They were developed for use at high temperatures where relatively severe mechanical stressing is encountered and where high surface stability is required. Restricted-field components, which raise the transition temperature, include antimony, arsenic, chromium, germanium, iridium, molybdenum, osmium, platinum, rhenium, rhodium, ruthenium, silicon, tantalum and tungsten ( Donaldson, 1986).Ĭobalt superalloys, a term generally applied to immensely strong, hard, wear-and corrosion-resistant alloys, were first introduced in the 1930s. Enlarged-field components, which lower the transition temperature, include aluminium, boron, carbon, copper, iron, manganese, niobium, nickel, tin, titanium and zirconium. Cobalt is also available as briquets, granules (99.5% cobalt), rondelles, powder (99.995% cobalt or 99.8% cobalt, 99.9% cobalt, 0.25–2 mm) ( Sax & Lewis, 1987 American Chemical Society, 1988 Aldrich Chemical Co., 1990).Ĭobalt alloys can be categorized into six broad types: superalloys (high-temperature alloys), magnetic alloys, hard-metal alloys, high-strength steels, electrodeposited alloys and alloys with special properties ( Donaldson, 1986).Įlements used in cobalt alloys are classified in terms of their effect on the transition from the cubic to the hexagonal form. Electrolytic coarse powder has a mean particle size of 4–10 μm ( Cobalt Development Institute, 1989). The ‘fine’, ‘extrafine’ and ‘superfine’ cobalt powders manufactured from the cathodes have a submicrometre mean particle size and contain both allotropic crystal forms in varying proportions for different applications. The cathodes measure 10–25 mm and weigh 20–50 g, with a purity greater than 99.5%. ![]() The Co 2+ ⇆ Co 3+ interconversion is important in many applications of cobalt compounds, including their use as catalysts and as paint driers and in the reactions of vitamin B 12 ( National Research Council, 1977 Donaldson, 1986 Donaldson et al., 1986a, b).Ĭobalt metal is available for industrial use as ‘broken’ or ‘cut’ cathodes or electrolytic coarse powder. In contrast, in an alkaline solution containing ammonium hydroxide or cyanide, Co 2+ is readily oxidized by air or hydrogen peroxide to the more stable Co 3+ complex. In acid solution, in the absence of such complexing ligands, Co 2+ is the stable form and Co 3+ is so unstable that it is reduced rapidly and spontaneously to Co 2+, oxidizing water to molecular oxygen. These electron-donor ligands strongly stabilize Co 3+ in solution, usually forming octahedral complexes, many of which can be isolated as stable salts. A few simple salts of cobalt in its + 3 oxidation state have been used commercially (e.g., Co 2O 3), and many Co(III) complexes with ligands such as NH 3, CN −, NO 2−, ethylene-diaminetetraacetic acid, phthalocyanines and azo dyes have been studied extensively. ![]() With the exception of the mixed oxide (Co 3O 4), the major commercial cobalt chemicals are all compounds of cobalt in its stable + 2 oxidation state. Single crystals show marked magnetic anisotropy up to about 250☌ ( Donaldson, 1986). The cubic form is magnetically anisotropic up to about 1000☌ and becomes paramagnetic at 1121 ☌. It forms a mixed oxide-sulfide scale in air containing sulfur dioxide ( Donaldson et al., 1986a).Ĭobalt also has magnetic properties. Cobalt forms a protective layer of sulfide scale when reacted with sulfur at temperatures below 877☌ or in an atmosphere of hydrogen sulfide. Cobalt metal does not combine directly with hydrogen or nitrogen but combines with sulfur, phosphorus and carbon when heated. Cobalt is stable to atmospheric oxygen, but when it is heated it is oxidized to the mixed oxide, Co(II,III) oxide (CO 3O 4) at temperatures above 900 ☌, Co(II) oxide (CoO) is the end-product. The main oxidation states of cobalt are Co(2 +) and Co(3 +). ![]() The free energy change is low, however, so that transformation from the face-centred cubic back to the hexagonal close-packed form is slow and may be inhibited by physical form (e.g., grain size or presence of other metals) ( Donaldson, 1986). The hexagonal close-packed form is more stable at temperatures below 417☌, and the face-centred cubic form at higher temperatures (from 417☌ to the melting-point Considine, 1974). Bergman established cobalt as an element ( Donaldson, 1986).Ĭobalt exists in two allotropic forms. Cobalt metal was isolated by the Swedish scientist G.
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