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Aqueous corrosion studies of a fluorozirconate glass
Mat. R e s . B u l l . , Vol. 17, p p . 1203-1210, 1982. P U n t e d in the USA. 0025-5408/82/091203-08503.00/0 C o p y ~ g h t (c) 1982 Pergamon P r e s s L t d .
AQUEOUS
CORROSION
FLUOROZIRCONATE Catherine Joseph
J.
H.
Simmons,
Simmons
STUDIES
OF A
GLASS Herbert
and Danh
Sutter,
C. Tran
V i t r e o u s State L a b o r a t o r y The C a t h o l i c U n i v e r s i t y of A m e r i c a Washington, D.C. 20064 Optical Sciences Division Naval R e s e a r c h L a b o r a t o r y Washington, D.C. 20375
(Received
July
14, 1982; Communicated
by W. B. White)
ABSTRACT The e f f e c t s of aqueous corrosion on a f l u o r o z i r c o n a t e glass c o n t a i n i n g ZrF 4, BaF 2, A I F 3, LaF. and LiF were i n v e s t i g a t e d using soaking s o l u t i o n analysis, infrared t r a n s m i s s i o n and reflectance s p e c t r o s c o p y and scanning e l e c t r o n microscopy. The d i s s o l u t i o n of the glass was n e a r l y congruent. The leached surface of the glass was covered w i t h hydroxide crystals of Zr and Ba. The outer leached layer was thick, highly hydrated and cracked w h i l e the glass below was also hydrated and cracked indicating that no p r o t e c t i v e surface layer was formed. Drying studies of leached samples showed the appearance of an infrared absorptance peak at 7 um (1440 cm -I) caused by bridging oxygen bonds formed during dehydration.
Introduction Fluoride glasses are being developed and studied in many l a b o r a t o r i e s for use as infrared t r a n s m i t t i n g optical components such as fiber waveguides, l a s e r w i n d o w s and IR lenses. Since these g l a s s e s are likely to come into c o n t a c t with water at some point during their use, it is desirable to understand the mechanisms which control their aqueous corrosion, and the effects of c o r r o s i o n on their IR t r a n s m i s s i o n characteristics. 1203
1204
C.J.
SIMMONS, et al.
Vol. 17, No. 9
A wide range of studies, extending over many years, have been conducted to discover the mechanisms which control chemical attack and dissolution in a variety of silicate glass systems. These glasses are primarily composed of glass forming elements (e.g. Si, B, AI) linked together by bridging oxygen atoms, and a much smaller number of non-bridging oxygens associated with network modifiers (e.g. alkali ions). Dissolution of silicates has been found to occur by two different mechanisms (1)-(3); electrophilic attack and nucleophilic attack. The first, occurring at low to neutral pH, is simply an ion exchange reaction involving the diffusion of alkali ions out of the glass in exchange for protons from solution, resulting in the formation of a dealkalized layer. This layer frequently provides some measure of protection against further attack. Corrosion by nucleophilic attack begins to occur rapidly at pH>9 and results in a breakdown of the Si-O-Si linkages and subsequent removal of any protective film which may have formed. The structure of fluoride glasses is quite different and less well defined than that of silicates. Fluorine, being univalent, does not form a clear bridging bond, and intermolecular bonding appears to be due to excess charges from high coordination numbers around the glass formers (e.g. Zr) (4). For this reason, the mechanisms governing aqueous dissolution should differ from those observed for silicates. The glass studied here, (mole %) 51.8 ZrF4, 20.0 BaF^, 5 . 3 L ~ , 3.3 AIF 3, 19.6 LiF, is of interest for several rea%ons: ~ its composition was selected to maximize the difference between its crystallization and glass transition temperatures, T~ and T-, in order to avoid crystallization during cooling; (29 it e~nibits a high degree of transparency over a broad range of frequencies in the mid-IR; (3) it contains both aluminum and lithium which are elements whose effect on leaching behavior is well understood in silicates. Experimental The fluoride glass was prepared from component fluorides with addition of excess NH HF~. It was melted at 800°C for one hour in capped platlnum crucibles in an argon atmosphere, and then annealed at around Tg (275°C) without being removed from the crucible. Flat plates (Ixl×0.2 cm) were polished with a 1200 grit diamond wheel using water-free oil, rinsed briefly in benzene and stored in a desiccator. The leach tests were conducted in polymethylpentene (PMP) containers, using a pre-leached teflon basket to support the samples. Most plastic containers previously used in leach tests either contribute ions to solution or adsorb ions from solution (6). PMP, however, appears to be inert. Teflon, on the other hand, leaches several ions into solution includin-~ S-~-, several transftion metal ions and, most fmportant, fluorine. Extraneous F would not only make solution analysis impossible to interpret, it would also affect the
Vol. 17, No. 9
FLUOROZIRCONATE GLASS
1205
s o l u t i o n pH and, therefore, the leach rate. To p r e v e n t this, the teflon b a s k e t s were s u b j e c t e d to a r i g o r o u s c l e a n i n g p r o c e d u r e (7) p r i o r to use. Test s a m p l e s were immersed in i00 ml of d e i o n i z e d w a t e r at 25°C for i, 5, 18, 24 and 72 hours s u c c e s s i v e l y for a total time of 5 days. All of the w a t e r was removed and r e p l a c e d after each time period in order to simulate high d i l u t i o n c o n d i t i o n s and avoid s a t u r a t i o n of the solution by the leached species. The leachate s o l u t i o n s were analyzed by D.C. plasma spectroscopy (Zr, Ba, La, AI), a t o m i c a b s o r p t i o n (Li) and ion s e l e c t i v e electrode (F). L e a c h rates were c a l c u l a t e d from Eq. (i): x.v - ~g cm-2d -l s. At w h e r e x = ppm in solution, v = s o l u t i o n area (cm2), and At = s o a k time (days).
(i) volume
(ml),
s = surface
S u r f a c e layer f o r m a t i o n w a s studied by scanning e l e c t r o n m i c r o s c o p y and infrared t r a n s m i s s i o n and r e f l e c t i o n s p e c r o s c o p y .
Results and D i s c u s s i o n Solution analysis results are shown in Fig. 1 w h e r e the n o r m a l i z e d leach r a t e s (g-cm-2d -I) for individual elements are p l o t t e d as a f u n c t i o n of time. Here all results have been norm a l i z e d to c o m p o s i t i o n by d i v i d i n g the leach rates o b t a i n e d in Eq. (i) by the w e i g h t fraction of each e l e m e n t in the glass. In this way it is p o s s i b l e to compare the relative leach rates of the d i f f e r e n t g l a s s c o m p o n e n t s , since, if c o n g r u e n t d i s s o l u t i o n w e r e to occur, all p o i n t s would lie on a single line. There are several features worth noting. First, it is a p p a r e n t that some s e l e c t i v e l e a c h i n g has o c c u r r e d , e s p e c i a l l y in the case of La w h i c h is released into solution i0 to i00 times more slowly than the major c o m p o n e n t s . This result is s i m i l a r to that seen in silicate leaching (8) w h e r e m u l t i - v a l e n t ions g e n e r a l l y remain behind, r e a d s o r b e d in the leached s u r f a c e layer. Second, all o t h e r e l e m e n t s appear to leach nearly congruently. A l t h o u g h Li leaches at a s l i g h t l y h i g h e r rate than Zr or Ba, the d i f f e r e n c e is m u c h less p r o n o u n c e d than that usually seen for a l k a l i m e t a l s in silicate glasses. Finally, c o n t r a r y to s i l i c a t e behavior, the a d d i t i o n of 2% A1 to the fluoride glass does not seem to have improved the d u r a b i l i t y and, in fact, the leach rate for A1 is only slightly b e l o w Li. It was noted d u r i n g the course of this study that the pH of the solutions d r o p p e d from an initial value of 5.8 to a value of 3-4 d e p e n d i n g on the soak time. This d e c r e a s e in pH is due to a Ffor O H - a n i o n e x c h a n g e w h i c h occurs d u r i n g dissolution. This drop in pH m a y be r e s p o n s i b l e for the high A1 leach rates observed, since the s o l u b i l i t y of A1 i n c r e a s e s at low pH.
1206
C.J.
SIMMONS, et al.
Vol. 17, No. 9
'0 10-2 E 0
1 0 -3
.~ lO-4
\
.,J
O~O\La
10- 5 N
E
ZBLAL Glass
3 10-6 Z
10-7
s
i
|
I
10-0.01
0.1
1.0
10
Time
(days)
FIG. Normalized 2.5
Leach
3.0 .
.
.
ZBLAL
.
i
4.0 . . . . .
'
"',
1 Rates
MICRONS ....
;
' ....
vs.
5.0 '
Time. 6.O
. . . .
'
....
7.0
' ....
' ....
'
8.0 ' ' " ' '
Glass
i z
4000
3500
3000
2500
2000
WAVENUMBER
FIG.
18C0
1600
14(X)
1200
(CM "t )
2
I n f r a r e d t r a n s m i s s i o n spectra showing the d e v e l o p m e n t of absorption peaks due to OH , H20, and b r i d g i n g o x y g e n as a result of h y d r a t i o n and d e h y d r a t i o n of a l e a c h e d surface layer. The spectra are v e r t i c a l l y d i s p l a c e d for clarity.
Vol. 17, No. 9
FLUOROZIRCONATE GLASS
1207
A series of infrared s p e c t r o s c o p y studies was c o n d u c t e d to d e t e r m i n e the effect of aqueous c o r r o s i o n and surface layer form a t i o n on the IR t r a n s m i s s i o n c a p a b i l i t i e s of fluoride glasses. The results (Fig. 2) show the formation of a large a b s o r p t i o n peak, c e n t e r e d at 2.9 um (3400 cm -I) due to OHstretching vibrations, and a second absorption at 6.1 um (1640 cm-l), attributed to HOH b e n d i n g vibrations.(9) In addition, a third a b s o r p t i o n band at 7um (1440 cm -I) was o b s e r v e d when the samples were dried. It was felt that this could be due either to the formation of oxide complexes in the surface layer or the presence of c a r b o n a t e s formed by d i s s o l v e d CO 2 in the water. In order to e l i m i n a t e one of these alternatives, a systematic study was performed in which a sample was exposed to a v a r i e t y of e n v i r o n m e n t a l conditions. Curve A in Fig. 2 represents the spectra obtained when the sample was s u c c e s s i v e l y e x p o s e d at 80°C to (i) vacuum for 21 hours, (2) a partial atmosphere of p u r e CO 2 gas for 2 hours, (3) 100% relative humidity (RH) for 2.5 hours, and (4) 100% RH w i t h CO 2 gas for 20 hours. No liquid w a t e r was allowed to condense on the sample during this test. No c h a n g e s were observed in the spectra. The sample was then soaked for 1 hour (Curve B) and 4 hours (Curve C) in w a t e r w h i c h had been freed of C O ~ by b o i l i n g and p u r g i n g w i t h N 2. The f o r m a t i o n and growth of the a b s o r p t i o n bands at 2.9 um, due to F-/OH- anion e x c h a n g e causing surface hydration, and at 6.1 um, due to surface a d s o r b e d w a t e r was observed. There was no sign of an a b s o r p t i o n at 7 um. The sample was then dried in vacuum at 30°C for 42 hours, causing d e h y d r a t i o n of the surface layer and giving rise to the 7 um band (Curve D). Finally, the sample was soaked in water s a t u r a t e d w i t h COm. Within 1 hour, the 7um a b s o r p t i o n p e a k had nearly v a n i s h e d ~Curve E), proving c o n c l u s i v e l y that this a b s o r p t i o n is due to oxide formation, and further, that it is a bridging oxygen, since it does not appear in the spectra of samples known to contain hydroxyl groups. Our results confirm the conclusions reported by D r e x h a g e and coworkers (i0) that the absorbance shoulder observed at 7 um in some of their g l a s s e s is due to the presence of oxide impurities in some of their melts. The IR r e f l e c t i o n spectra taken during our study show a strong m e t a l - f l u o r i d e p e a k at ~17 um (600 cm -I) in the polished sample which gradually disappears as leaching p r o g r e s s e s and h y d r a t i o n of the surface layer occurs.
Microscopic
Surface
Study
An e x a m i n a t i o n of the sample surface w i t h a scanning electron m i c r o s c o p e showedprecipitated crystal deposits over the e n t i r e surface (Fig. 3A). The crystals appear to be the hydroxide forms of Zr and Ba. Below the p r e c i p i t a t e d crystals, the glass surface was heavily hydrated and cracked over a depth of several m i c r o n s (Fig. 3B). No protective layer, commonly seen in the silicates, was observed. .The hydrated surface layer r e a d i y cracked away from the bulk in drying, and where pieces
1208
C.J.
SIMMONS, et al.
Vol. 17, No. 9
FIG. 3A Crystal deposits formed during leaching and drying.
FIG. 3B Partially dehydrated surface layer peeled off to expose glass beneath
Vol. 17, No. 9
FLUOROZIRCONATE GLASS
1209
had peeled off, further chemical attack and cracking was observed extending into the glass. These results support the IR data showing extensive hydration of the glass surface by OHexchange for F ions. Summary and Conclusions The mechanisms and effects of aqueous corrosion of a fluorozirconate glass containing ZrF , BaF2, AIF3, LaF 3 and LiF were investigated by soaking solution analysis, infrared transmission spectroscopy, infrared surface reflection spectroscopy and scanning electron microscopy. In fluorozirconate glasses, the most mobile species is the fluoride ion, in contrast to oxide silicates where the alkali metal ions are the most mobile. During leaching a decrease in pH was observed arising from F-/OH- exchange. The dissolution of the glass was nearly congruent, except for lanthanum, due to the high solubility of the fluoride components. The leached surface of the glass was covered with hydroxide crystals of Zr and Ba and the outer leached layer of the glass was thick, highly hydrated and cracked. Where pieces of the outer layer were removed, the glass below also appeared hydrated and cracked, indicating that no protective surface layer was formed in contrast to most oxide silicate glasses. Infrared transmission studies of the glass sample soaked in water showed the rapid appearance of both the OH stretching and the HOH bending absorption bands, reflecting the high degree of surface hydration. The same studies of samples exposed to a very humid atmosphere (100% RH), but avoiding condensation of water on the sample, revealed no noticeable hydration, thus demonstrating the need for solution contact to assure chemical corrosion. Infrared surface reflection studies showed a rapid disappearance of a metal fluoride absorption when the samples were soaked in water thus further supporting the conclusion that the surface hydration occurs by F-/OH- exchange. Finally, drying studies of leached samples which were heavily hydrated showed the appearance of an infrared absorption peak at 7 um (1440 cm -I) which was demonsrated to be caused by the vibration of bridging oxygen bonds arising from dehydration of the surface during drying. References i.
R. W. Douglas and T. M. EI-Shamy, "Reactions of Glass with Aqueous Solutions," J. Amer. Ceram. Soc., 50 (i), 1-8 (1967).
1210
C.J.
SIMMONS, et al.
Vol. 17, No. 9
2.
J. H. Simmons, A. Barkatt and P. B. Macedo, "Mechanisms That Control Aqueous Leaching of Nuclear Waste Glass," Nuclear Tech., 56 (2), 265-70 (1982).
3.
L. L. Hench, J. Non-Cryst.
Solids, 25, 343 (1977).
4.
C. A. Angell, "Structure and Mobility of Fluoride and Chloride Glasses by Computer Simulation", First International Symposium of Halide and Other Nonoxide Glasses, Cambridge, E n g l a n d (1982).
5.
D. C. Tran, R. Ginther and G. Sigel, Jr., "Fluorozirconate Glasses with Improved Viscosity Behavior for Continuous Fiber Drawing," to be published by Mat. Res. Bull. (1982).
6.
A. Barkatt,
7.
P. B. Macedo and A. Barkatt, "Evaluation of Bulk Properties of Radwaste Glass and Ceramic Container Materials to Determine Long Term Stability", Contract Report to NRC (1982).
8.
A. Barkatt, J. H. Simmons and P. B. Macedo, "Evaluation of Chemical Stability of Vitrification Media for Radioactive Waste Products," Phys. Chem. Glasses, 22 (4), 73-85 (1981).
9.
M. Robinson, R. C. Pastor, R. R. Turk, D. P. Devor and M. Braunstein, "Infrared Transparent Glasses Derived from the Fluorides of Zirconium, Thorium and Barium," Mat. Res. Bull., 15, 735-42 (1980).
i0.
M. G. Drexhage, C. T. Moynihan, B. Bendow, E. Gboji, K. H. Chung and M. Boulos, "Influence of Processing Conditions on IR Edge Absorption in Fluorohafnate and Fluorozirconate Glasses," Mat. Res. Bull., 16, 943-47 (1981).
private communication.
AQUEOUS
CORROSION
FLUOROZIRCONATE Catherine Joseph
J.
H.
Simmons,
Simmons
STUDIES
OF A
GLASS Herbert
and Danh
Sutter,
C. Tran
V i t r e o u s State L a b o r a t o r y The C a t h o l i c U n i v e r s i t y of A m e r i c a Washington, D.C. 20064 Optical Sciences Division Naval R e s e a r c h L a b o r a t o r y Washington, D.C. 20375
(Received
July
14, 1982; Communicated
by W. B. White)
ABSTRACT The e f f e c t s of aqueous corrosion on a f l u o r o z i r c o n a t e glass c o n t a i n i n g ZrF 4, BaF 2, A I F 3, LaF. and LiF were i n v e s t i g a t e d using soaking s o l u t i o n analysis, infrared t r a n s m i s s i o n and reflectance s p e c t r o s c o p y and scanning e l e c t r o n microscopy. The d i s s o l u t i o n of the glass was n e a r l y congruent. The leached surface of the glass was covered w i t h hydroxide crystals of Zr and Ba. The outer leached layer was thick, highly hydrated and cracked w h i l e the glass below was also hydrated and cracked indicating that no p r o t e c t i v e surface layer was formed. Drying studies of leached samples showed the appearance of an infrared absorptance peak at 7 um (1440 cm -I) caused by bridging oxygen bonds formed during dehydration.
Introduction Fluoride glasses are being developed and studied in many l a b o r a t o r i e s for use as infrared t r a n s m i t t i n g optical components such as fiber waveguides, l a s e r w i n d o w s and IR lenses. Since these g l a s s e s are likely to come into c o n t a c t with water at some point during their use, it is desirable to understand the mechanisms which control their aqueous corrosion, and the effects of c o r r o s i o n on their IR t r a n s m i s s i o n characteristics. 1203
1204
C.J.
SIMMONS, et al.
Vol. 17, No. 9
A wide range of studies, extending over many years, have been conducted to discover the mechanisms which control chemical attack and dissolution in a variety of silicate glass systems. These glasses are primarily composed of glass forming elements (e.g. Si, B, AI) linked together by bridging oxygen atoms, and a much smaller number of non-bridging oxygens associated with network modifiers (e.g. alkali ions). Dissolution of silicates has been found to occur by two different mechanisms (1)-(3); electrophilic attack and nucleophilic attack. The first, occurring at low to neutral pH, is simply an ion exchange reaction involving the diffusion of alkali ions out of the glass in exchange for protons from solution, resulting in the formation of a dealkalized layer. This layer frequently provides some measure of protection against further attack. Corrosion by nucleophilic attack begins to occur rapidly at pH>9 and results in a breakdown of the Si-O-Si linkages and subsequent removal of any protective film which may have formed. The structure of fluoride glasses is quite different and less well defined than that of silicates. Fluorine, being univalent, does not form a clear bridging bond, and intermolecular bonding appears to be due to excess charges from high coordination numbers around the glass formers (e.g. Zr) (4). For this reason, the mechanisms governing aqueous dissolution should differ from those observed for silicates. The glass studied here, (mole %) 51.8 ZrF4, 20.0 BaF^, 5 . 3 L ~ , 3.3 AIF 3, 19.6 LiF, is of interest for several rea%ons: ~ its composition was selected to maximize the difference between its crystallization and glass transition temperatures, T~ and T-, in order to avoid crystallization during cooling; (29 it e~nibits a high degree of transparency over a broad range of frequencies in the mid-IR; (3) it contains both aluminum and lithium which are elements whose effect on leaching behavior is well understood in silicates. Experimental The fluoride glass was prepared from component fluorides with addition of excess NH HF~. It was melted at 800°C for one hour in capped platlnum crucibles in an argon atmosphere, and then annealed at around Tg (275°C) without being removed from the crucible. Flat plates (Ixl×0.2 cm) were polished with a 1200 grit diamond wheel using water-free oil, rinsed briefly in benzene and stored in a desiccator. The leach tests were conducted in polymethylpentene (PMP) containers, using a pre-leached teflon basket to support the samples. Most plastic containers previously used in leach tests either contribute ions to solution or adsorb ions from solution (6). PMP, however, appears to be inert. Teflon, on the other hand, leaches several ions into solution includin-~ S-~-, several transftion metal ions and, most fmportant, fluorine. Extraneous F would not only make solution analysis impossible to interpret, it would also affect the
Vol. 17, No. 9
FLUOROZIRCONATE GLASS
1205
s o l u t i o n pH and, therefore, the leach rate. To p r e v e n t this, the teflon b a s k e t s were s u b j e c t e d to a r i g o r o u s c l e a n i n g p r o c e d u r e (7) p r i o r to use. Test s a m p l e s were immersed in i00 ml of d e i o n i z e d w a t e r at 25°C for i, 5, 18, 24 and 72 hours s u c c e s s i v e l y for a total time of 5 days. All of the w a t e r was removed and r e p l a c e d after each time period in order to simulate high d i l u t i o n c o n d i t i o n s and avoid s a t u r a t i o n of the solution by the leached species. The leachate s o l u t i o n s were analyzed by D.C. plasma spectroscopy (Zr, Ba, La, AI), a t o m i c a b s o r p t i o n (Li) and ion s e l e c t i v e electrode (F). L e a c h rates were c a l c u l a t e d from Eq. (i): x.v - ~g cm-2d -l s. At w h e r e x = ppm in solution, v = s o l u t i o n area (cm2), and At = s o a k time (days).
(i) volume
(ml),
s = surface
S u r f a c e layer f o r m a t i o n w a s studied by scanning e l e c t r o n m i c r o s c o p y and infrared t r a n s m i s s i o n and r e f l e c t i o n s p e c r o s c o p y .
Results and D i s c u s s i o n Solution analysis results are shown in Fig. 1 w h e r e the n o r m a l i z e d leach r a t e s (g-cm-2d -I) for individual elements are p l o t t e d as a f u n c t i o n of time. Here all results have been norm a l i z e d to c o m p o s i t i o n by d i v i d i n g the leach rates o b t a i n e d in Eq. (i) by the w e i g h t fraction of each e l e m e n t in the glass. In this way it is p o s s i b l e to compare the relative leach rates of the d i f f e r e n t g l a s s c o m p o n e n t s , since, if c o n g r u e n t d i s s o l u t i o n w e r e to occur, all p o i n t s would lie on a single line. There are several features worth noting. First, it is a p p a r e n t that some s e l e c t i v e l e a c h i n g has o c c u r r e d , e s p e c i a l l y in the case of La w h i c h is released into solution i0 to i00 times more slowly than the major c o m p o n e n t s . This result is s i m i l a r to that seen in silicate leaching (8) w h e r e m u l t i - v a l e n t ions g e n e r a l l y remain behind, r e a d s o r b e d in the leached s u r f a c e layer. Second, all o t h e r e l e m e n t s appear to leach nearly congruently. A l t h o u g h Li leaches at a s l i g h t l y h i g h e r rate than Zr or Ba, the d i f f e r e n c e is m u c h less p r o n o u n c e d than that usually seen for a l k a l i m e t a l s in silicate glasses. Finally, c o n t r a r y to s i l i c a t e behavior, the a d d i t i o n of 2% A1 to the fluoride glass does not seem to have improved the d u r a b i l i t y and, in fact, the leach rate for A1 is only slightly b e l o w Li. It was noted d u r i n g the course of this study that the pH of the solutions d r o p p e d from an initial value of 5.8 to a value of 3-4 d e p e n d i n g on the soak time. This d e c r e a s e in pH is due to a Ffor O H - a n i o n e x c h a n g e w h i c h occurs d u r i n g dissolution. This drop in pH m a y be r e s p o n s i b l e for the high A1 leach rates observed, since the s o l u b i l i t y of A1 i n c r e a s e s at low pH.
1206
C.J.
SIMMONS, et al.
Vol. 17, No. 9
'0 10-2 E 0
1 0 -3
.~ lO-4
\
.,J
O~O\La
10- 5 N
E
ZBLAL Glass
3 10-6 Z
10-7
s
i
|
I
10-0.01
0.1
1.0
10
Time
(days)
FIG. Normalized 2.5
Leach
3.0 .
.
.
ZBLAL
.
i
4.0 . . . . .
'
"',
1 Rates
MICRONS ....
;
' ....
vs.
5.0 '
Time. 6.O
. . . .
'
....
7.0
' ....
' ....
'
8.0 ' ' " ' '
Glass
i z
4000
3500
3000
2500
2000
WAVENUMBER
FIG.
18C0
1600
14(X)
1200
(CM "t )
2
I n f r a r e d t r a n s m i s s i o n spectra showing the d e v e l o p m e n t of absorption peaks due to OH , H20, and b r i d g i n g o x y g e n as a result of h y d r a t i o n and d e h y d r a t i o n of a l e a c h e d surface layer. The spectra are v e r t i c a l l y d i s p l a c e d for clarity.
Vol. 17, No. 9
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A series of infrared s p e c t r o s c o p y studies was c o n d u c t e d to d e t e r m i n e the effect of aqueous c o r r o s i o n and surface layer form a t i o n on the IR t r a n s m i s s i o n c a p a b i l i t i e s of fluoride glasses. The results (Fig. 2) show the formation of a large a b s o r p t i o n peak, c e n t e r e d at 2.9 um (3400 cm -I) due to OHstretching vibrations, and a second absorption at 6.1 um (1640 cm-l), attributed to HOH b e n d i n g vibrations.(9) In addition, a third a b s o r p t i o n band at 7um (1440 cm -I) was o b s e r v e d when the samples were dried. It was felt that this could be due either to the formation of oxide complexes in the surface layer or the presence of c a r b o n a t e s formed by d i s s o l v e d CO 2 in the water. In order to e l i m i n a t e one of these alternatives, a systematic study was performed in which a sample was exposed to a v a r i e t y of e n v i r o n m e n t a l conditions. Curve A in Fig. 2 represents the spectra obtained when the sample was s u c c e s s i v e l y e x p o s e d at 80°C to (i) vacuum for 21 hours, (2) a partial atmosphere of p u r e CO 2 gas for 2 hours, (3) 100% relative humidity (RH) for 2.5 hours, and (4) 100% RH w i t h CO 2 gas for 20 hours. No liquid w a t e r was allowed to condense on the sample during this test. No c h a n g e s were observed in the spectra. The sample was then soaked for 1 hour (Curve B) and 4 hours (Curve C) in w a t e r w h i c h had been freed of C O ~ by b o i l i n g and p u r g i n g w i t h N 2. The f o r m a t i o n and growth of the a b s o r p t i o n bands at 2.9 um, due to F-/OH- anion e x c h a n g e causing surface hydration, and at 6.1 um, due to surface a d s o r b e d w a t e r was observed. There was no sign of an a b s o r p t i o n at 7 um. The sample was then dried in vacuum at 30°C for 42 hours, causing d e h y d r a t i o n of the surface layer and giving rise to the 7 um band (Curve D). Finally, the sample was soaked in water s a t u r a t e d w i t h COm. Within 1 hour, the 7um a b s o r p t i o n p e a k had nearly v a n i s h e d ~Curve E), proving c o n c l u s i v e l y that this a b s o r p t i o n is due to oxide formation, and further, that it is a bridging oxygen, since it does not appear in the spectra of samples known to contain hydroxyl groups. Our results confirm the conclusions reported by D r e x h a g e and coworkers (i0) that the absorbance shoulder observed at 7 um in some of their g l a s s e s is due to the presence of oxide impurities in some of their melts. The IR r e f l e c t i o n spectra taken during our study show a strong m e t a l - f l u o r i d e p e a k at ~17 um (600 cm -I) in the polished sample which gradually disappears as leaching p r o g r e s s e s and h y d r a t i o n of the surface layer occurs.
Microscopic
Surface
Study
An e x a m i n a t i o n of the sample surface w i t h a scanning electron m i c r o s c o p e showedprecipitated crystal deposits over the e n t i r e surface (Fig. 3A). The crystals appear to be the hydroxide forms of Zr and Ba. Below the p r e c i p i t a t e d crystals, the glass surface was heavily hydrated and cracked over a depth of several m i c r o n s (Fig. 3B). No protective layer, commonly seen in the silicates, was observed. .The hydrated surface layer r e a d i y cracked away from the bulk in drying, and where pieces
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FIG. 3A Crystal deposits formed during leaching and drying.
FIG. 3B Partially dehydrated surface layer peeled off to expose glass beneath
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had peeled off, further chemical attack and cracking was observed extending into the glass. These results support the IR data showing extensive hydration of the glass surface by OHexchange for F ions. Summary and Conclusions The mechanisms and effects of aqueous corrosion of a fluorozirconate glass containing ZrF , BaF2, AIF3, LaF 3 and LiF were investigated by soaking solution analysis, infrared transmission spectroscopy, infrared surface reflection spectroscopy and scanning electron microscopy. In fluorozirconate glasses, the most mobile species is the fluoride ion, in contrast to oxide silicates where the alkali metal ions are the most mobile. During leaching a decrease in pH was observed arising from F-/OH- exchange. The dissolution of the glass was nearly congruent, except for lanthanum, due to the high solubility of the fluoride components. The leached surface of the glass was covered with hydroxide crystals of Zr and Ba and the outer leached layer of the glass was thick, highly hydrated and cracked. Where pieces of the outer layer were removed, the glass below also appeared hydrated and cracked, indicating that no protective surface layer was formed in contrast to most oxide silicate glasses. Infrared transmission studies of the glass sample soaked in water showed the rapid appearance of both the OH stretching and the HOH bending absorption bands, reflecting the high degree of surface hydration. The same studies of samples exposed to a very humid atmosphere (100% RH), but avoiding condensation of water on the sample, revealed no noticeable hydration, thus demonstrating the need for solution contact to assure chemical corrosion. Infrared surface reflection studies showed a rapid disappearance of a metal fluoride absorption when the samples were soaked in water thus further supporting the conclusion that the surface hydration occurs by F-/OH- exchange. Finally, drying studies of leached samples which were heavily hydrated showed the appearance of an infrared absorption peak at 7 um (1440 cm -I) which was demonsrated to be caused by the vibration of bridging oxygen bonds arising from dehydration of the surface during drying. References i.
R. W. Douglas and T. M. EI-Shamy, "Reactions of Glass with Aqueous Solutions," J. Amer. Ceram. Soc., 50 (i), 1-8 (1967).
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2.
J. H. Simmons, A. Barkatt and P. B. Macedo, "Mechanisms That Control Aqueous Leaching of Nuclear Waste Glass," Nuclear Tech., 56 (2), 265-70 (1982).
3.
L. L. Hench, J. Non-Cryst.
Solids, 25, 343 (1977).
4.
C. A. Angell, "Structure and Mobility of Fluoride and Chloride Glasses by Computer Simulation", First International Symposium of Halide and Other Nonoxide Glasses, Cambridge, E n g l a n d (1982).
5.
D. C. Tran, R. Ginther and G. Sigel, Jr., "Fluorozirconate Glasses with Improved Viscosity Behavior for Continuous Fiber Drawing," to be published by Mat. Res. Bull. (1982).
6.
A. Barkatt,
7.
P. B. Macedo and A. Barkatt, "Evaluation of Bulk Properties of Radwaste Glass and Ceramic Container Materials to Determine Long Term Stability", Contract Report to NRC (1982).
8.
A. Barkatt, J. H. Simmons and P. B. Macedo, "Evaluation of Chemical Stability of Vitrification Media for Radioactive Waste Products," Phys. Chem. Glasses, 22 (4), 73-85 (1981).
9.
M. Robinson, R. C. Pastor, R. R. Turk, D. P. Devor and M. Braunstein, "Infrared Transparent Glasses Derived from the Fluorides of Zirconium, Thorium and Barium," Mat. Res. Bull., 15, 735-42 (1980).
i0.
M. G. Drexhage, C. T. Moynihan, B. Bendow, E. Gboji, K. H. Chung and M. Boulos, "Influence of Processing Conditions on IR Edge Absorption in Fluorohafnate and Fluorozirconate Glasses," Mat. Res. Bull., 16, 943-47 (1981).
private communication.