Durability of some gamma-irradiated alkali borate glasses

Radiar. Phys. Chem. Vol. 44, No. I/2. pp. 45-51, 1994 Copyright 0 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0969-806X/94 $6.00 + 0.00

Pergamon

DURABILITY

N.

‘National

Center

OF SOME GAMMA-IRRADIATED BORATE GLASSES A.

EL-ALAILY’,

for Radiation Department,

F. M. EZZ-ELDIN’

Research National

and H. A.

ALKALI

EL-BATAL’

and Technology, AEA, Cairo, Egypt and *Glass Research Research Center, Dokki, Cairo, Egypt

Abstract-Dissolution of glass coupons in acid solution with the general composition 80 B,O,, 10 Na,O and 10 RO (divalent oxide) has been monitored with and without y-irradiation. The increase in glass solubility upon y-ray irradiation is interpreted in relation to the expected radiation damage in the glass matrix, which is possibly due to the competition between the intrinsic defects and the induced defects incurred by irradiation. The decrease in glass durability at 100°C is related to the increase in the activation energy of the corrosion process which leads to the dissolution of B from the glass and the formation of a surface layer. Moreover, the role of alkaline earth metal is also discussed according to the influence of the size and activity coefficient of cations, as well as its ability to be adsorbed on the glass surface. Also the amount of Na released from the leaching solution for the different ternary glasses was estimated and evaluated.

oxides Zn, Ca, Sr, Cd and Ba were prepared. Details of the glass compositions studied are given in Table 1. The glasses were prepared from AR or pure chemicals according to standard methods of melting and refining. Sodium oxide was obtained from anhydrous sodium carbonate, B was obtained from orthoboric acid while Zn, Ca, Sr, Cd and Ba oxides were obtained from the corresponding carbonates. The prepared annealed glasses were then cut into cubes of 1 cm’.

INTRODUCTION

durability of glass is a property of great practical importance. The differences in the chemical durability of glass surfaces are believed to depend on the ability to exchange ions, particularly by the acceptance of hydrogen or hydronium ions from water or acids, and by the release and dissolution of the alkali ions such as Na ions into the solution. The extent of this reaction depends on the composition of the glass, the type of attacking solution and the conditions under which leaching occurs (Volf, 1961). The effects of y-irradiation on alkali borate glasses, including the formation of defect centers, have been studied by several authors (Tsai et al., 1988; Simnad ef al., 1985; Cotrell, 1985; Yokota, 1954). It has been assumed that these defect centers may not be identical due to the fact that the valencies and coordination numbers of the network formers are different. This study aims to investigate the corrosion behavior of alkali alkaline-earth borate glasses of different compositions, before and after irradiation with different doses of gamma-rays. The effects of low and high doses of y-irradiation on the durability of these ternary borate glasses are reported. A glass with a very simple composition has been selected in order to define how its leachability is modified by adding different divalent oxides. The objective is to find out the effects of various divalent oxides, together with prolonged irradiation on the mechanism of corrosion under different temperatures.

The chemical

Durability measurements The procedure of chemical corrosion summarized as follows:

The glass cubes were accurately weighed (+ I%), placed in a 250 ml polyethylene beaker and 20 ml of 0.1 N HCl for each g of glass was added to completely cover the glass specimens. The glass samples were left in the acid for 3, 6 and 24 h at room temperature (27 k 1°C). Other beakers containing glass samples of the same compositions were placed in a water bath at 100°C (+ 2°C) for the same periods of time. The glass specimens were washed, dried and weighed after each time interval (the weight error was *0.3%), while the pH of the solution was measured using an Orion Research pH meter. The Na released in the solution was determined by the atomic absorption technique.

EXPERIMENTAL

Irradiation procedure

Glass preparation Borate glasses containing

studies can be

The glass specimens were irradiated by a @‘Co y-source. The dose rate for irradiation was 1.35 Gy/s

one of the alkaline earth 45

46

N.

Table I. Glass comuosition

and the concentration

I 2 3 4 5

B,O,

Na,O

80 80 80 80 80

IO IO IO IO IO

RO IO cao IO zno IO SrO IO Cd0 IO BaO

of released Na,O

Non-lrradlated 300 312 336 346 325

(Gamma Chamber 4000A manufactured Atomic Energy Agency of India). RESULTS

EL-ALAILY

et ul

(tmm) from the mvestmated

After immersion for 24 h at 27 C

Compositions Glass No

A.

by

classes before and after irradiation

After immersion for I2h at 100 C

82.6 (kGy)

437.5 (kGy)

270 287 262 230 250

32 35 40 90 68

the

AND DISCUSSION

The reaction between glass and different aqueous solutions of various pHs has been of interest for many years (El-Shamy and Douglas, 1972; Clark et al., 1979). Chemical attack mechanisms may be based upon two concepts; the etching characteristic of alkaline solution, and the leaching characteristic of the acid attack. The leaching process, is a diffusioncontrolled ion-exchange process, involving the cxchange of protons or hydronium ions for the alkalis present in glass. In general, there will be a selective removal of elements present in the interstices of the glass network as glass modifiers, because they are weakly attached (Adams, 1973). As shown in Table 1 the mass of sodium-oxide passed over to the extracting solution was found to decrease sharply with the first irradiation dose. It decreased at a lower rate with the prolonged exposure to y-rays. This behavior is observed at both room temperature and at high temperature. However, from the data illustrated in Table I, there is a gradual decrease in Na extracted from the leaching solution with increasing radiation dose. This may be attributed to the weakness in the bond lattice caused by irradiation damage. This makes it easier for Na ions to be extracted in the solution. leaving a thin layer at the glass surface, blocking to some extent the release of further Na ions from the lattice.

Greaves (1990) has suggested that glass corrosion is governed, to a large extent. by the mobility of metal ions within the network. He also presented a modified random network model. which predicts that a modifying metal occupies percolation channels weaving between the network parts of the structure. Thcsc channels are the most likely routes for alkali diffusion, ion-exchange, infusion of water and also crack propagation. The presence of relatively large divalent cations is assumed to inhibit the diffusion of alkali, if they both utilize the same percolation channels. The relative effect of the different divalent oxides is expected to depend on the ionic radii, field strengths of the respective cations and the relative strengths of the bonds between these cations and the corresponding attached oxygens (Doremus, 1979: Adams, 1984).

Non-lrradlated 490 510 540 545 538

X2.6 (kGy)

437.5 (kGy)

470 492 527 512 518

2x2 295 380 490 302

Ca’+, Sr’+, or Ba’+ cations are normally housed in the interstices, like the Na’ ions, the groupings of BO, and/or BO, around the divalent cations are more stable than the groupings which might form around the monovalent cations. Consequently the structure tends to develop a series of interstices of more or less regular type surrounding the Ca’+, Sr*+ and Ba’ ’ cations with a proportion of large irregular interstices in which Nat have to be housed, possibly with more than one Na+ ion in each of the large irregular rings (El-Badry and El-Batal. 1972). As shown in Fig. I, Cd0 gave the maximum weight loss, followed by the oxides of Ba, Zn and Sr. Calcium oxide gave the minimum weight loss of all the divalent cations used. In all the corrosion data obtained on different immersion times, one can see that the curves consist of two segments characterizing two stages of leaching. The first segment is characteristic of the early stage of leaching in which the rate of corrosion is high. In the second segment, the slope of the curve becomes smaller and is characteristic of latter stage of leaching.

Fig. 1. The relation between the weight loss percentage and the square root of the immersion time in hours for the unirradiated glasses at 100 C.

Durability of y-irradiated alkali borate glasses Cd0 is expected to enter the structure partly as CdO, groups because of the relatively limited amount of oxygen ions available from the soda present. During leaching the CdO, groups are lessened because less oxygen can be available from the alkali oxide and higher proportions of Cd*+ ions become enclosed in the structural interstices or acted as bridges between structural building units. Imagawa (1969) notices that the Cd04 centers decrease as the basicity of the network modifier increases. The results obtained from the Cd-containing glasses could be explained by taking into consideration that the Cd-O bond is very weak and the increase of Cd0 content increases the leaching of the glass. Addition of CaO to a binary alkali borate glass increases its durability. This may be due to either microphase separation and/or the activity of CaO in these glasses. Possible explanations for this phenomenon are that, Ca released from the glass through the network breakdown is partly retained by absorption on the surface. Boron may also be preferentially extracted from the bulk of the leached layer formed on the surface of the glass (Paul, 1982). The presence of both Ca*+ and Na+ ions may also block the percolation channels and inhibit the corrosion. Effect of irradiation

It has been assumed (Tsai et al., 1988) that defects induced by y-irradiation in alkali borate glasses are due to electrons trapped by oxygen vacancies in neighboring alkali ions. The fact that these defects are very sensitive to the kind of alkali in the glass suggests that the electron-trap center must be in the vicinity of an alkali ion. This defect center appears to shift in position according to the alkali ion present. Figures 2 and 3 show the effect of increasing y-irradiation doses on the weight loss percentage at room temperature and at 100°C. It can be seen that: at the first irradiation dose of 9.7 kGy, the difference in loss percentage between the irradiated and unirradiated glasses is very small. At high doses the difference is more pronounced. At a higher temperature, IOO’C, a remarkable increase in the weight loss is obtained. Gamma-irradiation of glasses may lead to surface damage, unstable charging, and migration of mobile (non-network) cations. These defects may lead to acceleration of the leaching process. Increasing the irradiation dose (Fig. 3) resulted in increasing the solubility of the glasses investigated. This may be caused by the enhancement of the penetration of protons or hydronium ions from acid into the glass structure due to large vacancies incurred following exposure to y-irradiation. There is evidence that a sequence of these bond-breaking reactions may cause large molecular islands to dissolve from the glass surface. The more open surface structure of a transformed layer increases the dissolution rate of the B-O, lattice. The higher dissolution rate of the glasses exposed to more radiation may be due to the fine-

47

scale phase-separation of the alkali borate glasses. These glasses are believed to separate into pure boron matrix and fine spherical regions of alkali borate. The alkali borate glasses are quite soluble in acid, therefore, the phase-separation is expected to cause microporosity that increases accessibility of the B-O, lattice to acid. This fine phase-separation can perhaps again be considered to increase the effective surface area of the glass. Even if this phase-separation is in isolated regions, rather than being interconnected, it will increase the rate of dissolution of the glass. This is because of the presence of thin regions of the durable glass phase between droplets of less durable phase, as compared to uniform durable glass (Perrea and Doremus, 1991).

Effect of temperature

and time of leaching

The importance of temperature in glass corrosion is well established. In addition, the agitation of a heated solution caused by bubbling on the glass surface may lead to the removal of the corroded layer from the glass surface, exposing fresh glass to the corrosion solution (Tsuchihashis et al.. 1985). The durability of glass may be expressed as a function of both thermodynamic and kinetic stability of its component oxides. It has been shown that at temperatures below 3O’C, diffusion is predominant, while above 85”C, total dissolution is rate controlling (Zagar and Schillmoeller, 1960). At the temperature of 27°C used in this experiment, the system had thermal energy only to overcome the activation barrier, and the kinetic part was predominant. On the other hand, at IOO’C, the thermodynamic part, was more important. At 27‘C selective leaching is rate controlling. These changes in the corrosion mechanism may be due to the extraction of Na oxide and RO (alkaline earth oxide) from the glass surface, presumably through an ion exchange with protons or hydronium ions. In any event, NazO and RO still remain present at the surface, although the Na concentration has decreased significantly. However, it should be noted that the extraction of RO requires prior removal of Na (Paul, 1982). At IOOC, the weight loss was observed to be progressively increased at a more or less constant rate. This may be explained in accordance with the previous assumption that at high temperature, the thermodynamic part of the corrosion is effective. In addition, the mobility of the ionic species; ion-exchange; diffusion and the solubility limit of the formed compounds are all highly accelerated with the increased temperature. Moreover, the possibility of the formation of a protective surface layer becomes less as the result of agitation of the heated solution. The increase of time of leaching is expected to increase the rate of ion-exchange reaction. At room temperature the first sharp increase (first segment) suggests that the process of ion-exchange is diffusion controlled, i.e. the rate controlling process is at first

N.

48

A.

EL-ALAILY

cl al.

+ f

22

A 9.1 kGy 0

82.6

Unirradiated

A 9.1 kGy

Unirradiated _

0

X2.6

kGy

kGy

* 10

0

4

2

0

6

I

I

2

4

h

.ri

J-i

Unirradiated

+

Unirradiated

A 9.7 kGy 0

0

82.6

kGy

2

4

4

2

6

J-i

Ji-

+

Unirradiated

A 9.7 kGy 0 18

-

+

437.5

kGy

10 0

2

4

6

Ji

Fig. 2. The relation

between the weight loss percentage and the square root of the immersion for glasses irradiated with different irradiation doses at 27°C.

time in hours

h

+ f -

Unirradiated

A 9.7 kGy

+ Unirradiated A 9.7 kGy 0

+

82.6 kGy

#c437.5

kGy

/

39

;-

I-

j-

I

I

15

0

I

+ Unirradiated

+ Unirradiated A 9.7 kGy

+

25 -

20 -

15

0

1

3

5

J-i+ + Unirradiated _

Fig. 3. The relation

A 9.7 kGy

between the weight loss percentage and the square root of the immersion for glasses irradiated with different irradiation doses at 100°C.

time in hours

N. A. EL-ALAILY et ul

50 Table

2. The pH ralues

of the glass mvesttgated for several

leaching

Non-irradiated Glass

No.

before and after exposure

times.

temerpature

97kGy

to different 27 i

lrradlatmn

dose\ and

I C

X2.6 kCy

437.5 (kGy)

3h

6h

24 h

3h

6h

24 h

3h

6h

24 h

3h

hh

I

1.55

2.04

6.79

I .60

1.99

64

I 66

z II

6.4

I 34

I 50

2 30

2

I.71

I 97

6.71

2.04

2.47

5.74

I .74

I.6

2.9

I .40

I .70

2.20

24 h

3

I .44

I 70

I.17

1.7x

2.17

6.79

I.66

2.15

6.2

I .50

I .x0

6 IO

4

I.XI

2.23

5.43

2.02

2.3x

5.41

1.7

2.14

5.71

2 20

4.00

5

1.58

2.08

2.08

1.99

2.41

6.43

I .63

I 70

2.13

6.15

1.50

2.00

2.60

Table

3. The pH values of the glass investigated for several Non-irradiated

Glass

No.

before

leachmg

and after exposmg times.

to different

lrradtation

do\e\

and

at 100 k 2 C

9.7 kCy

82.6 kGy

3h

6h

I?h

3h

6h

I2h

3h

hh

I

6.30

6.6

6.02

6.40

1.35

5.9

6.10

6.30

2

6.55

6.6

5.80

I .4h

4.10

4.x

2.90

3

5.80

6.3

5.10

6.50

I.23

5.4

I .66

437.5 (kGy) 3h

6h

I?h

I .Y3

2 50

6.20

6.33

I.10

7 I5

4.40

5.YO

3 X6

2.15

6.2

I 50

I x0

6.10

l2h

4

2.60

5.7

4.90

3.10

I .5x

4.6

6.52

5 55

2.4

5.70

5.90

4.90

5

2.90

4.26

3.99

5.40

1.30

3.7

I .43

I.16

29

5 70

5.90

4.90

the diffusion of Na,O through the glass solution interface. The constant rate of glass dissolution observed on increasing time of leaching could be related to the following: 1. The decrease in the concentration of NazO in the glass surface may be due to the diffusion of Na ions from the inside part of the glass, through the Na-depleted layer to the leaching solution. The rate of extraction would be decreased with increasing diffusion distances. 2. The accumulation of the released products either as soluble compounds or in the form of a protective thin layer slow the diffusion rate. EfSect

at room

of leaching on

the solution pH

The effect of irradiation doses and the immersion time on the solution pH was studied. The data obtained at room temperature are given in Table 2, and that at the temperature of 100-C in Table 3. As is clear from the table. the pH values decrease with increasing the irradiation doses. At the higher temperature, the pH data at increasing time intervals reveal a progressive increase in pH values. With increasing the immersion time the pH value also increased. For a binary alkali borate glass, the surface composition profile approaches a steady state condition determined by the relative rate of leaching. The data in Tables 2 and 3 represent the change in pH of the solutions during the leaching process. However, it may be argued that the minor changes noted in solution pH at 100 C are the primary criteria in determining the surface composition profile. In particular, one would expect a preferential increase of leaching of Na and alkaline earth oxides at this more acidic pH. This should be reflected in both the composition profile and solution analysis. The higher solution pH leads to a higher corrosion rate for alkali

glasses which has higher polarity. Higher polarizing power is a result of ion exchange of the hydronium ions for alkali ions (Clark et rrl., 1979). The results demonstrate that dissolution of the glass is possible in the nearly neutral pH range. This suggests that steady-state dissolution corresponds to a condition whereby preferential leaching and network dissolution are equally probable. To render the two process equally probable this requires the formation of a leached layer on the surface to retard further extraction of modifier ions. The condition of steady-state dissolution defined here may correspond to the extraction of alkali from the glass with a linear time dependence. This is often interpreted to be a result of diffusion controlled ion-exchange between proton or hydronium ions in solution and Na ions in glass.

CONCLUSION

In conclusion, the durability of alkali borate glasses is considerably reduced as a result of exposure to y-irradiation which can cause damage in the glass matrix. Alkaline earth oxides added as glass modifiers, were found to affect the durability of glass to a varying degree depending on the size and activity coefficient of the cation involved and temperature.

REFERENCES

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Greaves G. N. (1990) Non-Crysr. Solids 120, 108. Imagawa H. (1969) Phys. Chem. Glasses J. 10, 187. Paul A. (1982) Chemistry of Glasses. Chapman and Hall, New York. Perrea G. and Doremus R. H. (1991) Am. Cer. Sot. J. 74, 1269. Appl.

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A. (1985)

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D. L. and Friebele

E. J. (1988) Phys.

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Y. (1985) Cer.

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