Molybdenum News Center

Molybdenum (Mo) is a silvery-white metal, occurring chiefly in the mineral molybdenite but also obtained as a by-product from copper ores. The metal has a specific gravity of 10.2 and a melting point of 2610°C. It is ductile, softer than tungsten, and is readily worked or drawn into very fine wire. It cannot be hardened by heat treatment, but only by working.

 

Its major use is in alloy steels, for example, as tool steels (<10% molybdenum), stainless steel, and armor plate. Up to 3% molybdenum is added to cast iron to increase strength. Up to 30% molybdenum may be added to iron-, cobalt-, and nickel-base alloys designed for severe heat- and corrosion-resistant applications. It may be used in filaments for light-bulbs, and it has many applications in electronic circuitry.

 

Molybdenum forms mirrors and films on glass when it is produced by gas-phase reduction or decomposition of volatile molybdenum compounds in glass tubes. Also, because the molybdenum metal surface binds strongly to oxygen without weakening the metal structure, and because the metal has such a low coefficient of thermal expansion — virtually equal to hard glass — molybdenum metal is an extraordinarily good material for use in glass-metal seals. Molybdenum trioxide (MoO3) dissolves in glass, allowing strong binding of molten glass with preoxidized metal surfaces. Annealing is very effective, with little or no difference in thermal expansion at the metal-glass interface. Molybdenum found early use in filaments for electric light bulbs and later in the construction of electronic devices (for example, in vacuum tubes, contacts, electrodes, and transistors).

 

Protection from Oxidation

 

At temperatures over about 538°C, unprotected molybdenum oxidizes so rapidly in air or oxidizing atmospheres that its continued use under these conditions is impractical. Uncoated molybdenum is, however, used satisfactorily where very short lives are involved (as in some missile parts) or where the surrounding atmosphere is nonoxidizing (as in hydrogen and vacuum furnaces). Protective coatings seem to be the answer where oxidation is a problem. Various coatings differing in maximum time-temperature capabilities and in physical and mechanical characteristics are available. Selection of the proper coating for a specific application involves consideration of a number of factors, foremost among which is the service temperature. For temperatures up to 1204°C, nickel-base alloys applied as cladding or sprayed coatings, and chromium-nickel electroplates appear most generally suitable. For temperatures up to 1538°C, or short periods at higher temperatures, modified chromized coatings and sprayed aluminum-chromium-silicon are predominant. For longer periods at higher temperatures, the choice would probably rest between siliconizing and ceramic coatings. Component tests have been found most reliable for final selection of suitable coatings.

 

 

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