Cobalt

Cobalt — Material Page

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Cobalt (Co, atomic number 27) is a hard, lustrous, silver-gray ferromagnetic transition metal in Group 9 of the periodic table, situated between iron and nickel — its two fellow ferromagnetic elements — in Period 4. With a density of 8.90 g/cm³ and a melting point of 1,495 °C, cobalt is one of only three naturally ferromagnetic elements at room temperature. It has an unusual dual-phase crystal structure: hexagonal close-packed (HCP, α-Co) is stable below 417 °C and face-centered cubic (FCC, β-Co) above, with the HCP phase's high magnetocrystalline anisotropy being central to its use in permanent magnets. Cobalt's Curie temperature of 1,115 °C is the highest of any ferromagnetic element — far exceeding iron (770 °C) and nickel (358 °C) — enabling cobalt-containing magnets and alloys to retain magnetic properties and high-temperature mechanical strength in environments where iron-based materials fail. Cobalt is monoisotopic in nature (⁵⁹Co, 100% natural abundance) and is classified by the EU and US as a critical raw material due to geographic supply concentration, with more than 70% of global mine production originating from the Democratic Republic of Congo.

In energy storage, cobalt is the enabling element in the lithium-ion battery cathode chemistries that power virtually all consumer electronics and a substantial fraction of electric vehicles. Lithium cobalt oxide (LiCoO₂, LCO) — first commercialized by Sony in 1991 — remains the dominant cathode material in smartphones, laptops, and wireless earbuds due to its high volumetric energy density (~700 Wh/L) and excellent cycle life. NMC cathodes (LiNiₓMnᵧCoᵤO₂) balance energy density, power, and cycle life with progressively lower cobalt content (NMC 111: 33% Co; NMC 622: 20% Co; NMC 811: 10% Co), while NCA (LiNi₀.₈Co₀.₁₅Al₀.₀₅O₂) is used in Tesla cylindrical cells. The drive to reduce cobalt dependence has produced cobalt-free alternatives (LFP, LMFP, LNMO), but higher-energy applications continue to rely on cobalt-containing chemistries for volumetric energy density and thermal stability that cobalt-free materials cannot yet match.

Cobalt's elevated-temperature strength, oxidation resistance, and hot corrosion resistance make it indispensable in aerospace superalloys and hard-facing applications where no iron- or nickel-based alternative performs equivalently. Co-base superalloys (Haynes 25/L-605, Mar-M 509, FSX-414) are used for gas turbine nozzle guide vanes, combustion liners, and first-stage turbine blades where their superior resistance to Type I (Na₂SO₄, >900 °C) and Type II (<900 °C) hot corrosion and their retention of strength near the solidus temperature are critical. Stellite alloys (Co-Cr-W-C and Co-Cr-Mo-C) — the archetypical hard-facing materials — achieve hardness of 40–60 HRC, outstanding sliding wear resistance, and corrosion resistance in aggressive process fluids, deposited by PTA, laser cladding, or HVOF onto valve seats, pump impellers, and cutting tools. The WC-Co cemented carbide system, consuming approximately 25% of world cobalt production, uses cobalt as the ductile metallic binder phase that holds tungsten carbide grains together — producing the hardest and most wear-resistant tooling material in widespread industrial use.

General Properties

Property	Value	Notes
Atomic Number	27	Group 9, Period 4; transition metal; between Fe (26) and Ni (28) — all three are ferromagnetic at room temperature; one of only three naturally ferromagnetic elements
Atomic Mass	58.933 u	Monoisotopic — ⁵⁹Co is the only stable isotope (100% natural abundance); this makes cobalt one of only 22 monoisotopic elements
Density (20 °C)	8.90 g/cm³	Intermediate between iron (7.87) and nickel (8.91); denser than most structural engineering metals; relevant to weight calculations in Co-base superalloy turbine components and WC-Co cemented carbide tooling
Melting Point	1,495 °C (1,768 K)	High melting point enables use in superalloys operating near 1,100 °C; slightly lower than nickel (1,455 °C) and iron (1,538 °C); Co-base alloys are selected over Ni-base where hot corrosion resistance outweighs the modest melting point advantage
Boiling Point	2,870 °C (3,143 K)	Wide liquid range (~1,375 °C); cobalt vapor pressure is significant above ~1,500 °C — relevant for vacuum arc melting and electron beam refining of high-purity Co
Thermal Conductivity	100 W/m·K	Moderate — similar to iron (80 W/m·K) and nickel (91 W/m·K); cobalt alloys used in turbine components require adequate thermal conductivity for cooling air distribution through internal passages
Electrical Resistivity	62.4 nΩ·m (20 °C)	Similar to iron (100 nΩ·m) and nickel (69 nΩ·m); increases anomalously near the Curie temperature (1,115 °C) due to magnetic scattering — the spin-disorder contribution to resistivity
Crystal Structure	HCP (α-Co, <417 °C); FCC (β-Co, >417 °C)	HCP phase has high magnetocrystalline anisotropy (K₁ = 4.5 × 10⁵ J/m³) enabling high coercivity in SmCo and Co-Pt permanent magnets; FCC phase is more ductile and prevalent in most commercial cobalt alloys quenched from high temperature
Curie Temperature	1,115 °C (1,388 K)	Highest Curie temperature of any ferromagnetic element — 345 °C above iron (770 °C) and 757 °C above nickel (358 °C); enables Co-containing magnetic materials and alloys to retain ferromagnetic properties and high-temperature strength in demanding thermal environments

Mechanical Properties

Property	Value	Notes
Hardness	Mohs ~5; ~125 HV (annealed)	Harder than iron (~60 HV annealed) and nickel (~68 HV annealed); work-hardens rapidly due to stacking fault energy considerations in HCP/FCC dual-phase structure; Stellite hard-facing alloys reach 40–60 HRC
Elastic (Young's) Modulus	209 GPa	Similar to iron (200 GPa) and nickel (200 GPa); slightly higher stiffness in the HCP phase than FCC; WC-Co cemented carbides achieve 500–700 GPa modulus due to the high WC content
Poisson's Ratio	0.31	Typical for metallic systems; used in finite element analysis of turbine blade and WC-Co insert thermal and mechanical stress distributions
High-Temperature Strength	Retains significant strength to ~1,000 °C	Co-base superalloys (Haynes 25, Mar-M 509) retain tensile strength of ~200–400 MPa at 900 °C — comparable to Ni-base superalloys but with superior hot corrosion resistance; relevant for nozzle guide vanes and combustion liners not cooled as effectively as turbine blades

Thermal & Environmental Properties

Property	Value	Notes
Corrosion Resistance	Moderate — forms passivating CoO/Co₃O₄ layer	Cobalt oxidizes slowly in air at room temperature; the mixed Co₃O₄ spinel oxide layer slows further oxidation but is less protective than Cr₂O₃; alloy additions of Cr and Al greatly improve oxidation resistance — Stellite alloys with 25–30% Cr are highly resistant to oxidizing acids and hot corrosion
Hot Corrosion Resistance	Superior to nickel superalloys (Type I >900 °C)	Co-base alloys resist Type I hot corrosion (Na₂SO₄-induced basic fluxing, >900 °C) significantly better than most Ni-base superalloys — critical for marine gas turbines and industrial turbines burning sulfur-containing fuels; Co has lower solubility in molten Na₂SO₄ than Ni, reducing the sulfidation attack rate
Oxidation States	+2 (primary), +3; also 0, +1, +4 in compounds	Co²⁺ (cobaltous) gives the characteristic pink/red color of hydrated salts; Co³⁺ (cobaltic) forms low-spin octahedral complexes — the basis of Werner's original coordination chemistry theory; Co²⁺/Co³⁺ redox couple (0.18 V vs. SHE) underpins LiCoO₂ battery cathode operation
Biocompatibility	Generally biocompatible (Co-Cr-Mo alloys)	Co-Cr-Mo (ASTM F75, F1537) and Co-Cr-W-Ni (ASTM F90) alloys are approved biomedical implant materials for hip and knee prostheses, dental prosthetics, and bone fixation devices — combining high wear resistance, corrosion resistance in physiological saline, and mechanical strength; concern exists about Co²⁺ ion release from metal-on-metal hip bearings causing adverse local tissue reactions

Chemical Properties

Property	Value / Behavior	Notes
Surface Oxide	Co₃O₄ (spinel, mixed Co²⁺/Co³⁺); CoO at high T	Co₃O₄ is a p-type semiconductor used as an electrocatalyst for oxygen evolution reaction (OER) in alkaline water electrolysis; transforms to CoO above ~900 °C in air; both are active catalysts in diesel soot combustion and hydrocarbon reforming
Acid Resistance	Dissolves in dilute HCl, H₂SO₄, HNO₃	Reacts with mineral acids forming Co²⁺ salts; resistant to alkalis; resistant to acetic acid and many organic acids at room temperature; concentrated HNO₃ passivates; aqua regia dissolves Co rapidly
Magnetic Properties	Ferromagnetic; saturation magnetization 1.42 T	Co has the highest saturation magnetization among the 3d ferromagnets after iron (2.16 T) and the highest magnetocrystalline anisotropy — SmCo₅ and Sm₂Co₁₇ permanent magnets exploit the strong uniaxial anisotropy of the hexagonal Co sublattice to achieve coercivities of 800–2,500 kA/m, the highest of any commercial permanent magnet system
Catalytic Activity	Active in Fischer-Tropsch, HDS, OER, hydrogenation	Co-based catalysts are used in Fischer-Tropsch synthesis (Co/TiO₂, Co/Al₂O₃ at 200–240 °C producing waxes and diesel from syngas), hydrodesulfurization (Co-Mo/Al₂O₃ in petroleum refining), and alkaline OER electrocatalysis for green hydrogen production; vitamin B₁₂ (cobalamin) contains a Co³⁺ center essential for methyl group transfer in human metabolism

Identifier	Value
Symbol	Co
Atomic Number	27
CAS Number	7440-48-4
UN Number	UN3089 (powder)
EINECS Number	231-158-0

Isotope	Type	Notes
⁵⁹Co	Stable	100% natural abundance; I = 7/2, NMR-active; cobalt is monoisotopic — ⁵⁹Co is the only naturally occurring isotope; ⁵⁹Co NMR is used to probe cobalt coordination chemistry in catalysts, metalloenzymes, and battery cathode materials
⁶⁰Co	Radioactive	t½ = 5.27 yr (β⁻); emits two high-energy gamma rays (1.17 and 1.33 MeV) via ⁶⁰Ni* daughter; produced by neutron activation of ⁵⁹Co in nuclear reactors; one of the two most widely deployed industrial gamma sources (alongside ¹³⁷Cs) — used in cancer teletherapy (Co-60 units), industrial radiography of welds and castings, food and medical device sterilization, and nucleonic gauges; also a significant activated corrosion product in nuclear reactor coolant circuits
⁵⁷Co	Radioactive	t½ = 271.8 days (electron capture); decays to ⁵⁷Fe* emitting a 14.4 keV gamma ray — the standard source for ⁵⁷Fe Mössbauer spectroscopy, used to characterize iron oxidation states, hyperfine fields, and coordination environments in minerals, steels, oxides, and biomolecules; ⁵⁷Co sources are produced by proton bombardment of natural iron or nickel targets in cyclotrons
⁵⁸Co	Radioactive	t½ = 70.9 days (β⁺/EC); 811 keV gamma emitter; produced in nuclear reactors by fast neutron reaction on ⁵⁸Ni; an activated corrosion product of Co-containing stainless steels in reactor coolant circuits; monitored in nuclear plant radiation surveys and liquid radwaste; also used as a radiotracer in geological and hydrological studies

Scientific & Research Applications

Use Case	Form Typically Used	Description
Permanent Magnet Research (SmCo)	High-purity Co metal, Co powder, SmCo alloy ingots	Samarium-cobalt permanent magnets (SmCo₅ and Sm₂Co₁₇) exploit the large magnetocrystalline anisotropy of the hexagonal cobalt sublattice to achieve coercivities of 800–2,500 kA/m and maximum energy products of 160–240 kJ/m³. SmCo magnets outperform NdFeB above ~150 °C and in corrosive environments, making them the choice for high-temperature motors, sensors, and aerospace actuators. Research focuses on reducing Sm content, improving grain boundary engineering, and exploring Co-Fe-based alternatives.
Battery Cathode Research	Co metal, Co₃O₄ precursor, CoSO₄ solution	Cobalt is the active redox element in LiCoO₂ (LCO) cathodes — the Co³⁺/Co⁴⁺ couple at ~3.9 V vs. Li/Li⁺ provides the high voltage and energy density of consumer electronics batteries. Research focuses on NMC cathodes (NMC 811, NMC 9-0.5-0.5) with progressively lower Co content to reduce cost and supply risk while maintaining structural stability at high states of charge; also Li-rich layered oxides (Li₁.₂Ni₀.₁₃Co₀.₁₃Mn₀.₅₄O₂) for next-generation high-energy cells.
Fischer-Tropsch Catalysis Research	Co/TiO₂, Co/Al₂O₃, Co/SiO₂ supported catalysts	Cobalt-based Fischer-Tropsch catalysts (Co/TiO₂ or Co/Al₂O₃, 5–20 wt% Co, promoted with Ru or Re) convert syngas (CO/H₂) to long-chain paraffins (synthetic diesel and wax) at 200–240 °C with high selectivity. Co catalysts give higher C₅+ selectivity and lower methane yield than iron-based catalysts and are preferred for natural gas-to-liquids (GTL) processes. Research focuses on nanoparticle size optimization, support interactions, and deactivation mechanisms by sintering and coking.
Mössbauer Spectroscopy Sources	⁵⁷Co sealed sources in Rh or Cu matrix	⁵⁷Co sealed sources (t½ = 271.8 days, decaying to ⁵⁷Fe*) are the standard gamma-ray source for ⁵⁷Fe Mössbauer spectroscopy — an essential analytical technique for characterizing iron oxidation states (+2/+3), spin states (high/low spin), coordination geometry, and hyperfine magnetic fields in minerals, meteorites, corrosion products, battery cathodes, and iron-sulfur protein active sites.
Hard Magnetic Thin Films & Spintronics	Co thin films, CoPt and CoCrPt alloy films, Co/Pt multilayers	Co-Pt alloy films (equiatomic L1₀ phase) have very high perpendicular magnetic anisotropy (Ku ~4.9 × 10⁶ J/m³), used in ultrahigh-density magnetic recording research. Co/Pt multilayers are standard perpendicular recording media and model spintronics systems. CoCrPt alloys were the dominant longitudinal hard disk recording medium before the transition to perpendicular recording; now used in next-generation HAMR (heat-assisted magnetic recording) research.
Industrial Gamma Radiography (⁶⁰Co)	⁶⁰Co sealed sources (ceramic or metallic Co pellets)	⁶⁰Co sealed gamma sources (1.17 and 1.33 MeV, t½ = 5.27 yr) are one of the two standard sources for industrial radiography — used for non-destructive testing of thick steel welds (>50 mm), large castings, and pressure vessels where the higher photon energy penetrates material that iridium-192 cannot. The long half-life reduces source replacement costs. ⁶⁰Co is also used for cancer teletherapy (Cobalt-60 units remain in widespread use in emerging-market radiotherapy centers) and large-volume food, medical device, and pharmaceutical sterilization.

Industrial & Commercial Applications

Sector	Form / Alloy Used	Description
Lithium-Ion Battery Cathodes	LiCoO₂ (LCO); NMC (LiNiₓMnᵧCoᵤO₂); NCA	Cobalt-containing cathode materials dominate high-energy lithium-ion batteries. LCO (LiCoO₂) provides the highest volumetric energy density (~700 Wh/L) for smartphones and laptops; NMC cathodes in 622 and 811 compositions power EV battery packs (Tesla, Panasonic, LG Energy Solution, CATL); NCA (LiNi₀.₈Co₀.₁₅Al₀.₀₅O₂) is used in Tesla cylindrical cells. Total cobalt demand for batteries exceeds 120,000 tonnes/year and is the primary driver of global cobalt market pricing.
Cemented Carbide (WC-Co) Cutting Tools	WC-Co powder (3–25 wt% Co binder)	Tungsten carbide–cobalt (WC-Co) cemented carbides are the dominant cutting tool material for metal machining, mining, and drilling — consuming approximately 25% of world cobalt production. Cobalt acts as the ductile metallic binder phase that bonds WC grains and provides toughness: 3–6% Co grades (high hardness, ~1,600 HV) for high-speed precision cutting; 10–25% Co grades (high toughness) for mining drill bits and rock excavation tools. WC-Co inserts machine essentially all hardened steel, cast iron, and non-ferrous metals in modern manufacturing.
Co-Base Superalloys (Aerospace)	Haynes 25 (L-605), Mar-M 509, FSX-414, Stellite 31	Cobalt-base superalloys are used for gas turbine nozzle guide vanes, combustion liners, and first-stage turbine blades where their superior Type I hot corrosion resistance (>900 °C, Na₂SO₄ flux) and high-temperature ductility are critical. Haynes 25 (Co-20Cr-15W-10Ni) achieves 200 MPa tensile strength at 900 °C. FSX-414 (Co-29Cr-10.5Ni-7W) is the standard nozzle guide vane alloy for many commercial and military turbofans. Mar-M 509 is used in directionally solidified and single-crystal nozzle applications.
Stellite Hard-Facing & Wear Alloys	Stellite 6, 12, 21 (Co-Cr-W-C); deposited by PTA, laser, HVOF	Stellite alloys (Haynes International / Kennametal) are Co-Cr-W-C and Co-Cr-Mo-C hard-facing alloys that achieve 38–65 HRC through carbide precipitation in an FCC cobalt matrix. Used on valve seats in oil, gas, and nuclear plant valves; pump impellers handling slurry and corrosive fluids; cutting tools for machining nickel superalloys; and wear surfaces in food processing equipment where corrosion resistance and FDA compliance are required alongside hardness.
Orthopedic & Dental Implants	Co-Cr-Mo (ASTM F75, F1537); Co-Cr-W-Ni (ASTM F90)	Cobalt-chromium-molybdenum alloys are the standard material for total hip and knee replacement bearing surfaces, femoral stems, and dental prosthetics — offering yield strengths of 500–1,000 MPa, outstanding corrosion resistance in chloride-containing physiological fluids, and wear rates in metal-on-polyethylene bearings an order of magnitude lower than stainless steel. Wrought Co-Cr-Mo (ASTM F1537) for femoral heads and tibial trays; cast Co-Cr-Mo (ASTM F75) for complex geometries; porous Co-Cr for bone ingrowth surfaces.
Magnetic Recording & Data Storage	CoCrPt alloy films, Co/Pd and Co/Pt multilayers	Cobalt-chromium-platinum alloy films (CoCrPt with SiO₂ segregant) are the active recording layer in conventional perpendicular magnetic recording (PMR) hard disk drives — the technology in essentially all HDDs produced since 2006. The hexagonal grain structure and perpendicular anisotropy of the granular CoCrPt film enable recording densities exceeding 1 Tb/in². Co/Pd and Co/Pt multilayers are used in magneto-optical and spintronic research devices.

Purity	Main Use
Co 99.6%	General research and metal alloys — suitable for cobalt master alloy additions to superalloy and cemented carbide melts, and laboratory synthesis of Co-containing compounds where sub-0.5% impurities (mainly Ni, Fe) are acceptable
Co 99.8%	Battery materials and electronic devices — appropriate for synthesis of LiCoO₂ and NMC cathode precursors (CoSO₄ dissolution), Co₃O₄ electrode materials, and hard magnetic thin-film alloy targets where controlled Ni and Fe content is required
Co 99.9%	Magnetic materials and analytical standards — used in SmCo permanent magnet alloy preparation, Mössbauer source matrix material, and primary standard solution preparation where <1,000 ppm total metallic impurities are required for accurate elemental analysis
Co 99.99%	Advanced research, semiconductor, and medical-grade applications — the standard purity for PVD sputtering targets for CoPt and CoCrPt magnetic recording film deposition, fundamental magnetic anisotropy and spintronics research, and biomedical implant alloy qualification where trace element control is required by ASTM F75/F1537

Synonym / Alternative Name	Context
Co	Chemical symbol
Cobalt metal	Standard commercial and regulatory designation for the elemental form; used in REACH, RoHS, and conflict minerals (3TG+Co) regulatory reporting
Elemental cobalt	General scientific term distinguishing pure metal from cobalt compounds (CoO, Co₃O₄, LiCoO₂, CoSO₄, etc.)
Cobalto	Spanish and Italian language equivalent; the name derives from the German Kobold (goblin or evil spirit) — miners in medieval Saxony named the ore koboldite because it was thought to be harmful and yielded no useful metal when smelted (the arsenic fumes were toxic); Georg Brandt identified cobalt as a distinct element in 1735
Kobalt	German language equivalent; the etymological root from which the English and Romance language forms derive
Cobalt blue	Common cultural reference to cobalt aluminate (CoAl₂O₄) pigment — a vivid blue colorant known since antiquity in Chinese blue-and-white porcelain, Meissen porcelain, and Renaissance glass; not a synonym for the metal itself but widely associated with cobalt in art and materials contexts

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