Glass alkali corrosion

The effect of temperature on alkali corrosion is illustrated by data from this paper in Table 18.10. There are various glass compositions in service and it is important to ensure that the lining is appropriate for the particular... 

The chloride is mixed on a laboratory scale with xs Ca (powder or chips) in an Fe tube in a high-T glass distillation vessel. The Fe tube protects the glass from corrosive attack by the alkali-metal vapors. The vessel is inclined and evacuated while slowly heating to 700-800°C. The liberated Rb or Cs distills onto the cooler upper walls of the vessel and runs into integral glass ampules, which are sealed under vacuum for storage. Further purification is achieved by repeated. vacuum distillation at 300°C. Yields arc theoretical. 

The alkali silicate glasses are easily corroded by aqueous acids and the main reaction is an exchange between H3O+ of the solution with the alkali M+ of the glass the corrosion rate increaseswith the ionic radii ofM+, that is, K > Na" " > Li" ". The chemical durability is greatly increased by addition of divalent CaO or trivalent AI2O3 oxides, leading to commercial soda-lime-silica compositions. 

Glass fibre/resin reinforced bars used for the reinforcement of concrete are subject to alkali corrosion by the alkali present in concrete and this is a major cause of deterioration of the tensile properties of reinforced concrete. 

Solid glass and glass fibers are attacked by acids, alkalis, and also by water. Here, the attack mechanisms of acids and alkalis are fundamentally different. While glass fiber corrosion by acids is diffusion-controlled following 7f-kinetics, attack by water and alkali is more linear and controlled by interfacial properties. 

Alkali corrosion is a surface attack on laminates, because, for example, caustic soda has lower wetting properties compared to water and therefore penetrates the resin to a much smaller degree. First, water penetrates and preferably accumulates at the interfaces. This water accumulation facilitates liber damage by the alkaline medium, because the water prepares surface diffusion or capillary paths along the glass fibers. 

Glasses for electrical and electronic components are represented by the lead tubing and cathode-ray-tube screen and cone glasses. These glasses do not operate under severe corrosion conditions, but surfaces must not leach excessive alkali under damp conditions or electrical breakdown can occur. The glass compositions are formulated to give the maximum electrical resistivity and moisture resistance compatible with other necessary properties. 

More generally, Valez et al. have reviewed the corrosion behaviour of silicate and borate glasses in contact with alkali metals and molten salts, as well as in aqueous conditions. 

As a future alternative to glassed steel there is ceramics-coated steel which is resistant to abrasion, corrosion and high temperatures. The base metal is coated with silicon nitride formed in situ. Silicon nitride has resistance to both acid and alkali and it is durable at temperatures up to 1 000°C, suggesting a promising future coating in aggressive operating environments. 

Studies on hot water tank enamelsin media of varying pH demonstrate a minimum corrosion rate at pH value of 4. In citric acid (pH 2), IR measurements indicate that ion exchange is the principal mode of corrosion. Distilled water (pH 7) showed evidence of a bulk dissolution mechanism with no silica enrichment of the surface layer. In neutral solutions, the first stage of attack is leaching of alkali ions, raising the pH of solution, which subsequently breaks down the glass network of the acidic oxides. 

There are a number of mechanisms that pose potential problems to predicting dissolution rate kinetics as the system approaches saturation. Part of this conundrum originates from current models of glass corrosion kinetics that cannot yet incorporate these unanticipated phenomena into a mathematical equation that is consistent with the constraints of thermodynamics or kinetics. These phenomena include (1) alkali-hydrogen exchange (2) dissimilar reactivity of... 

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