We continue to review some of the most important materials in heat treatment and metallurgy.
Cobalt (chemical Symbol: Co)
Cobalt has been used for centuries as a pigment and as a coloring for glass and ceramic. Cobalt compounds impart a deep-blue color to the substrate. It has been found in ancient Persian jewelry and Egyptian sculpture, in the ruins of Pompeii and in China's Ming and Tang Dynasties. Starting in the Middle Ages, cobalt has been used in the manufacture of smalt, a blue colored glass produced by melting a mixture of the roasted mineral smaltite, quartz and potassium carbonate, yielding a dark-blue silicate glass that is finely ground. Smalt was also widely used as pigment for paintings.
In 1735, Swedish chemist Georg Brandt (1694-1768) analyzed a dark-blue pigment found in copper ore. Brandt demonstrated that the pigment contained a new element, later named cobalt. He was able to show that cobalt was the source of the blue color in glass, which previously had been attributed to the bismuth found with cobalt. Cobalt was named after Kobold, a German word meaning "goblin" or "evil spirit," which was a term was used by miners to describe a mineral that was very difficult to mine and was damaging to their health. When the mineral was heated during smelting, it released a toxic gas that caused illness and was later identified as arsenic trioxide, which often occurs with cobalt in nature.
Cobalt is located between iron and nickel on the periodic table and shares some properties of both. It is one of only three naturally occurring magnetic metals. The other two being iron and nickel.
In the early 1900s, wear-resistant cobalt alloys were developed. These stellite alloys contain chromium, as well as varying amounts of tungsten and carbon and are very hard and wear-resistant. One of the largest uses of cobalt is in modern superalloys – high-strength alloys that are stable at very high temperatures. They are common in gas turbine blades (Fig. 1) and jet aircraft engines, along with nickel superalloys.
Some high-speed steels contain cobalt for increased heat and wear-resistance. Cobalt-based alloys are also (along with titanium) used in medical orthopedic implants. Special cobalt-chromium-molybdenum alloys like Vitallium are used for prosthetic hip and knee replacements. Cobalt alloys are also used as a substitute for nickel in dental prosthetics when the patient is allergic to nickel. Cobalt is widely used in batteries and in electroplating. Radioactive cobalt-60 is used as a gamma ray source in radiation therapy to treat cancer. Cobalt is essential to many living creatures and is a component of vitamin B12. Due to its magnetic property, cobalt is used in samarium-cobalt rare-earth magnets. In 1966, the first samarium-cobalt rare-earth magnets were developed, and in 1972, they were improved by Albert Gale and Dilip K. Das of Raytheon Corporation. They are similar in strength to neodymium magnets but have higher temperature resistance and coercivity (resistance to demagnetization).
Here are some important facts about cobalt.
- Periodic table classification: Transition metal
- Atomic number: 27
- Atomic weight: 58.9332
- Melting point: 1495°C, 1768K
- Density @ 20°C: 8.90 g/cm3
- Siemens (www.siemens.com)