Refractory metals pyrophoricity

Caution. Finely divided titanium, zirconium, and other refractory metals are often pyrophoric and should be handled with a minimum exposure to air. Weighings should be made in a closed container. 

The reaction of Ca or Mg with UO2 produces uranium metal powder, a consequence of the refractory character of the CaO or MgO reaction products that interfere with the formation of a molten metal billet. Ca is preferred over Mg , which results in the formation of a U powder of finer particles at a lower chemical yield the smaller powder size increases the pyrophoricity hazard. Metal powder is also formed in the electrolysis of UCI3 or UF4 dissolved... 

Properties. Uranium metal is a dense, bright silvery, ductile, and malleable metal. Uranium is highly electropositive, resembling magnesium, and tarnishes rapidly on exposure to air. Even a poHshed surface becomes coated with a dark-colored oxide layer in a short time upon exposure to air. At elevated temperatures, uranium metal reacts with most common metals and refractories. Finely divided uranium reacts, even at room temperature, with all components of the atmosphere except the noble gases. The silvery luster of freshly cleaned uranium metal is rapidly converted first to a golden yellow, and then to a black oxide—nitride film within three to four days. Powdered uranium is usually pyrophoric, an important safety consideration in the machining of uranium parts. The corrosion characteristics of uranium have been discussed in detail (28). 

Early transition metal carbides are catalyticaUy active for a variety of reactions. Since they are refractory and of high surface area, they could also be promising alternatives to oxides as supports for heterogeneous catalysts. Only a few investigations have used them for this purpose. And in these limited investigations, the metals were loaded onto intentionally passivated Mo C powders due to their high pyrophoricity. 

Evidence for the existence of boron hydrides appeared late in the nineteenth century. However, means of handling compounds that are toxic and, especially, pyrophoric were not available, and so serious experimention was precluded. The first study giving actual chemical formulas was reported in 1912 by the German chemist, Alfred Stock. Stock had reacted magnesium metal with boron oxide to form the refractory substance, magnesium boride,... 

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