CiteSeerX — Influence of Accelerated Weathering on the Corrosion of Low-Alloy Steels

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VoL 127, No. I K I N E T I C S O F Z I N C E L E C T R O D E 15 18. W. F. Libby, A b s t r a c t s of papers p r e s e n t e d at the

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(1978).

Influence of Accelerated Weathering on the Corrosion of Low-Alloy Steels

H. Schwitter and H. B~hni

Swiss Federal Institute of Technology, Institute o~ Materials Chemistry and Corrosion, 8093 Zifrich, Switzerland

A B S T R A C T

I n l o w - a l l o y steels u n d e r accelerated atmospheric corrosion conditions t h e type of w e a t h e r i n g , p a r t i c u l a r l y the ratio of the time of wetness to the d r y i n g time, has a g r e a t e r influence on the corrosion b e h a v i o r t h a n does the composition of the alloys. Electrochemical m e a s u r e m e n t s show t h a t the r u s t p r o - tection is n o t o n l y due to the f o r m a t i o n of compact macroscopic protective layers, b u t also to p a s s i v a t i o n effects. P a s s i v a t i o n occurs for a p r o l o n g e d period o n l y at f a v o r a b l e accelerated w e a t h e r i n g conditions. We propose a m o d e l for the i n t e r p r e t a t i o n o~ the corrosion behavior of l o w - a l l o y steels.

W e a t h e r i n g steels exposed to the a t m o s p h e r e form, as is well k n o w n , compact a n d w e l l - a d h e r i n g p r o t e c - tive layers w h i c h i n h i b i t a f u r t h e r attack b y corrosion.

D e p e n d i n g o n t h e composition of the alloy a m o r e or less decreasing corrosion rate c a n be observed. N u m e r - ous papers have b e e n p u b l i s h e d which s u p p o r t this o b - s e r v a t i o n (1-3).

A l t h o u g h these steels h a v e b e e n used t h r o u g h o u t the world for a long period of time, the corrosion p r o - tective properties of t h e f o r m e d r u s t layers a r e not well e n o u g h understood. Most of the a t m o s p h e r i c corrosion tests p e r f o r m e d u n t i l today do n o t allow a n y c o n c l u - sions i n this direction, because the different p a r a m e t e r s , which a r e i m p o r t a n t for the corrosion behavior, h a v e n o t b e e n v a r i e d s y s t e m a t i c a l l y i n these tests.

The practical experiences w i t h this m a t e r i a l as well as the results of l o n g - t i m e tests o b t a i n e d b y L a r r a b e e a n d C o b u r n (4) i n the U S A a n d E d u a r d s (5) i n E n - g l a n d show, however, t h a t c o n s i d e r a b l e differences can occur b e t w e e n the different test sites. I n E n g l a n d com- p a r a b l e steels corroded 5-10 times faster t h a n i n t h e USA, suggesting that the type of w e a t h e r i n g is at least as i m p o r t a n t as the alloy composition.

It was t h e r e f o r e the a i m of this s t u d y to e x a m i n e the influence of w e a t h e r i n g as well as the i m p o r t a n c e of t h e a l l o y i n g e l e m e n t s on the corrosion b e h a v i o r of l o w - a l l o y steels b y m e a n s of a p p r o p r i a t e accelerated w e a t h e r i n g tests. A d d i t i o n a l l y , n a t u r a l exposure tests, light a n d e l e c t r o n - o p t i c a l studies, x - r a y diffraction, a n d electrochemical m e a s u r e m e n t s were carried out.

Experimental

Materials.--Materials e x a m i n e d w e r e p l a i n c a r b o n steel a n d l o w - a l l o y steels with v a r i a b l e composition.

Table I shows the analysis of these steels.

All l o w - a l l o y steels tested have the same basic composition. I n three different groups, however, the a l l o y - Key words: accelerated weathering tests, weathering steel, atmospheric corrosion, wet/dry cycles, rust layers.

i n g e l e m e n t s copper, c h r o m i u m , a n d phosphorus w e r e s y s t e m a t i c a l l y varied.

Before exposure the steels were s a n d b l a s t e d a n d de- greased i n benzene.

Weathering.--The samples were exposed to three dif- f e r e n t types of accelerated w e a t h e r i n g tests. F i g u r e 1 gives a schematic s u r v e y of these w e a t h e r i n g conditions. I n the first w e a t h e r i n g (3x) the samples were w e t t e d a n d dried three times w i t h i n 12 hr. I n one w e e k the samples were exposed 12 times to these 3x cycles a n d twice i n 12 h~" to a SO~-containing atmosphere. I n the w e a t h e r i n g type 2x the specimens were exposed to two d r y / w e t cycles w i t h i n 12 hr, i n w e a t h e r i n g type l x they were w e t t e d a n d dried o n l y once i n the same time. The average total time of wetness i n c r e a s e d f r o m 46% i n the 3x cycle to 77% i n the l x cycle.

The specimens were w e t t e d b y m e a n s of aerosol s p r a y i n g a n d s p r i n k l i n g , dried at 42~ a n d at a r e l a t i v e h u m i d i t y of 55%. D u r i n g the SO2-exposure the SO2- c o n c e n t r a t i o n was 20 p p m a n d the r e l a t i v e h u m i d i t y 85%. Before each SO2-exposure the samples w e r e also dried. For the n a t u r a l exposure test i n a s u b u r b a n area n e a r Z u r i c h the samples were o r i e n t e d towards the south at a n angle of 45 ~

ElectrochemicM experiments.--The electrochemical e x p e r i m e n t s were carried out i n a n a i r s a t u r a t e d 0.1M Na2SO4 solution. The c u r r e n t - p o t e n t i a l curves were obtained p o t e n t i o s t a t i c a l l y by the u s u a l electronic de- vices (potentiostat, f u n c t i o n generator, p o t e n t i a l - m e t e r ) . The sweep r a t e was 15 m V / m i n . As r e f e r e n c e electrode a s a t u r a t e d calomel electrode was used. The surface of t h e sample e x a m i n e d was 4 cm 2.

Results

Weight losses.--Figure 2 gives a s u r v e y of the results o b t a i n e d b y the accelerated w e a t h e r i n g tests. The cop- p e r as well as the c h r o m i u m series c l e a r l y show the influence of w e a t h e r i n g . The samples w e a t h e r e d w i t h

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16 J. EIectrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y January 1980 Table I. Chemical analysis of the steels used

S t e e l C S Si Mn Cr P CR Ni

Plain carbon steel Low alloy

S 0.02 0.011 0.005 0.29 0.015 0.042 0.018 0.04

E 0,09 0.009 0.36 0.50 0.56 0,061 ~ 0.27

P 0.10 0.008 0.37 0.42 0.50 0.074 ~ 0.28

M 0.11 0.010 0.34 0.49 0.54 0,081 0.30

H 0.10 0.006 0.34 0.42 ~ 0.079 0.34 0.29

F 0.11 0.010 0.34 0.49 l ~ 8 ] 0.067 0.34 0.30

K 0.11 0,007 0.42 0.50 0.075 0.45 0.31

J 0.10 0.007 0.36 0.45 0.54 ~ 0.44 0.31

Q 0,11 0.011 0.39 0.50 0.51 1~01~4 I 0.41 0.30

N 0.11 0.012 0.38 0.50 0.51 0.47 0.30

the 3x cycle h a v e s m a l l e r w e i g h t losses t h a n those w i t h 2x a n d l x cycle. T h e difference i n the 2x a n d l x type weathering, however, is n o t v e r y pronounced. I n the w e a t h e r i n g 3x the influence of the alloying elements is weak. I n addition, the u n a l l o y e d steel S shows only a v e r y small corrosion rate after 9 weeks. The influence of the alloying e l e m e n t s i n the w e a t h e r i n g types 2x a n d l x is slightly greater. I n Fig. 2 the results of the most h i g h l y alloyed steels a r e at the l o w e r limit, the ones of the less alloyed steels at the u p p e r l i m i t of the v a l u e band.

F i g u r e 3 shows a d d i t i o n a l l y t h e w e i g h t loss of the S a n d M - t y p e steels as a f u n c t i o n of total t i m e - o f - w e t - ness t* i n s t e a d of exposure time. The results i n d i c a t e t h a t the lower w e i g h t losses of the 3 x - t y p e w e a t h e r i n g a r e n o t o n l y due to the shorter t i m e - o f - w e t n e s s d u r i n g these w e a t h e r i n g conditions b u t also due to the m o r e protective properties of the r u s t layer.

F r o m these results the following s t a t e m e n t c a n be made: F r e q u e n t d r y i n g / w e t t i n g changes i n the 3x w e a t h e r i n g h a v e a f a v o r a b l e influence on the corrosion behavior. A f t e r a large corrosion r a t e at the b e g i n n i n g of the exposure the samples corrode o n l y slightly. This b e h a v i o r is almost i n d e p e n d e n t of the c o n c e n t r a t i o n of

the a l l o y i n g elements. F o r the u n a l l o y e d steel S the corrosion rate decreases significantly also.

W h e n the samples a r e w e a t h e r e d w i t h l o n g e r w e t t i n g periods (2x, l x ) , the corrosion rate does n o t decrease so much. Moreover, the total m e t a l loss is two to three times as high compared to w e a t h e r i n g 3x. D u r i n g t h e s e

200

O,J

L ~

I00'

I x ( S , E , P , M )

2x (S,E,P,M)

3x ( S , E , P,M)

, i i i i I i i i __ T i m e [ d a y s ]

14 28 4 2 5 6

pl

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i I _ I I

Ni~l ~

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)rJnk~ng 8

-j ⁱ Weathering

5 x

I E , Time of wetness

I'-'

I l l I

ng )_h

5

I I '12h

Weathering 2x

= 4 6 %

Time of wetness = 5 8 %

";""'Y"~""""'"'~I Weathering i x

o I

o I . = _

I I I I I I t i l l I

4 8 12h

Wet period [ I Dry period

Fig. |. Schematic survey of the types of accelerated weathering

Time of wetness = 7 7 %

P"I

200

kiI

- | 0 0 ._~

Cr - Series b)

P - S e r i e s

S ^ix (S,H,F,K)

3x (S,H, F,K) ( S, J,Q,N)

, J ~ ~

, , , , I --Time[days]

14 2 8 4 2 5 6

Fig. 2. Results of the accelerated weathering tests: a) tappet- series; b) chromium and phosphorus series.

160

E

"-- 120 _{c ~}

~

⁸⁰

o

9 ~

4o

/SIx

_ //~M1x

- ~ "

~

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] I I I 1

28 56 ⁼

t" [doys]

Fig. 3. Weight loss, x-axis reduced to time of wetness t*

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VoL 127, No. 2 A C C E L E R A T E D W E A T H E R I N G 17

280 240 200

" 160 12o

~= 8O 40

Plain carbon steel S ~ I f "~=

/ / / ~ E,H,J

/ P,F,Q

/ ..

M,K,N

/ i ! ! . . i:=

~ steels

r i I ~ ~ r i t - Time[days]

200 400 600 800

Fig. 4. Results of the natural weathering in suburban area w e a t h e r i n g conditions a beneficial influence of the al=

l o y i n g e l e m e n t s is observed.

F i g u r e 4 shows the results of the n a t u r a l exposure tests. I n spite of the large scatter a s i m i l a r influence of the a l l o y i n g e l e m e n t s is o b t a i n e d as i n the case of the accelerated w e a t h e r i n g l x a n d 2x. G e n e r a l l y , the most alloyed steels h a v e t h e lowest a n d the u n a l l o y e d car- b o n steels h a v e the highest w e i g h t losses. Moreover, a f t e r two years exposure n o n e of the steels tested f o r m e d the completely protective r u s t layers observed d u r i n g 3 x - t y p e accelerated w e a t h e r i n g .

Metallographic tests.--For t h e description of the f u r t h e r tests o n l y the results of the c a r b o n steel S a n d the copper c o n t a i n i n g steel M are used. Also, the 2x w e a t h e r i n g is n o t m e n t i o n e d a n y m o r e because the r e - sults are s i m i l a r to those i n l x w e a t h e r i n g .

E x a m i n e d macroscopically, the samples of the 3x- type w e a t h e r i n g have a r e l a t i v e l y dark, h o m o g e n e o u s - looking surface. No real r u s t blisters are visible. On the o t h e r hand, the r u s t layers f o r m e d o n the I x - t y p e samples have a l i g h t e r color. Moreover the surface is covered w i t h n u m e r o u s big blisters.

Scanning electron microscopy (SEM).--Figure 5 shows the surface of the samples M3x a n d S3x t a k e n w i t h SEM. Here the rust l a y e r is v e r y homogeneous a n d compact. T h e single blisters are v e r y small a n d of a b o u t the same size. The surface of the M3x sample has no visible cracks a n d o n t h e S3x s a m p l e o n l y f e w t h i n cracks can be seen.

O n t h e other hand, the pictures of the M l x a n d S l x samples show a r a t h e r i n h o m o g e n e o u s r u s t surface, as m a y be seen i n Fig. 6. Beside n u m e r o u s small r u s t blisters there a r e also big ones covered w i t h r e l a t i v e l y large cracks.

Light microscope tests.--Microscopic observations show the s t r u c t u r e o~ the rust layer. Yigure 7 i l l u s t r a t e s the m e t a l l o g r a p h i c m i c r o g r a p h s of samples M3x a n d S3x.

F i g u r e 7a is t a k e n by m e a n s of a light microscope, Fig.

7b u n d e r polarized light a n d crossed nicols. The r u s t l a y e r of the M3x s a m p l e consists of two layers: the d a r k one covering t h e m e t a l surface a n d the light one above it. The optical b e h a v i o r of the d a r k l a y e r is iso- tropic. The other l a y e r shows a n anisotropic b e h a v i o r caused b y double r e f r a c t i o n a n d is b r i g h t o r a n g e - colored i n the microscope. E l e c t r o n probe m i c r o a n a l y s i s showed t h a t i n the d a r k l a y e r the alloying e l e m e n t s a n d s u l f u r are accumulated. X - r a y diffraction analysis i n d i c a t e d that a large a m o u n t of r u s t has a n x - r a y amorphous structure. F r o m these results can be coa-

Fig. 5. Rust surface, weather- ing type 3x, scanning electron microscope.

Fig. 6. Rust surface, weather- ing type lx, scanning electron microscope.

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18 J. E~ectrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y January I980

Fig. 7. Cross sections of the rust layers, weathering type 3x: a) light microscope; b) polarized light and crossed nicols c l u d e d t h a t the d a r k p a r t s in the cross sections consist

of t h e a m o r p h o u s F e O x ( O H ) 3 - 2 x as was a l r e a d y d e - s c r i b e d b y M i s a w a (6).

The s u r f a c e of the M3x s a m p l e is c o v e r e d at 80-90%

w i t h the d a r k , a m o r p h o u s l a y e r . The cross section t h r o u g h the r u s t l a y e r of s a m p l e S3x shows a s l i g h t l y less o r d e r e d structure. T h e l i g h t a n i s o t r o p i c l a y e r is a l i t t l e t h i c k e r c o m p a r e d w i t h t h e d a r k l a y e r . Yet, t h e m e t a l s u r f a c e is l a r g e l y c o v e r e d w i t h the g r a y r u s t phase.

F i g u r e 8 shows cross sections of t h e r u s t l a y e r of t h e s a m p l e s M l x a n d S l x . I n t h e case of l x - t y p e w e a t h e r - ing, the r u s t l a y e r does not h a v e t h e s a m e o r d e r e d s t r u c t u r e as w i t h the 3 x - t y p e w e a t h e r i n g . The l i g h t r u s t phases also a p p e a r d i r e c t l y on t h e m e t a l surface.

T h e d a r k r u s t l a y e r covers h e r e o n l y 50-60% of the w h o l e m e t a l surface. F u r t h e r m o r e , t h e t h i c k n e s s of the r u s t l a y e r is not uniform. The r u s t l a y e r b e s i d e t h e big b l i s t e r s is u s u a l l y v e r y thin a n d consists m a i n l y of the l i g h t - c o l o r e d a n i s o t r o p i c r u s t phase. A d d i t i o n a l l y , s u l -

Fig. 8. Cross sections of the rust layers, weathering type lx: a) light microscope; b) polarized tight and crossed nicols

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Vol. I27, No. 1 A C C E L E R A T E D W E A T H E R I N G 19

f a t e p o c k e t s a r e f o r m e d as can be seen in Fig. 8 ( S l x ) r i g h t u n d e r n e a t h t h e big r u s t blister.

W h e n p r e p a r i n g t h e m e t a l l o g r a p h i c cross sections, t h e l i g h t p h a s e s p r o v e d to be v e r y b r i t t l e and t e n d e d to b r e a k out easily. T h e d a r k l a y e r s on the o t h e r h a n d a r e v e r y c o m p a c t a n d can e a s i l y be h a n d l e d . This d i f - f e r e n c e i n the m e c h a n i c a l p r o p e r t i e s is v e r y e v i d e n t a n d p o i n t s to t h e p r o t e c t i v e effect of t h e g r a y i s o t r o p i c l a y e r .

Electrochemical measurements.--Potential decay during immersion.--The d r y s a m p l e s w e r e i m m e r s e d in the e l e c t r o l y t e a n d the p o t e n t i a l was c o n t i n u o u s l y r e c o r d e d .

F i g u r e 9 shows the p o t e n t i a l - t i m e curves o f the d i f - f e r e n t l y w e a t h e r e d a l l o y e d a n d u n a l l o y e d s a m p l e s a f t e r i m m e r s i o n . T h e p o t e n t i a l s of t h e l x - t y p e w e a t h e r e d s a m p l e s d e c r e a s e in b o t h cases w i t h i n a f e w m i n u t e s b y 300-400 m V a n d q u i c k l y r e a c h a constant value. I n the w e l l - w e a t h e r e d s a m p l e s this v a l u e is r e a c h e d o n l y a f t e r s e v e r a l hours. The constant p o t e n t i a l v a l u e is n e v e r t h e l e s s a p p r o x i m a t e l y the s a m e in all the e x p e r i - ments. A r a p i d p o t e n t i a l d e c a y is also a c h i e v e d b y s c r a t c h i n g the s u r f a c e of the w e l l - w e a t h e r e d s a m p l e

(3x) w i t h a n e e d l e ( d o t t e d l i n e in Fig. 9).

F u r t h e r m o r e , t h e l o w - a l l o y steel M as w e l l as t h e u n a l l o y e d steel S e x p o s e d to n a t u r a l w e a t h e r i n g conditions s h o w q u i t e s i m i l a r p o t e n t i a l - t i m e c u r v e s as the l x w e a t h e r e d s a m p l e s ( c u r v e s M A a n d S A in Fig.

9). This confirms t h e r e s u l t s a l r e a d y o b t a i n e d b y w e i g h t loss m e a s u r e m e n t s w h e r e t h e n a t u r a l e x p o s u r e tests w e r e also c o m p a r a b l e w i t h t h e l x a n d 2 x - t y p e a c c e l e r a t e d w e a t h e r i n g .

Anodic polarization.--After h a v i n g b e e n i m m e r s e d i n t h e e l e c t r o l y t e for 20 min, the s a m p l e s w e r e a n o d i c a l l y p o l a r i z e d .

F i g u r e 10 shows such p o l a r i z a t i o n c u r v e s w h i c h a r e s i m i l a r to c u r r e n t - p o t e n t i a l curves of p a s s i v e m e t a l s . It can be d e m o n s t r a t e d t h a t the m o r e the p o t e n t i a l d e - creases d u r i n g i m m e r s i o n , t h e l a r g e r is the v a l u e of the p e a k c u r r e n t in the anodic p o l a r i z a t i o n curve. P o o r l y w e a t h e r e d s a m p l e s a l w a y s s h o w a c c o r d i n g l y h i g h p e a k - c u r r e n t densities, w h e r e a s i n w e l l - w e a t h e r e d s a m p l e s t h e y a r e e x t r e m e l y s m a l l o r do n o t e x i s t a t all.

F i g u r e 11 shows the anodic p o l a r i z a t i o n c u r v e of the s a m p l e M3x w h i c h was s c r a t c h e d d u r i n g i m m e r s i o n . T h e p e a k c u r r e n t d e n s i t y for t h e p a s s i v a t i o n of the s a m p l e s is r e m a r k a b l y h i g h e r t h a n in the s a m p l e w h i c h was o n l y i m m e r s e d . The p a s s i v a t i o n p o t e n t i a l ep h o w e v e r has t h e s a m e value.

T h e r e d u c t i o n curves, s t a r t i n g in t h e p a s s i v e state, also s h o w a n a c t i v a t i o n process in the s a m e p o t e n t i a l range, as is shown in Fig. 12. As is e x p e c t e d f r o m t h e anodic p o l a r i z a t i o n curves, this p h e n o m e n o n a p p e a r s m o r e c l e a r l y in p o o r l y w e a t h e r e d s a m p l e s t h a n i n w e l l - w e a t h e r e d ones.

9

~ 0,2

g ql g,

/ \

/ ^\

/ / \

/ / - - . , - - . S 3 x

I I /, ,/ ~ / /-M3x

, I I , / .

~--§

-400 0 + 400 + 800 '~ Potenlial

[mVsc E ]

Fig. 10. Anodic current density-potential-curve after 20 rain of immersion in O.]M Na2SO~, sweep rate 15 mV/min.

20C

E 0

~ -200

no

-400

o)

- Mix MSx

I I I = t [ m i n i

60 120 180

t,6

"" 1,2

~ 0,8 g

0,4

-,x v I

f',o: \ ~,"J I

" ~ : ' ~ F-M 5x (scratched)

|

/

F -M 3 X zz

/ i I . ~ - - ~ - - - ~ - - ~- - - - - ~ J I

- 4 0 0 0 ~p+400 + 800 ,. Potential

[ m Vsc E]

Fig. 11. Anodic current density-potential-curve of a scratched and an undamaged sample, 3x type weathering.

0•

^b) ^+6ol

'% 20 +4o

no 400~- ~ ~ S A S3x

- - --40

I ~ i ~ - t [ r n i n ]

60 120 180

Fig. 9. Potential decay during immersion: a) weathering steel M;

b) plain carbon steel S.

I I

/imox. = 8~juA/c

-200

\

I I ( ~ Potential [ mVsc E ]

0 -F200

Fig. 12. Cathodic current density-potentlai-curve of different weathered (lx, 3x) steels immediately after immersion in 0.1M Na2S04, sweep rate 15 mV/min., start at + 4 0 0 inV.

(6)

20 J. Electrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y January 1980

Discussion

F r o m the r e s u l t s of t h e e l e c t r o c h e m i c a l e x p e r i m e n t s one can conclude t h a t t h e w e a t h e r e d s a m p l e s a r e in a t r u e p a s s i v e s t a t e a t t h e b e g i n n i n g of t h e i m m e r s i o n period. But w i t h t i m e the p o t e n t i a l d e c r e a s e s to l o w e r v a l u e s a n d r e a c t i v a t i o n of t h e p a s s i v e s u r f a c e occurs.

In o r d e r to m a i n t a i n p a s s i v i t y , an e a s i l y r e d u c i b l e o x i d i z i n g a g e n t has to be p r e s e n t in t h e r u s t of t h e w e a t h e r e d samples. This o x i d i z i n g a g e n t keeps t h e p o t e n t i a l of the s y s t e m in t h e p a s s i v e r a n g e u n t i l i t is u s e d up. N o t h i n g can be said f r o m this e x p e r i m e n t c o n c e r n i n g the q u a n t i t y a n d t h e t y p e of this o x i d i z i n g agent. As t h e d i f f e r e n t a l l o y i n g e l e m e n t s do not influence the p a s s i v a t i o n w i t h i n the d i f f e r e n t t y p e s of w e a t h e r i n g , i t m a y b e c o n c l u d e d t h a t a n e a s i l y r e d u c i - b l e p a r t of t h e o r d i n a r y r u s t is r e s p o n s i b l e for m a i n - t a i n i n g passivity. I n this e x p e r i m e n t such a r u s t p h a s e could be d e t e r m i n e d n e i t h e r b y c a t h o d i c r e d u c t i o n curves nor b y x - r a y diffraction analysis. This i n d i c a t e s t h a t t h e a m o u n t is e i t h e r v e r y s m a l l a n d / o r t h a t the s t r u c t u r e of t h e o x i d i z i n g a g e n t is not m u c h different f r o m the s t r u c t u r e of the o r d i n a r y rust.

The test w i t h s c r a t c h e d s a m p l e s s h o w e d t h a t b y i n - c r e a s i n g the a c t i v e s u r f a c e the anodic c u r r e n t i n c r e a s e s as well. T h e r e f o r e i t can be s u p p o s e d t h a t t h e a c t i v e s u r f a c e of t h e p o o r l y w e a t h e r e d s a m p l e s l x is m u c h l a r g e r c o m p a r e d to t h e w e l l - w e a t h e r e d ones. This fact e x p l a i n s also w h y the p o t e n t i a l of t h e s a m p l e s l x d e - c a y e d so m u c h f a s t e r to l o w e r values. P a s s i v i t y is m a i n t a i n e d h e r e o n l y u n t i l t h e o x i d i z i n g a g e n t is u s e d up.

M o r e o v e r the m e t a l l o g r a p h i c e x a m i n a t i o n s s h o w t h a t t h e r u s t l a y e r of t h e l x w e a t h e r e d s a m p l e s is c o v e r e d b y n u m e r o u s cracks, in c o n t r a r y to the r u s t o b t a i n e d b y 3 x - t y p e w e a t h e r i n g w h i c h is a l m o s t f r e e f r o m cracks. I n this case the m e t a l s u r f a c e is m o s t l y c o v e r e d w i t h the d a r k c o m p a c t r u s t phase. The c r a c k s in t h e r u s t l a y e r a r e s u p p o s e d to be the sites of a c t i v e c o r r o - sion on t h e m e t a l surface. T h e b e t t e r t h e steel is w e a t h e r e d , t h e m o r e it is c o v e r e d w i t h t h e p r o t e c t i v e l a y e r , i.e., the l o n g e r the oxidizing a g e n t lasts to m a i n - t a i n passivity. Thus t h e p o t e n t i a l decreases slower.

D u r i n g the d r y i n g p e r i o d s r e d u c e d r u s t is r e o x i d i z e d a g a i n b y t h e a t m o s p h e r i c o x y g e n , so t h a t t h e s a m p l e s a r e p a s s i v e again.

W i t h t h e r e s u l t s o b t a i n e d so f a r the f o l l o w i n g m o d e l for t h e f o r m a t i o n of corrosion p r o t e c t i v e l a y e r s on l o w - a l l o y steels can be p r o p o s e d : W h e n the m e t a l s u r f a c e is e x p o s e d to f r e q u e n t d r y / w e t cycles, t h e

w h o l e s u r f a c e c o r r o d e s r a t h e r evenly. I n case of s h o r t w e t p e r i o d s no l a r g e s u l f a t e p o c k e t s are formed. Due to p r e c i p i t a t i o n of i n s o l u b l e c o r r o s i o n p r o d u c t s a n d aging t h e m e t a l s u r f a c e is g r a d u a l l y c o v e r e d b y a r a t h e r c o m p a c t a n d m a i n l y a m o r p h o u s r u s t l a y e r w h i c h p r o t e c t s most os the s u r f a c e f r o m f u r t h e r a t t a c k . i n a l l y the r e d u c t i o n c u r r e n t m a i n t a i n e d b y r e d u c i b l e r u s t c o m p o n e n t s is sufficiently l a r g e to p a s s i v a t e t h e r e s i d u a l a c t i v e a r e a s d u r i n g t h e w e t periods. T h e g r o w t h of l a r g e r u s t b l i s t e r s is t h e r e f o r e i n h i b i t e d and t h e o v e r - a l l c o r r o s i o n r a t e d e c r e a s e s continuously.

This process occurs not o n l y on l o w - a l l o y b u t also on u n a l l o y e d steels.

~vhen the m e t a l s u r f a c e is e x p o s e d to long w e t p e - riods, t h e c o r r o s i o n r a t e i n c r e a s e s a n d l a r g e s u l f a t e p o c k e t s a r e formed. The s u r f a c e is o n l y p a r t i a l l y c o v - e r e d b y the corrosion i n h i b i t i n g a m o r p h o u s r u s t l a y e r . I n this case t h e r e s i d u a l active s u r f a c e a r e a s a r e con- s i d e r a b l y l a r g e r a n d can be p a s s i v a t e d in t h e long w e t p e r i o d o n l y for a s h o r t p e r i o d of time. I n this w a y localized c o r r o s i o n processes a r e s t i m u l a t e d a n d due to the f o r m a t i o n of v o l u m i n o u s corrosion p r o d u c t s l a r g e c r a c k s a r e formed. F i n a l l y the w h o l e r u s t l a y e r is p e e l e d off. T h e c o r r o s i o n i n h i b i t i n g effect of these l a y e r s is t h e r e f o r e small.

M a n u s c r i p t s u b m i t t e d Nov. 15, 1978; r e v i s e d m a n u - s c r i p t r e c e i v e d M a y 18, 1979.

A n y discussion of this p a p e r w i l l a p p e a r i n a D i s - cussion Section to be p u b l i s h e d in t h e D e c e m b e r 1980 JOURNAL. A l l discussions for t h e D e c e m b e r 1980 D i s - cussion S e c t i o n s h o u l d b e s u b m i t t e d b y Aug. 1, 1980.

Publication costs of this article were assisted by the Swiss Federal Institute of Technology.

R E F E R E N C E S

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2. W. F r i e h e a n d U. T e n h a v e n , Stahl Eisen., 92, 277 (1972).

3. S. G. V e d e n k i n , Prot. Met., 11, 259 (1975).

4. C. P. L a r r a b e e a n d S. K. Coburn, F i r s t I n t e r n a t i o n a l Congress on M e t a l l i c Corrosion, p. 276, L o n d o n (1962).

5. A. M. E d w a r d s , Proc. S y m p . on D e v e I o p m e n t s in M e t h o d s of P r e v e n t i o n a n d C o n t r o l of C o r r o s i o n in Buildings, B r i t i s h I r o n a n d S t e e l F e d e r a t i o n , L o n d o n (1966).

6. T. Misawa, K. Hashimoto, a n d S. S h i m o d e i r a , Corros. Sci., 14, 181 (1974),