We continue to review some of the most important materials in heat treatment and metallurgy.
Rhenium (chemical symbol: Re)
Rhenium (Fig. 1) is a rare, silvery-white, lustrous, dense metal that is resistant to corrosion and oxidation, but it will slowly tarnish in the presence of moist air. Rhenium has one of the highest melting points of all the elements (only surpassed by carbon and tungsten) and is also one of the densest (exceeded only by iridium, osmium and platinum). Despite being so dense, rhenium is rather ductile and malleable.
Rhenium is one of the five major refractory metals, which are defined by their very high resistance to heat and wear. Rhenium was the final element to be discovered that had at least one stable isotope. Future discoveries were for elements with no stable isotopes; that is, elements that would undergo radioactive decay.
Dmitri Mendeleev first predicted rhenium in 1869, noting that it would have similar properties to manganese. Later, in 1925, German scientists Walter Noddack, Ida Tacke and Otto Berg discovered trace amounts of the new element in platinum ores and in the mineral columbite, which they had subjected to X-ray analysis.
Rhenium is quite rare. Noddack and Berg were only able to extract about 1 gram of the element from 660 kg of molybdenite (molybdenum disulfide). Rhenium does not occur freely in nature nor does it exist as a compound, but it is widely spread throughout the Earth’s crust at about 0.001 ppm. Commercial rhenium is obtained as a byproduct of refining molybdenum and copper.
Uses of rhenium range from flash lamps for photography, filaments in mass spectrographs/ion gauges and as an alloying agent in both tungsten and molybdenum. Due to rhenium’s high heat and wear resistance, it is commonly used as an electrical contact material since it withstands arc corrosion. Rhenium is also used in combination with platinum as a catalyst during the production of lead-free, high-octane gasoline.
In metallurgy, the vast majority (75%) of all rhenium used is in the manufacturing of superalloys in combination with iron, cobalt or nickel. Rhenium’s ability to withstand high temperatures and oxidizing makes it an ideal addition to superalloys used in making jet-engine parts and gas-turbine engines (Fig. 2).
Here are a few important facts about rhenium.
- Atomic number: 75
- Atomic weight: 186.207
- Melting point: 3458 K (3185°C or 5765°F)
- Boiling point: 5869 K (5596°C or 10105°F)
- Density: 20.80 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
- Period number: 6
- Group number: 7
- Group name: Transition metals
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