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Corrosion products encountered on glass surfaces
SHORTCOMMUNICATION Corrosion Products Encountered on Glass Surfaces B u l e n t A r m a n a n d Baha K u b a n Turkiye Sise ve Cam Fabrikalari A.S. Research Center, Istambul, Turkey
type 2, crystals with holes in them; type 3, pencil-type crystals; and type 4, bugshaped crystals with curved arms. Examples of all these types can be seen in Figs. 1-4. Although they can be observed separately, it is also possible to observe different types coexisting on the glass surface. Figure 3 shows one such example of type 1 crystals together with type 3 crystals. Figure 1(a-d) shows a series of micrographs of increasing magnification, culminating in an example of the precise rhombohedral-shaped crystal of CaSiO3 designated type 1. Energy dispersive analysis by x-ray was used to determine that the composition of these crystals satisfied the exact stoichiometry requirements. The individual crystals themselves appear to be a little less than 1 p,m in size. At the lower magnifications, the crystals can be seen to have grown into branched structures several tens of crystals long and easily discernible from their jagged edges. The composition of the glass surface immediately adjacent to the well-formed crystals has the approximate composition of 85% SiO2-6c/( K20-5% CaO-4% Na20, indicating Si enrichment of the surface. Another feature of the corroded glass surface is the cracking and peeling of the Si-rich regions [Fig. l(b)]. This is due to the process of dealkalization and concomitant surface stresses and has been well documented by other researchers [5-10]. This type of growth would seem to point to a slow corrosion reaction controlled by the rate of diffusion of alkali ions. The obser-
INTRODUCTION
The interactions between soda-lime-silica glasses and various atmospheres have been the subject of a number of investigations [1-4]. In the present case, the surfaces of glass of soda-lime-silica composition were found to be corroded severely as a result of storage in a humid environment and to contain well-crystallized surface corrosion products. This article describes a short study of the morphology and identity of these corrosion products carried out as a preliminary to a more extensive project to characterize the corrosion mechanisms and segregation processes that take place on glass surfaces.
SPECIMEN PREPARATION
The specimens examined were cut from corroded glasses of the approximate composition 73% SIO2-12% Na20-8% CaO6% K20. The corroded specimens were sputter-coated with a 50-nm-thick Au film to enhance them for photographic purposes but were left uncoated for chemical composition analyses. The electron microscopy and microprobe analysis was performed using a JEOL 733 Superprobe.
OBSERVATIONS
Four different types of corrosion products or surface morphologies were observed: type 1, rhombohedral crystals of CaSiO3; 49 ~'Elsevier Science Publishing Co., Inc., 1992 655 A v e n u e of the Americas, N e w York, NY 10010
MATERIALS CHARACTERIZATION 29:49 53 (1992) 1{}44 58{}3,'92,'$5.I)0
B. Arman and B. Kuban
50
(a)
(b)
(c)
(d) FIc. 1. Type 1 crystals of rhombohedral CaSi03.
I
A
~n
52
B. Arman and B. Kuban
FIG. 3. Coexistence of types 1 and 3 crystals.
(a)
(b)
FIG. 4. Type 4 crystals growing from preformed crystals underneath.
53
Corrosion Products on Glass Surfaces vation, in Fig. l(b), that CaSiO3 crystals appear to be crossing crack boundaries indicates that crystal formation has preceded the dealkalization cum cracking process. It is also of interest to note that, with this type of surface corrosion product, the arms of the larger crystals have grown at an almost fixed angle from each other. Figure 2(a) shows an example of the type 2 morphology, crystals with holes in them. These crystals were found to have the approximate composition of 87% S I O 2 - 5 % Na20-5% K20-3% CaO, very similar to that of the dealkalized Si-rich glass surface. Type 3 crystals [Figs. 2(b) and 3] have a chemical composition of approximately 50% S I O 2 - 4 0 % Na20-5% CaO-4% K 2 0 . This indicates some sort of complex silicate, the high Na content of which probably points to a w e t - d r y cycle and a small amount of available solution. It is unlikely that this type of corrosion product could form under wash-down conditions. It was not possible to obtain compositional measurements on type 4 crystals seen in Fig. 2(c) because of their small sizes in general. Tentative measurements on some of the larger ones suggested a high Si:Na ratio of approximately 3:1. By way of contrast, the approximate composition of the larger crystals from which they seem to have grown is 77% SIO2-13% CaO-9% K20-1% Na20. The Na depletion in these underlying crystals may be explained by the relatively high Na content tentatively identified in the type 4 crystals and points possibly to a three-step growth process. In conclusion, the glass corrosion process is a complex one, exhibiting itself in a variety of surface morphologies. The corrosion products, apart from the type 1 rhombohedral CaSiO3, do not seem to have a precise stoichiometry but are silicates of varied Ca, Na, and K contents. Because of the lack of access to corrosive solution in large amounts, the Na depletion
observed in the glass surface composition is reflected in surface corrosion products with different Na contents. A more systematic investigation involving highly controlled conditions would have to be conducted in order to permit elaboration of the corrosion reaction mechanisms and better characterization of the various surface morphologies. The authors express their gratitude to S1SECAM for permission to publish the results Off this study.
References 1. S. M. Budd, The mechanism of chemical reaction between silicate glass and attacking agents, Phys. Chem. Glasses 2:111-114 (1961). 2. C. R. Das and R. W. Douglas, Studies on the reaction between water and glasses part 3, Phys. Chem. Glasses 8:178-184 (1967). 3. R. W. Douglas and T. M. M. E1-Shamy, Reactions of glasses with aqueous solutions, J. Am. Ceram. Soc. 50:1-8 (1967). 4. F. Moser, Glass surface deterioration by moisture, Glass Ind. 42:244-248, 286-287 (1961). 5. D. E. Clark, W. A. Acree, and L. L. Hench, Electron microprobe analysis of corroded soda-limesilica glass, J. Am. Ceram. Soc. 59:463-464 ~'1976). 6. D. E. Clark, M. F. Dilmore, E. C. Ethridge, and L. L. Hench, Aqueous corrosion of soda-silica and soda-lime-silica glass, J. Am. Ceram. Soc. 59:62-65 (1976). 7. D. M. Sanders and L. L. Hench, Mechanisms of glass corrosion, J. Am. Ceram. Soc. 56:373-377 (1973). 8. D. M. Sanders, W. B. Person, and L. L. Hench, New methods for studying glass corrosion kinetics, Appl. Spectroscopy 26:530-536 (1972). 9. M. A. Rana and R. W. Douglas, The reaction between glass and water, Phys. Chem. Glasses 2:179195 (1961). 10. D. E. Clark, C. G. Pantano, and L. L. Hench, eds., Corrosion ~ff Glass, Books for Industry, New York (1979).
Received October 1991; accepted February 1992.
type 2, crystals with holes in them; type 3, pencil-type crystals; and type 4, bugshaped crystals with curved arms. Examples of all these types can be seen in Figs. 1-4. Although they can be observed separately, it is also possible to observe different types coexisting on the glass surface. Figure 3 shows one such example of type 1 crystals together with type 3 crystals. Figure 1(a-d) shows a series of micrographs of increasing magnification, culminating in an example of the precise rhombohedral-shaped crystal of CaSiO3 designated type 1. Energy dispersive analysis by x-ray was used to determine that the composition of these crystals satisfied the exact stoichiometry requirements. The individual crystals themselves appear to be a little less than 1 p,m in size. At the lower magnifications, the crystals can be seen to have grown into branched structures several tens of crystals long and easily discernible from their jagged edges. The composition of the glass surface immediately adjacent to the well-formed crystals has the approximate composition of 85% SiO2-6c/( K20-5% CaO-4% Na20, indicating Si enrichment of the surface. Another feature of the corroded glass surface is the cracking and peeling of the Si-rich regions [Fig. l(b)]. This is due to the process of dealkalization and concomitant surface stresses and has been well documented by other researchers [5-10]. This type of growth would seem to point to a slow corrosion reaction controlled by the rate of diffusion of alkali ions. The obser-
INTRODUCTION
The interactions between soda-lime-silica glasses and various atmospheres have been the subject of a number of investigations [1-4]. In the present case, the surfaces of glass of soda-lime-silica composition were found to be corroded severely as a result of storage in a humid environment and to contain well-crystallized surface corrosion products. This article describes a short study of the morphology and identity of these corrosion products carried out as a preliminary to a more extensive project to characterize the corrosion mechanisms and segregation processes that take place on glass surfaces.
SPECIMEN PREPARATION
The specimens examined were cut from corroded glasses of the approximate composition 73% SIO2-12% Na20-8% CaO6% K20. The corroded specimens were sputter-coated with a 50-nm-thick Au film to enhance them for photographic purposes but were left uncoated for chemical composition analyses. The electron microscopy and microprobe analysis was performed using a JEOL 733 Superprobe.
OBSERVATIONS
Four different types of corrosion products or surface morphologies were observed: type 1, rhombohedral crystals of CaSiO3; 49 ~'Elsevier Science Publishing Co., Inc., 1992 655 A v e n u e of the Americas, N e w York, NY 10010
MATERIALS CHARACTERIZATION 29:49 53 (1992) 1{}44 58{}3,'92,'$5.I)0
B. Arman and B. Kuban
50
(a)
(b)
(c)
(d) FIc. 1. Type 1 crystals of rhombohedral CaSi03.
I
A
~n
52
B. Arman and B. Kuban
FIG. 3. Coexistence of types 1 and 3 crystals.
(a)
(b)
FIG. 4. Type 4 crystals growing from preformed crystals underneath.
53
Corrosion Products on Glass Surfaces vation, in Fig. l(b), that CaSiO3 crystals appear to be crossing crack boundaries indicates that crystal formation has preceded the dealkalization cum cracking process. It is also of interest to note that, with this type of surface corrosion product, the arms of the larger crystals have grown at an almost fixed angle from each other. Figure 2(a) shows an example of the type 2 morphology, crystals with holes in them. These crystals were found to have the approximate composition of 87% S I O 2 - 5 % Na20-5% K20-3% CaO, very similar to that of the dealkalized Si-rich glass surface. Type 3 crystals [Figs. 2(b) and 3] have a chemical composition of approximately 50% S I O 2 - 4 0 % Na20-5% CaO-4% K 2 0 . This indicates some sort of complex silicate, the high Na content of which probably points to a w e t - d r y cycle and a small amount of available solution. It is unlikely that this type of corrosion product could form under wash-down conditions. It was not possible to obtain compositional measurements on type 4 crystals seen in Fig. 2(c) because of their small sizes in general. Tentative measurements on some of the larger ones suggested a high Si:Na ratio of approximately 3:1. By way of contrast, the approximate composition of the larger crystals from which they seem to have grown is 77% SIO2-13% CaO-9% K20-1% Na20. The Na depletion in these underlying crystals may be explained by the relatively high Na content tentatively identified in the type 4 crystals and points possibly to a three-step growth process. In conclusion, the glass corrosion process is a complex one, exhibiting itself in a variety of surface morphologies. The corrosion products, apart from the type 1 rhombohedral CaSiO3, do not seem to have a precise stoichiometry but are silicates of varied Ca, Na, and K contents. Because of the lack of access to corrosive solution in large amounts, the Na depletion
observed in the glass surface composition is reflected in surface corrosion products with different Na contents. A more systematic investigation involving highly controlled conditions would have to be conducted in order to permit elaboration of the corrosion reaction mechanisms and better characterization of the various surface morphologies. The authors express their gratitude to S1SECAM for permission to publish the results Off this study.
References 1. S. M. Budd, The mechanism of chemical reaction between silicate glass and attacking agents, Phys. Chem. Glasses 2:111-114 (1961). 2. C. R. Das and R. W. Douglas, Studies on the reaction between water and glasses part 3, Phys. Chem. Glasses 8:178-184 (1967). 3. R. W. Douglas and T. M. M. E1-Shamy, Reactions of glasses with aqueous solutions, J. Am. Ceram. Soc. 50:1-8 (1967). 4. F. Moser, Glass surface deterioration by moisture, Glass Ind. 42:244-248, 286-287 (1961). 5. D. E. Clark, W. A. Acree, and L. L. Hench, Electron microprobe analysis of corroded soda-limesilica glass, J. Am. Ceram. Soc. 59:463-464 ~'1976). 6. D. E. Clark, M. F. Dilmore, E. C. Ethridge, and L. L. Hench, Aqueous corrosion of soda-silica and soda-lime-silica glass, J. Am. Ceram. Soc. 59:62-65 (1976). 7. D. M. Sanders and L. L. Hench, Mechanisms of glass corrosion, J. Am. Ceram. Soc. 56:373-377 (1973). 8. D. M. Sanders, W. B. Person, and L. L. Hench, New methods for studying glass corrosion kinetics, Appl. Spectroscopy 26:530-536 (1972). 9. M. A. Rana and R. W. Douglas, The reaction between glass and water, Phys. Chem. Glasses 2:179195 (1961). 10. D. E. Clark, C. G. Pantano, and L. L. Hench, eds., Corrosion ~ff Glass, Books for Industry, New York (1979).
Received October 1991; accepted February 1992.