Adams - Record of the Past

8/3/2019 Adams - Record of the Past

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Journal of Non-Crystalhne Sohds 67 (1984) 193-205 193North-Holland, Amsterdam

Sectton IV. Corroston and leachmg

G L A S S C O R R O S I O NA r e c o r d o f t he pa s t ? A p r e d i c to r o f t he f u tu r e ?

P. B . A D A M S

Cormng Glass Works, Cormng, New York, USA

The pnnclples o f chenucal corroston o f glasses are reviewed. The influence of temperature,glass composition, glass homogeneity and corrosion medta are related to ob served corrosion rates.A comparison o f the durabthty o f man-m ade and natural glasses ~s offered and the corrosion ratesof tekUtes and obstdians are d~scussed. Natural glasses are n ot hkely to advance o ur presentunderstanding o f corrosion theory

1 . I n t r oduc t io n

S i n c e g l a s s w a s f i r s t e n c o u n t e r e d , m a n ' s u s e o f g la s s h a s b e e n i n f l u e n c e d -w h e t h e r h e r e a li z e d it o r n o t - b y i ts c o r r o s i o n r e s i s ta n c e . T h e b u l k f e a t u r e s o fg l a s s te ll u s m u c h a b o u t t h e p r o c e s s o f it s f o r m a t i o n a n d h e n c e it s o r ig i n . T h es u r f a c e o f a g la s s it e m a l s o h a s a s t o r y t o t e ll, s o m e t i m e s a b o u t i ts o r i g i n b u tm o s t o f t e n a b o u t i t s i n t e r v e n i n g e n v i r o n m e n t a l h i s t o r y, R a i n w a t e r a n d d i s hw a t e r w a s h t h e s u r f a c e o f gla ss . W e t t i n g a n d d r y i n g p r o c e e d . Te m p e r a t u r e r is e sa n d f a l l s . S o m e o f t h e s e c h a n g e s a f f e c t g l a s s o n l y s l i g h t l y o r n o t a l a l l ; o t h e r sa f f e c t g l a s s d r a s t i c a l l y. S o m e a f f e c t t h e m u n i f o r m l y ; o t h e r s a f f e c t t h e m s e l e c -t ively.

T h e g l a s s t e c h n o l o g i s t h a s w a t c h e d , p u z z l e d , a n d t h e o r i z e d a b o u t t h ec h a n g e s t h a t c h e m i c a l r e a c t i o n s p r o d u c e o n g l a s s . I n t h e p a s t h e h a s r e l i e dh e a v i l y o n e m p i r i c a l t e s ts t o h e lp h i m d e f i n e g l a s s c o r r o s i o n p r o c e s s e s . H ek n o w s f r o m t h e s e s t u d i e s t h a t c o r r o s i o n c a n g e n e r a l l y b e d e s c r i b e d e i t h e r a s a n

e t c h i n g p r o c e s s , o r a s a l e a c h i n g p r o c e s s , o r as a c o m b i n a t i o n o f b o t h .R e c e n t l y, e f f o r t s t o d e s c r i b e t h e g l a s s c o r r o s i o n p r o c e s s h a v e b e e n a c c e l -e r a te d . G l a s s a p p e a r s t o b e t h e b e s t h o s t f o r t r a p p i n g n u c l e a r w a s t e a n di n s u r i n g i t s s a f e d i s p o s a l . T h e u r g e n c y o f t h i s i s s u e h a s f o c u s e d t h e e f f o r t s o fm a n y c o m p e t e n t s c i e n t i s t s o n r e f i n i n g g l a s s c o r r o s i o n t h e o r y i n o r d e r t op r e d i c t h o w a n u c l e a r w a s t e g la s s w i ll s u r v i v e i n a n a t u r a l e n v i r o n m e n t f o rt h o u s a n d s o f y e a r s.

T h i s p a p e r b r i e f ly r e v ie w s t h e p r in c i p le s o f g la s s c o r r o s i o n a n d c o m m e n t so n t h e r o le o f g l a ss c o m p o s i t i o n . I t d e a l s p r i m a r i l y w i t h o b s i d i a n s a n d t e k ti te ss in c e lu n a r g la s se s h a v e n o t b e e n e x p o s e d t o a h i s t o r y o f c o r r o s i o n . M u c h o f

t h e d is c u s s i o n is ta k e n f r o m m o d e r n c o m m e r c i a l g la s s te c h n o l o g y e x p e ri e n ce ,r e c o g n i z i n g t h a t m a n - m a d e g l a ss e s r e p r e s e n t a m e r e f e w h u n d r e d y e a r s o fc o r r o s i o n h i s t o r y, c o m p a r e d t o s e v e r a l m i l l i o n y e a r s f o r n a t u r a l g l a s s e s .

0022-3093/84/$03.00 Elsevaer Science Pubhshers B.V(North-Holland Physics Pubhsbang Division)


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194 P B. Ada ms / Glass corroston

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2 . Pr inc ip l e s o f cor ros ion

C o r r o s i o n m e c h a n i s m s c a n b e c l a s s i f i e d i n t o t w o c a t e g o r i e s , e t c h i n g o rl each in g [1 -8 ] . E tch in g i s u su a l ly ch a rac t e r i zed a s a lk a l in e a t t ack ( f ig . 1 ) . Asth e s i l i c a n e two rk i s d e s t ro y ed , o th e r g l a ss co m p o n en t s a r e r e l ea sed . I f t h e r e i sn o a c c u m u l a t i o n o f r e a c t io n p r o d u c t s o n t h e r e m a i n i n g g l as s s u r f ac e a n d n oc h a n g e i n t h e a c t i v it y o f th e c o n t a c t i n g s o l u t i o n , t h e p r o c e s s p r o c e e d s a t ac o n s t a n t r a t e . I n t h e i d ea l c as e , t e m p e r a t u r e i n f l u e n c e s c o r r o s i o n r a t e a sd esc r ib ed b y th e Ar rh en iu s eq u a t io n . F ig . 2 sh o ws em p i r i ca l cu rv es r e l a t in gt e m p e r a t u r e t o t h e c o r r o s i o n r a t e o f a b o r o s i l i c a t e g l a s s . T h e s l o p e o f t h e s ecu rv es in d ica t e s th a t co r ro s io n r a t e in c r ease s b y a f ac to r o f 2 to 2 p e r 1 0 Ct e m p e r a t u r e r is e .

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Fig. 2. Solubihty of a borosl hcate glass m vanou s solutions versus temperature.


3/13

P B Ad ams / Glass corrosion 195

A c c u m u l a t i o n o f r e a c t io n p r o d u c t s i n s o l u ti o n m a y d e p r e s s t h e r e ac t i o nra t e . I n f a c t s il ic a s a t u r a t i o n c an d ep re s s t h e r a t e t o e s s en t i a l ly ze ro . Th ep r e s e n c e o f o t h e r e l e m e n t s m a y m i t m t e p r e c i p i t a ti o n o f s a l ts w h i ch i n t u r n m a yr e s t o r e t h e a c t i v i t y o f t h e c o n t a c t i n g s o l v e n t o r m a y a d h e r e t e n a c i o u s l y t o t h eg l a s s s u r f a c e , s h i e l d i n g i t f r o m a t t a c k . I t h a s b e e n s h o w n t h a t m a n y e l e m e n t ss u c h a s b e r y l l i u m ( f i g . 3 ) , z i n c , a l u m i n u m , l e a d , c a l c i u m , s t r o n t i u m a n d b a r i u ms t r o n g l y i n h i b i t a l k a l i n e a t t a c k [9 ]. E t c h i n g l e a v e s a s m o o t h s u r f a c e i f d i s s o lu -t i o n is to t a l a n d n o r e a c t io n p r o d u c t s r e m a i n . H o w e v e r , t h e s u r f a ce c o a t e d w i t hr e a c t io n p r o d u c t s , o r g r o s s ly r o u g h e n e d , i s p r o b a b l y m o r e f r e q u e n t ly e n c o u n -t e r e d i n r e a l e n v i r o n m e n t s .

L e a c h i n g ~ s u s u a l l y c h a r a c t e r i z e d b y a c i d a t t a c k a s s c h e m a t i c a l l y s h o w n i nf i g . 4 . H y d r o g e n i o n s , o r m o r e p r o b a b l y h y d r o n i u m i o n s , e x c h a n g e f o r a l k a l ia n d o t h e r p o s i t i v e l y c h a rg e d m o b i l e i o n s in t h e b u l k g l a ss . T h e r e m a i n i n g g l a s s

n e t w o r k , p r i n c i p a l l y s i li ca , u s u a l l y r e t a i n s i ts in t e g r i ty. I t m a y b e c o m e h y d r a t e di f th e n e t w o r k i s re l a ti v e ly u n s ta b l e ; o r t t m a y b e c o m e e v e n m o r e d e n s e a n ds t a b l e t h a n t h e p a r e n t g l a s s .

I f t h e l e a c h e d l a y e r i s n o t s u b s t a n t i a l l y a l t e re d , t h e r e a c t i o n r a t e d e c r e a s e sw i t h t i m e a n d i s p r o p o r t i o n a l t o t h e s q u a r e r o o t o f ti m e . I n t h i s s i m p l e c a s e,t h e l a y e r b e c o m e s , i n e ss e n ce , a p r o t e c t i o n a g a i n s t c o n t i n u i n g a t t a c k ( fi g. 5) .O f t e n t h is l a y e r c o n s o l i d a t e s , s h r i n k i n g t h e r e a c t i o n r a t e t o z e r o ( f ig . 6 ). I f t h el a y e r i s l o o s e l y c o n s t i t u t e d , i t m a y d e t a c h i t s e l f s o t h a t i t s p r o t e c t i v e e f f e c t i sl o s t. A c i d s r e a c t m u c h m o r e s l o w l y w i t h g l a s s t h a n s t r o n g a l k a l i a n d t h e p He f f e c t is m u c h l es s p r o n o u n c e d . T h e r a t e a t l o w P h m a y n o t b e n o t i c e a b l yd i f f e r e n t f r o m p H 5 o r 6.

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Fig 3 Effect of berylhum on corrosion rate F~g. 4. Schem atic of leachmg processb y 0 5 N N a O H a t 1 0 0 C .


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A t t a c k b y w a t e r a n d o t h e r n e a r n e u t r a l s o l u t i o n s i s l e s s t h a n f o r a l k a l i a n do f t e n i s si m i l a r t o a c i d a t t a c k . U s u a l l y i t i n v o l v e s b o t h i o n e x c h a n g e a n d s d i c an e t w o r k d e s t r u c t io n . I n c l o s e d s y s t e m s , o r in s y s t e m s w h e r e t h e r e is n o

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F i g . 7 . R a t e o f a t t a c k b y v a r i o u s c o n c e n t r a t i o n so f p o t a s s m m c h l o n d e [5].

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F i g . 8 . S c h e m a t i c o f w e a t h e r i n g p r o c e s s.


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P B Ad am s / Glass corroston 197

d e t a c h m e n t o f r e a c t i o n p r o d u c t s, w a t e r a t ta c k m a y i n f a c t b e c o m e a l k a li a t t a c ka s a l k a l i is e x t r a c t e d i n t o s o l u ti o n . I f r e a c t i o n p r o d u c t s a r e r e m o v e d , i t r e ta i n si t s i o n e x c h a n g e c h a r a c t e r. O f t e n i t i s m o r e p r o p e r l y d e s c r i b e d a s a h y b r i ds y s t e m w h e r e i o n e x c h a n g e a n d s i l i c a n e t w o r k d e s t r u c t i o n p r o c e e d s i m u l t a -n e o u s l y. D i f f u s i o n i s t h e i n it ia l r a t e c o n t r o l l i n g f a c t o r s o t h a t t h e u s u a l s q u a r er o o t d e p e n d e n c e o n t i m e i s f o u n d . A s t h e s d i c a l a y e r t h i c k e n s , t h e r e a c t i o nw i t h s i li c a m a y b e c o m e e q u i v a l e n t to a l ka l i e x t r a c t i o n a n d t h e r e a c t i o n t e n d s t ob e s t r i c t ly p ro p o r t io n a l t o t~m e . A t t ack b y sa l t s m ay b e , i n so m e case s , s im i l a rt o w a t e r. H o w e v e r , th e d e v i a t i o n s a b o u n d , a n d t h e y a r e n o t a l w a y s r e a d i l yex p la in ed ( f ig . 7 ).

We a t h e r i n g o f g l a s s g e n e r a l l y r e f e r s t o r e a c t i o n s t h a t o c c u r b e t w e e n g l a s sa n d v a r i o u s a t m o s p h e r i c c o m p o n e n t s , p r i n c i p a l l y w a t e r [ 10 ]. F i g. 8 i ll u s tr a t e sth e p ro cess . I t i n v o lv es a v a r i e ty o f s t ep s , i n c lu d in g th o se p r ev io u s ly d i scu ssed ,

w h i c h o c c c u r in s e q u e n c e o r s ~ m u l ta n e o u sl y, s u c h a s a d s o r p t i o n o f w a t e r , i o ne x c h a n g e o f h y d r o n i u m i o n f o r al k a li i o n in t h e g la ss , h y d r a t i o n t o f o r m a t h i nl a y e r , as w e l l a s p h y s i c a l c h a n g e s r e s u l t i n g f r o m w e t t i n g a n d d r y i n g .

Wi t h o u t a t m o s p h e r i c w a t e r , t h e r e i s n o w e a t h e r i n g . A m o n g c o m m e r c i a lg l a s s e s , n o n e b u t t h e p o o r l y d u r a b l e g l a s s e s w i l l s h o w a n y d e t e c t a b l e w e a t h e r -i n g b e l o w a b o u t 2 0 t o 3 0 % r e l a t i v e h u m i d i t y ( f i g . 9 ) . A t t h e o t h e r e x t r e m e , a th u m i d i t ie s a b o v e 9 0 t o 9 5% R H , w e a t h e r i n g m a y a l s o c e a s e b e c a u s e o f d i lu t i o na n d r u n - o f f o f r e a c t i o n p r o d u c t s ( f ig . 10 ),

6 B

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Flg. 9. Sodmm oxide found on surface of asoda-hme glass versus relatwe hurm&ty andtime [10]

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F]g 10. Water and s odm m oxade found onsurface of a soda-brae glass versus relativehumadity [10].


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198 P B Ad am s / Glass corroston

Ta b l e 1C o r r o s n o n o f t y p ic a l c o m m e r c i a l g l a s se s

G l a s s t y p e W e i g h t l o s s ") m :

5 % N a O H 5 % H C 1S o d a - h m e 2 0 .0 2Boros f l ] ca te 5 0 . 005H i g h z i r c o m a 0 . 4 0 0 lA l u r m n o s l h c a te 4 0 . 2H igh s i l i ca 2 0 . 001Vi t r e o u s s i h c a 2 0 O 0 0 X

a) m g / c m 2 a f t e r 2 4 h a t 9 5 o C .

3 . C o m p o s i t io n a n d c o r r o s i o n r e s is t a n c e

Th e co r ro s io n r a t e s o f s ix co m m erc ia l g l asse s, a so d a - l im e , a b o ro s i l i c a t e , ah igh z i rcon ia s i l ica te , an a lumino s i l ica te , a h igh s i l ica and a 100% si l ica te g lassa r e s h o w n i n t a b l e 1 . T h e i r r e s is t a n c e t o s t r o n g a l k a l i d if f e r s a b o u t 1 0 - fo l d ,wh e reas r e s i s t an ce to ac id v a r i e s ab o u t 1 0 0 0 - fo ld . I n q u a l i t a t iv e t e rm s , t h i sr e l a t io n sh ip , a s ex h ib i t ed fo r t h e b ro ad f am i ly o f s i l ic a t e g l a sse s , i s sh o wn inf ig . 11 . Ex p e r i e n ce in d ica t e s th a t a d u ra b le c o m m erc ia l g l ass w i ll su rv iv e w a te ra t t a c k f o r a l o n g p e r i o d o f t im e . O n e w o u l d p r e d i c t t h a t a c h e m i c a l l y r e s i s ta n tb o r o s i l ic a t e g la s s w o u l d d i s so l v e a t t h e r a t e o f a b o u t o n e m i l l i m e t e r in 2 5 0 0 0y e a r s o r m o r e . B u t t h i s i s s t i l l s e v e r a l o r d e r s o f m a g n i t u d e f a s t e r t h a n i sb e l i e v e d to b e t h e c a s e f o r m a n y n a t u r a l g la s se s .

T h e p r e d o m i n a n t c o m p o s i t i o n a l v a r i a b l e i n f l u e n c i n g s o l u b i l i t y i s g e n e r a l l ys il ic a c o n t e n t . T h e r e a s o n i s e v i d e n t f r o m t h e s t a b i l it y d i a g r a m o f f u s e d s il ic ain aq u eo u s so lu t io n a t v a r io u s p H ( fig. 1 2 ) [11] . N o t in g th a t a l o w ac t iv i ty o f

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F i g. 11 . R e l a t l o n s l u p b e t w e e n p H a n d c o r r o - F i g . 1 2 . S t a b i h t y c h a g r a m o f f u s e d s d l c a ms i o n r a t e f o r a v a r i e ty o f g l a s se s , a q u e o u s s o l u U o n v e r s u s p H [ 11 ]


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P. B Ada ms / Glass corroston 199

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F ng . 13 . S t a b t h t y & a g r a m o f a l u m i n a m a q u e o u s s o l u t i o n v e r s u s p H [11].

- 4 i s e q u i v a l e n t t o l es s t h a n o n e p p mS I O 2 i n s o l u t i o n , i t b e c o m e s a p p a r e n tw h y, i n t h e a b s e n c e o f i o n e x c h a n g e , a h i g h s i l i c a g l a s s i s s o e x t r e m e l yre s i s t an t .

S t a b i l i t y d i a g r a m s f o r o t h e r g l a s s c o m p o n e n t s p r o v i d e a d d i t i o n a l i n s i g h ti n t o g la s s c o r r o s i o n r e s is t a n c e . F ig . 1 3 s ho w s a l u m i n a , c o m m o n l y f o u n d i na lm o s t a l l n a tu r a l g l a sse s [ 11 ] . As p r ed ic t ed h e r e , t h e g l a ss t e ch n o lo g i s t k n o wst h a t a l u m i n a e n h a n c e s d u r a b i l i t y i n n e u t r a l s o l u t i o n , a t t h e e x p e n s e o f r e s i s -t a n c e t o s t r o n g a l k a l i a n d s t r o n g a c i d . T h u s , a g l a s s t h a t h a s g o o d w e a t h e r i n gr e s i s t a n c e m a y b e a p o o r l a b o r a t o r y v e s s e l .

T h e a l k a l i n e e a r t h e l e m e n t s w h i c h a r e c o m m o n t o a l m o s t a l l n a t u r a l g l a s s e sa l s o i m p a r t e x c e p t i o n a l s t a b i l i t y i n n e u t r a l s o l u t i o n t o a s i l i c a t e g l a s s . H y d r o -g e n i o n e x c h a n g e i s s t r o n g l y d e p e n d e n t o n p H a n d C a 2 + a c t i v i t y ( fi g. 1 4 ) [11].C a l c i u m h y d r o x i d e i s e x t r e m e l y i n s o l u b l e in w a t e r s o t h a t r e a c t i o n s a r es u p p r e s s e d . F u r t h e r m o r e , s i l i c i c a c i d p r o b a b l y p r o v i d e s a d i f f u s i o n b a r r i e r a tl o w p H . S o t h e e f f e c t o f t h e a l k a l in e e a r t h s d e r i v e s m o r e f r o m t h e i r p r e s e n c e i ns o l u t i o n t h a n t h e i r p r e s e n c e i n t h e g la ss .

A d s o r p t i o n o f w a t e r , a k e y f a c t o r in w e a t h e r i n g , is d e p e n d e n t o n g la s sc o m p o s i t i o n . A c o m m e r c i a l s o d a - li m e g l as s r e s p o n d s q u i t e d i f f e r e n t l y i n t h isr e sp e c t t h a n a b o ro s i l i c a t e g l a ss ( fig . 1 5 ). I n f ac t t h e r e is a w id e sp ec t ru m o f

) -I '-

7-I- -. 3

oM

4

- 4 - S I L IC A

I I I I I ~ I I2 6 pH I0 14

F ro g 1 4 . S t a b i li t y d n a g r a m o f c a l c m m s l h c a t e i n a q u e o u s s o l u t i o n v e r s u s p H [11].


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200 P.B. Ad am s / Glass corrosion

1 2 / S O D A - L I M E

r /

-;- J=. /&.o

3 0 5 0 7 5 9 0% RELATIVE HUMIDITY

Fig. 15. Water adso rbed o n so me glasses after 7 days v ersus relative hunu dity [10].

r e s i st a n c e t o w e a t h e r i n g ( fig . 16 ) s u c h t h a t h i g h a l u m i n a g l a ss e s m a y b eh y d r o p h o b i c . Va r i a n c e i n h y d r a t i o n r a te , a s a c o n s e q u e n c e o f d i f f e re n c e s ins il ic a c o n t e n t , a s f o r o b s i d i a n is a m o s t i m p o r t a n t f a c t o r in n a t u r a l w e a t h e r i n g(f ig . 17) [12] .

A n c i e n t m a n - m a d e g l a s s e s a r e s o m e t i m e s v i e w e d a s a p o t e n t i a l s o u r c e o f

W O R S T G L A S S E S

E " ~ , , , '

i I I " / / d ~ ? / ~ ' X , , ~ 2 + ~ ~

o L I / / / / . $ ~ . ~ , o . . ~ . , % I F I / / / / J~ ' ,~ ,, , _ , " ~ ~ i

~t " N G

A F T 7 I I I I

0 2 4 6 8 I0 12TIME (WEEKS)

F i g .16. Weathering o f various glasses at t, gh hum idity


9/13

P B Ad ams / Glass corroston 201

1 5 0 0

o o o . Jx ,q ,

I [ I I I I

o 5 Io IS 2 0 2 5 3 0T E M P E R A T U R E ( % G )

F i g . 1 7 . H y d r a t i o n r a t e s o f o b s i d i a n w i t h a r a n g e o f s l h c a c o n t e n t [ 1 2]

information about how and why silicate glass survives in the natural environ-ment. The earliest of these glasses date to about 1500 BC, so that they haves u r v i v e d o n l y a b r i e f 3 5 0 0 y e a r s , r a t h e r i n s i g n i f i c a n t w h e n c o m p a r e d t o t h em i l l i o n s o f y e a r s f o r s o m e t e k ti t e s. M o s t a n c i e n t g l a s s e s h a v e w e a t h e r e ds i g n i f i c a n t l y [ 1 3 ] . A f e w h a v e s u r v i v e d v i r t u a l l y u n a f f e c t e d . C o m p o s i t i o n a U yt h e y a r e a l m o s t a l l s o d a - l i m e g l as s es , w h i c h a r e r e l a t i v e ly e a s y t o m e l t a n d n o th i g h l y d u r a b l e .

F o r t h e m o s t p a r t , t h o s e e x c a v a t e d i n p l a c e s w i t h d r i e r c l i m a t e s s u c h a sE g y p t , a r e l es s w e a t h e r e d t h a n s i m i l a r gl a s se s f r o m - w e t t e r a r e a s t h r o u g h o u t t h eN e a r E a s t a n d E u r o p e . S o m e b e a r t h i c k cr u s ts o f w e a t h e r i n g p r o d u c t s w h i l eo t h e r s a r e s i m p l y i r i d e s c e n t . T h e r e a r e a f e w p e c u l i a r g l a s s e s f r o m I n d i a a n dt h e B l a c k S e a a r e a w h i c h h a v e s u r v i v e d n o t i c e a b l y b e t t e r t h a n o t h e r g l a s s e sf r o m t h e s a m e r e g i o n s . I n t e r e s t i n g l y, t h e s e a r e s o d a - l i m e g l a s s e s o f r e l a t i v e l y

h i g h A 1 2 0 3 c o n t e n t , a b o u t 6 - 1 0 % . E s t i m a t e s o f l o n g t e r m , a m b i e n t c o r r o s i o na r e s h o w n m t a b l e 2. S o m e c o m m e r c i a l g la s s es sh o u l d s u r v i v e f o r l o n g p e r i o d so f t i m e , e s p e c i a ll y i f d u r i n g t h a t t i m e t h e r e a r e p e r i o d s w h e n w a t e r i s a b s e n t ,w h e n t e m p e r a t u r e s a r e l o w o r w h e n s o l v e n t s a r e s a t u r a t e d .

Table 2Calculated corroston rate m pure w ater

Loss m thickness a)(ram per 106 y)

Soda-hme 1000

Borosihcate 10High slhca 1

a ) A t 2 5 o C


10/13

202 P B Ad am s / Glass corrosion

Table 3Typical composttlons a) of some commeroal and natural glasses

SO2 A1203 RO b) R 2 c) FeO + B203Fe203

Soda hme 73 2 12 13 0 0Borosdlcate 81 2 0 5 0 13High slhca 96 1 0 0 0 3Alununo slhcate 65 25 10 0 0 0Obsidians 50-75 10-20 0-15 5-10 0-12 0Tektites 50-88 5-35 1-25 0-7 0-12 0

a) In wt%.b) Includes MgO, CaO, and BaOc) Includes Na20, K20 and Li20.

Table 3 shows a few commercial and natural glass compositions [14,15].Seldom do we find high A1203 or high alkaline earth commercial glasses unlesssignificant amounts of alkali or boron - needed for ease of melting - are alsopresent. On the other hand, the natural glasses invariably contain no boron,little alkali and relatively large amounts of A1203 and/or alkaline earth oxides,all of which impart exceptional stability in neutral solutions. Only on rareoccasions do we find a commercial glass of such composition, because technol-ogists are usually trying to design a material for a modem environment thatincludes acids and bases. But if a glass was needed to survive for a long timewhere near neutral conditions prevailed the technologist would start with silicaand add generous amounts of alumina and lime or magnesia.

Tektites in general, have apparently survived much longer than obsidians;and obsidians are often reported to be substantially hydrated at the surface, incontrast to tektites. This may relate to structural differences associated withoriginal water content when formed. The extremely low water content oftektites may make them virtually inpregnable to hydration.

4 . l n h o m o g e n e i t y

All glass products formed by man or nature are inhomogeneous on somescale. Surface composi tion often differs from bulk composition. Sometimes, thesurface will nucleate subsurface crystal growth. Reboil leaves the surfacepeppered with tiny bubbles. Also present are bulk inhomogeneities, such asbubbles, cords, stones and crystals. Some inhomogeneities may be erased bystrong corrosive action. But other fairly subtle inhomogeneities may influencethe course of corrosion, or persist in spite of it. Hence, the inhomogeneities inthe original glass will leave their mark. Then, there is the corrosion environ-

men t itself. Even in a lab ora tory situation, it is rarely homo gen eou s unlessmeticulously controlled. The pr oduct s of reaction continual ly alter the reactioninterface chemistry. A glass of water left on the shelf is changing as alkali is


11/13


12/13

204 P. B Ad ams / Glass corroston

SOLUTIONH

/ /*- GLA SS SURFACE / / ='-- TRANSITION

/ / Z O N E/ /

/ // /

HYDRATED GEL LAYER ~ / / DRY GLASS/ // // /

/ /

Fig. 19. Representationof aqueous attack process [19].

c o n t a c t w i t h a n a q u e o u s s o l u t i o n ( f i g . 1 8 ) . T h e y c o n c l u d e d t h a t " I n t h ea b s e n c e o f a p p r e c i a b l e n e t w o r k d i s s o l u t i o n . . , g r o w t h o f t h e g e l l a y e r b e c o m e s

e v e n t u a l l y p r o p o r t i o n a l t o t h e s q u a r e r o o t o f ti m e ; h o w e v e r a s l o n g a sp r o c e s s e s i n t h e t r a n s i ti o n z o n e a r e r a t e c o n t r o ll i n g , g r o w t h i s p r o p o r t i o n a l t ot im e . " T lu s i s o f i n t e r e s t s in ce i t co in c id es w i th th e l i n ea r h y d ra t io n r a t e f o u n dfo r o b s id i an s .

P l o d i n e c e t a l . [ 1 8 ] h a v e e x t e n d e d t h e t h e r m o d y n a m i c a p p r o a c h t o a w i d ev a r i e ty o f g l a sse s in c lu d in g o b s id i an . Th e i r r e su l t s a r e r ep o r t ed in th e sep r o c e e d i n g s . T h i s a p p r o a c h s e e m s e x c e p t i o n a l l y p r o m i s i n g f o r r a n k i n g g l as s esin o rd e r o f r eac t iv i ty.

P e s c a t o r e a n d M a c h i e l s [ 1 9 ] h a v e c o n s o l i d a t e d t h e m a t h e m a t i c s o f " f o u rB a s i c M e c h a n i s m s , w h i c h a r e p o t e n t i a l l y r a t e c o n t r o l l i n g " , i n t o a s in g le a p -p r o a c h r e l y in g o n t h e r m o d y n a m i c s a n d k i n e t ic p r i n c ip l e s ( fi g. 1 9) . T h e r e s u l t isa c o m p u t e r p r o g r a m , w h i c h i n c o r p o r a t e s a d e s c r i p t i o n o f t h e s o l u t i o n r a t e a tth e g l a ss su r f ace , r ed ep o s i t i o n o f sp ec ie s b ack o n to th e su r f ace d i s so lu t io n o ft h e s o l i d n e t w o r k a t t h e i o n e x c h a n g e f r o n t , a n d i o n e x c h a n g e o f s p e c i e s i n t oth e b u lk g l ass .

I n t h e f in a l a n a ly s i s a c o m p r e h e n s i v e v ie w o f n e a r n e u t r a l c o r r o s i o n m u s tc o n s i d e r c o m p o s i t i o n , k in e t ic s , w a t e r a d s o r p t i o n r a te s , c h a n g i n g s o l u t i o n a c ti v -i ty an d , a l so , m ech an ica l ev en t s wh ich d i s tu rb o r d e s t ro y p ro tec t iv e l ay e r s , a sw e l l a s t h e c h e m i s t r y a n d p h y s i c s o f " s o i l " r e t e n t i o n o n t h e s u r f a c e o f a

m a te r i a l [ 20 ]. Th e l a t t e r i n c lu d es th e e f f ec t s o f p a r t i c l e s ize , p H , r e l a t iv eh u m i d i t y a n d g la s s c o m p o s i t i o n . F i n a ll y, t h e r e a r e a l so b i o l o g i c a l a g e n t s w h i c hin te rac t wi th g lass [21] .

6 . S u m m a r y

T h e e s s en t ia l m e c h a n i s m s o f g la s s c o r r o s i o n h a v e b e e n u n d e r s t o o d f o r s o m et im e . T e n s o f t h o u s a n d s o f e x p e r i m e n t a l t es t s le a v e fe w f r o n t i e r s o f c o m p o s i -t i o n r e s e a r c h u n t o u c h e d . G l a s s t e c h n o l o g i s t s u n d e r s t a n d q u i t e w e l l h o w t o

f o r m u l a t e a c o m b i n a t i o n o f o x i d e s t o m a k e a g l a s s t h a t w i ll re s is t n e u t r a lc o r r o s i o n . S o , i t d o e s n o t a p p e a r t h a t t h e s e n a t u r a l g l as s es c a n t e a c h u s m u c ha b o u t c o r r o s i o n t h e o r y.


13/13

P B Ad am s / Glass corroston 205

Nature has, however, demonstrated that certain glasses can resist herchemical forces for extremely long periods of time, perhaps longer than wewould have dared pred ict . This is no t because we d id not und ers ta nd corros iontheory, but because we did not and stdl do not have an accurate record of theen vi ro nm en t to which these glasses have been exposed.

I t seems that we are often puzzled by the different appearance of similarmaterials and attempt to relate them to some pecuharity of the glass. I t may bethat the record on these natural glasses for which we should be looking is therecord of climate and weather. Perhaps we can make better use of thesedifferences as clues to past environment.

The dramatic difference between tektite survival and obsidian survival,despite the fact that ma ny are similar in gross com po sm on , deserves a f inalcomment . Tekt i tes repor ted ly contain a few ppm water. Obsid ians contain a t

least a few tenths of a per cent of water. This may result in a different structurewhich is more amenable to diffusion of water.

In spite of the fact that corro sion theory is well docume nted , pred icting a 30milhon year life for a typical tektite stretches our theories. But those theoriesare now be ing f ine tuned by a group of nuclear waste dtsposal explorers. Let ushope that they can certify them, perhaps with the aid of these ancientmaterials, so that they can insure that a million years from n ow there will beother scientists who will study the nu clear waste glasses being forme d todayand conclude that the Twentie th Ce ntur y scien tis t knew his business well.

R e f e r e n c e s

[1] L. Holland, The Properties of Glass Surfaces (Chapman and Hall, London, 1964).[2] C R. Das, Glass. Ind. (1969) 422.[3] S M. Bond and J. Fracklewlcz, Phys. Chem. Glasses 2 (4) (1961) 111[4] R W. Douglas and T.M.M.' EI-Shamy, J. Amer. Cer. Soc. 50 (1) (1967) 1[5] E.B. Shand, Glass Engineenng Handbook (McGraw-Hdl, New York)[6] P.B. Adams, Ultra Purity (Dekker, New York) p 293.[7] D M. Saunders and L.L. Hench, J Am. Cer. Soc. 56 (7) (1973) 373.[8] M B Volf, Techmcal Glasses (Pitman, London, 1961)[9] G.A Hudson and F R. Bacon, Bull Amer. Cer. Soc 37 (4) (1958) 185

[10] H V Waiters and P B. Adams, J Non-Crystalhne Sohds 19 (1975) 183.[11] A. Paul and A. Youssefi, J. Mat. Scl 13 (1978) 97; A. Paul, J. Mat. Scl 12 (1977) 2246[12] R.E Taylor, Advances m Obsidian Glass Studies (Noyes Press, 1976)[13] R.H Bnll, Archeology 14 (1) (1961) 18.[14] B.P Glass, Report CR 170518 NASA Contract NAS 526273 (1982)[15] J A O'Keefe, Tektites and Their Ong/n (Elsevier, New York, 1976).[16] R.H. Bnll, Sol Am. 209 (5) (1963) 120[17] T.M Sulhvan and A.J. Machlels, presented at Am. Cer. Soc 85th Mtg, Clucago (1983)[18] J. Plodmec, C.M. Jantzen and G.C. Wicks, presented at Am. Cer. Soc 85th Mtg, Cbacago

(1983).

[19] C Pescatore and A J Machtels, presented at Am. Cer. Soc 85th Mtg, Clucago (1983).[20] E.F Cuddlhy, Theoretical Consideration of Soil Reaction, Jet Prop. Lab, Cal. Tech, Pasadena

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