Materials Degradation Caused by Acid Rain - American Chemical

9 Environmental Factors Affecting Corrosion of Weathering Steel Fred H . Haynie Atmospheric Sciences Research Laboratory, U.S. Environmental Protection Agency,

Downloaded by AUBURN UNIV on September 29, 2017 | http://pubs.acs.org Publication Date: September 25, 1986 | doi: 10.1021/bk-1986-0318.ch009

Research Triangle Park, NC 27711

Weathering steel samples were exposed for periods of up to 30 months at nine air monitoring sites in the St. Louis, Missouri area. Climatic and air quality data were recorded during the exposure period and subjected to a rigorous evaluation to eliminate recording errors and to estimate missing values. Weight loss was used as the measure of steel corrosion. Corrosion rate was evaluated with respect to, 1) flux of pollutants (sulfur oxides, nitrogen oxides, oxidants, and particles) to the steel during both wet and dry periods, 2) temperature, and 3) exposure history. Different definitions of when the steel was wet were evaluated to determine the most likely "critical relative humidity." Non-linear multiple regression techniques were used to determine the statistical significance of each factor and develop a theoretically consistent environmental damage function. From t h e f a l l of 1974 t o t h e s p r i n g o f 1977, EPA c o n d u c t e d an a i r p o l l u t i o n modeling s t u d y i n S t . L o u i s , M i s s o u r i (1)· N i n e o f 25 c o n t i n u o u s a i r m o n i t o r i n g s i t e s were s e l e c t e d f o r s t u d y i n g t h e e f f e c t s o f p o l l u t a n t s on e i g h t t y p e s o f m a t e r i a l s (2^). W e a t h e r i n g s t e e l was one o f t h e m a t e r i a l s . T h i s paper p r e s e n t s t h e r e s u l t s o f a n a l y z i n g the c o r r o s i o n o f weathering s t e e l w i t h r e s p e c t t o environmental d a t a . Theoretical

Considerations

Many m e t a l s f o r m c o r r o s i o n p r o d u c t f i l m s as t h e y c o r r o d e . These f i l m s tend t o r e s t r i c t t h e r a t e o f c o r r o s i o n . In general, the rate of c o r r o s i o n i s i n v e r s e l y p r o p o r t i o n a l t o t h e t h i c k n e s s o f t h e c o r r o s i o n p r o d u c t f i l m ( r a t e c o n t r o l l e d by d i f f u s i o n t h r o u g h t h e film). When t h e f i l m i s i n s o l u b l e and does n o t change s t r u c t u r e w i t h time, the c o r r o s i o n - t i m e f u n c t i o n i s p a r a b o l i c (c = a / t ) . Many m e t a l c o r r o s i o n p r o d u c t s have s o l u b i l i t i e s t h a t a r e p r o p o r tional to acidity. Thus, i n v e r y a c i d s o l u t i o n s , f i l m s a r e v e r y This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

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

MATERIALS

164

DEGRADATION

C A U S E D BY ACID RAIN

t h i n and the r e s u l t i n g c o r r o s i o n - t i m e f u n c t i o n i s almost l i n e a r . In a t m o s p h e r i c e x p o s u r e s , t h e r e a r e many s e t s o f e n v i r o n m e n t a l c o n d i t i o n s where t h e s e two mechanisms a r e competing. The r e s u l t s i s the e m p i r i c a l l y o b s e r v e d r e l a t i o n s h i p s , c - A t , where the exponent, n, most o f t e n has a v a l u e between 0.5 and 1.0 (2-4). T h e o r e t i c a l l y , the amount of c o r r o s i o n ( c ) i s the sum o f the m e t a l accumulated i n the c o r r o s i o n p r o d u c t f i l m ( T ) and the amount of m e t a l s o l u b i l i z e d from t h a t f i l m w i t h time ( 3 t ) . n

w

c = Τ + 3t

(1)

w

where t i s time-of-wetness and 3 i s a s o l u b i l i z a t i o n r a t e which i s a f u n c t i o n of a c i d i c p o l l u t a n t f l u x e s . The r a t e o f f i l m t h i c k ness growth i n u n i t s of m e t a l c o r r o d e d i s ;


w

dT/dt

= α/Τ -

w

3

(2)

where α i s a f u n c t i o n o f d i f f u s i v i t i e s through the f i l m . Pollu t a n t s may a l s o a f f e c t t h i s c o e f f i c i e n t . Under c o n s t a n t c o n d i t i o n s , i n t e g r a t i o n of e q u a t i o n (2) y i e l d s ; -3 t

- 3T + a l n ( l - 3T/a)

2

w

s u b s t i t u t i n g e q u a t i o n (1) i n t o e q u a t i o n (3) the t r a n s c e n d e n t a l e q u a t i o n : c - 3t

w

(3) and r e a r r a n g i n g

+ a(l-exp(-3C/a))/3

produces

(4)

which i s e q u i v a l e n t t o : c - 3t

w

(5)

+ a/(dc/dt ) w

A l e a s t squares f i t o f e q u a t i o n ( 4 ) cannot be used t o determine the e f f e c t s o f e n v i o r n m e n t a l f a c t o r s on t h e c o e f f i c i e n t s α and 3 be cause as a/ 3 becomes l a r g e the e q u a t i o n approaches b e i n g an identity. When 3c/α i s l a r g e , exp(-3c/a) approaches zero and e q u a t i o n (4) becomes; c = 3t

w

+ a/3

(6)

At e a r l y s t a g e s of c o r r o s i o n , however, from the e m p i r i c a l l y o b s e r v e d r e l a t i o n s h i p , d c / d t ^ n c / t which y i e l d s ; w

c - 3t

w

w

(7)

+ at /nc w

D u r i n g t h e f i r s t few y e a r s o f c o r r o s i o n of w e a t h e r i n g s t e e l , n e a r l y a l l o f t h e c o r r o s i o n i s accumulated i n the f i l m and α/nc i s much g r e a t e r than 3, which would i n d i c a t e t h a t t h e c o r r o s i o n time f u n c t i o n s h o u l d be n e a r l y p a r a b o l i c . The c o r r o s i o n p r o d u c t f i l m on w e a t h e r i n g s t e e l , however, has the p r o p e r t y t h a t , g e n e r a l l y , a g i v e n f i l m t h i c k n e s s becomes more p r o t e c t i v e w i t h t i m e . An e x c e p t i o n i s when the f i l m s t a y s wet. I n t h a t c a s e , the c o r r o s i o n b e h a v i o r of w e a t h e r i n g s t e e l i s not much b e t t e r than carbon s t e e l s . T h i s means t h a t η i n the e m p i r i c a l e q u a t i o n A t can be l e s s than n


9.

HAYNIE

165

Environmental Factors Affecting Corrosion of Steel

0,5 and i s e x p e c t e d t o i n c r e a s e w i t h f r a c t i o n o f t i m e - o f - w e t n e s s (f). Measurement and Data A n a l y s i s E n v i r o n m e n t a l d a t a . Hern, e t a l . , (_5) d e s c r i b e s the d a t a c o l l e c t i o n and e v a l u a t i o n system f o r the e n v i r o n m e n t a l parameters a t the nine s i t e s . S u b s e q u e n t l y , the r e s u l t i n g R e g i o n a l A i r P o l l u t i o n Study (RAPS) d a t a base was r e v i s e d s e v e r a l times u s i n g b e t t e r v a l i d a t i o n t e c h n i q u e s . The p o r t i o n s of t h a t d a t a base used i n t h i s s t u d y a r e h o u r l y averages of t e m p e r a t u r e , dew p o i n t , windspeed, wind d i r e c t i o n , t o t a l o x i d a n t , Ν 0 , S 0 , and 24-hour t o t a l suspended p a r t i c u l a t e m a t t e r (TSP) samples. The v a l i d a t e d d a t a base c o n t a i n s a l o t of m i s s i n g h o u r l y a v e r a g e s and the system was not o p e r a t i n g d u r i n g the f i r s t month o r the l a s t f i f t e e n days i n which w e a t h e r i n g s t e e l samples were exposed. A methodology was d e v e l o p e d (6) to e s t i m a t e t h e s e m i s s i n g v a l u e s u s i n g t o t a l system r e l a t i o n s h i p s and r e l a t i o n s h i p s between the system and c l i m a t o l o g i c a l d a t a from Lambert F i e l d ( a i r p o r t m e t e o r o l o g i c a l s t a t i o n l o c a t e d about 16 k i l o m e t e r s northwest from the S t . L o u i s c e n t r a l b u s i n e s s d i s t r i c t ) . Rainfall was not r e c o r d e d a t the exposure s i t e s . With r a i n f a l l e x p e c t e d t o be an i m p o r t a n t parameter, d a t a from Lambert F i e l d were used i n this study.


χ

X

Weathering S t e e l C o r r o s i o n . T r i p l i c a t e specimens were exposed f o r p e r i o d s v a r y i n g f r o m t h r e e t o t h i r t y months w i t h exposures s t a r t e d at each of t h e f o u r seasons d u r i n g the f i r s t y e a r . The e x p e r i m e n t a l p r o c e d u r e and exposure s c h e d u l e a r e documented i n an EPA r e p o r t (2). The r e s u l t s a r e 153 s e t s of t r i p l i c a t e weight l o s s d a t a w i t h s i t e , exposure time and i n i t i a l exposure s e a s o n as p r i m a r y v a r i a b l e s . A n a l y s i s of E n v i r o n m e n t a l D a t a . A l t h o u g h the methodology f o r a n a l y z i n g the d a t a has been p r e v i o u s l y r e p o r t e d (6) t h e r e a r e some d i f f e r e n c e s t h a t s h o u l d be n o t e d . F i r s t , a l a t e r v e r s i o n o f the RAPS d a t a base was used as an i n i t i a l s o u r s e . Second, t i m e - o f - w e t n e s s i n t h i s paper i s d e f i n e d d i f f e r e n t l y , t h u s , a r e l a t i o n s h i p t o c a l c u l a t e r e l a t i v e h u m i d i t y from temperature and dew p o i n t i s based on d a t a f o r dew p o i n t s g r e a t e r than 0°C. Third, deposition v e l o c i t i e s a r e c a l c u l a t e d from boundary l a y e r t h e o r y r a t h e r than e m p i r i c a l relationships· T h i s l a t e r v e r s i o n o f RAPS d a t a base e l i m i n a t e d many e r r o r s but produced more m i s s i n g d a t a . Time-of-wetness as p r e v i o u s l y d e f i n e d was the time e x c e e d i n g some c r i t i c a l r e l a t i v e h u m i d i t y (6^). I n t h i s paper t i m e - o f - w e t n e s s i s the time a c r i t i c a l r e l a t i v e h u m i d i t y i s exceeded and t h e dew p o i n t i s g r e a t e r than 0°C, p l u s any time the c r i t i c a l h u m i d i t y i s not exceeded and i t i s r a i n i n g . For t h a t r e a s o n a r e g r e s s i o n r e l a t i n g r e l a t i v e h u m i d i t y t o dew p o i n t s above 0°C and temperature was used t o c a l c u l a t e r e l a t i v e h u m i d i t y f o r each h o u r . This r e l a tionship i s : RH - 100 exp{[-0.0722 + 0.0002 5(T

+ DP)][T-DP]}


(8)

166

MATERA ILS DEGRADATO IN CAUSED BY ACID RAIN

where RH i s r e l a t i v e h u m i d i t y , Τ i s temperature and DP i s dew p o i n t , the l a t e r b o t h i n °C. D e p o s i t i o n v e l o c i t i e s were c a l c u l a t e d f r o m windspeed d a t a . Windspeeds a t tower h e i g h t can be used t o c a l c u l a t e windspeeds a t specimen r a c k h e i g h t u s i n g t h e r e l a t i o n s h i p f o r rough s u r f a c e s (J)* V

+

= 8.5 + 2.5 l n ( Z / e )

(9)


where v+ i s t h e d i m e n s i o n l e s s v e l o c i t y and ζ and e a r e m e a s u r i n g h e i g h t and roughness h e i g h t r e s p e c t i v e l y . The r a c k h e i g h t was about t h r e e meters from the ground. A t t h r e e o f t h e s i t e s t h e m e t e o r o l o g i c a l towers were 10 meters w h i l e a t t h e o t h e r s they were 30 m e t e r s . The average ground roughness h e i g h t was assumed t o be 0.1 m. Thus, t h e r a c k h e i g h t windspeed i s 0.7 5 times t h e windspeed at 30 meters o r 0.85 times t h e windspeed a t 10 m e t e r s . From a n o l o g y w i t h momentum t r a n s p o r t , gases w i t h a Schmidt number o f a p p r o x i m a t e l y one t h a t r e a d i l y r e a c t a t a s u r f a c e , have a d e p o s i t i o n v e l o c i t y of : 2

u - V* /V

(10)

where u i s t h e d e p o s i t i o n v e l o c i t y , V* i s t h e f r i c t i o n v e l o c i t y and V i s t h e average windspeed. The f r i c t i o n v e l o c i t y i s e q u a l t o V / f / 2 where f i s t h e f r i c t i o n f a c t o r . From boundary l a y e r t h e o r y f o r smooth f l a t p l a t e s ( 7 ) : f

= 0.03/(RE )

1 / 7

(11)

L

where R E = LV/ v, L = l e n g t h o f s u r f a c e over which t h e a i r f l o w s , and ν i s t h e k i n e m a t i c v i s c o s i t y o f a i r ( 0 . 1 5 cm^/sec). L i s assumed t o be t h e g e o m e t r i c mean o f t h e p a n e l dimensions (/10.2 χ 15.2 = 12.45 cm). Thus: L

u = 0.35 V 5 u = 0.31 V 5

8 6

x

8 6

3

(12a) (12b)

ANA

V^Q * V30

w i t h u i n cm/sec and i n m/sec. D e p o s i t i o n v e l o c i t i e s were c a l c u l a t e d on an h o u r l y b a s i s and averaged over exposure p e r i o d s . Because windspeeds a r e n o t n o r m a l l y d i s t r i b u t e d , t h e average d e p o s i t i o n v e l o c i t y i s about 91% of t h e d e p o s i t i o n v e l o c i t y c a l c u l a t e d from average windspeed f o r an expo sure p e r i o d . Pollutant Fluxes. H o u r l y d e p o s i t i o n v e l o c i t i e s were m u l t i p l i e d by h o u r l y 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 get h o u r l y p o l l u t a n t f l u x e s . These were summed over exposure p e r i o d s f o r hours o f wetness w i t h d i f f e r e n t c r i t i c a l r e l a t i v e h u m i d i t y c r i t e r i a ( 7 5 t o 90% i n 5% intervals). Average f l u x e s were then c a l c u l a t e d by d i v i d i n g by t h e time-of-wetness. The r e s u l t s were compared w i t h f l u x e s c a l c u l a t e d by m u l t i p l y i n g average d e p o s i t i o n v e l o c i t i e s f o r a p e r i o d by t h e average p o l l u t a n t c o n c e n t r a t i o n d u r i n g times o f wetness. The v a l u e s by t h e two methods were f a i r l y c o n s i s t e n t .


HAYNIE

9.

167


F l u x e s o f TSP were c a l c u l a t e d by m u l t i p l y i n g average TSP by two-tenths o f t h e average d e p o s i t i o n v e l o c i t y f o r gases ( a c t u a l deposition v e l o c i t i e s vary considerably with the s i z e d i s t r i b u t i o n of p a r t i c l e s ) . R a i n f l u x e s were c a l c u l a t e d by d i v i d i n g t h e amount of r a i n f o r an exposure p e r i o d by time o f w e t n e s s . S t a t i s t i c a l A n a l y s i s o f Data An i n i t i a l l i n e a r r e g r e s s i o n was performed on t h e d a t a t o d e t e r m i n e the r e l a t i v e s i g n i f i c a n c e o f each o f t h e f a c t o r s . The form o f t h e model was: In(C)=OoIn(t)+Σ o ^ l n f +30+Σy P


±

i

(13)

i

where C i s amount o f s t e e l c o r r o s i o n , t i s t o t a l time o f e x p o s u r e , f i s f r a c t i o n o f time o f wetness f o r d i f f e r e n t c r i t i c a l r e l a t i v e h u m i d i t i e s , Q i s a temperature f a c t o r (1000[l/(273.16+T°C)-1/ (273.16+T°C a v g ] ) ( 6), and the P ^ a r e t h e f l u x e s o f p o l l u t a n t s and rain. B o t h f o r w a r d and backward s t e p w i s e r e g r e s s i o n s were done. The r e s u l t s i n d i c a t e d t h a t f r a c t i o n o f time o f wetness ( f ) f o r a c r i t i c a l r e l a t i v e h u m i d i t y o f 85% was t h e most s i g n i f i c a n t o f the f v a l u e s , t h e temperature f a c t o r was somewhat l e s s s i g n i f i c a n t , and o n l y ozone f l u x was n o t a s i g n i f i c a n t f a c t o r . TSP and S O 2 had p o s i t i v e c o e f f i c i e n t s w h i l e the N O 2 and r a i n c o e f f i c i e n t s were negative. Because t h e amount o f c o r r o s i o n o f s t e e l i s e x p e c t e d t o f o l l o w the e m p i r i c a l form C = A t , where b o t h A and η a r e v a r i a b l e s w i t h changing e n v i r o n m e n t a l f a c t o r s , a l i n e a r r e g r e s s i o n on t h e l n - l n form was performed on t h e f o l l o w i n g model: n

w

l n ( C ) = a + a l n ( t ) + a f l n ( t ) + a 3 l n ( f ) + 3 Q + ly V± 0

1

w

2

w

(14)

±

where t and f a r e time-of-wetness and f r a c t i o n o f t i m e - o f wetness r e s p e c t i v e l y , f o r a c r i t i c a l r e l a t i v e h u m i d i t y o f 85%. The r e s u l t s are g i v e n i n T a b l e I . w

Table I .

Results of the Linear Regression of Equation: ln(C)=a ^a ln(t )+a 0

Variable ln(C) ln(t ) f ln(t ) ln(f) Q TSP F l u x S0 Flux N0 Flux Rain Flux w

w

2

2

1

Units ln(y) ln(years) ln(years)

w

t

( w)

Coefficient

2

3

= = = =

+ c x

3

l n

(

f

E

p

TL i

Standard e r r o r

4.245 0.3116 0.8 583 0.4399 3 = -0.7023 ^1 = 0.8729 Ύ2 - 0.1152 Ύ = -0.0370 -4 -3.18x10 ο 1 α α

α

3

mg/cm^ year mg/cm2 year mg/cm year cm/year

f l n 2

0.0444 0.1561 0.0819 0.1809 0.1004 0.0299 0.0128 -4 1.24x10

P a r t i a l F**

49.19 30.23 28.82 15.07 75.62 14.87 8.33 6.58


168

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

* Q = 1000(1/273.16+T)-1/(273.16+Tavg) where Τ i s i n °C and Tavg i s o v e r a l l average o f average temperatures d u r i n g t i m e s - o f wetness ( 1 3 . 5 5 ° C ) . **A measure o f t h e s t a t i s t i c a l s i g n i f i c a n c e o f a d d i n g t h e s p e c i f i c v a r i a b l e t o a l l of the o t h e r s . The p e r c e n t o f v a r i a b i l i t y e x p l a i n e d by r e g r e s s i o n i s ' 95.85%. The f r a c t i o n o f time-of-wetness a f f e c t s t h e v a l u e o f η i n t h e e m p i r i c a l e q u a t i o n C = A t , (n=0.312+0.858f), w h i c h means t h a t t h e f i l m becomes more p r o t e c t i v e w i t h time when f i s low, b u t l e s s p r o t e c t i v e w i t h time when f i s h i g h . A l l o f t h e o t h e r terms i n e q u a t i o n 14 a r e c o n s i d e r e d t o be a p a r t o f t h e A c o e f f i c i e n t . A pseudo S t e e l c o r r o s i o n £ ^ J Ç x a s c a l c u l a t e d by d i v i d i n g s t e e l c o r r o s i o n by f " ^(0-312+0.858f) (- .702Q). T h i s v a l u e was r e g r e s s e d a g a i n s t t h e p o l l u t a n t and r a i n f l u x e s t o determine n o n e x p o n e n t i a l c o e f f i c i e n t s t h a t make up t h e A term. The r e s u l t s a r e given i n Table I I . n

w

w

0

4

4


e x p

Table I I .

0

R e s u l t s o f R e g r e s s i o n o f Pseudo C o r r o s i o n Rate A g a i n s t Pollutant Fluxes

Variable Pseudo C o r r o s i o n Rate TSP F l u x S0 Flux N0 Flux Rain Flux 2

2

Units

Coefficient

V

r 85.13 A = 10.67

2

2

2

Partial F

65.63

A

mg/cm y e a r mg/cm y e a r mg/cm y e a r cm/year

Standard error

A = -3.51 A = -0.0302 3

4

82.26 18.88 11.36 12.51

9.39 2.46 1.04 0.0085 n

The r e s u l t i n g e m p i r i c a l damage f u n c t i o n , C = A t , has t h e c o efficients: w

A - (65.63+85.13TSP+10.67S0 -3.51N0 -.03Rain)f 2

η

2

u,44

e x p ( - . 7 Q ) (15a)

- 0.3 1 2+0.858f

where t h e p o l l u t a n t f l u x e s have t h e u n i t s i n T a b l e I I . t i o n a c c o u n t s f o r 95.93 p e r c e n t o f t h e v a r i a b i l i t y .

(15b) T h i s equa

Evaluation of data with respect to theory. The s i g n i f i c a n t f a c t o r s i n t h e e m p i r i c a l e q u a t i o n s 15a and 15b may a f f e c t e i t h e r o r b o t h α and 3 c o e f f i c i e n t s i n e q u a t i o n 7. The r e l a t i v e e f f e c t s of each parameter on α and 3 were d e t e r m i n e d by r e g r e s s i n g s t e e l c o r r o s i o n / t i m e - o f - w e t n e s s (c/tw) a g a i n s t a l l o f t h e f l u x e s , 1/nCp (where C = A t with the c o e f f i c i e n t s c a l c u l a t e d u s i n g equations 15a and 15b), e x p ( - . 7 Q ) / n C , f / n C , and a l l o f t h e p r o d u c t s o f f l u x e s and 1/nCp. Stepwise r e g r e s s i o n was used i n t h e o r d e r o f most s i g n i f i c a n t t o l e a s t s i g n i f i c a n t v a r i a b l e and i n c l u d i n g o n l y those v a r i a b l e s w i t h a 0.95 p r o b a b i l i t y o f s i g n i f i c a n c e . A total of 13 independent v a r i a b l e s were c o n s i d e r e d . Table I I I gives the results· n

w

p

p


9.

Environmental

HAYNIE

Factors Affecting

Corrosion

169

of Steel

T a b l e I I I . R e g r e s s i o n C o e f f i c i e n t s f o r T h e o r e t i c a l Model o f Weathering S t e e l C o r r o s i o n C / t = + ^a ? /nC w

Variable

c/t

Units μ/year

w

TSP/nC f/nC l/nC N0 Flux S0 Flux Rain Flux* EXP(-.7Q)/nC„ p

a =1573 a =1381 3^-8.85 3 =18.32 33=-0.0798 a y 1104 Q

mg/cm^ y e a r mg/cm year cm/year

2

2

p

Partial F

Q

2

p

Standard e r r o r

±

3 =39.12

αχ-2292 p


Coefficient

±

2

175 262 308 1.85 4.62 0.0227 335

171.10 36.05 20.15 22.88 15.71 12.37 10.85

* R a i n F l u x i s e x p r e s s e d i n c u b i c c e n t i m e t e r s o f r a i n p e r square c e n t i m e t e r o f s u r f a c e p e r y e a r o f wet t i m e . V a r i a b l e s a r e l i s t e d i n t h e o r d e r i n which they were e n t e r e d i n t o the r e g r e s s i o n . The r e s u l t i n g damage f u n c t i o n ( C = ( Z 3 P + E a P / n C ) t ) c a n a c c o u n t f o r 95.65% o f t h e v a r i a b i l i t y . i

i

Discussion

1

i

p

w

of Results

The e m p i r i c a l damage f u n c t i o n w i t h c o e f f i c i e n t s c a l c u l a t e d u s i n g e q u a t i o n s 15a and 15b p r o v i d e s t h e b e s t f i t o f t h e d a t a ( g r a p h i c a l l y presented i n Figure 1). The gaseous p o l l u t a n t f l u x e s a r e based on h o u r l y c o n c e n t r a t i o n s and d e p o s i t i o n v e l o c i t i e s d u r i n g p e r i o d s of wetness. These do n o t d i f f e r d r a m a t i c a l l y from f l u x e s c a l c u l a t e d f r o m exposure p e r i o d a v e r a g e s o f d e p o s i t i o n v e l o c i t i e s and c o n c e n trations. The TSP f l u x e s a r e c a l c u l a t e d from exposure p e r i o d averages o f d e p o s i t i o n v e l o c i t i e s and c o n c e n t r a t i o n s . The r a i n f l u x e s a r e t h e amounts o f r a i n d i v i d e d by times o f w e t n e s s . Frac t i o n o f time when wet ( f ) i s f o r a c r i t i c a l r e l a t i v e h u m i d i t y o f 85%. T h i s e q u a t i o n c a n be used t o p r e d i c t w e a t h e r i n g s t e e l c o r r o s i o n as a f u n c t i o n o f e n v i r o n m e n t a l c o n d i t i o n s . The c o e f f i c i e n t s i n T a b l e 3 p r o v i d e a b e t t e r t h e o r e t i c a l u n d e r s t a n d i n g o f how t h e d i f f e r e n t f a c t o r s a f f e c t t h e c o r r o s i o n of weathering s t e e l . The 3 c o e f f i c i e n t s a f f e c t t h e s o l u b i l i t y o f the p r o t e c t i v e o x i d e l a y e r and t h e α c o e f f i c i e n t s a f f e c t t h e d i f f u s i v i t y through the l a y e r . The l a r g e r a t i o o f a/3 c o n f i r m s t h e r e l a t i v e I n s o l u b i l i t y o f t h e r u s t on w e a t h e r i n g s t e e l i n most environments. S u l f u r dioxide increases the s o l u b i l i t y of the f i l m w h i l e N 0 and r a i n d e c r e a s e t h e s o l u b i l i t y . R a i n a p p a r e n t l y washes away a c i d i c components ( d e p o s i t e d d u r i n g dew f o r m a t i o n ) t h a t i n c r e a s e the s o l u b i l i t y . The α c o e f f i c i e n t f o r TSP i s h i g h l y s i g n i f i c a n t . Accumulation of p a r t i c l e s i n t h e o x i d e l a y e r appears t o i n c r e a s e t h e d i f f u s i v i t y of i o n s o r t h e e l e c t r i c a l c o n d u c t i v i t y o f t h e f i l m . TSP does n o t appear t o enhance t h e s o l u b i l i t y o f t h e f i l m . Increasing the f r a c t i o n o f t i m e - o f - w e t n e s s appears t o i n c r a s e the d i f f u s i v i t y t h r o u g h t h e l a y e r . T h i s c o u l d a c t u a l l y be a time f u n c t i o n o f a matter of hours. The d i u r n a l c y c l e s u g g e s t s t h a t w h i l e t h e f i l m i s wet i t becomes l e s s p r o t e c t i v e w i t h time; when i t i s d r y i t becomes 2


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


170

PREDICTED CORROSION -

Figure

1.

Micrometers

F i t of weathering s t e e l c o r r o s i o n data to A t ^

where A and η a r e f u n c t i o n s

RAIN

of environmental

n

parameters


model,

9.

HAYNIE


171

more p r o t e c t i v e w i t h t i m e . When f i s l a r g e t h e f i l m a p p a r e n t l y i s not d r y l o n g enough t o r e a c h a d e s i r e d l e v e l o f p r o t e c t i v i t y . The temperature e f f e c t i s as e x p e c t e d ; d i f f u s i o n i n c r e a s e s as temperature increases.


Conclusions W e a t h e r i n g s t e e l c o r r o s i o n c a n be d e s c r i b e d as competing mechanisms of f o r m a t i o n and d i s s o l u t i o n o f a p r o t e c t i v e o x i d e l a y e r d u r i n g p e r i o d s o f wetness. E m p i r i c a l l y , the best f i t of the c o r r o s i o n data suggests that time-of-wetness o f t h e s t e e l i s b e s t d e f i n e d as t h e time when t h e dew p o i n t exceeds 0°C and t h e r e l a t i v e h u m i d i t y exceeds 85% p l u s the time d u r i n g r a i n when t h e r e l a t i v e h u m i d i t y does n o t exceed 85%. Three v a r i a b l e s i n c r e a s e t h e d i f f u s i v i t y t h r o u g h t h e o x i d e f i l m ; 1) f r a c t i o n o f time when wet, 2) t e m p e r a t u r e , and 3) TSP flux. S o l u b i l i t y o f t h e o x i d e f i l m i s i n c r e a s e d by i n c r e a s i n g t h e f l u x o f s u l f u r o x i d e s d u r i n g p e r i o d s o f wetness. D i s s o l u t i o n o f t h e o x i d e f i l m i s reduced by i n c r e a s i n g t h e f l u x e s o f n i t r o g e n o x i d e s and r a i n . Ozone appears t o have no s i g n i f i c a n t e f f e c t on t h e c o r r o s i o n of w e a t h e r i n g s t e e l .

Literature Cited 1. Schiermeier, F. A. Environmental Sci. Technol. 1978, 12, 644. 2. Mansfeld, F. "Regional Air Pollution Study: Effects of Airborne Sulfur Pollutants on Materials"; EPA-600/4-80-007, 1980. 3. Haynie, F. H.; Upham, J. B. Materials Protection and Performance 1971, 10, 18. 4. Mattsson, E. Materials Performance 1982, 21, 9. 5. Hern, D. H.; Taterka, M. H. "Regional Air Monitoring System Flow and Procedures Manual"; EPA Contract 68-02-2093, Rockwell International, Creve Coeur, Mo. 1977. 6. Haynie, F. H. Durability of Building Materials 1982/1983, 1, 241. 7. Knudson, J. G.; Katz, D. L. "Fluid Dynamics and Heat Transfer"; University of Michigan, Ann Arbor, Michigan, 1954, p. 38 and p. 149. RECEIVED January 2, 1986


Materials Degradation Caused by Acid Rain - American Chemical

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