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Being essentially insoluble in tin these two oxides will enter into the glass either by diffusion or by exchange for calcium oxide [6.55]. Calcium oxide is also insoluble in molten tin and will therefore form a deposit on the bottom surface of the glass. The deposit can be washed from the bottom surface of the glass by a vinegar solution.* Thus iron that enters the tin at the hot end of the bath will reenter the glass at the cold end, setting up an equilibrium concentration of iron in the tin. This equilibrium can be altered if the glass composition is changed from one of high iron content to one of lower iron content (or vice versa).
Although the interaction layer thickness is quite small, the presence of tin in the surface of the glass ribbon causes some secondary fabrication problems. Many fabrication methods require that the flat glass piece be bent. This is carried out by reheating the glass on a metal frame and allowing the glass to sag to the desired shape. This reheating process can provide additional oxidation of the tin (from stannous to stannic oxide) in the bottom surface. This oxidation is accompanied by an expansion of the tin-rich layer causing a microwrinkled surface. This wrinkled surface becomes visible as a faint iridescent haze—known as the defect bloom [6.56]. This phenomenon can also occur when glass is reheated for tempering.
6.3 BOROSILICATE GLASSES
The durability of borosilicate glasses has been extensively investigated by the nuclear waste glass community. No attempt will be made here to review all the literature related to nuclear waste glasses; however, the article by Jantzen [6.28] described quite well the use of Pourbaix diagrams in predicting the dissolution of nuclear waste glasses. Jantzen
* The deposit of calcium oxide reacts with atmospheric carbon dioxide forming calcium carbonate on the glass surface that is insoluble in water and must be washed off with a vinegar solution.
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has performed a very thorough job in explaining the int errelationship of pH, Eh, activity, free energy of hydration, and glass dissolution. It was shown that solution Eh had an effect upon network dissolution that was 20 times less than that of pH. But when redox-sensitive elements were leached from the glass, the solution Eh could have a much larger effect. Jantzen also concluded that less durable glasses had a more negative free energy of hydration and thus released more silicon and boron into solution. Higher boron release over that of silicon was attributed to the greater solution activity of vitreous boria compared to that of vitreous silica at any given pH. Refs. 5.28–5.32 listed at the end of the previous chapter are a good source of information for the reader interested in the aqueous attack upon borosilicate glasses and nuclear waste materials in general.
In borosilicate glasses requiring a heat treatment step after initial melting and cooling to produce phase separation, a surface layer is formed by selective evaporation of Na2O and B2O3. These surface layers have been observed by several workers. This silica-rich surface layer can influence the subsequent leaching process that would be needed to produce Vycor™*-type glass [6.57]. If the hydrated surface layer were removed before heat treatment, the silica-rich layer would be almost entirely eliminated.
The leaching rate in 3 N HCl solution for borosilicates glasses with an interconnected microstructure was shown by Takamori and Tomozawa [6.58] to be dependent upon the composition of the soluble phase. The composition and size of this interconnected microstructure was also dependent upon the temperature and time of the phase separation heat treatment process. Taylor et al. [6.59] have shown that phase separated low soda borosilicate glasses form a less durable Na2O plus B2O3-rich phase dispersed within a more durable
* Vycor™ is manufactured by Corning, Inc.
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silica-rich phase. The overall durability in distilled deionized water was strongly dependent upon the soda content and was best for a composition containing about 3 mol% Na2O. The durability was also dependent upon the SiO2/B2O3 ratio, with the higher silica content glasses being more durable. In a study of soda borosilicate glasses, Kinoshita et al. [6.60] related the effects of the Si/B ratio to the dissolution rates. At low Si/B ratios, the glasses dissolved congruently at rapid constant rates at a pH=2 in HCl/glycine solutions. Higher Si/B ratios caused the selective leaching of sodium and boron leaving behind a silica-rich layer that caused the dissolution rate to decrease with time.
In a study closely related to borosilicate glasses, El-Hadi et al. [6.61] investigated the addition of soda to B2O3 and the effect upon durability, which is generally very poor for borate glasses. Increased durability toward both acids and bases was related to the change in coordination of the boron from three to four as the alkali level was increased. Alkali borate glasses also increased in density as the alkali content was increased, suggesting that the change in coordination caused a more compact, more difficult to leach, structure. Addition of various divalent metal oxides to a lithium borate glass also increased the durability in the order:
CdO>ZnO>PbO>SrO>BaO. Tait and Jensen [6.62] found an order-of-magnitude increase in durability (in deionized water) of a sodium borosilicate glass containing 8.5 mol% ZnO. CaO and Al2O3 also increased the durability.
The attack by various acids was studied by Katayama et al. [6.63], who determined that the corrosion of a barium borosilicate glass decreased in the order acetic, citric, nitric, tartaric, and oxalic acid, all at a pH of 4 at 50°C. The mechanism of attack by orthophosphoric acid was shown to vary with temperature by Walters [6.64]. The considerable degradation above 175°C was attributed to acid dehydration. At higher temperatures, the acid condensed and reacted with the glass forming a protective layer of SiP2O7. The formation of this barrier layer formed sufficient stresses to produce strength loss and caused mechanical failure.
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Metcalfe and Schmitz [6.65] studied the stress corrosion of E-glass (borosilicate) fibers in moist ambient atmospheres and proposed that ion exchange of alkali by hydrogen ions led to the development of surface tensile stresses that could be sufficient to cause failure.
The effect of dissolved water content upon the resistance of borosilicate glasses to acid vapor attack (over boiling 20% HCl) was investigated by Priest and Levy [6.66]. Increasing water contents correlated with increasing corrosion resistance.
The use of borosilicate foamed glass blocks to line the outlet ducts of coal burning power plants was reported by Koch and Syrett [6.67] to perform better than silicate cement gunite, as well as nickel-based or titanium alloys in an 18-month test. This was attributed to the high concentration of aluminum in the outlet flue gas that formed soluble complexes with fluorine that are not detrimental to borosilicate glass.
Fast ion conduction glasses, such as lithium-borate and lithium-chloroborate glasses, were studied by Velez et al. [6.68] to determine their resistance to molten lithium at temperatures between 180 and 250°C. They found that those compositions with a minimum B2O3 content resulted in the best resistance to attack.
Recently, Conzone et al. [6.69] reported the development of borate glasses for use in treatment of rheumatoid arthritis, as these glasses are potentially more reactive with physiological liquids. Borate glasses containing only alkali ions dissolved uniformly (i.e., congruently) in simulated physiological liquids at temperatures ranging from 22 to 75°C. When the borate glasses contain other cations (such as Ca, Mg, Fe, Dy, Ho, Sm, and Y) in amounts ranging from 2 to 30 wt.% dissolution was nonuniform (i.e., incongruent) with the formation of new compounds. Day [6.70] gave an example of Dy2O3-containing borate solid glass microspheres that reacted to form hollow spheres, shells of concentric layers, or microspheres filled with homogeneous gel-like material depending upon the Dy2O3 content. The dissolution mechanism involved the selective leaching of lithium and boron allowing the rare earth (i.e.,
Copyright © 2004 by Marcel Dekker, Inc.