Resistance to Molten Glass

The exudation potential can be checked by firing discs or bars of the material. The change in volume indicates the exudation. The glass phase will come onto the surface of the sample as exudate and will increase the volume. The standard for this test is ASTM C1223-98 [9]. When the increase in volume is beyond 2%, those bricks will cause defects in the glass. 

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Although molybdenum is resistant to molten glass, except leaded, molybdenum components not coated with glass but exposed to the oxidising furnace atmosphere corrode rapidly due to volatilisation of molybdenum oxide above 370°C. To overcome this, stirrers etc. for use in glass plant are physically clad with platinum sheet in vulnerable areas. Modern plating techniques have enabled dense platinum coatings to be put onto the surface of the molybdenum and it is expected that this technique will be exploited further in the near future. 

It must be emphasized that boron nitride has an exceptionally good resistance to most molten salts and glasses, probably due to its resistance to wetting. It is inert to a number of corrosive molten salts, such as alkali halides, lithium borate and cryolites, and has a good resistance to molten glasses. However, it does react with molten alkali carbonates and hydroxides. 

In addition, molybdenum has high resistance to a number of alloys of these metals and also to copper, gold, and silver. Among the molten metals that severely attack molybdenum are tin (at 1000°C), aluminum, nickel, iron, and cobalt. Molybdenum has moderately good resistance to molten zinc, but a molybdenum—30% tungsten alloy is practically completely resistant to molten zinc at temperatures up to 800°C. Molybdenum metal is substantially resistant to many types of molten glass and to most nonferrous slags. It is also resistant to hquid sulfur up to 440°C. 

I. Penkov, R. Pascova and I. Drangajova, A New Glass Ceramic Material with High Resistance to Molten Aluminium, 7. Mat. Sci. Lett. 16, 1544-1546(1997). 

HM>9) Gray amorphous powder or crystals. Corrosion-resistant to molten metals such as Al, Pb, Zn, Cd, Bi, Rb, and Sn, and molten salts NaCl-KCl, NaF, and sihcate glasses. Corroded by molten Mg, Ti, V, Cr, Fe, Co, cryolite, KOH, and NaaO. Silicon nitride (Nitrasil )... 

Molybdenum is extremely resistant to attack by molten glass up to 1 000° C if sulphur is present, and up to 1 400°C in its absence. In addition, any reaction products formed by the two materials are colourless. Consequently molybdenum is used in the production of optical and soda lime glasses. Molybdenum is, however, corroded by lead glasses which lose their gloss as a result of the interaction between the two materials. ... 

The principal applications of the outstanding stability of platinum and its alloys at high temperatures lie in their use as materials of construction for equipment to handle molten glass and as electrical resistance windings for high-temperature furnaces. 

PTFE, known under the trade names Teflon and Fluon, is resistant to all chemicals, except molten alkalies and fluorine, and can be used at temperatures up to 250°C. It is a relatively weak material, but its mechanical strength can be improved by the addition of fillers (glass and carbon fibres). It is expensive and difficult to fabricate. PTFE is used extensively for gaskets and gland packings. As a coating, it is used to confer non-stick properties to surfaces, such as filter plates. It can also be used as a liner for vessels. 

Hot, molten glass is thick and cohesive it can be shaped, and, as it cools down, it hardens while keeping its shape. Solid glass is extremely tough, withstands compression better than steel, is impervious to liquids, and is resistant to chemical attack. All this makes glass useful for making utilitarian artifacts, such as containers for solids and liquids, as well as ornamental and decorative objects (Tite et al. 2002 Tait 1991). 

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