Glasses acid attack

Chlorosulfuric acid attacks brass, bronze, lead, and most other nonferrous metals. From a corrosion standpoint, carbon steel and cast Hon are acceptable below 35°C provided color and Hon content is not a concern. Stainless steels (300-series) and certain aluminum alloys are acceptable materials of constmction, as is HasteUoy. Glass, glass-lined steel, or Teflon-lined piping and equipment are the preferred materials at elevated temperatures and/or high velocities or where trace Hon contamination is a problem, such as in the synthetic detergent industry. 

Because borosilicate is a brittle material, its design stress is restricted to less than 7 N/mm. Borosilicate glass is attacked by hydrofluoric acid even when a solution contains only a few parts per million of fluoride ions, and at... 

In the glass-bottle industry the bottles can be cooled in a dilute SOj/SO, atmosphere to increase chemical resistance. A similar effect has been noted with vitreous enamel. It has been postulated that a thin layer of —OH groups or — OH—HjO (hydronium) ions is adsorbed on the surface of a fired enamel. These ions are transformed into — OSO, or —OSO3 in the presence of oxides of sulphur which are more resistant to further acid attack. It is known that the acid resistance of a recently fired enamel improves on ageing, probably due to the enamel reaction with SOj/SO, in the atmosphere and it is quite common for the grading to improve from Class A to Class AA (BS 1344). 

Figure 5.10 In these glasses, the main phase is depleted in calcium and fluoride, which reduces Its reactivity. Acid attack occurs selectively at the phase-separated droplets which are rich in calcium and fluoride (Hill Wilson, 1988a).

On mixing the cement paste, the calcium aluminosilicate glass is attacked by hydrogen ions from the poly(alkenoic acid) and decomposes with liberation of metal ions (aluminium and calcium), fluoride (if present) and silicic acid (which later condenses to form a silica gel). 

The latter interpretation of data is more in accord with the recent Al and Si NMR findings of Ellison Warrens (1987), who found that the structure of an appreciable fraction of the glass changed under acid attack with some loss of aluminium including all in fivefold coordination (see Section 5.9.2). Thus, acid attack was not entirely confined to the surface layer of a glass particle. If this is so then silicic acid as well as ions must migrate from the body of the particle and it is reasonable to suppose that silicic acid deposits as siliceous gel at the particle-matrix interface. 

Crisp, Lewis Wilson (1980) found that these same three species were released, in greater amounts but in roughly the same proportions, under acid attack. The association of silica with fluoride suggests, perhaps, that it is principally the glass particles that are attacked rather than the matrix... 

Crisp, Lewis Wilson (1980) made a chemical study of the erosion of a glass polyalkenoate cement under acid attack. They found that the chief species eluted were sodium and fluoride ions and silicic acid suggesting that attack occurred mainly on the glass particles rather than on the matrix. 

Figure 6.18 A stereoscan of a fracture surface of a dental silicate cement. The debonded glass particle is to be identified by its pitted surface, the result of selective acid attack. Note the particulate nature of the matrix (Wilson et at., 1972).

Some of these conclusions may require revision, since recent findings of Ellison Warrens (1987) on the related glass-ionomer cement (Section 5.9.6) suggest that acid attack occurs throughout the body of the glass particle and not just at the surface layer. In that case the silica gel layer is not a relict but a zone of gelation. This view is more in accord with ideas on the decomposition of aluminosihcate glasses. 

Despite the failing of the dental silicate cement under acid conditions it is more resistant to acid attack than all other dental cements with the notable exception of the glass polyalkenoate cement (Norman, Swartz Phillips, 1959 Walls, McCabe Murray, 1985 Beech Bandyopadhyay, 1983 Kuhn, Setchell Teo, 1984 Wilson et al., 1986a). These studies have been confirmed by in vivo observations (Norman et al., 1969). A clinical study carried out by Robinson (1971) over many years showed that when carefully prepared and placed, the dental silicate cement was capable of giving good performance. Many of the failures of this material must be attributed to faulty preparation. 

Systematic attempts to formulate improved materials have met with no success (Manly et al., 1951 Rockett, 1968). The last and, in some ways, most promising attempt at improving the dental silicate cement was made by Pendry (Pendry Cook, 1972 Pendry, 1973) who improved its resistance to acid by adding indium to both powder (5-8 %) and liquid (5-65 %). The cement, however, lacked suflScient translucency, and by this time the glass-ionomer cement had arrived with its advantages of translucency and resistance to staining and acid attack. 

Cements based on phytic add set more quickly than their glass polyalkenoate or dental silicate cement cormterparts, but have similar mechanical properties (Table 8.2). They are unique among add-base cements in being impervious to acid attack at pH = 2-7. Unfortunately, they share with the dental silicate cement the disadvantage of not adhering to dentine. They do bond to enamel but this is by micromechanical attachment - the cement etches enamel - and not by molecular bonding. Lack of adhesive property is a grave weakness in a modern dental or bone... 

White powder, hexagonal graphite-like form or cubic crystal cubic form similar to diamond in its crystal structure, and reverts to graphite form when heated above 1,700°C density 2.18 g/cm melts at 2,975°C (under nitrogen pressure) sublimes at 2,500°C at atmospheric pressure insoluble in water and acid attacked by hot alkalies and fused alkali carbonates not wetted by most molten metals or glasses. 

The use of hydrofluoric add and the fluorides.—Hydrofluoric acid attacks quartz and siliceous substances, glass, etc., forming silicon fluoride Si02+4HF... 

When all the solid has been added, the solution is gently heated until the evolution of hydrogen chloride has ceased care must be taken not to overheat it otherwise some product may be lost through volatilization. The material is then distilled, using an air condenser, until the temperature of the vapor reaches 165°C the receiver consists of a 250-ml round flask protected from moisture. As the acid attacks cork and rubber readily an all-glass apparatus must be used. The crude product is redistilled from the receiver and the colorless liquid is collected at 149-154°C pure chloro-... 

The cooler part of the combustion air preheater - the tail end of the flue-gas-heat-recovery train - is more likely to corrode due to sulfur dioxide (S02) condensing from the flue gas. In this area, cast iron or glass will resist the acid attack. Carbon steel preheater tubes, joined with 1.5 to 2 meters of SS 304 tubes at the cold end of the tube sheet, can ensure reasonable service life. Typically, the flue gas temperature to the stack is maintained above the dew point of S02 to prevent condensation. During startup and shutdown, condensation of S02 will occur88. 

CO can be liberated from pyrex glass by mineral acid attack at 200 °C during acid dissolution experiments47. ... 

The addition of B2O3 to a silicate glass markedly reduces the thermal expansion coefficient and consequently enhances the resistance to thermal shock. At the same time, the chemical durability, especially the resistance to acid attack, is greatly improved. 

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