Brandt discovered cobalt around 1735. It occurs in the minerals cobaltite, smaltite and erythrite and is often associated with nickel, silver, lead, copper and iron ores, from which it is most frequently obtained as a by-product. It is also present in meteorites.
Cobalt (Co) is a brittle, hard metal white in appearance resembling nickel (and iron) but with a bluish tinge instead of the yellow of nickel. It is rarer and more valuable than nickel. It is diamagnetic and has magnetic permeability approximately two thirds that of iron and three times that of nickel.
Cobalt exists as two allotropes over a wide temperature range. The β-form a close-packed hexagonal crystal is stable and predominates below approximately 417 °C (782 °F), and the α-form a cubic crystal is stable and predominates above this temperature until the melting point.
Although allied to nickel, it is more active chemically than nickel. Cobalt dissolves in dilute sulphuric acid, nitric or hydrochloric acid and is slowly attacked by alkalis. The oxidation rate of pure cobalt is twenty five times that of nickel. Cobalt’s ability as a whitening agent against copper alloys is inferior to that of nickel. However, small amounts in nickel-copper alloys will neutralise the yellowish tinge of the nickel and make them whiter.
Cobalt imparts red-hardness to tool steels. It can harden alloys to greater extent than nickel, especially in the presence of carbon and can form more chemical compounds in alloys than nickel.
Natural cobalt is cobalt 59, which is stable and non-radioactive, but other isotopes 54 to 64 are all radioactive (table 1), emitting beta and gamma radiation. Other isotopes not listed in table 1 have short half-lives.
Table 1. Cobalt isotopes and their half-lives.
Cobalt 60 has a number of applications. These include:
- Radiographic inspection
- A gamma ray source
- A tracer
- A radiotherapeutic agent
- Irradiation of plastics
- A catalyst for the sulphonation of paraffin oil. In this application the gamma rays emitted by the cobalt cause the reaction of sulphur dioxide and liquid paraffin.
Other uses for cobalt are:
- In superalloys for aircraft gas turbine engines
- It is a key elemental ingredient in magnet steels, by which it increases residual magnetism and coercive force and in nonferrous-base magnetic alloys
- Cobalt is an important element in numerous glass-to-metal sealing alloys as well as low expansion alloys
- Alloys for dental and surgical applications because they are not attacked by physiological fluids. An example of which is ‘Vitallium’ which is used to replace bone. Such alloys are ductile enough to permit anchoring of dentures on neighbouring teeth and contain up to 65% cobalt
- High-speed, heavy duty, high temperature cutting tools, and dies
- Gas turbine generators
Cobalt salts are used as a source of brilliant permanent blue colour in porcelain, glass, pottery, tiles and enamels.