s chromate t r e a t m e n t , T a b l e s VI and V I I . XPS a n a l y s i s shows both C r and C r p r e s e n t i n the c o r r o s i o n f i l m f o l l o w i n g a 1-raonth exposure at the New York s i t e ; C r was as much as 6 times more abundant i n the c o r r o s i o n f i l m than C r . Lead was a minor c o n s t i t u e n t o f the c o r r o s i o n f i l m , p r o b a b l y as PbO or Pb^O^. T h r e e peaks were found f o r oxygen; a r e l a t i v e l y s m a l l one f o r the chromium o x i d e s ; one f o r Z n C 0 ; and a t h i r d i d e n t i f i e d as Z n ( 0 H ) . ISS shows as much as 13 at pet Cr i n the o u t e r 1 nm of the c o r r o s i o n f i l m s and s m a l l amounts of aluminum. Beyond 1 nm i n t o the c o r r o s i o n f i l m , the oxygen c o n c e n t r a t i o n , a f t e r c o r r e c t i n g f o r the amount combined w i t h Cr i n C r 0 and A l i n A 1 0 , was s t o i c h i o r a e t r i c a l l y e q u i v a l e n t t o ZnO. The c o r r o s i o n f i l m on g a l v a n i z e d s t e e l may be s i m i l a r t o t h a t on z i n c where t h e r e appears to be a ZnO l a y e r on the o u t e r s u r f a c e , perhaps as a consequence of d r y i n g , and Z n ( 0 H ) i s a c o n s t i t u e n t deeper i n t o the c o r r o s i o n f i l m . These r e s u l t s c l e a r l y show that Cr i s present i n s u b s t a n t i a l amounts on the s u r f a c e o f the g a l v a n i z e d s t e e l c o r r o s i o n f i l m and e v i d e n t l y s e r v e s t o i n h i b i t the c o r r o s i o n of the z i n c c o a t i n g . SEM e x a m i n a t i o n o f g a l v a n i z e d s t e e l exposed 3 y e a r s at the N o r t h C a r o l i n a s i t e shows v e r y d i f f e r e n t c o r r o s i o n f i l m s on the groundward and skyward s i t e s , F i g u r e 9. The l a r g e s o l u t i o n - a l t e r e d f e a t u r e s on the skyward s i d e look not u n l i k e those f o r z i n c exposed at the same s i t e . However, the s m a l l n o d u l a r s t r u c t u r e observed on z i n c was not p r e s e n t i n the c o r r o s i o n f i l m on g a l v a n i z e d s t e e l and t h e r e were areas (one appears as a smooth white a r e a i n the lower r i g h t c o r n e r o f F i g u r e 9A) which are u n c o r r o d e d . These areas are presumed t o be due t o the c o n t i n u e d p a s s i v a t i o n of the s u r f a c e by the r e s i d u a l chromate c o a t i n g . The groundward s i d e , F i g u r e 9B, i s c h a r a c t e r i z e d by l a r g e areas of u n c o r r o d e d s u r f a c e . S m a l l areas of l o c a l i z e d breakdown are d i s t r i b u t e d over t h i s s u r f a c e i n c l u d i n g a r e a s where c o r r o s i o n has proceeded a l o n g s c r a t c h marks. It is p r o b a b l e t h a t these s c r a t c h e s o c c u r r e d a f t e r the chromate t r e a t m e n t and p e n e t r a t e the p r o t e c t i v e chromate l a y e r . By removing the c h r o mate i n t h e s e areas the s c r a t c h e s have made the z i n c c o a t i n g more s u s c e p t i b l e to c o r r o s i o n . 3

3


3 +

6 +

3 +

6 +

3

2

3

2

2

3

2

G i v e n the n o n u n i f o r m d i s t r i b u t i o n o f c o r r o s i o n on the s u r f a c e of the g a l v a n i z e d s t e e l i t i s p o s s i b l e t h a t the ISS and XPS r e s u l t s c o n c e r n i n g ZnO and Z n ( 0 H ) are not i n c o n f l i c t ; two d i s t i n c t l y d i f f e r e n t areas may have been examined. Perhaps a l a r g e l y u n c o r r o d e d s u r f a c e , as seen i n F i g u r e 9B, was examined by ISS. T h i s area would then be c h a r a c t e r i z e d as p a s s i v a t e d by Cr and c o n s i s t i n g of a t h i n ZnO l a y e r . On the o t h e r hand, c o r r o s i o n product from a more a c t i v e l y c o r r o d i n g s u r f a c e , as i n F i g u r e 9A, may have been examined by 2


7.

FLINN ET AL.


143

XPS. T h i s a r e a would be c h a r a c t e r i z e d as c o n t a i n i n g s u b s t a n t i a l water and Z n ( O H ) and Z n C 0 as w e l l as ZnO. T h r e e - y e a r c o r r o s i o n f i l m s on g a l v a n i z e d s t e e l from the New J e r s e y s i t e d i d not p o s s e s s s o l u t i o n - a l t e r e d f e a t u r e s on the skyward side. T h i s was t r u e a l s o f o r the c o r r e s p o n d i n g z i n c samples. There were, however, s u b s t a n t i a l areas of uncorroded s u r f a c e on both the skyward and groundward s i d e s . W h i l e s h o r t - t e r m c o r r o s i o n r a t e s are d i f f e r e n t between z i n c and g a l v a n i z e d s t e e l , i n long-term exposures t h e r e are c l o s e p a r a l l e l s i n the development of the weathered c o r r o s i o n f i l m on both m a t e r i a l s .


2

3

Galvalume. A p l o t of f i l m weight v e r s u s weight l o s s showed a s c a t t e r of p o i n t s w i t h a p o o r l y d e f i n e d upper l i m i t l i k e t h a t f o r g a l vanized s t e e l . C o r r o s i o n f i l m w e i g h t s were o f t e n l e s s than the weight of m a t e r i a l which had d i s s o l v e d t o form the c o r r o s i o n f i l m . T h i s i n d i c a t e s t h a t Galvalume, l i k e the o t h e r z i n c c o n t a i n i n g mater i a l s , l o s e s c o r r o s i o n product t o r u n o f f d u r i n g exposure. SEM photomicrographs of an unexposed Galvalume b l a n k and Galvalume exposed 3 years at the New J e r s e y s i t e are shown i n F i g u r e 10. Galvalume i s a 2-phase A l Z n c o a t i n g which c o n s i s t s of an A l r i c h d e n d r i t i c phase and a Z n - r i c h i n t e r d e n d r i t i c phase ( 3 3 ) . These s t r u c t u r e s are c l e a r l y e v i d e n t i n F i g u r e 10A f o r the b l a n k , w i t h most of the s u r f a c e c o v e r e d by the o x i d e on the A l - r i c h phase, and w i t h i s o l a t e d windows through t h i s d e n d r i t i c phase e x p o s i n g the Znr i c h phase. C o r r o s i o n appears t o be c o n f i n e d l a r g e l y t o those areas where the Z n - r i c h phase i s exposed, F i g u r e 10B. Average c o r r o s i o n r a t e s were l e s s than 1 ym/y i n 1-month exposures at the New Jersey s i t e from May 1982 through May 1984 (j6). A c t u a l c o r r o s i o n r a t e s would be s u b s t a n t i a l l y h i g h e r than t h i s i f based on the a r e a that i s c o r r o d i n g , i . e . , the i n t e r d e n d r i t i c phase. The v o i d s t h a t are p r o duced i n t h i s a r e a leave a s u r f a c e which, on a m i c r o s c o p i c s c a l e , i s q u i t e rough and can r e a d i l y t r a p f i n e dust p a r t i c l e s , a e r o s o l s , and c o r r o s i o n product. 110 Copper. P l o t s of f i l m weight v e r s u s metal l o s s f o r 1- and 3month exposures o f 110 copper g e n e r a l l y show a l i n e a r r e l a t i o n s h i p between f i l m weight and m e t a l l o s s . I f 1-year exposures are i n c l u d e d i n the p l o t they w i l l be s c a t t e r e d and f a l l below the l i n e e s t a b l i s h e d by the 1- and 3-month e x p o s u r e s . T h i s may be an i n d i c a t i o n of r u n o f f l o s s e s i n l o n g e r e x p o s u r e s . The l i n e s based on s h o r t - t e r m d a t a had s l o p e s of 1.12, 1.19, 1.10, and 1.08 f o r the N o r t h C a r o l i n a , New J e r s e y , New York, and D i s t r i c t of Columbia s i t e s , r e s p e c t i v e l y , when based on g r a v i m e t r i c d a t a , E q u a t i o n 6. The D i s t r i c t of Columbia d a t a p o i n t s e x h i b i t e d somewhat g r e a t e r s c a t t e r then the o t h e r s i t e s , which c o u l d r e p r e s e n t i n c r e a s e d r u n o f f l o s s f o r these samples. P l o t s of f i l m weight v e r s u s the copper determined by wet chemi s t r y from the s t r i p p i n g s o l u t i o n s , E q u a t i o n 8, are l i n e a r . The s l o p e s — 1 . 1 2 , 1.15, 1.14, and 1.20 f o r the North C a r o l i n a , New J e r s e y , New York, and D i s t r i c t o f Columbia s i t e s — g e n e r a l l y c o n f i r m the r e s u l t s from the g r a v i m e t r i c d a t a . No s i g n i f i c a n t s i t e - t o - s i t e d i f f e r e n c e s were apparent i n the s l o p e s . The D i s t r i c t of Columbia s i t e had a s l i g h t l y h i g h e r s l o p e u s i n g the wet c h e m i c a l analyses, but i t a l s o had the lowest s l o p e by g r a v i m e t r i c methods. The s l o p e s f e l l between the t h e o r e t i c a l v a l u e s f o r pure CuO and C u 0 , 2



144


RAIN

F i g u r e 9. Skyward (a) and groundward (b) s i d e s of g a l v a n i z e d s t e e l microsample G51 exposed 3 y e a r s a t NC s i t e . Magnification: 80X.

F i g u r e 10. Galvalume b l a n k (a) and skyward s i d e of microsample F38 (b) exposed 3 y e a r s a t NJ s i t e . Magnification: 720X.


7.

F L I N N ET A L .

145


T a b l e I I I , i n d i c a t i n g t h a t the f i l m i s composed p r i m a r i l y o f t h e s e two c o n s t i t u e n t s . These r e s u l t s are c o n s i s t e n t w i t h o b s e r v a t i o n s o f M a t t s o n ( 3 4 ) , who i n d i c a t e d t h a t C u 0 was the p r i m a r y f i l m c o n s t i t u e n t f o r short-term exposures. ISS a n a l y s i s o f copper f i l m s c o n f i r m e d t h a t the f i l m i s composed m o s t l y o f oxygen and c o p p e r . The atomic p e r c e n t a g e o f c h l o r i n e or s u l f u r i s r e l a t i v e l y s m a l l and d i m i n i s h e s r a p i d l y w i t h d i s t a n c e i n t o the f i l m . Thermal a n a l y s i s r e s u l t s f o r 1-month and 1y e a r e x p o s u r e s , T a b l e IV, r u l e out the p o s s i b i l i t y t h a t l a r g e amounts o f s u l f u r , c h l o r i n e , or c a r b o n a t e c o n t a i n i n g compounds a r e p r e s e n t i n the f i l m . A slow, c o n s t a n t r a t e o f weight l o s s was o b s e r v e d at temperatures between 110° and 350° C by TGA which was not t y p i c a l o f the t h e r m a l d e c o m p o s i t i o n r e p o r t e d f o r any i n o r g a n i c copper compounds ( 2 3 ) . I n measurements on pure CuO, no weight change was observed i n t h i s temperature range. I n t e r a c t i o n of carbonaceous m a t e r i a l d e p o s i t e d on the f i l m w i t h CuO t o produce C u 0 and C 0 c o u l d p o s s i b l y e x p l a i n the slow weight l o s s from samples d u r i n g TGA. X - r a y d i f f r a c t i o n r e s u l t s , T a b l e V, show the p r e s e n c e o f C u 0 i n samples both b e f o r e and a f t e r TGA. The SEM photomicrographs i n F i g u r e 11 show t h a t copper c o r r o d e s l o c a l l y and n o n u n i f o r m l y . F i g u r e 1 IB shows a sample, exposed f o r 1 y e a r at the N o r t h C a r o l i n a s i t e , which has had i t s c o r r o s i o n f i l m stripped. F l a t u n c o r r o d e d a r e a s , i d e n t i f i e d by the r o l l i n g marks from t h e o r i g i n a l copper s u r f a c e , are p r e s e n t . F i g u r e 11A shows a s i m i l a r sample w i t h the c o r r o s i o n f i l m i n t a c t . T h i c k mounds formed i n the c o r r o s i o n f i l m appear t o be a s s o c i a t e d w i t h the a r e a s o f l o c a l i z e d m e t a l a t t a c k seen i n F i g u r e 11B. XPS was used t o study f i l m s on copper samples from s e v e r a l s i t e s f o r d i f f e r e n t exposure p e r i o d s . As i n d i c a t e d i n T a b l e V I I I major changes i n the c o m p o s i t i o n o f the c o r r o s i o n f i l m s u r f a c e appear t o be o c c u r r i n g w i t h t i m e . D i f f e r e n t s i t e s gave s i m i l a r r e s u l t s f o r the same exposure p e r i o d , a l t h o u g h t h e r e was some v a r i a t i o n i n the C l S peaks among the s i t e s . This could i n d i c a t e that the n a t u r e o f the c a r b o n compounds b e i n g d e p o s i t e d on the f i l m v a r i e s from s i t e t o s i t e . The C u 2 P peak was i n t e r p r e t e d u s i n g d a t a from Schon (35) t o d i s t i n g u i s h CuO from Cu and C u 0 . I t was not p o s s i b l e t o s e p a r a t e the c o n t r i b u t i o n s o f Cu and C u 0 . The 1-month samples showed l a r g e amounts o f both CuO and Cu or C u 0 . At l o n g e r exposures the amount o f CuO i n c r e a s e d w h i l e the amount o f the l e s s o x i d i z e d s p e c i e s d e c r e a s e d . The groundward s i d e s of the samples tended t o have l e s s CuO and more C u 0 or Cu than the skyward. The r e s u l t s i n d i c a t e t h a t a l a y e r o f CuO i s produced i n the o u t e r 10 nm o f the f i l m s u r f a c e w i t h i n c r e a s i n g exposure time and t h a t t h i s p r o c e s s proceeds f a s t e r on the skyward s i d e than the groundward side.


2

2

2

2

1 / 2

3 / 2

2

2

2

2

3003-H14 Aluminum. The Al-Mn 3003 a l l o y c o r r o d e s at r a t e s l e s s t h a n 0.1 pm/y i n exposures o f 1, 3, and 12 months. Rates at the New J e r s e y s i t e a r e 2 t o 3 times those at the o t h e r s i t e s . SEM examinat i o n showed p i t t i n g i n exposures o f 12 and 36 months, F i g u r e 12. The p i t s were i n the e a r l y s t a g e s of development w i t h a s m a l l , deeper c e n t r a l a r e a surrounded by s h a l l o w l o c a l i z e d a t t a c k r a d i a t i n g outwards, p a r t i c u l a r l y a l o n g g r a i n b o u n d a r i e s . T h i s type o f a t t a c k i s s i m i l a r t o t h a t d e s c r i b e d by S o w i n s k i ( 3 6 ) . P i t s were more f u l l y d e v e l o p e d at the New J e r s e y s i t e , encompassing a l a r g e r a r e a o f



146

MATERIALS DEGRADATION C A U S E D BY ACID

RAIN

F i g u r e 11. Skyward s i d e of 110 copper exposed 1 y e a r at NC s i t e , (a) microsample D231 w i t h c o r r o s i o n f i l m i n t a c t ; (b) w i e g h t - l o s s p a n e l D l - 9 1 a f t e r removing c o r r o s i o n f i l m . Magnification: 240X.

F i g u r e 12. Skyward s i d e o f aluminum 3003-H14 w e i g h t - l o s s p a n e l B2-73 exposed 1 y e a r a t NJ s i t e a f t e r removing c o r r o s i o n f i l m . Magnification: 720X.



FLINN ETAL.

Table V I I I .

X-ray photoelectron spectroscopic analysis of c o r r o s i o n f i l m on m i c r o a n a l y s i s samples


Metal/Exposure

HO

C o p p e r / l month

110 Copper/1

year

XPS Peak

Cu2P

Observation*

3 / 2

cis

1 / 2

ois

1 / 2

Cu2P

3 / 2

Cu, C u 0 : major CuO: major L e s s Cu as CuO on groundward s i d e t h a n skyward s i d e . 2

C-H, C-0,

C-C: C=0:

major minor

Oxide [CuO, C u 0 ] : h y d r o x i d e , water: C=0, C - 0 : minor

major minor

2

Cu, C u 0 : major CuO: major CuO > C u , C u 0 2

2

cis

110 Copper/3

year

1 / 2

CulP

3 / 2

C=0, C-H,

C-O: C-C:

major minor

skyward s i d e - C u O : major groundward s i d e - C u O : major Cu, C u 0 : minor 2

cis

1 / 2

ois

1 / 2

C«=0, C - 0 : C-H, C-C:

major minor

C=0, C - O : major o x i d e [CuO, C u 0 ] : 2

Phases

i n brackets

minor

i n d i c a t e probable i d e n t i t i e s of

peaks.


148

MATERIALS

DEGRADATION

CAUSED

BY ACID

RAIN

l o c a l i z e d breakdown and i n v o l v i n g more i n t e n s e d i s s o l u t i o n of g r a i n s and g r a i n b o u n d a r i e s . P i t t i n g was more e v i d e n t on the skyward s i d e t h a n the groundward s i d e of the samples. A l s o p r e s e n t on the s k y ward s i d e was a f i n e l y s t r u c t u r e d n o n u n i f o r m g e n e r a l a t t a c k which t e x t u r e d the s u r f a c e i n exposures o f 3 y e a r s and o b s c u r e d o r i g i n a l s u r f a c e d e t a i l such as r o l l i n g marks and f i n e s c r a t c h e s . This a t t a c k may be a s s o c i a t e d w i t h s m a l l i n t e r m e t a l l i c F e A l g and FeMnAlg p r e c i p i t a t e s d i s p e r s e d i n the 3003 a l l o y . S i m i l a r t e x t u r i n g o f the groundward s i d e had not o c c u r r e d i n 3-year exposures and much o f the s u r f a c e d e t a i l was r e t a i n e d .


Conclusions F o r purposes of a n a l y s i s , growth of an a t m o s p h e r i c c o r r o s i o n f i l m was viewed s c h e m a t i c a l l y as a t h r e e s t e p p r o c e s s c o n s i s t i n g o f : (1) f o r m a t i o n of a c o r r o s i o n p r o d u c t ; (2) w e a t h e r i n g o f t h a t p r o d u c t w h i l e c o n s e r v i n g mass of the m e t a l ; and (3) c o r r o s i o n product l o s s e s through r u n o f f and s p a l l i n g . One or more o f these s t e p s may be o c c u r r i n g d u r i n g any stage i n f i l m growth depending upon the mater i a l and the environment. C o r r o s i o n f i l m s formed on m e t a l s and m e t a l - c o a t e d s t e e l p r o d u c t s exposed by the Bureau o f Mines at f i e l d s i t e s i n the e a s t e r n U.S. f o r times from 1 t o 36 months had: (1) c h e m i s t r i e s t h a t were independent of environment i n s h o r t - t e r m expos u r e s ; (2) m o r p h o l o g i e s t h a t were, i n some c a s e s , h i g h l y a l t e r e d by s o l u t i o n p r o c e s s e s ; and (3) r u n o f f l o s s e s f o r z i n c that were dependent upon h y d r o g e n i o n l o a d i n g . C o r r o s i o n F i l m C h e m i s t r y . A l i n e a r r e l a t i o n s h i p e x i s t s between the mass of c o r r o s i o n p r o d u c t formed on c a r b o n s t e e l , Cor-Ten A, z i n c , g a l v a n i z e d s t e e l , and copper and the mass o f m e t a l i n the c o r r o s i o n film. T h i s r e l a t i o n s h i p i s independent of s i t e and the wide v a r i a t i o n i n e n v i r o n m e n t a l parameters between the s i t e s i n s h o r t - t e r m exposures of 1 and 3 months. The r a t i o o f the two masses i s r e l a t i v e l y s e n s i t i v e t o the c o m p o s i t i o n of the c o r r o s i o n f i l m . The independence of t h i s r a t i o from s u b s t a n t i a l v a r i a t i o n s i n a i r q u a l i t y , m e t e o r o l o g y , and r a i n c h e m i s t r y i s i n t e r p r e t e d as i n d i c a t i n g , at l e a s t f o r the major c o n s t i t u e n t s , t h a t the c o m p o s i t i o n o f the c o r r o s i o n f i l m i s independent o f the environment i n s h o r t - t e r m exposures. The c o r r o s i o n f i l m on Cor-Ten A c o n t a i n s between 30 and 70 wt pet FeOOH, about 4-12 wt pet l o o s e l y - b o u n d water, about 1-5 wt pet water o f h y d r a t i o n , and the b a l a n c e h y d r a t e d F e 0 . No s i g n i f i c a n t c o n c e n t r a t i o n s o f FeO, F e 0 , F e 0 , FeSO^, and F e ( 0 H ) are p r e s e n t . The c o r r o s i o n f i l m on carbon s t e e l i s s i m i l a r t o t h a t on Cor-Ten A i n short exposures. The c o r r o s i o n f i l m on z i n c c o n t a i n s about 65 wt pet Z n C 0 and 35 wt pet Z n ( 0 H ) . The c o r r o s i o n f i l m from the Washington, DC, site may a l s o c o n t a i n ZnO. The c o r r o s i o n f i l m on g a l v a n i z e d s t e e l c o n s i s t s of Z n C 0 , Zn(OH)^, and ZnO. The p r e s e n c e of the ZnO i s p r o b a b l y due t o the s t a b i l i z a t i o n o f an i n i t i a l p a s s i v e ZnO f i l m by C r ^ which p e r s i s t s over p a r t s o f the s u r f a c e i n exposures up to 3 years. The c o r r o s i o n f i l m on copper c o n t a i n s m o s t l y C u 0 and some CuO. The c o r r o s i o n f i l m s from the Washington, DC, s i t e c o n t a i n a h i g h e r p e r c e n t a g e of CuO, i . e . , 40 wt pet C u 0 and 60 wt pet CuO. The 2

2

3

3

3

1 +

2

3

2

3

+

2

2


7. FLINN ET A L .


149


f i l m s do not c o n t a i n l a r g e amounts o f the copper s u l f i d e s , s u l f a t e s , hydroxides, carbonates, c h l o r i d e s , or combinations o f these compounds. C o r r o s i o n F i l m Weathering. S i g n i f i c a n t s p a l l i n g and r u n o f f l o s s e s o c c u r f o r a l l o f t h e metals i n exposures o f 1 y e a r . Z i n c and g a l v a n i z e d s t e e l e x h i b i t r u n o f f l o s s e s i n 3- and 12-raonth e x p o s u r e s . M a s s i v e r e o r g a n i z a t i o n o f t h e c o r r o s i o n f i l m s on carbon s t e e l , C o r Ten A, z i n c , and g a l v a n i z e d s t e e l o c c u r s which produces new morpholo g i e s i n t h e weathered f i l m . These morphologies a r e dependent on s i t e , exposure t i m e , and o r i e n t a t i o n (skyward and groundward). They d e v e l o p over extended p e r i o d s o f time, i n d i c a t i n g t h a t t h e s o l u t i o n and p r e c i p i t a t i o n p r o c e s s e s p r o d u c i n g them do not o c c u r p r i m a r i l y d u r i n g p r e c i p i t a t i o n e v e n t s , when washing would t o t a l l y remove d i s s o l v e d c o r r o s i o n p r o d u c t s , but d u r i n g the d r y i n g phase o f p r e c i p i t a t i o n events and when m o i s t u r e c o l l e c t s on t h e s u r f a c e w i t h l i t t l e or no r u n o f f , as w i t h dew. The s o l u t i o n p r e s e n t on t h e s u r f a c e d u r i n g the d r y i n g phase i s a p o w e r f u l s o l v e n t , a l t e r i n g f e a t u r e s o f the c o r r o s i o n f i l m which appear u n a f f e c t e d by the c o n t i n u o u s washing o f rain. A b s o r p t i o n o f a c i d i c gases from the atmosphere i s p r o b a b l y important t o the f o r m a t i o n o f t h i s s o l v e n t . With c o n t i n u i n g e v a p o r a t i o n o f the s o l u t i o n , d i s s o l u t i o n o f c o r r o s i o n p r o d u c t d i m i n i s h e s and p r e c i p i t a t i o n b e g i n s . D i f f e r e n c e s e x i s t i n the d e t a i l s o f t h i s p r o c e s s f o r t h e i n d i v i d u a l m e t a l s , e.g., Cor-Ten A and z i n c . Corros i o n r a t e s f o r a l l o f the m e t a l s except Galvalurae and aluminum d e c r e a s e w i t h i n c r e a s i n g time, i n d i c a t i n g t h e f o r m a t i o n o f a more p r o t e c t i v e c o r r o s i o n f i l m i n the l o n g e r e x p o s u r e s . C o n d i t i o n s which a f f e c t t h e w e a t h e r i n g p r o c e s s e s d e s c r i b e d here w i l l have a marked e f f e c t on the c o r r o s i o n o f the metals themselves, as t h e i r a b i l i t y t o a c h i e v e a s t a b l e , low c o r r o s i o n r a t e i n long-term exposures depends e n t i r e l y on the development o f the c o r r o s i o n f i l m . R u n o f f L o s s e s Due t o Hydrogen I o n L o a d i n g . Runoff l o s s e s from z i n c , and presumably g a l v a n i z e d s t e e l , a r e dependent upon hydrogen i o n l o a d i n 3- and 12-month e x p o s u r e . Such an e f f e c t i s not apparent i n 1-month e x p o s u r e s . Hydrogen i o n d i s s o l v e s z i n c c a r b o n a t e , perhaps the major c o n s t i t u e n t o f the c o r r o s i o n f i l m , by the r e a c t i o n ZnC0

3

+ H

+

Zn

2 +

+ HC0 " 3

A d d i t i o n a l c o r r o s i o n f i l m i s l o s t i n the r u n o f f due t o the l i m i t e d s o l u b i l i t y o f t h e c o r r o s i o n product i n r a i n . The r e l a t i v e c o n t r i b u t i o n o f t h e s e e f f e c t s t o the r u n o f f i n 3- and 12-raonth exposures was 55 pet d i s s o l u t i o n and 45 pet hydrogen i o n l o a d i n g . To m a i n t a i n t h e s t a b l e c o r r o s i o n f i l m t h a t d e v e l o p s on z i n c i n l o n g - t e r m e x p o s u r e s , i t i s e v i d e n t t h a t z i n c must c o r r o d e at a r a t e s u f f i c i e n t t o r e p l a c e the c o r r o s i o n product l o s t i n r u n o f f . Acknowledgment T h i s r e s e a r c h has been funded by the Bureau o f Mines and t h e N a t i o n a l A c i d P r e c i p i t a t i o n Assessment Program through a c o s t s h a r i n g I n t e r a g e n c y Agreement between t h e Bureau and t h e E n v i r o n m e n t a l P r o t e c t i o n Agency. Use o f t r a d e names or company names does not imply endorsement by t h e Bureau o f Mines.


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Literature Cited 1. Flinn, D. R.; Cramer, S. D.; Carter, J. P.; Lee, P. K; Sherwood, S. I. "Acidic Deposition and the Corrosion and Deterioration of Materials in the Atmosphere: A Bibliography. 1880-1982"; PB83-126091; National Technical Information Service, July 1983. 2. Haynie, F. H.; Upham, J. B. In "Corrosion in Natural Environments"; STP 558; American Society for Testing and Materials: Philadelphia, 1974; 33-51. 3. Mansfeld, F. B. "Effects of Airborne Sulfur Pollutants on Materials"; PB81-126351; National Information Service, January 1980. 4. Lipfert, F. W.; Benarie, M.; Daura, M. L. Derivation of Metallic Corrosion Damage Functions For Use in Environmental Assessments. Draft Report, available from senior author, Dept. of Energy and Environment, Brookhaven National Laboratory, Upton, NY 11973. 5. Franey, J. P.; Graedel, T. E.; Kammlott, G. W. In "Atmospheric Corrosion"; Ailor, W. H., Ed.; John Wiley and Sons: New York, 1982; 383-392. 6. Flinn, D. R.; Cramer, S. D.; Carter, J. P.; Spence, J. W. Durability of Building Materials, 1985, 3(2), 147-175. 7. Bureau of Mines. Quality Assurance Project Plan. Interagency Agreement AD-14-F-1-452-0 between Bureau of Mines and Environmental Protection Agency, October 1983. (Available from D. R. Flinn, Bureau of Mines, Avondale Research Center, Avondale, MD 20782-3393.) 8. Reddy, M. M.; Sherwood, S. I. Limestone and Marble Dissolution by Acid Rain. In this book. 9. Youngdahl, C. Α.; Doe, B. R.; Sherwood, S. I. Roughening Recession and Chemical Alteration of Marble and Limestone Sample Surfaces After Atmospheric Exposure in the Northeastern United States. In this book. 10. Cohen, M.; Hashimoto, K. J. Electrochem. Soc., 1974, 121(1), 42-45. 11. Inouye, K.; Ichimura, K.; Kaneko, K.; Ishikawa, T. Corrosion Sci., 1976, 16, 507-517. 12. Kameko, K.; Inouye, K. Bull. Chem. Soc. of Japan, 1976, 49(12), 3689-3690. 13. Suzuki, I.; Masuko, N.; Hisamatsu, Y. Corrosion Sci., 1979, 19, 521-535. 14. Cramer, S. D.; Carter, J. P.; Covino, B. S., Jr. "Atmospheric Corrosion Resistance of Steels Prepared from the Magnetic Fraction of Urban Refuse"; U.S. Bureau of Mines, RI 8477, 1980. 15. Suzuki, I.; Hisamatsu, Y.; Masuko, N. J. Electrochem. Soc., 1980, 127(10), 2210-2215. 16. Matijevic, E. Pure and Applied Chemistry, 1980, 52(5), 11791193. 17. Spedding, D. J.; Sprott, A. J. Proc. 8th Intern. Congr. Met. Corr.; Dechema: Frankfurt, 1981; Vol. 1, 329-335. 18. de Meybaum, B. R.; Ayllon, E. S.; Bonard, R. T.; Granesse, S. L.; Ikeha, J. L. Proc. 8th Intern. Congr. Met. Corr.; Dechema: Frankfurt, 1981; Vol. 1, 317-322.


7. FLINN ET AL. 19. 20. 21. 22.

23.


24. 25. 26. 27. 28. 29. 30. 31. 32. 33.

34. 35. 36.


Keiser, J . T . ; Brown, C. W.; Heidersbach, R. H. Corrosion, 1982, 38(7), 357-360. Leidheiser, H . , Jr.; Czako-Nagy, I. Corrosion S c i . , 1984, 24(7) 569-577. Cleary, H. J. Corrosion, 1984, 40(11), 606-608. Albrecht, P.; Naeemi, A. H. "Performance of Weathering Steel in Bridges"; National Cooperative Highway Research Program Report 272, Transportation Research Board, National Research Council: Washington, DC, 1984. Liptay, G. "Atlas of Thermoanalytical Curves"; Heyden and Sons: New York, 1971; Vol. 2, Section 89. Evans, U. R. "The Corrosion and Oxidation of Metals"; Ε. Arnold: London, 1960; p. 25. Horton, J . B. Proc. San Francisco Regional Technical Meeting; American Iron and Steel Institute: Washington, DC, November 18, 1965; 171-195. Anderson, Ε. Α.; Fuller, M. L. Metals and Alloys, 1939; Vol. 10, pp. 292-287. Biestek, T. In "Atmospheric Corrosion"; Ailor, W. H . , Ed.; John Wiley and Sons: New York, 1982; 631-643. Handbook of Chemistry and Physics; 60th Ed.; CRC Press: Boca Raton, FL, 1979; B-142. Ageno, F . ; Valla, E. Hydrolysis. Atti Accad. Lincei, 1911; Vol. 20, Part II, 706-712. Barton, K. "Protection Against Atmospheric Corrosion"; Wiley and Sons: New York, 1973; 49. Williams, L. F. G. Plating, 1971, 59(10), 931-938. Duncan, J . R. Surface Technology, 1982, 16, 163-173. Zoccola, J . C . ; Townsend, H. E.; Borzillo, A. R.; Horton, J . B. In "Atmospheric Factors Affecting the Corrosion of Engineering Materials"; STP 646; Coburn, S. Κ., Ed.; American Society for Testing Materials: Philadelphia, 1978; 165-184. Mattson, E.; Holm, R. In "Atmospheric Corrosion"; Ailor, W. H., Ed.; John Wiley and Sons: New York, 1982; 365-381. Schon, G. Surface S c i . , 1973, 35, 96-108. Sowinski, G.; Sprowls, D. O. In "Atmospheric Corrosion"; Ailor, W. H . , Ed.; John Wiley and Sons: New York, 1982; 297328.

RECEIVED January 2, 1986


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