Corrosion of Valve Metals - ACS Publications - American Chemical

7 Corrosion of Valve Metals J. E. D R A L E Y

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 19, 2015 | http://pubs.acs.org Publication Date: January 31, 1979 | doi: 10.1021/bk-1979-0089.ch007

Argonne National Laboratory, Argonne, I L 60439

I had a little problem deciding what kind of t a l k to give you. I concluded that it should not be a gene r a l survey of everything I can f i n d that is r e l a t e d to corrosion of valve metals or film-forming metals; that it should not contain j u s t one or two topics within that subject--about which I could talk enough to give you some depth--but a compromise between these two. Secondly, I had to decide whether to t a l k t h e o r e t i c a l l y or mathematically, on the one hand, or phenomenologica l l y on the other, and I have chosen the l a t t e r approach. My intent is to give you some perspective about the way these materials corrode. I have selected some examples that I think are s u f f i c i e n t l y general and i l l u s t r a t e the p r i n c i p l e s well enough to do t h i s . Many of the s l i d e s are taken from our own work of some years ago. I t must be acknowledged that the unique corrosion c h a r a c t e r i s t i c s of a number of the valve metals are not i d e n t i f i e d , and behavior in a number of common environments is not addressed. F i r s t of a l l , I have found the t i t l e valve metals to be confusing f o r a number of people; f o r example, someone asked i f they are the metals of which valves are made. Let me give you a few guidelines. F i r s t , the valve metals form r e l a t i v e l y perfect oxide f i l m s . That means they are r e l a t i v e l y perfect with respect to protection against corrosion, which we s h a l l amplify a little. Secondly, they are r e l a t i v e l y perfect insofar as there is not much l o c a l breakdown or leakage when they are anodized. P a r t l y r e l a t e d to that there are high f i e l d s in these oxides during the period of growth--fields greater than about 10 volts/cm. In the f i r s t figure, we see i l l u s t r a t e d a r e l a t i o n ship often drawn between the energy of the bond between oxygen and the metal and the f i e l d required to produce an anodic f i l m . In t h i s instance, the f i e l d is that 6

0-8412-0471-3/79/47-089-185$12.50/0 © 1979 American Chemical Society

In Corrosion Chemistry; Brubaker, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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40

Γ

MO BOND ENERGY, eV Journal of the Electrochemical Society Figure 1.

Field required to sustain an ionic current density of 2 X JO" A · cm' for anodic oxides as function of metaV-oxide bond energy (1) 3



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which corresponds t o a g i v e n s m a l l c u r r e n t in a n o d i z i n g , or production of the oxide f i l m . The f a c t o r s respons i b l e f o r sound and p r o t e c t i v e f i l m s are not w e l l known. I t is s t i l l a b i t o f a m y s t e r y w h y s o m e s y s t e m s form better films than others. A number o f i n d i v i d u a l princ i p l e s have been brought up. P i l l i n g and Bedworth y e a r s ago w r o t e t h a t i f t h e volume o f t h e o x i d e prod u c e d o n a m e t a l s u r f a c e is l e s s t h a n t h e v o l u m e of m e t a l f r o m w h i c h i t is p r o d u c e d , t h e r e is l i t t l e chance t h a t t h e o x i d e f i l m w i l l be p r o t e c t i v e . As s t a t e d , t h a t is a g o o d g u i d i n g p r i n c i p l e , b u t o f a l l t h e syst e m s f o r w h i c h t h e r e is e x p a n s i o n in t h e f o r m a t i o n o f oxide, the r u l e doesn t p r e d i c t r e l i a b l y which oxides w i l l be p r o t e c t i v e and w h i c h w i l l not. Strong bonding between the oxide and the m e t a l s u b s t r a t e seems t o be e s s e n t i a l t o t h e f o r m a t i o n o f highly protective films. T h a t ' s one p r i n c i p l e I t h i n k you can h o l d on t o . The o t h e r s d o n t work v e r y w e l l . There are a l o t of p e c u l i a r i t i e s ; for example, in s o m e c a s e s t h e o x y g e n d i s s o l v e s s i g n i f i c a n t l y in t h e surface layers of the metal. This doubtless reduces the e f f e c t i v e s i z e o f the s u r f a c e m e t a l atoms and allows adjustment of the p o s i t i o n s of the surface atoms o f t h e s u b s t r a t e so t h e y w i l l n e a r l y m a t c h the p o s i t i o n s o f t h e c o n t i g u o u s c a t i o n s in t h e oxides. F i g u r e 2 shows t h a t w h i c h m e t a l s f o r m g o o d f i l m s depends not o n l y on the bond energy b u t a l s o on the s o l u t i o n in w h i c h t h e f i l m s a r e f o r m e d . These curves a r e f o r a n o d i z i n g a l u m i n u m in a s e r i e s o f solutions. The top 5 c u r v e s a r e f o r a f a m i l y o f s o l u t i o n s in which the f i l m s remain r e s i s t a n t to greater thicknesses t h a n f o r t h e two l e s s i d e a l s o l u t i o n s . In other solut i o n s , o n e c a n t g e t a n o d i z e d f i l m s at a l l . The r e s i s t i v i t y o n t h e o r d i n a t e is f o r t h e f i l m i t s e l f . I t h i n k i t is i m p o r t a n t t o n o t e t h a t m a n y m e t a l s form h y d r a t e d o x i d e s , in w h i c h h y d r o x i d e i o n s a r e constituents. As a g e n e r a l r u l e , h i g h l y p r o t e c t i v e f i l m s are a n h y d r o u s ; i t is o f t e n s p e c u l a t e d t h a t t h e h i g h f i e l d present during the formation of the f i l m (vide supra) contributes to the formation of anhydrous r a t h e r than the hydrated oxide. I t is e a s y t o s e e t h a t a h i g h f i e l d o p e r a t e s when one a n o d i z e s by t h e a p p l i c a t i o n o f a p o t e n t i a l ; i t is l e s s o b v i o u s , b u t e v i d e n t l y e q u a l l y true during unassisted thin-film oxidation. Local f i e l d s from m u l t i p l y - c h a r g e d cations might contribute to the formation of compact, anhydrous oxides; the best films u s u a l l y contain cations w i t h a charge of 3 or more. One m o r e t h i n g a b o u t f i l m s : they often are not v e r y s t a b l e in w a t e r o r in a q u e o u s s o l u t i o n s . This tends to cause confusion about the p r o t e c t i v e n e s s of f

f

f




188

Barrier Film Thickness ( Â xlO ) 2

100

3

6

9

Τ

I

1

15

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1

(I) Sulphuric Acid (2) Oxalic Acid (3) Phosphoric Acid

80

(4) Malonic Acid (5) Chromic Acid (6) Tartaric Acid (7) Ammonium Borate (IO%Solutions)

60

120

180

240

300

Quantity of Electricity (mQ/cm ) 2

Electrochimica Acta Figure 2.

Differential specific resistance for growing barrier films on aluminum β)



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oxide films. The f i l m i t s e l f may be a v e r y g o o d o n e , b u t i t may d e t e r i o r a t e , s o m e t i m e s s l o w l y , sometimes r a p i d l y ; sometimes r a t h e r g e n e r a l l y and sometimes locally. I t s p r o t e c t i v e n e s s t h u s may b e t e m p o r a r y or i m p e r f e c t as a f u n c t i o n of t i m e . One o f t h e conundrums about such p r o t e c t i v e f i l m s has always been how t o r a t i o n a l i z e t h e d e m o n s t r a t e d r e q u i r e m e n t t o a d s o r b s p e c i e s o n some m e t a l s u r f a c e s o r o n some m e t a l oxide surfaces to provide p r o t e c t i o n . I t appears to me t h a t t h e f u n c t i o n o f t h e a d s o r b a t e is t o protect the oxide f i l m against the h y d r a t i n g or s o l v a t i n g effect of the water. T h e f i l m i t s e l f is g o o d e n o u g h , then, to provide p r o t e c t i o n . I n F i g u r e 3 we s e e t h e r e l a t i o n s h i p b e t w e e n m e t a l oxide bond energy and the T a f e l s l o p e - - t h e r e l a t i o n s h i p between p o t e n t i a l and l o g c u r r e n t d u r i n g oxide film formation. I t s a y s , in e f f e c t t h a t w h e n t h e b o n d e n e r g y is h i g h e r , i t t a k e s m o r e v o l t a g e i n c r e a s e f o r a g i v e n i n c r e a s e in c u r r e n t , w h i c h is t o s a y t h a t w h e n t h e b o n d e n e r g y is h i g h e r i t is m o r e d i f f i c u l t t o p a s s e x t r a c u r r e n t t h r o u g h a more p r o t e c t i v e film. C o r r o s i o n and o x i d a t i o n of v a l v e metals g e n e r a l l y c o n s i s t both of the growth and degradation of oxide films. I ' d l i k e to begin d i s c u s s i o n of the growth of o x i d e f i l m s w i t h F i g u r e 4, t a k e n f r o m H a u f f e ( 3 ) . In t h i s c a s e i r o n is o x i d i z e d d i r e c t l y t o f e r r i c oxide in a n i t r i c - n i t r o u s a c i d s o l u t i o n . The a u t h o r has suggested the existence of concentration gradients for cation l a t t i c e defects, oxygen v a c a n c i e s , and electrons. D u r i n g t h e g r o w t h o f t h e f i l m , i t is p r o posed that e l e c t r o n s , c a t i o n s , and anions migrate. A l a r g e space charge v a r i a t i o n is proposed w i t h i n the o x i d e f i l m — a c a s e w h i c h was i g n o r e d f o r y e a r s by p e o p l e who s t u d i e d o x i d a t i o n . The r e s u l t a n t c o m b i n a t i o n of f i e l d s from surface charges and from the space charge l e a d s t o n o e l e c t r i c a l f i e l d at t h e i n t e r f a c e between Z o n e s I a n d I I at w h i c h l o c a t i o n n e w o x i d e is proposed to form. O t h e r more c o m p l i c a t e d models have been d e v e l o p e d ; I h a v e c h o s e n t o show t h i s b e c a u s e o f i t s generally applicable principles. I n F i g u r e 5 we s e e t h a t s o m e t i m e s s e e m i n g l y perfect t h i n f i l m s are f a r from perfect. T h i s is f o r a n a l u m i n u m a l l o y t h a t , f r o m some t e c h n i q u e s , w o u l d a p p e a r to carry a perfect film. We s e e s o m e o f a s e r i e s of p l a t e l e t s g r o w i n g o u t in t h i s t r a n s m i s s i o n e l e c t r o n micrograph t a k e n across an edge. F i g u r e 6 shows a c a s e in w h i c h o x i d e n u c l e i g r o w o n a m e t a l s u r f a c e . This o n e is f o r i r o n in l o w p r e s s u r e o x y g e n ; s i m i l a r r e s u l t s have been observed f o r other m e t a l s . If we l o o k at this surface during oxidation, we s e e only a very thin



190 CORROSION C H E M I S T R Y



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Surface Charge Plenum Press Figure 4. Schematic of the concentration of the free electrons and ionr-defect positions in the homogeneously structured passive layer Fe O with space-charge inversion (3) 2

s

Academic Press Figure 5.

Oxide platelets viewed by electron silhouette on aluminum after corrosion for 30 hoursinsteamat540° C (4)



192


Maruzen Co., Ltd. Figure 6.

Oxide nuclei on (111) iron foil after oxidation at 540°C and 1.1 X 10~ Ton for 55 minutes (5)


5

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f i l m u n t i l n u c l e i show up at the same t i m e at many p l a c e s o v e r the s u r f a c e . These c r y s t a l s t h e n grow at m e a s u r a b l e r a t e s . T h e i r o r i e n t a t i o n can be d e t e r m i n e d and r e l a t e d t o the o r i e n t a t i o n o f the m e t a l s u b s t r a t e . I d now l i k e t o show you some u n p u b l i s h e d c o l o r p h o t o g r a p h s o f a s e r i e s o f i r o n samples w h i c h were o x i d i z e d f o r t h r e e days in w a t e r at e l e v a t e d temperat u r e s . We've changed the p o t e n t i a l f o r o x i d a t i o n by u s i n g d i f f e r e n t c o n c e n t r a t i o n s o f oxygen in w a t e r . W i t h 290 ppm oxygen c o n c e n t r a t i o n at 260°C., a h i g h l y p e r f e c t f e r r i c o x i d e f i l m formed showing a l i t t l e t i n t o f i n t e r f e r e n c e c o l o r — t h a t is o f t e n v a l u a b l e f o r g i v i n g some c l u e s . The c o l o r i n d i c a t e s t h a t the t h i c k n e s s is u n i f o r m o v e r a b i g enough a r e a t o d e v e l o p the i n t e r f e r ence c o l o r ; in a few m i n u t e s , w e ' l l see t h a t t h i c k n e s s e s sometimes v a r y from g r a i n t o g r a i n . A t 35 ppm oxygen a r e a s o f breakdown a r e o b s e r v e d . The n e x t s l i d e shows some p h o t o g r a p h s o f samples at 150°C., but o t h e r w i s e at the same c o n d i t i o n s . Y o u ' l l see t h a t f o r the same oxygen c o n c e n t r a t i o n s in the w a t e r the p r o t e c t i o n is n o t as good. A g r e a t e r o x i d i z i n g p o t e n t i a l (oxygen c o n c e n t r a t i o n ) is r e q u i r e d f o r a h i g h l y p r o t e c t i v e f i l m as the t e m p e r a t u r e is l o w e r e d ; the t r e n d c o n t i n u e s t o at l e a s t as low as 100°C ( t h i r d s l i d e ) at w h i c h tempera t u r e 540 ppm O2 p r o v i d e d an e x c e l l e n t f i l m ( & ) . To the b e s t o f my knowledge, nobody has u s e d t h i s system t o m a i n t a i n the p o t e n t i a l l y e x c e l l e n t aqueous c o r r o s i o n r e s i s t a n c e o f i r o n . The n e x t s l i d e shows a range o f i n t e r f e r e n c e c o l o r s f o r c o n t i g u o u s g r a i n s on a s p e c i men o f i r o n , under c o n d i t i o n s i d e n t i c a l t o t h o s e shown earlier. I f t h e r e is a d e f i c i e n c y o f o x y g e n — i n s u f f i c i e n t oxygen t o p r o v i d e good p r o t e c t i o n — s i z e a b l e p i t s a r e formed in p u r e w a t e r . A l e s s o n f r o m t h i s is t h a t p i t t i n g doesn't o c c u r o n l y in s t r o n g e l e c t r o l y t e s o r selected electrolyte solutions. I f the c o m p o s i t i o n o f the a l l o y is changed the c o m p o s i t i o n o f the o x i d e changes, and i t is p o s s i b l e t o make the o x i d e c o n s i d e r a b l y more p r o t e c t i v e on a m e t a l l i k e i r o n . The n e x t s l i d e shows the e f f e c t o f a l i t t l e b i t o f chromium. C r o l o y - 5 c o n t a i n s 5% c h r o mium and some molybdenum; you can see t h a t at 260°C w i t h 35 ppm 0 c o r r o s i o n r e s i s t a n c e is s u b s t a n t i a l l y b e t t e r t h a n t h a t o f i r o n . As shown in F i g u r e 7 the c o r r o s i o n b e h a v i o r o f s u c h a l l o y s a l s o seems f a v o r a b l y i n f l u e n c e d by h i g h oxygen c o n c e n t r a t i o n in w a t e r . I n F i g u r e 8 i t is seen t h a t t h e r e is c o n s i d e r a b l e chromium e n r i c h m e n t in the o x i d e f i l m s on o x i d i z e d i r o n - c h r o m i u m a l l o y s ; e v i d e n t l y the g r e a t e r p r o t e c t i v e n e s s o f the f i l m s f o r the chromium-bearing a l l o y s is r e l a t e d t o t h i s composition. F i g u r e 9 shows t h a t the r a t i o o f oxygen t o f


193

2



194


Corrosion Figure 7.

Corrosion of Croloy 2 k steelinwater at 260°C (6) x



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CHEMICAL ANALYSIS 24 Cr

14 Cr 10 Cr

-o- —

5Cr 20

40

60

SPUTTERING TIME , min Journal of the Electrochemical Society Figure 8.

Cr/Fe ratio as a function of distance from surface for four oxidized iron—chromium alloys (7). Ratios for alloys on right.

40

80

120

SPUTTERING TIME, min Journal of the Electrochemical Society Figure 9.

O/M ratio as a function of depth in oxideiniron and iron-chromium alloys (7)


196

CORROSION

m e t a l a l s o v a r i e s w i t h i n the f i l m . That c o r r e s p o n d i n g t o M 0 is shown. As one g e t s c l o s e r t o the m e t a l , shown by s u r f a c e a n a l y s i s a f t e r s p u t t e r i n g some materi a l o f f the s u r f a c e , the o x i d e is seen t o have l e s s oxygen in i t . I t is a n o t h e r p e c u l a r i t y o f some o f t h e s e systems t h a t in the t h i n f i l m r a n g e , the s t o i e h i ometry o f the o x i d e is n o t as e x p e c t e d . As most o f you know, i f n i c k e l is added t o c h r o mium a l l o y s a s e r i e s o f s t a i n l e s s s t e e l s is p r o d u c e d w i t h q u i t e good c o r r o s i o n r e s i s t a n c e . These a l l o y s have a w i d e s p r e a d use; t h e s e days t h o s e w i t h composit i o n s c l o s e t o 18 Cr-8 N i are i n c r e a s i n g l y b e i n g u s e d in many p r a c t i c a l a p p l i c a t i o n s . I guess I s h o u l d warn you t h a t i f you l i s t e n e d t o Roger S t a e h l e you h e a r d t h a t t h e r e a r e a number o f i n s t a n c e s when t h e y a l s o crack unexpectedly a f t e r a p e r i o d of exposure. So t h e y a r e by no means p e r f e c t l y r e s i s t a n t t o c o r r o s i o n ; n e v e r t h e l e s s , t h e y o f f e r a v e r y s u b s t a n t i a l improvement in the p e r f o r m a n c e o f low t o moderate c o s t m a t e r i a l s . F i g u r e 10 is made up t o show t h a t i f aluminum is added t o 304 s t a i n l e s s s t e e l - - t h a t is r o u g h l y the s i m p l e 18 and 8 s t a i n l e s s s t e e l - - t h e c o r r o s i o n p r o t e c t i o n p r o v i d e d by the o x i d e t o s u p e r h e a t e d steam is v e r y markedly increased. T h i s element a l s o i n c r e a s e s r e s i s t a n c e t o o x i d a t i o n by g a s e s - - a i r , oxygen, and c a r b o n d i o x i d e . Aluminum is an e f f e c t i v e a l l o y i n g c o n s t i t u e n t f o r imp r o v i n g the c o r r o s i o n - o x i d a t i o n r e s i s t a n c e o f i r o n as w e l l . 'The p r o t e c t i v e f i l m is e n r i c h e d in aluminum as compared t o the m e t a l c o m p o s i t i o n . In both systems, the a l l o y s c o n t a i n i n g aluminum t e n d t o be b r i t t l e . I have been t a l k i n g t o you about the f o r m a t i o n o f o x i d e f i l m s ; f o r c o m p l e t e n e s s i t is a p p r o p r i a t e t o m e n t i o n t h a t f i l m s a l s o grow o r f o r m by r e c r y s t a l l i z a t i o n o f a s u b s t r a t e l a y e r o r by h y d r a t i o n o f a materi a l t h a t was o r i g i n a l l y formed in an anhydrous form. As a g e n e r a l r u l e , t h o s e c a s e s produce f i l m s t h a t a r e not h i g h l y p r o t e c t i v e . There is a p r a c t i c a l e x c e p t i o n t o t h a t : i f one a n o d i z e s aluminum in some e n v i r o n m e n t s , n o t a b l y s u l f u r i c a c i d , the c o a t i n g is r e l a t i v e l y p o r o u s , w i t h the o x i d e o n l y p a r t i a l l y h y d r a t e d . By b o i l i n g in w a t e r , h y d r a t i o n is i n c r e a s e d , the volume o f the o x i d e is i n c r e a s e d and the p o r e s a r e s e a l e d , and an e f f e c t i v e p r o t e c t i v e l a y e r is formed. I'd l i k e t o s w i t c h now and t a l k t o you about degr a d a t i o n of f i l m s . I n o r d e r t o t a l k about c o r r o s i o n one has t o c o n s i d e r b o t h the growth o f f i l m s and t h e i r degradation. As one s t u d i e s them, one f i n d s t h a t the c o m b i n a t i o n o f the two is the key t o g e t t i n g a measure o f u n d e r s t a n d i n g o f the b e h a v i o r o f the system. Some o f the r e s u l t s a r e a l i t t l e u n e x p e c t e d at f i r s t g l a n c e . 2


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%AI

Metal Loss, mg/cm

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18.0

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8.8

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Samples exposed in as cast condition, surfaces electropolished. Corrosion Figure 10.

Corrosion of aluminum-modified Type 304 S S for 28 days in steam contg. 30 ppm 0 at 650°C (8) 2

20

40

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IOO

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TIME, days Figure 11.

Parabolic corrosion of experimental aluminum alloy A288 (Al + 1% Ni, 0.5% Fe, 0.1% Ti)instagnant waterat260°C (9)


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I t is w e l l f o r us t o be a l e r t f o r such t h i n g s . L e t me make an o b v i o u s p l a t i t u d e f o r y o u about a f i l m t h a t degrades at a c o n s t a n t r a t e . I t is o b v i o u s t h a t t h e m e t a l w i l l c o r r o d e at a c o n s t a n t r a t e i f d e g r a d a t i o n is u n i f o r m a l l o v e r t h e s u r f a c e , a l t h o u g h i t m i g h t t a k e some time t o r e a c h t h a t s t a g e w h i l e i t b u i l d s up a film. That is n o t a r a r e phenomenon. I t is more common, I t h i n k f o r d e g r a d a t i o n n o t t o be t h a t u n i f o r m o v e r t h e s u r f a c e . We're g o i n g t o t a l k about some o f those cases. F i r s t , l e t us c o n s i d e r a r a t h e r s i m p l e c a s e : t h e f i l m d i s s o l v e s in w a t e r . When i r o n o r s t e e l c o r r o d e s in h i g h t e m p e r a t u r e w a t e r w i t h o u t oxygen, t h e r a t e o f c o r r o s i o n is d e t e r m i n e d by t h e r a t e at w h i c h t h e f i l m d i s s o l v e s and is l o s t from t h e s u r f a c e . T h i s r a t e remains c o n s t a n t i f t h e r e is some p r o c e d u r e t h a t c l e a n s up t h e w a t e r and keeps i t from becoming s a t u r a t e d . One such p r o c e d u r e is passage t h r o u g h a l o o p c o n t a i n i n g a l o w e r t e m p e r a t u r e septum in w h i c h some o f t h e m a t e r i a l p r e c i p i t a t e s . A n o t h e r is passage t h r o u g h an i o n exchange r e s i n o r some o t h e r d e v i c e t h a t w i l l p u r i f y the w a t e r . Such l o o p systems o f t e n c o n t a i n suspended s o l i d c o r r o s i o n p r o d u c t in t h e w a t e r . This m a t e r i a l in n u c l e a r r e a c t o r systems is c a l l e d c r u d and i t has been t h e s o u r c e o f a l o t o f i r r i t a t i o n and money. These come about because t h e c r u d is r a d i o a c t i v e and d e p o s i t s in a l l p a r t s o f t h e system, c o m p l i c a t i n g maintenance. I f one chooses t h e t e m p e r a t u r e and t h e a l l o y ( e . g . A288 c o n t a i n i n g 1% N i ) aluminum forms in w a t e r a f i l m such t h a t t h e r a t e o f f o r m a t i o n is 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 . That g i v e s what's c a l l e d t h e p a r a b o l i c g r o w t h l a w ; d a t a f o r such b e h a v i o r a r e shown in F i g u r e 11 ( t o g e t h e r w i t h a dashed c u r v e f o r a n o t h e r experiment). I n t h e k i n d o f system in râiich f r e s h w a t e r is c o n t i n u o u s l y added and t h e e x c e s s a l l o w e d s i m p l y t o l e a k o u t , at l e a s t a p a r t i a l l y s a t u r a t e d s o l u t i o n is l o s t a l l o f t h e t i m e . The two specimens o f F i g u r e 12 have been f i t t e d by c u r v e s t h a t have t h e same growth r a t e c o n s t a n t as in F i g u r e 11 and d i f f e r e n t d i s s o l u tion rates. They were in t h e same a u t o c l a v e at s l i g h t l y d i f f e r e n t l o c a t i o n s ; probably the water contained a l i t t l e more d i s s o l v e d aluminum at one specimen t h a n at t h e o t h e r . The e x p r e s s i o n f o r t h e p a r a l i n e a r b e h a v i o r in i t s s i m p l e form is: dt - L-(g+ft)

'



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Paralinear corrosion of aluminum alloy A288 in refreshed water at 260°C(9)

1 CORROSION OF ALLOY 255 IN 350 · WATER • —CONTROL SAMPLES • - 0 . 2 MILS OXIDE REMOVED Ο - 0.4 MILS OXIDE REMOVED

—

1

ol0

20

1

*-

8 I

£

40 TIME,

1

60

1

80

days Journal of Nuclear Materials

Figure 13.

Influence of corrosion-oxide removal on the corrosion of an O.lTi alloy (10)


Al-lNi-

CORROSION


200

CHEMISTRY

where L is t h e amount o f m e t a l l o s s ( d e t e r m i n e d t h r o u g h t h e use o f a s p e c i a l m e t a l t h i c k n e s s gauge), kp t h e p a r a b o l i c growth c o n s t a n t , f t h e r a t e o f d i s s o l u t i o n f o r t h e specimen, and g t h e amount o f d i s s o l u t i o n t h a t o c c u r s e a r l y in t h e e x p o s u r e . A t l o n g t i m e s t h e c u r v e f o r L v e r s u s time r e s e m b l e s a s t r a i g h t l i n e w i t h slope f. P a r a l i n e a r c o r r o s i o n ( r e l a t e d to 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 ) does n o t o c c u r f o r a l l aluminum a l l o y s in w a t e r at a l l h i g h t e m p e r a t u r e s . I n F i g u r e 13 are p l o t t e d d a t a f o r an a l l o y ( A l , 1% N i , 0.1% T i ) c o r r o d e d in w a t e r at 350°C ( 1 0 ) . The c o r r o s i o n r a t e was low and c o n s t a n t , as shown b e t t e r in o t h e r f i g u r e s in the same p u b l i c a t i o n . F o r some specimens in t h e f i g u r e 1/3 o r 2/3 o f t h e c o r r o s i o n p r o d u c t was removed m e c h a n i c a l l y a f t e r t h e f i r s t exposure p e r i o d . There was no d i s c e r n i b l e e f f e c t on subsequent c o r r o s i o n , ind i c a t i n g that control of corrosion probably resided c l o s e to the m e t a l - o x i d e i n t e r f a c e . Similar experiments f o r t h e a l l o y s and t e m p e r a t u r e s where p a r a l i n e a r b e h a v i o r o c c u r s showed t h a t removing some o f t h e p r o d u c t caused an i n c r e a s e in subsequent c o r r o s i o n r a t e . The use o f F i g u r e 14 b e g i n s d i s c u s s i o n about low t e m p e r a t u r e c o r r o s i o n o f aluminum in w a t e r . Again we 11 see t h a t the t o t a l f i l m is n o t c o n t r o l l i n g . This is t h e k i n d o f c u r v e o b t a i n e d in a c o u p l e o f t e s t s t h a t ran f o r a l o n g t i m e in c o n t i n u o u s l y r e f r e s h e d systems. Note t h a t t h e r a t e is d e c r e a s i n g c o n t i n u a l l y w i t h time f o r at l e a s t a few y e a r s — w e ' l l a n a l y z e t h a t c u r v e shape s t a r t i n g w i t h F i g u r e 15. A t t h e b e g i n n i n g o f the t e s t s ( f r o m 0.1 t o n e a r l y 10 h o u r s ) and subsequent t o about 100 h o u r s , t h e w e i g h t g a i n and t h e m e t a l c o r r o d e d ( d e t e r m i n e d t h r o u g h t h e use o f a s e n s i t i v e m e t a l t h i c k ness gauge) v a r i e d as t h e l o g a r i t h m o f t i m e . The i n i t i a l f i l m is boehmite, t h e same p a r t l y h y d r a t e d o x i d e t h a t forms at h i g h t e m p e r a t u r e s . When t h i s f i l m b r e a k s down, b e g i n n i n g in h a l f a day at t h e s e c o n d i t i o n s , we f i n d t h a t t h e t o t a l amount o f c o r r o s i o n and the t o t a l o x i d e p r e s e n t q u i c k l y i n c r e a s e s e v e r a l f o l d . The new p r o d u c t is t h e c o m p l e t e l y h y d r a t e d o x i d e b a y e r i t e . I f one runs t h e t e s t a l o n g time ( F i g u r e 16) one f i n d s t h a t t h e l o g a r i t h m i c r a t e law h o l d s . The dashed l i n e s i n d i c a t e t h a t the i n i t i a l p e r i o d is s e n s i t i v e t o t h e t e s t p r o c e d u r e t h a t is used. The h e i g h t o f t h e p l a t e a u v a r i e s i n v e r s e l y w i t h how w e l l r e f r e s h e d t h e s o l u tion is. The r e q u i r e m e n t f o r measurement s e n s i t i v i t y in t h i s t e s t was s u b s t a n t i a l . As a m a t t e r o f f a c t , t h e (eddy c u r r e n t ) gauge l i m i t a t i o n came n o t by i t s s e n s i t i v i t y (about t e n Angstrom u n i t s in d i a m e t e r f o r 1



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U.S. Atomic Energy Commission and European Atomic Energy Society Figure 14.

Corrosion experiments in water at two oxygen concentrations (< 0.4 and 19mg/L) (11)

0.1

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U.S. Atomic Energy Commission and European Atomic Energy Society Figure 15.

Early stagesincorrosion of aluminum (11)


1000


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a r o u n d specimen) b u t because o f the m e t a l l u r g i c a l changes t h a t o c c u r r e d in the samples. Now I'd l i k e t o a n a l y z e what happens t o the o x i d e d u r i n g the second l o g a r i t h m i c c o r r o s i o n p e r i o d . Fig u r e 17 shows the amount o f aluminum d i s s o l v e d f r o m a specimen (1100 aluminum) and c a r r i e d away in the d i s charge w a t e r . T h i s was d e t e r m i n e d by v e r y c a r e f u l l y c o l l e c t i n g the e f f l u e n t s o l u t i o n , b o i l i n g o f f the w a t e r , c o l l e c t i n g the aluminum, and a n a l y z i n g the m i c r o g r a m q u a n t i t i e s . D i s s o l u t i o n (amount = ξ) oc c u r r e d at a c o n s t a n t r a t e d u r i n g the second l o g a r i t h mic p e r i o d . Combining t h e s e d a t a w i t h w e i g h t g a i n (G) and m e t a l l o s t f r o m t h e s i n g l e specimen (L) , the amount o f boehmite (a) and t h e amount o f b a y e r i t e (b) shown in F i g u r e 18 were c a l c u l a t e d ( 1 3 ) . The c u r v e f o r boehmite l a y e r growth c l o s e t o the m e t a l c l o s e l y f o l lows the c u r v e f o r the amount o f c o r r o s i o n ( b o t h even showing an a t y p i c a l i n c r e a s e in s l o p e n e a r t h e end), w h i l e the amount o f b a y e r i t e s l o w l y d e c r e a s e s (presum a b l y by d i s s o l u t i o n ) . The e v i d e n c e is t h a t in t h i s system boehmite makes a p r o t e c t i v e ( r a t e - c o n t r o l l i n g ) l a y e r w h i l e the b a y e r i t e d i s s o l v e s and o b s c u r e s the truth. I w o u l d l i k e t o t e l l you about a n o t h e r k i n d o f u n u s u a l k i n e t i c b e h a v i o r , t h a t o f z i r c o n i u m . An a l l o y c a l l e d Z i r c a l o y 2 c o n t a i n s m i n o r amounts o f i r o n , chromium and n i c k e l and a l i t t l e t i n . The a l l o y was i n v e n t e d by a c c i d e n t a l l y c o n t a m i n a t i n g a z i r c o n i u m tin alloy with stainless steel. Since t h a t time p e o p l e have been making i t on p u r p o s e . The w e i g h t g a i n s l o p e s on t h e l o g - l o g p l o t in F i g u r e 19 a r e a p p r o x i m a t e l y 1/3 and t h e r a t e e x p r e s s i o n t h a t is n e a r l y f o l l o w e d is c a l l e d c u b i c o x i d a t i o n . I t o c c u r s i n i t i a l l y in w a t e r ; i t is f o l l o w e d by f i l m breakdown and an i n c r e a s e in o x i d a t i o n r a t e w i t h new k i n e t i c s (showing s t r a i g h t l i n e s and s l o p e s v e r y c l o s e t o 1 on t h i s p l o t ) . A l o g - l o g s t r a i g h t l i n e w i t h a s l o p e o f one is u n u s u a l , s i n c e i t r e q u i r e s the p a r t i c u l a r s t r a i g h t l i n e on c a r t e s i a n c o o r d i n a t e s t h a t p a s s e s t h r o u g h t h e o r i g i n . A t the t i m e o f "breakaway" t h e r e is a r e c r y s t a l l i z a t i o n o f the z i r c o n i u m o x i d e t o a l e s s coherent f i l m . That is an a d d i t i o n a l f o r m o f degradation. L e s t you b e l i e v e t h i n g s a r e too s i m p l e d u r i n g " c u b i c o x i d a t i o n I've chosen t o show t h i s s l i d e ( F i g u r e 20) b a s e d on d a t a by Bob Shannon at H a n f o r d , Washington, w h i c h shows t h a t i n d i v i d u a l specimens c o r r o d e in s e r i e s o f waves. P a u l P e m s l e r once c h r i s tened those the P i c t u r e s q u e H i l l s o f Shannon d u r i n g a m e e t i n g . What t h i s i l l u s t r a t e s is t h a t we're n o t 1 1




204

Journal of the Electrochemical Society Figure 17.

Corrosion product dissolved from specimen of Figure 18(13)



7. DRALEY Corrosion of Valve Metals


205


206


10,000 F

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Westinghouse Atomic Power Division Figure 19.

Corrosion of beta-quenched Zircaloy-2 in water and steam (14)


Corrosion

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DRALEY

Figure 21.

Corrosion of an aluminum alloy (1100 Al + 1% Ni) in high-temperature water (9)


CORROSION


208

CHEMISTRY

d e a l i n g w i t h continuous f i l m s c o n t i n u o u s l y growing w i t h o u t breakdown. I t happens t h a t t h e breakdown on t h e s e specimens o c c u r r e d o v e r much o f t h e s u r f a c e at the same t i m e so t h e phenomenon is v i s i b l e . If i t o c c u r r e d l o c a l l y and n o t at t h e same t i m e at d i f f e r e n t p l a c e s , y o u w o u l d n o t see t h e " h i l l s " , b u t odd k i n e t i c s such as r o u g h l y c u b i c . The phenomenon is by no means unique t o zirconium. F i g u r e 21 shows s i m i l a r b e h a v i o r by aluminum, a l s o in h i g h t e m p e r a t u r e w a t e r . The o x i d a t i o n o f z i r c o n i u m in oxygen at e l e v a t e d temperatures f o l l o w s near-cubic k i n e t i c s f o r awhile, t h e n p a r a b o l i c k i n e t i c s . A number o f e f f o r t s have been made t o e x p l a i n t h e n e a r - c u b i c b e h a v i o r , w i t h l o c a l i z e d o r l i n e d i f f u s i o n as t h a t perhaps g e n e r a l l y p r e f e r r e d . I ' l l d e s c r i b e f o r y o u my own t h i n k i n g , p u t t o g e t h e r and p r e s e n t e d i n f o r m a l l y in 1967. I t has n o t been publ i s h e d . A t 700°C t h e r e is an i n i t i a l l a y e r o f z i r c o nium o x i d e t h a t is r e l a t i v e l y p e r f e c t , t h a t forms i n t e r f e r e n c e c o l o r s on s m a l l a r e a s ; t h e r a t e o f growth o f f i l m and t h e r a t e o f d i f f u s i o n o f oxygen i n t o t h e m e t a l depend on t h e o r i e n t a t i o n o f t h e m e t a l c r y s t a l . P o l y c r y s t a l l i n e specimens p r e p a r e d m e t a l l u r g i c a l l y in d i f f e r e n t ways, always p u r e and e q u a l l y c a r e f u l l y t r e a t e d , o x i d i z e at q u i t e d i f f e r e n t r a t e s i n i t i a l l y , but t h e r a t e s become e q u a l at l o n g ( p a r a b o l i c ) t i m e s . To f i t t h e s e f a c t s I d e v e l o p e d t h e f o l l o w i n g model. The f i l m t h a t grows on t h e m e t a l s u r f a c e is p r o p o s e d t o be u n i q u e and d i f f e r e n t from t h e one t h a t is s t a b l e on t h e o u t s i d e and at l o n g t i m e s . X-ray d i f f r a c t i o n p a t t e r n s t a k e n in o u r l a b o r a t o r y d u r i n g i n i t i a l o x i d a t i o n ( s p e c i a l apparatus) suggest t h a t t h i s oxide is t e t r a g o n a l r a t h e r t h a n t h e commonly found monoc l i n i c oxide. A t any s m a l l a r e a on t h e s u r f a c e , f o r example f o r a s i n g l e c r y s t a l s u r f a c e , t h e i n i t i a l f i l m grows and oxygen d i f f u s e s i n t o t h e m e t a l at r a t e s t h a t are unique f o r that area u n t i l the f i l m t h i c k n e s s r e a c h e s t h e v a l u e s ( p r o p o s e d t o be t h e same f o r a l l areas). The i n i t i a l f i l m t h e n t r a n s f o r m s n e a r l y ins t a n t a n e o u s l y i n t o f i n a l f i l m , and t h e i n i t i a l f i l m t h e n grows a g a i n in a second c y c l e , a g a i n t o t h e l i m i t i n g t h i c k n e s s s. The p a r a b o l i c r a t e c o n s t a n t f o r t h e f i n a l f i l m is c ( v a l u e t h e same f o r a l l a r e a s ) and t h e r a t e o f growth o f i n i t i a l f i l m at a p a r t i c u l a r a r e a is t



7.

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Metals

209

where η is t h e number o f t h e growth c y c l e at t i m e t and kj_ is t h e l o c a l d i f f u s i o n c o n s t a n t f o r oxygen in m e t a l . An e s t i m a t e o f t h e t y p e o f o x i d a t i o n c u r v e t o be e x p e c t e d f r o m t h i s model was d e v e l o p e d f r o m an i n i t i a l case where two t y p e s o f s u r f a c e s (A and B) were i d e n t i f i e d , and r a t e c o n s t a n t s a s s i g n e d t h a t were n o t in c o n f l i c t w i t h any known d a t a . Weight g a i n s e x p e c t e d f o r t h e f i r s t thousand m i n u t e s a r e shown in F i g u r e 22 f o r a r e a s A and Β and f o r a specimen c o n s i s t i n g o f 80% t y p e A and 20% t y p e B. The ends o f t h e f i r s t growth c y c l e f o r each a r e a c a n be i d e n t i f i e d . A number o f w e i g h t - g a i n p o i n t s f o r t h i s h y p o t h e t i c a l specimen a r e p l o t t e d in F i g u r e 23 a l o n g w i t h a r e p r o d u c e d w e i g h t gain recorder t r a c i n g (the s o l i d l i n e ) . A perfect c u b i c l i n e is a l s o shown f o r c o m p a r i s o n (dashed l i n e ) . The f i t is so good t h a t i t seems h i g h l y l i k e l y t h a t t h e use o f more t y p e s o f a r e a s t o smooth o u t t h e average in t h e model w o u l d l e a d t o e x c e l l e n t f i t t i n g of real-sample data. I t i s n ' t p o s s i b l e t o s a y much more at t h i s t i m e ; I do b e l i e v e t h a t t h e w e i g h t - g a i n c u r v e f o r t h e s i n g l e c r y s t a l w a f e r (more t h a n 8 0 % o f a r e a o f one o r i e n t a t i o n ) shown in F i g u r e 24 d i s p l a y s b r e a k s and s l o p e s o f t h e r i g h t o r d e r f o r t h e segments on a l o g - l o g p l o t . My p e r s o n a l c o n v i c t i o n is t h a t some k i n d o f s t a t i s t i c a l models a r e g o i n g t o be r e q u i r e d t o f i t c o r r e c t l y much o f t h e c o r r o s i o n and o x i d a t i o n d a t a of the valve metals. You've h e a r d e l e c t r o c h e m i s t r y o f c o r r o s i o n as a l e c t u r e ; I s h o u l d n ' t spend much t i m e on i t b u t I ' d l i k e t o d e s c r i b e some e l e c t r o c h e m i c a l e f f e c t s f o r f i l m formers. F i r s t the general p r i n c i p l e s . I f you put a good e l e c t r o n i c c o n d u c t o r ( a m e t a l ) in an aqueous s o l u t i o n , y o u w i l l t y p i c a l l y f i n d t h a t an e l e c t r i c a l p o t e n t i a l is d e v e l o p e d between t h e p i e c e o f c o n d u c t o r and the s o l u t i o n . When i o n s o f t h e m e t a l e n t e r t h e s o l u t i o n and l e a v e e x t r a e l e c t r o n s b e h i n d a n e g a t i v e p o t e n t i a l is d e v e l o p e d . A l l o x i d a t i o n r e a c t i o n s o c c u r r i n g on t h e s u r f a c e a r e e x p e c t e d t o produce t h i s r e s u l t . Similarly, r e d u c t i o n r e a c t i o n s t h a t use e l e c t r o n s f r o m t h e m e t a l a r e e x p e c t e d t o p r o d u c e a more p o s i t i v e p o t e n t i a l in the m e t a l . The s o l u t i o n p o t e n t i a l o f t h e m e t a l i n f l u ences t h e r a t e o f an e l e c t r o c h e m i c a l h a l f - c e l l r e a c t i o n in a c c o r d a n c e w i t h L e C h a t e l i e r ' s P r i n c i p l e , so i t is p o s s i b l e t o p r e d i c t t h r o u g h t h e use o f t h e N e r n s t Equa t i o n t h e p o t e n t i a l t h a t w i l l e x i s t when t h e o n l y s i g n i f i c a n t l y r a p i d r e a c t i o n s a r e t h e o x i d a t i o n and r e d u c t i o n p a r t s o f a r e v e r s i b l e r e a c t i o n . When more t h a n one p o t e n t i a l l y r e v e r s i b l e process occurs, the r a t e of o x i d a t i o n w i l l be e x p e c t e d t o exceed t h e r a t e o f r e d u c t i o n f o r at l e a s t one and t h e c o n v e r s e f o r at l e a s t one. A t



210 CORROSION C H E M I S T R Y



DRALEY Corrosion of Valve Metals


212

CORROSION

CHEMISTRY

a s t e a d y - s t a t e p o t e n t i a l , the sum o f the r a t e s o f a l l o f the a n o d i c r e a c t i o n s w i l l e q u a l the sum o f the r a t e s o f a l l the c a t h o d i c r e a c t i o n s . F o r f i l m f o r m e r s , a t y p i c a l a n o d i c r e a c t i o n is M + H0

•> MOH

2

+ H

+

+ e" ,


so an o x i d e is formed and t h e s o l u t i o n becomes a c i d . The most common c a t h o d i c r e a c t i o n s a r e H0

+ e"

1/2

0

2

2

1/2

H

2

+ OH"

+ 2H 0 + 2e~ + 20H" 2

and ,

so c a t h o d i c r e a c t i o n s produce a l k a l i . Directly related t o the pH a r e the s t a b i l i t i e s o f the v a r i o u s s p e c i e s f o r the c o r r o d i n g m e t a l . Thus f o r i r o n , the s o - c a l l e d P o u r b a i x Diagram f o r i r o n in F i g u r e 25 shows p o t e n t i a l pH zones in w h i c h F e 0 o r F e ( 0 H ) a r e s t a b l e and thus in w h i c h p r o t e c t i v e f i l m s o f t h e s e s u b s t a n c e s m i g h t f o r m at a t o t a l i o n i c c o n c e n t r a t i o n o f 10" M. When a f i l m is p r e s e n t , the hydrogen p r o d u c e d from the second r e a c t i o n above is n o t n e c e s s a r i l y a l l l i b e r ated d i r e c t l y i n t o the water or s o l u t i o n . Some o f i t may be l i b e r a t e d b e n e a t h the f i l m as shown in F i g u r e 26. The r e s u l t may be l o c a l r u p t u r i n g o f the o x i d e f i l m - a form of degradation--the f o r m a t i o n of metal h y d r i d e , o r the e n t r y o f hydrogen i n t o t h e m e t a l , depending on w h i c h a r e f e a s i b l e o r most f a v o r a b l e . I b e l i e v e t h e r e a r e a number o f cases where f i l m r u p t u r e o c c u r s , a l though they a r e o f t e n n o t easy t o i d e n t i f y . We have d e c l a r e d the b e l i e f t h a t i t is i m p o r t a n t in the c o r r o s i o n o f aluminum a l l o y s below the b o i l i n g p o i n t o f w a t e r ( 1 9 ) . To p r o v i d e e v i d e n c e o f t h i s , we r a n a ser i e s o f e x p e r i m e n t s t o d e t e r m i n e the l o g a r i t h m i c c o r r o s i o n r a t e c o n s t a n t f o r 1100 aluminum at 70°C w i t h pot e n t i a l s c o n t r o l l e d by a s p e c i a l i n t e r r u p t i n g p o t e n t i o s t a t ( 2 0 ) . The r e s u l t s , in F i g u r e 27, show t h a t a n o d i c p o l a r i z a t i o n (diamond-shaped p o i n t s ) caused lower c o r r o s i o n r a t e s t h a n the u n p o l a r i z e d runs ( c i r c u l a r p o i n t s ) . A r e d u c t i o n in c a t h o d i c damage t o the f i l m is s u g g e s t e d . The p o t e n t i a l above w h i c h hydrogen s h o u l d n o t be l i b e r a t e d cannot be i d e n t i f i e d because the l o c a l pH d u r i n g a n o d i c p o l a r i z a t i o n c o u l d be cons i d e r a b l y below the 6+ o f the d i s t i l l e d w a t e r at 70°. We s h a l l see something e n l i g h t e n i n g on t h i s l a t e r . For pH 6 the p o t e n t i a l f o r the r e v e r s i b l e hydrogen l i b e r a t i o n r e a c t i o n is c a l c u l a t e d t o be about -0.4 v o l t . Most n o t a b l y at h i g h e r t e m p e r a t u r e s , aluminum a l s o s u f f e r s f r o m the e n t r y o f c o r r o s i o n p r o d u c t i n t o the 2

3

2

6



7.

DRALEY

Corrosion

of Valve

213

Metals

Marcel Dekker, Inc. Figure 25.

Pourbaix diagram for the system Fe-H 0 at 25° C (17) 2


214


CORROSION

CHEMISTRY


-0.5

Schematic of corrosion processes (IS)

-0.4

-0.3

-0.2

SOLUTION POTENTIAL,

Figure 27.

V.

-0.1 Vs.

STD.

0 HYD.

+0.1 ELEC.

Effect of controlled-solution potential on logarithmic corrosion-rate constant for aluminum (12)



7.

DRALEY

Corrosion

of

Valve

215

Metals

metal. I n F i g u r e 28 a r e shown specimens o f commerc i a l l y p u r e aluminum a f t e r two weeks exposure t o w a t e r at 275°C. The b l i s t e r i n g p r o g r e s s e s w i t h more s e v e r e exposure c o n d i t i o n s , as shown in F i g u r e 29 (66 h o u r s , 300°C). Some o f the b l i s t e r s a r e h o l l o w b e f o r e the w a t e r g a i n s a c c e s s and b e f o r e they become o x i d e - m e t a l m i x t u r e s . F i g u r e 30 shows what happens at a h i g h e r t e m p e r a t u r e ; t h i s exposure was f o u r hours at 315°C. I f v a r y i n g amounts o f m a t e r i a l a r e e t c h e d from t h e s u r f a c e o f a s e r i e s o f samples c o r r o d e d f o r a b r i e f p e r i o d , and each r e m a i n i n g sample is a n a l y z e d f o r hydrogen c o n t e n t , the hydrogen in t h e e t c h e d - o f f l a y e r s can be c a l c u l a t e d . The r e s u l t s in F i g u r e 31 show t h a t the hydrogen c o n t e n t o f the s u r f a c e l a y e r s i n c r e a s e d q u i t e a b i t , and demonstrate t h a t the gas t h a t formed t h e b l i s t e r s was hydrogen. I f t h e h y d r o g e n is produced l a r g e l y at a p o s i t i o n remote from the m e t a l s u r f a c e , as in F i g u r e 32, the s e v e r e damage is p r e v e n t e d . In t h i s c a s e , the aluminum is s i m p l y b o l t e d t o a p i e c e of s t a i n l e s s s t e e l . Exposure c o n d i t i o n s were as in F i g u r e 30. I f t o the w a t e r a r e added i o n s t h a t a r e r e d u c i b l e t o m e t a l ( l a r g e l y at a c t i v e cathode s p o t s ) m e t a l d e n d r i t e s a r e formed. The n i c k e l d e n d r i t e s in F i g u r e 33 were formed in t h i s way; no s e v e r e c o r r o s i o n o f 1100 aluminum was o b s e r v e d d u r i n g c o r r o s i o n exposure t o s t a b l e n i c k e l s a l t s o l u t i o n s at e l e v a t e d temperat u r e s . F i g u r e 34 s u g g e s t s t h a t i f d e p o s i t s o f somet h i n g l i k e t h e n i c k e l - a l u m i n u m compound N i A l were u s e d t h e y w o u l d a c t as v e r y e f f e c t i v e cathodes f o r hydrogen l i b e r a t i o n . F o r t h i s r e a s o n , we made aluminumn i c k e l a l l o y s in w h i c h N i A l p r e c i p a t e d . As i n d i c a t e d in F i g u r e 35, some 1% n i c k e l a l l o y s showed e x c e l l e n t c o r r o s i o n r e s i s t a n c e in h i g h t e m p e r a t u r e w a t e r . I won't d i s c u s s d e t a i l s o f c o m p o s i t i o n and m e t a l l u r g i c a l p r e p a r a t i o n ; t h e y were f o u n d t o be i m p o r t a n t . Uranium c o r r o d e s in o x y g e n - f r e e w a t e r at a c o n s t a n t r a t e t o f o r m U0 in t h e form o f a r e l a t i v e l y u n p r o t e c t i v e l a y e r ; F i g u r e 36 shows such c o r r o s i o n r a t e s on an A r r h e n i u s p l o t . When the r a t e g e t s v e r y l a r g e at e l e v a t e d t e m p e r a t u r e s , u r a n i u m h y d r i d e can be found m i x e d in w i t h t h e o x i d e powder. I f oxygen is p r e s e n t in the water, f o r a long p e r i o d p r o t e c t i v e oxide f i l m s are formed; t h e s e e v e n t u a l l y break down l o c a l l y and s p r e a d . F i g u r e 37 shows t h a t t h e whole s u r f a c e e v e n t u a l l y becomes bad. We b e l i e v e t h a t some h y d r i d e was r e g u l a r l y formed b e n e a t h t h e o x i d e b o t h in d e a e r a t e d and a e r a t e d w a t e r , and t h a t the h y d r i d e s u b s e q u e n t l y was c o n v e r t e d t o t h e more s t a b l e o x i d e . T h i s is b e l i e v e d t o be a case o f f i l m d e g r a d a t i o n by t h e f o r m a t i o n o f h y d r i d e beneath i t . For s e l e c t e d uranium a l l o y s , oxide f i l m s 3

3

2



216



Typical appearance of 1100 aluminum after about two weeksindis tilled waterat275°C (IS)

Corrosion Figure 29.

Surface of "normal" 1100 aluminum sample after 66 hours in distilled waterat300°C., 20χ (21)



DRALEY

Corrosion

of Valve

217

Metals


1100 aluminum after four hoursindistilled waterat315°C (22)

TREATMENT

AVERAGE METAL ETCHED AWAY

HYDROGEN CONTENT (WHOLE SAMPLE)

-

12.1 ppm

AS CORRODED

ESTIMATE OF HYDROGEN IN INCREMENTS OF ETCHED METAL

-

-

7.3

ETCHED (DILUTE HNO3-HF)

0.016 mm

3.0

ETCHED

0.041

1.8

59

ETCHED

0.071

1.1

24

ETCHED

0.120

1.0

5

ETCHED

0.22

0.8

3

-

0.6

-

AS STRIPPED

BLANK (NO CORROSION)

280 ppm

Argonne National Laboratory Figure 31.

Hydrogen analyses of 1100 aluminum after two days corrosion in waterat290°C (23)



218



1100 aluminum, coupled to 347 stainless steel, after four hours in distilled waterat315°C (22)

Figure 33.

Dendritic nickel deposited on 1100 aluminum from N i S 0 (50ppmNi )250X (21)

Corrosion 4

solution

++


Corrosion

DRALEY

219

of Valve Metals

FeAI Cυ Α Ι ÛJ -.5

2

3

Α--^^ FeNiAI,

-.4


Ο

1 *

Ni AU Α ^ -

> ω -.2

3

Α

'

à.-— A - " ^ — A

TEMPERATURE: 2 9 0 · C MEDIUM: DISTILLED WATER

£

0

20 CATHODIC

Figure 34.

30

1

I

I

I 10

40

50

60

POLARIZING CURRENT, ^α/cm

70

80

2

Cathodic polarization curves for some aluminum intermetallic com pounds (9)

A

Ί

Γ

'X800l-350°C LINE REPRESENTS ~b DATA OF DILLON '

ί U\A/_CIQ/1Q >

AQUEOUS CORROSION OF ALUMINUM ALLOY Δ288 IAEA Conf. Corrosion of Reactor Materials Figure 35.

Aqueous corrosion of aluminum alloy A288 (Al -\- 1% Ni, 0.5% Fe, 0.1% Ti) (24)



220



Corrosion rate of uraniuminhydrogen-saturated water (25)



7. D R A L E Y

Corrosion

of Valve

Metals

221


Corrosion of uraniuminaerated distilled water (26)




222

a r e p r o t e c t i v e even in the absence o f oxygen, and f o r v e r y l o n g p e r i o d s . F i g u r e 38 shows some c o r r o s i o n r a t e s w h i c h a r e r e s p e c t a b l y low. The maximum tempera t u r e shown is 400°C. These f i l m s a l s o b r e a k down e v e n t u a l l y , and t h i s appears t o i n v o l v e the f o r m a t i o n o f some u r a n i u m h y d r i d e l o c a l l y . The breakdown c e r t a i n l y i n v o l v e s the b u i l d u p o f hydrogen in the m e t a l . F o r the p a r t i c u l a r a l l o y and t e m p e r a t u r e in F i g u r e 39 specimens l a s t e d much l o n g e r b e f o r e damage o c c u r r e d when they were p r e t r e a t e d in a vacuum b e f o r e c o r r o d i n g . When z i r c o n i u m is o x i d i z e d in w a t e r , a c o n s i d e r a b l e f r a c t i o n o f the c o r r o s i o n p r o d u c t hydrogen e n t e r s the m e t a l . There is e v i d e n c e t h a t the o x i d e r e c r y s t a l l i z a t i o n and the t r a n s i t i o n in k i n e t i c s t h a t were shown in F i g u r e 19 a r e r e l a t e d t o a b u i l d u p o f h y d r o gen in the s u r f a c e o f the m e t a l . I t has been s u g g e s t e d t h a t at t h a t time some z i r c o n i u m h y d r i d e forms b e n e a t h the f i l m . The a d d i t i o n o f a l l o y i n g elements w h i c h form a c t i v e c a t h o d e s f o r hydrogen l i b e r a t i o n have r e duced hydrogen u p t a k e , have d e l a y e d t r a n s i t i o n , and have r e s u l t e d in the f o r m a t i o n o f more c o h e r e n t o x i d e after transition. There has been no c l e a r r e s o l u t i o n as t o mechanisms ( 2 9 ) . I t is p o s s i b l e t o c o n s i d e r gaseous o x i d a t i o n p r o d u c i n g a s t a b l e o x i d e f i l m as an e l e c t r o c h e m i c a l p r o cess in w h i c h o x i d a t i o n o c c u r s at the m e t a l - o x i d e i n t e r f a c e where m e t a l i o n s l e a v e the m e t a l (see F i g u r e 4) and r e d u c t i o n o c c u r s at the o u t e r s u r f a c e o f the o x i d e where e l e c t r o n s combine w i t h oxygen. On the b a s i s o f t h i s l i n e o f r e a s o n i n g i t is p o s s i b l e t o p r e d i c t (a) the f o r m a t i o n o f a p o t e n t i a l d i f f e r e n c e between m e t a l and o x i d e e x t e r i o r f o r t h o s e systems in w h i c h the r e s i s t a n c e o r r e t a r d a n c e t o the passage o f i o n s t h r o u g h the g r o w i n g o x i d e is n o t much g r e a t e r t h a n the r e t a r d a n c e t o the passage o f e l e c t r o n s , and (b) a change in o x i d a t i o n r a t e f r o m the a p p l i c a t i o n o f an e l e c t r i c a l p o t e n t i a l between m e t a l and o x i d e e x t e r i o r . An i l l u s t r a t i o n is the b e h a v i o r o f z i r c o n i u m f o r w h i c h a p o t e n t i a l in e x c e s s o f one v o l t can be measured ( F i g u r e 40) and whose o x i d a t i o n r a t e at p o i n t s of e l e c t r i c a l c o n t a c t can be m a r k e d l y i n f l u e n c e d ( F i g u r e 4 1 ) . The c o n t a c t c o n s i s t e d o f p o i n t s at w h i c h the specimen r e s t e d upon c o n d u c t i n g powder. S i n c e the a r e a o f c o n t a c t d i m i n i s h e d under s h a r p e n i n g p o i n t s o f a n o d i c a l l y s t i m u l a t e d growth and i n c r e a s e d where g r o w t h was r e t a r d e d c a t h o d i c a l l y , the r a t e o f w e i g h t g a i n f o r the e n t i r e specimen in the f i g u r e was c o n s i d e r a b l y more r e d u c e d by a p p l i e d c a t h o d i c c u r r e n t t h a n i t was a c c e l e r a t e d by a p p l i e d a n o d i c c u r r e n t . fl

,f



DRALEY

Corrosion

of Valve

223

Metals

1000/T ( K) e

Figure 38.

Corrosion of uranium alloysinwater (27)

TEST TIME, HOURS Journal of the Electrochemical Society Figure 39.

Effect of gas removal on corrosion in water at 290°C of U-5% Zr1V Nb alloy (28) 2


CORROSION CHEMISTRY

224


1.6,


Potential developed during oxidation of zirconium at 700°C

(30)


Variation in oxidation rate with applied potential: zirconium in oxygen at 700°C (30)



7.

DRALEY

Corrosion

of

Valve

Metals

225

B e f o r e we f i n i s h , I'd l i k e t o spend a few m i n u t e s on p i t t i n g . As you know, t h e r e a r e l o c a l s i t e s at w h i c h c o r r o s i v e a t t a c k o c c u r s f o r some systems p r e f erentially. I f t h i s l o c a l attack continues, quite deep p i t s can form. T h i s is one o f the i n s i d i o u s k i n d s o f c o r r o s i o n a t t a c k f o r a m a t e r i a l . Some o f the f i l m - f o r m i n g m e t a l s t e n d t o be q u i t e s u s c e p t i b l e to p i t t i n g a t t a c k under a p p r o p r i a t e c o n d i t i o n s . One o f the c h a r a c t e r i s t i c r e q u i r e m e n t s f o r i t is the format i o n o f low pH w i t h i n a p r o t e c t e d p i t — p r o t e c t e d in some way f r o m the g e n e r a l e n v i r o n m e n t . We t h i n k t h a t commonly t h e r e a r e l o c a l cathodes at w h i c h hydrogen l i b e r a t i o n is v e r y g r e a t , at w h i c h f i l m r u p t u r e o c c u r s . T h i s is made e a s i e r by the f a c t t h a t the cathodes a r e o f t e n a second, c a t h o d i c phase so t h e r e is o f t e n an i m p e r f e c t i o n in the o x i d e at the p o i n t anyhow. A t f i r s t t h e r e is a h i g h a n o d i c r e a c t i o n r a t e n e x t t o the cathode because o f the p r o x i m i t y . When the a n o d i c r e a c t i o n has u n d e r c u t the c a t h o d i c p a r t i c l e d e s t r o y i n g e l e c t r i c a l c o n t a c t , the two h a l f - c e l l r e a c t i o n s w i l l no l o n g e r be v e r y c l o s e t o g e t h e r and t h e r e w i l l be pH changes so t h a t the anode a r e a w i l l become a c i d . The r e s u l t o f t h i s is t h a t a p r o t e c t i v e f i l m doesn't form as a p r i m a r y p r o d u c t o f the r e a c t i o n ; i n s t e a d , m e t a l i o n s a r e formed in s o l u t i o n . As t h e s e i o n s d i f f u s e out t o the s u r f a c e o f the o x i d e f i l m , the environment becomes more n e a r l y n e u t r a l and o x i d e s p r e c i p i t a t e . T h i s l e a d s t o the c h a r a c t e r i s t i c b a r n a c l e d appearance of a p i t , w i t h p r e c i p i t a t e d oxide over i t . In t h i s way the s o l u t i o n w i t h i n the p i t is i s o l a t e d f r o m the b u l k s o l u t i o n , and the a c i d i t y can be g r e a t . I f the b a r n a c l e becomes a s u f f i c i e n t l y e f f e c t i v e b a r r i e r t o the f l o w o f the i o n i c c u r r e n t w h i c h must pass t h r o u g h the s o l u t i o n from remote c a t h o d e s , the p i t s t o p s g r o w i n g . F i g u r e 42 shows the c l e a n e d s u r f a c e o f a ground p i e c e o f 1100 aluminum a f t e r about 4 h o u r s in o x y g e n - s a t u r a t e d d i s t i l l e d w a t e r . There a r e o c c a s i o n a l s m a l l p i t s ( b l a c k in the p h o t o g r a p h ) . After nearly two y e a r s exposure ( F i g u r e 4 3 ) , t h e r e a r e no l a r g e p i t s ; the appearance is as i f e s s e n t i a l l y a l l o f the s u r f a c e had in t u r n s e r v e d as the l o c a t i o n f o r m i c r o p i t formation. The low c o n d u c t i v i t y o f the w a t e r m i g h t have c o n t r i b u t e d t o e a r l y s t i f l i n g o f p i t g r o w t h A t h i g h e r m a g n i f i c a t i o n the p i t s ( F i g u r e 44) a r e n o t u n l i k e l a r g e r ones seen in o t h e r systems. Presumably a n o t h e r l e c t u r e r in t h i s s e r i e s w i l l g i v e o r has g i v e n you some d e t a i l s o f p r a c t i c a l p i t t i n g problems and the most p r o m i s i n g approaches t o c o n t r o l them. A number o f i n v e s t i g a t o r s have made e f f o r t s t o measure the pH in p i t s ; v a l u e s as low as 1.5 have




226

U.S. Atomic Energy Commission and European Atomic Energy Society Figure 42. Cleaned 1100 aluminum sur face after 4 A hours in oxygen-saturated distilled waterat70°C., 25χ (11) l

U.S. Atomic Energy Commission and European Atomic Energy Society Figure 43. Cleaned 1100 aluminum sur face after 704 days corrosion in oxygensaturated distilled water at 70°C., 25X (11)



7.

DRALEY

Corrosion

Figure 44.

Electron micrograph of corroded surface of "commercially pure" aluminum (12)

of Valve

Metals


227


228


been r e p o r t e d . Even exposed t o the b u l k w a t e r , we found, t h r o u g h the use o f a s p e c i a l e l e c t r o d e a few t e n t h s o f a m i l l i m e t e r in t i p d i a m e t e r ( F i g u r e 4 5 ) , t h a t the pH n e a r specimens o f 1100 aluminum c o r r o d i n g in d i s t i l l e d w a t e r r e a c h e d v a l u e s below 3 ( F i g u r e 4 6 ) . Some y e a r s ago, Howard F r a n c i s , t h e n o f the A r mour R e s e a r c h F o u n d a t i o n , made some t i m e - l a p s e m o t i o n p i c t u r e s o f a p o t e n t i a l map o f the s o l u t i o n n e x t t o s t e e l and aluminum a l l o y s p i t t i n g in s a l t w a t e r . The p r e s e n c e o f some a c t i v e cathode p o i n t s and g r o w i n g p i t s was r e a d i l y d i s p l a y e d . I s u g g e s t e d t h a t he r u n the f i l m backwards t o see whether the a c t i v e p i t s had been a c t i v e cathodes j u s t b e f o r e t h e i r i n i t i a t i o n . He l a t e r t o l d me he had done so, and t h e y had been w i t h few o r no e x c e p t i o n s . A t h i g h t e m p e r a t u r e s f i l m breakdown and p i t t i n g f o r aluminum a l l o y s t a k e s an u n u s u a l form e s p e c i a l l y when t h e r e is a h i g h r a t e o f f l o w o f w a t e r p a s t the metal surface. I f t h e r e a r e a l o t o f specimens in the system, the c o r r o s i o n r a t e is l o w e r t h a n i f the a r e a is s m a l l . The r e s u l t s o f some e x p l o r a t o r y e x p e r i m e n t s a r e shown in F i g u r e 47. A l a r g e a r e a ( f a c t o r o f 20) o f aluminum a l l o y i n h i b i t e d c o r r o s i o n w h i l e a l a r g e area of s t a i n l e s s s t e e l d i d not. The e f f e c t is c e r t a i n l y r e l a t e d t o c o r r o s i o n p r o d u c t in the system somehow. I n F i g u r e 48 one can see t h a t the c o r r o s i o n p r o d u c t l o s t f r o m the specimen was s u b s t a n t i a l l y in e x c e s s o f t h a t w h i c h d i s s o l v e d in the system. We t h o u g h t t h a t , at l o c a l p i t s and b r e a k s the c o r r o s i o n p r o d u c t , as i t r e a c h e d the o x i d e s u r f a c e and was app r o x i m a t e l y n e u t r a l i z e d , was swept away as p a r t i c l e s o f oxide. We t h o u g h t t h a t the a d d i t i o n o f c o l l o i d a l p a r t i c l e s t o the s o l u t i o n w o u l d t e n d t o " p l u g " openi n g s , r e d u c e the l o s s o f o x i d e , and lower the c o r r o s i o n r a t e . F i g u r e 49 shows t h a t when a h y d r a t e d c o l l o i d was i n j e c t e d in t o the system, a v e r y low c o r r o s i o n r a t e was o b t a i n e d . I n the same system the e f f e c t s o f p o l a r i z i n g c u r r e n t on c o r r o s i o n r a t e ( F i g u r e 50) a r e s i m i l a r t o what I showed you at low temp e r a t u r e in F i g u r e 27: a n o d i c p r o t e c t i o n and c a t h o d i c stimulation. The message I'd l i k e t o l e a v e w i t h you t o n i g h t is t h a t some f i l m s a r e v e r y good and v e r y p r o t e c t i v e , some f i l m s have b r e a k s in them, and they a r e moderatel y good, some become bad by some o f the s t r a n g e s t mechanisms: an u n d e r s t a n d i n g o r c o r r o s i o n phenomena sometimes r e q u i r e s q u i t e a b i t o f i n g e n u i t y .



7.

DRALEY

Corrosion

of Valve

229

Metals


Glass, silver/silver iodide pH microelectrode (SI)


230

CORROSION

CHEMISTRY

7 2 mm FROM UPSTREAM EDGE

•-··-·"

j

——2mm FROM DO\ VNSTREAM ED(SE w

^^CENTER

5


X CL

3

I Ο

40

80

TIME, mins

120

160

200


Effect of position on pH of50°C oxygen-saturated water 0.1 mm from corroding 1100 aluminum (SI)

Figure 47.

Effect on aqueous corrosion of 8001 aluminum at 260°C., 7 m/sec velocity of added surface of aluminium or stainless steel (S2)

Argonne National Laboratory


Corrosion

of Valve

231

Metals

Normal (small) Area, 21-day Exposure

Large Area, 28-day Exposure

Average Metal Loss

13.1 mg/cm

4.60 mg/cm

Estimated AI2O3 · H2O Produced (from metal loss)

29.1 mg/cm

10.2 mg/cm

Actual Average Product Remaining

11.6 mg/cm

7.9 mg/cm

Corrosion Product Lost

17.5 mg/cm

2.3 mg/cm

Corrosion Product PresentatEnd of Test Compared with Total Produced

40%

78%

Aluminum Area

70 cm

2

2

2

2

2

2

2

2

1470 cm

2

2

Maximum Dissolved Corrosion Product, from Solubility Data (2 χ 10" g of Al 0 per liter at 1.5-liters/hr refreshment)

2.4 mg/cm

0.14 mg/cm

Ratio Dissolved to Actual Loss

0.14

0.06

Corrosion RateinLast 7-day Period

40mdd

5.8 mdd

4

2

3

2

2


Comparison of two dynamic corrosion tests. Water at 260°C., 7 m/sec velocity (32).

20 18 16 ^ 14

METAL CORRODED, mç


7. D R A L E Y

_ DISTILLED ^i.JV'' *

(run between colloid experiments below)

00

jç*"^

WATER

HYDRATED ALUMINUM OXIDE COLLOID INJECTED:

UPSTREAM _ -

Λ

Λ

Μ

Α

1

β

β

Α

Μ

Ο „

DOWNSTREAM Q η

c

ο-ο» 1

5

°(

Ο" 1

ΙΟ

1

15

Ό

Q

^ 1

I

20 25 TIME, days

1

30

1

35

1

40


Influence of 35-ppm colloid on the aqueous corrosion of 8001 alumi numat260°C., 7 m/sec velocity (32)



232

CORROSION

CHEMISTRY

18

14 DISTILLED WATER

150

50 100 CATHODE

50

INITIAL APPLIED CURRENT,

100 ANODE

150

μα/cm

2


Effects of applied current on corrosion of 8001 aluminumat260°C in flowing water, 5.6 m/sec (32)


7.

DRALEY

Corrosion

of Valve

Metals

233

L i t e r a t u r e Cited 1. 2. 3.


4.

5.

6. 7. 8. 9. 10. 11.

12. 13. 14. 15.

16. 17. 18. 19.

V i j h , Α. Κ., J. Electrochem. Soc. (1972) 116, 972. Tajima, S., Baba, Ν., and Shimura, F. Μ., E l e c t r o chimica Acta (1967) 12, 955. Hauffe, K a r l , " O x i d a t i o n of Metals," p. 419, Plenum Press, New York, New York, 1965. Hart, R. Κ., "Morphology of Corundum Films on Alum inum," Fifth Int. Cong. f o r Electron Microscopy, Philadelphia, Pennsylvania, Aug. 29 - Sept. 5, 1962, Paper No. C-10, Academic Press, New York, New York, 1962. Hart, R. Κ., and Maurin, J. Κ., "Growth of Oxide Nuclei on Iron," Sixth Int. Cong. f o r Electron Microscopy, Kyoto, Japan, Aug. 28 - Sept. 4, 1966, p. 539, Maruzen Co., Ltd., Tokyo, Japan, 1966. Ruther, W. E., and Hart, R. Κ., Corrosion (1963) 19, 127t. Frankenthal, R. P., and Malm, D. L., J. E l e c t r o chem. Soc. (1976) 123, 186. Ruther, W. E., Schleuter, R. R., Lee, R. Η., and Hart, R. Κ., Corrosion (1966) 22, 147. Ruther, W. Ε., and Draley, J. E., unpublished work. Draley, J. E., Ruther, W. E., and Greenberg, S., J. Nucl. Matl. (1962) 6, 157. Draley, J. E., TID-7587, "Aqueous Corrosion of 1100 Aluminum and of Aluminum-Nickel A l l o y s , " Int. Conf. on Aqueous Corrosion of Reactor Mat'ls., Brussels, Oct. 14-17, 1959, p. 165, U. S. Atomic Energy Commission and European Atomic Energy So c i e t y , July 1960. Draley, J. Ε., Mori, S., and Loess, R., unpub lished figure. Mori, S., and Draley, J. E., J. Electrochem. Soc. (1967) 114, 352. WAPD-MRP-107, "Pressurized Water Reactor (PWR) Project Technical Progress Report October 24, 1963January 23, 1964," Westinghouse Atomic Power Div. Griggs, B., Maffei, H. P., and Shannon, D. W., HW-67818, "Multiple Rate Transitions in the Aque ous Corrosion of Z i r c a l o y , " Hanford Laboratories, General E l e c t r i c Company, December 20, 1960. Draley, J. E., unpublished work. Diggle, John W. (Ed.), "Oxides and Oxide Films," Vol. 1, p. 38, Marcel Dekker, Inc., New York, New York, 1972. Draley, J. E., and Ruther, W. E., J. Electrochem. Soc. (1957) 104, 329. Draley, J. Ε., and Mori, Shiro, and Loess, R. E., J. Electrochem. Soc. (1963) 110, 622.


CORROSION CHEMISTRY

234

20. 21. 22.


23.

24.

25.

26. 27. 28. 29.

30. 31. 32.

Youngdahl, C. Α., and Loess, R. E., J. E l e c t r o chem. Soc. (1967) 114, 489. Draley, J. E., and Ruther, W. E., Corrosion (1956) 12, 480t. Draley, J. E., and Ruther, W. E., Corrosion (1956) 12, 441t. Draley, J. E., and Ruther, W. E., ANL-5658 "Exper iments in Corrosion Mechanism: Aluminum at High Temperatures," Argonne National Laboratory, A p r i l 1957. Draley, J. E., and Ruther, W. E., "The Corrosion of Aluminum A l l o y s in High Temperature Water," IAEA Conf. Corrosion of Reactor Materials, Salzburg, June 1962, Vol. I, p. 477. McWhirter, J. W., and Draley, J. E., ANL-4862 "Aqueous Corrosion of Uranium and A l l o y s : Survey of Project L i t e r a t u r e , " Argonne National Labora tory, May 14, 1952. Mollison, W. Α., English, G. C., and Nelson, F., CT-3055 "Corrosion of Tuballoy in Distilled Water," Argonne National Laboratory, June 23, 1945. Greenberg, S., and Draley, J. E., unpublished figures. Draley, J. E., Greenberg, S., and Ruther, W. Ε., J. Electrochem. Soc. (1960) 107, 732. Parfenov, B. G., Gerasimov, V. V., and Venediktova, G. I., "Corrosion of Zirconium and Zirconium A l l o y s , " Translated from Russian by Ch. Nisenbaum, I s r a e l Program f o r S c i e n t i f i c Translations, Jerusalem, 1969. Bradhurst, D. Η., Draley, J. E., and Van Drunen, C. J., J. Electrochem. Soc. (1965) 112, 1171. Mori, S. Loess, R. E., and Draley, J. E., Corro sion (1963) 19, 165t. Draley, J. E., and Ruther, W. E., ANL-7227, "Corrosion of Aluminum A l l o y s by Flowing High Temperature Water," Argonne National Laboratory, January 1967. Acknow1edgement

This paper was prepared with the support of the U. S. Energy Research and Development Administration. RECEIVED September 1,

1978.


Corrosion of Valve Metals - ACS Publications - American Chemical

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