CORROSION STUDIES OF GLASS USING CONVENTIONAL AND GLANCING ANGLE EXAFS

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CORROSION STUDIES OF GLASS USING

CONVENTIONAL AND GLANCING ANGLE EXAFS

G. Greaves

To cite this version:

G. Greaves. CORROSION STUDIES OF GLASS USING CONVENTIONAL AND GLANC- ING ANGLE EXAFS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-819-C8-824.

�10.1051/jphyscol:19868157�. �jpa-00226060�

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C o l l o q u e C 8 , s u p p l e m e n t au n o 1 2 , Tome 47, d e c e m b r e 1 9 8 6

CORROSION STUDIES OF GLASS USING CONVENTIONAL AND GLANCING ANGLE EXAFS

G.N. GREAVES

SERC, Daresbury Laboratory, GB-Warrington W A 4 4 A D , Great-Britain

A b s t r a c t - Glass c o r r o s i o n i s p r i m a r i l y governed by t h e d i f f u s i o n of c a t i o n s t o t h e s u r f a c e . This i s modified by any chemical changes t a k i n g p l a c e a t t h e i n t e r f a c e . We demonstrate t h e v a l u e of normal i n c i d e n c e and g l a n c i n g a n g l e EXAFS measurements f o r e l u c i d a t i n g t h e c o r r o s i o n p r o p e r t i e s of g l a s s , t a k i n g s t u d i e s of phosphate and b o r o s i l i c a t e g l a s s e s a s examples.

I n t r o d u c t i o n

The c o r r o s i o n p r o p e r t i e s of g l a s s a r e c h i e f l y a f u n c t i o n of t h e t r a n s p o r t of c a t i o n s i n t h e bulk and a t t h e i n t e r f a c e with t h e c o r r o d i n g medium. Many oxide g l a s s e s a r e s u s c e p t i b l e t o aqueous a t t a c k , r e l e a s i n g modifying c a t i o n s a t t h e s u r f a c e i n exchange f o r water r a d i c a l s . This may r e s u l t i n chemical change a t t h e g l a s s s u r f a c e , f o r example t h e c r e a t i o n of a g e l l a y e r , which might impede f u r t h e r c o r r o s i o n . The l o c a l environments of metal atoms - i n p a r t i c u l a r t h e metal-oxygen p a i r d i s t r i b u t i o n f u n c t i o n s - give c l u e s a s t o t h e l i k e l y p e r c o l a t i n g s p e c i e s and can be measured u s i n g conventional EXAFS techniques. I t i s c l e a r l y a l s o important t o examine modifier environments i n t h e v i c i n i t y of t h e s u r f a c e i t s e l f t o s e e i f and how t h e s e a l t e r a s c o r r o s i o n advances. S u r f a c e s e n s i t i v i t y can be achieved u s i n g g l a n c i n g a n g l e geometry t o measure EXAFS (1). I n t h i s paper we w i l l d e s c r i b e both approaches, t a k i n g examples from l e a d phosphate and b o r o s i l i c a t e g l a s s e s . Both a r e c a n d i d a t e s f o r n u c l e a r waste d i s p o s a l g l a s s f o r which c o r r o s i o n r e s i s t a n c e i s obviously a prime requirement. However, a s t h e m o d i f i e r s a r e heavy atoms, t h e s e g l a s s e s a r e a l s o p a r t i c u l a r l y a t t r a c t i v e f o r fundamental EXAFS s t u d i e s .

E f f e c t of I r o n on t h e Corrosion of Lead Phosphate G l a s s

The i n t r o d u c t i o n of i r o n t o l e a d phosphate g l a s s d r a m a t i c a l l y improves i t s

r e s i s t a n c e t o aqueous a t t a c k ( 2 ) . The Pb-0 p a i r d i s t r i b u t i o n f u n c t i o n s d e r i v e d from Zb LIII EXAFS f o r a range of g l a s s e s are p r e s e n t e d i n f i g . 1, t h e arrow i n d i c a t i n g increasing i r o n c o n t e n t . These d i s t r i b u t i o n s a r e broader t h a n simple c r y s t a l l i n e oxides l i k e l e a d dioxide b u t a r e s i m i l a r i n width t o c r y s t a l l i n e polyphosphates l i k e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868157

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Fe- P b- Po3

Fig. 1

Pb-0 p a i r d i s t r i b u t i o n f u n c t i o n s

l e a d pyrophosphate. Atomic d i s t r i b u t i o n s f o r Pb02 and Pb2P207 a r e included i n f i g . 1 f o r comparison. P K-edge XANES f o r iron-lead-phosphate g l a s s e s confirms t h e

presence of polyphosphate groups ( 3 ) .

Fe-0 p a i r d i s t r i b u t i o n f u n c t i o n s however a r e much narrower t h a n t h o s e of l e a d shown i n f i g . 1 ( 4 ) . A t y p i c a l width (FWHM) i s 0.2 A f o r i r o n compared t o 0.5 A f o r l e a d w i t h a mean Fe-0 bond l e n g t h of 1.95 A c o n t r a s t i n g with an average Pb-0 bond l e n g t h of 2.46 A . Compared t o i r o n s i t e s i n s i l i c a t e g l a s s e s r e p o r t e d elsewhere ( 5 ) t h e bond l e n g t h s i n t h e p r e s e n t g l a s s e s a r e up t o 0.09 A longer. ~ l t h o u g h t h e Fe K-edge t h r e s h o l d s i n d i c a t e t h e o x i d a t i o n s t a t e i s ~ e ~ + , t h e c o o r d i n a t i o n number

(between 6 and 7) and t h e bond l e n g t h of 1.95 a a r e c o n s i s t e n t with well-defined o c t a h e d r a l s i t e s ( 4 ) . Hence, i n t h e s e phosphate g l a s s e s i r o n occupies network

modifying p o s i t i o n s r a t h e r t h a n network forming p o s i t i o n s a s it does i n t h e s ~ l i c d c r g l a s s e s r e f e r r e d to.

The changes i n shape of t h e . i r o n and l e a d p a i r d i s t r i b u t i o n f u n c t i o n s i n phosphate g l a s s e s a s t h e modifier b a l a n c e i s changed a r e f a i r l y s m a l l ( f i g . 1 ) . The major conclusion from t h i s EXAFS s t u d y has been t h e comparative compositional i n v a r i a n c e of i r o n and l e a d s i t e s . Indeed, i f t h e two m o d i f i e r s a r e t o a f i r s t

Fig.2 Fig.3

Corrosion r a t e v e r s u s pH f o r Radial d i s t r i b u t i o n f u n c t i o n of U o b t a ~ r , . rl phosphate and b o r o s i l i c a t e g l a s s from L EXAFS measured a t ec/2

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approximation s t r u c t u r a l l y decoupled, t h e n t h e c o r r o s i o n r e s i s t a n c e of iron-lead- phosphate g l a s s e s must i n v o l v e one C a t i o n e f f e c t i v e l y blocking t h e d i f f u s i o n path- ways of t h e o t h e r . This i s most e a s i l y v i s u a l i s e d i n terms of a modified random network. O r i g i n a l l y devised t o model t h e EXAFS and d i f f u s i o n p r o p e r t i e s of s i l i c a t e g l a s s e s ( 6 ) , f o r phosphate g l a s s e s t h e network component should b e r e p l a c e d by randomly convoluted polyphosphate c h a i n s , c r o s s - l i n k e d by modifying c a t i o n s . I n t h e c o n t e x t of iron-lead-phosphate g l a s s e s t h e t i g h t e r environment of i r o n i o n s cause t h e p e r c o l a t i o n pathways t o "neck" t h e r e b y impeding t h e d i f f u s i o n of t h e l a r g e r l e a d i o n s .

Aqueous Corrosion a s a Function of pH

The r e s i s t a n c e of i r o n l e a d phosphate g l a s s e s t o aqueous a t t a c k v a r i e s c o n s i d e r a b l y with pH ( 7 ) . It i s e v i d e n t from f i g . 2 t h e b e s t chemical r e s i s t a n c e i s e x h i b i t e d f o r a n e u t r a l pH of around 7. However, t h e l e a c h i n g of anions and c a t i o n s i s n o t always congruent. The anomalous behaviour e v i d e n t i n f i g . 2 can be understood by

c o n s i d e r i n g t h e s i z e of t h e m i g r a t i n g s p e c i e s a t t h e w a t e r / g l a s s i n t e r f a c e . I n t h e s i m p l e s t model t h e modifying c a t i o n s w i l l exchange with H ~ O + and t h e phosphate groups w i t h OH'. A s l e a d and hydronium a r e of s i m i l a r s i z e w i t h r a d i i of around 1 A t h e y should exchange r e a d i l y . F e r r i c i o n s on t h e o t h e r hand with a r a d i u s of 0.7 A a r e l e s s well-matched and from fig.;! c l e a r l y d i s s o l v e l e s s r e a d i l y . I n t h e most a c i d i c s o l u t i o n s where both modifying c a t i o n s a r e drawn i n t o s o l u t i o n , c r o s s l i n k s i n t h e (PO,,) c h a i n s w i l l be broken which w i l l i n e v i t a b l y l e a d t o a l o s s of

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phosphorus and t h e congruent l e a c h i n g of c a t i o n s and a n i o n s a s observed ( f i g . 2 ) . A t high pH, however, t h e s c a r c i t y of H ~ O + w i l l diminish c a t i o n exchange l e a v i n g anion exchange comparatively enhanced. I f phosphorus is l o s t from chain ends, a s i s l i k e l y , modifying m e t a l s w i l l n o t be c a s u a l t i e s and t h e incongruent l e a c h i n g behaviour seen i n f i g . 2 w i l l occur.

U Environments a t t h e S u r f a c e of B o r o s i l i c a t e G l a s s e s

The c o r r o s i o n r e s i s t a n c e of a b o r o s i l i c a t e g l a s s i s a l s o included in fig.2.

The response t o pH i s obviously d i f f e r e n t and it i s n o t expected t h e c o r r o s i o n behaviour w i l l be t h e same a s what we have proposed f o r iron-lead-phosphate g l a s s . I n any e v e n t , d i r e c t measurement of metal environments a t t h e g l a s s s u r f a c e a s a f u n c t i o n of l e a c h i n g i s c l e a r l y d e s i r a b l e . I n t h i s r e s p e c t FLEXAFS i n glancing a n g l e geometry has proved p a r t i c u l a r l y u s e f u l . This t e c h n i q u e was f i r s t demon- s t r a t e d by Heald and coworkers ( 8 ) f o r t h i n Au d e p o s i t s on g l a s s and has been adapted r e c e n t l y t o examine changes i n t h e l o c a l s t r u c t u r e of U a t t h e s u r f a c e of a b o r o s i l i c a t e g l a s s with p r o g r e s s i v e l e a d i n g (I). By v a r y i n g t h e a n g l e of t h e incoming x-rays t o t h e specimen s u r f a c e c l o s e t o t h e c r i t i c a l angle f o r t o t a l e x t e r n a l r e f l e c t i o n , t h e depth of p e n e t r a t i o n of x-rays, z, can be changed from t e n s t o thousands of angstrtlms. F i g u r e 3 shows t h e Fourier transform of

Fig. 4

U-0 p a i r d i s t r i b u t i o n f u n c t i o n f o r d i f f e r e n t l e a c h i n g t i m e s

U LIIIEXAFS f o r a v i r g i n b o r o s i l i c a t e g l a s s specimen measured a t an a n g l e of Oc/2, f o r which z

-

A t an a n g l e of i n c i d e n c e of 20c, t h e depth of p e n e t r a t i o n i s i n excess of 1000 A and a s changes i n t h e U L EXAFS a r e s t i l l observed it i s c l e a r t h e

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p r o c e s s p e n e t r a t e s t h i s f a r . We can s e e t h i s i n f i g . 4 where t h e U-0 p a i r d i s t r i b u - t i o n f u n c t i o n o b t a i n e d from f i t t i n g t h e k3-weighted spectrum i s compared f o r d i f f e r - e n t amounts of l e a c h i n g a t 100°C i n water. Note how t h e double peak breaks up a s t h e g l a s s corrodes. The d o u b l e t i n t h e v i r g i n g l a s s with peaks a t 1.8 A and 2.2 i s r e m i n i s c e n t of an i s o l a t e d uranyl r a d i c a l . This comprises a d i s t o r t e d octahedron with f o u r p l a n a r oxygens and two s h o r t e r orthogonal oxygen bonds. Hydration i s expected t o r e s u l t i n an i n c r e a s e i n t h e U-0 bond l e n g t h of about 0.2 A.

In e a r l i e r conventional U LIII EXAFS s t u d i e s by Knapp and co-workers it was demonstrated t h a t i f t h e U c o n c e n t r a t i o n was i n c r e a s e d i n a b o r o s i l i c a t e g l a s s t h i s r e s u l t e d i n t h e c l u s t e r i n g of uranyl r a d i c a l s i n t o i s l a n d s a s expected i n a modified random network ( 6 ) . It i s noteworthy t h a t a t t h e s u r f a c e peaks B and C of t h e U s t r u c t u r e ( f i g . 3 ) a r e l o c a t e d i n t h e F o u r i e r t r a n s f o r m a t roughly J2 and 2 times t h e U-0 d i s t a n c e r e s p e c t i v e l y - s u g g e s t i n g a square p l a n a r arrangement. Moreover we f i n d t h e s e two peaks ( n o t a b l y B) i n c r e a s e with l e a c h i n g i n d i c a t i n g p r o g r e s s i v e c l u s t e r i n g of u r a n y l r a d i c a l s near t h e s u r f a c e (1). It i s a l s o worth mentioning t h a t t h e c o n c e n t r a t i o n of U i n t h e bulk g l a s s was 3 wt.%

-

Glass

u n

'e' M o d e l

3 5

r (A1

Fig. 5

U-U d i s t r i b u t i o n f o r v i r g i n g l a s s compared t o random square p l a n a r model d e r i v e d from U-0 d i s t r i b u t i o n .

F i n a l l y , by assuming random s q u a r e p l a n a r u r a n y l i s l a n d s , it i s p o s s i b l e t o p r e d i c t t h e shape of t h e U-U d i s t r i b u t i o n from t h e U-0 p a i r d i s t r i b u t i o n f u n c t i o n . This i s done i n f i g . 5 f o r t h e s t r u c t u r e of t h e v i r g i n g l a s s measured a t 8 /2. The agree- ment i s convincing and t o g e t h e r with t h e r e s u l t s p r e s e n t e d i n f i g . 4 t h i s work demonstrates t h e e f f i c a c y of glancing a n g l e geometry f o r r e g i s t e r i n g t h e s u r f a c e s t r u c t u r e of g l a s s under c o r r o s i o n c o n d i t i o n s .

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R e f e r e n c e s

F.R. T h o r n l e y , N.T. B a r r e t t , G.N. G r e a v e s and G.M. A n t o n i n i , J. P h y s . C (in p r e s s ) ; N.T. B a r r e t t , G.M. A n t o n i n i , F.R. T h o r n l e y and G.N. G r e a v e s , t h i s conference.

B.C. S a l e s and L. A. B o a t n e r , Science 226, 45 ( 1 9 8 4 ) . G.N. G r e a v e s ( u n p u b l i s h e d r e s u l t s )

.

G.N. G r e a v e s , S.J. G u r m a n and N. Jenkins, t h i s conference.

N. B i n s t e d , G.N. G r e a v e s and C.M.B. H e n d e r s o n , t h i s conference.

G.N. G r e a v e s , E X A F S and N e a r E d g e Structure I1 ( S p r i n g e r V e r l a g , B e r l i n , 1 9 8 2 1 , p . 2 4 8 ; G.N. G r e a v e s , J. N o n - C r y s t . S o l i d s 11, 2 0 3 ( 1 9 8 5 ) .

B.C. Sales and L.A. B o a t n e r , O a k Ridge N a t i o n a l Laboratory R e p o r t

-

S.M. H e a l d , E. K e l l e r and E. Stern, P h y s . L e t t . E, 155 ( 1 9 8 4 ) .

G . S . K n a p p , B.W. V e a l , D.J. Lam, A.P. P a u l i k a s and H.K. P a n , mter. L e t t . 2,

253 ( 1 9 8 4 ) .