COBALT

Name: Cobalt

Symbol: Co

Group: 9

Period: 4

Block: d

Category: Transition Metals

Atomic Number: 27

Atomic Mass: 58.933 u

Electrons per Shell: 2, 8, 15, 2

Electronegativity: 1.88

Density: 8.9 g/cc

Melting Point: 1495ºC

Boiling Point: 2927ºC

Thermal Conductivity: 100 W (m·K)

Electrical Conductivity: 1.7 × 10^7 S/m

Magnetic Order: Ferromagnetic

Ordinary State: Solid

Oxidation States: -3, -1, 1, 2, 3, 4, 5

Mohs Hardness: 5

Vickers hardness: 1043 MPa

Brinell hardness: 700 MPa

Most stable isotopes: Co-59 (100%)

Discoverer: Georg Brandt, Swedish (1735)

Cobalt (Co) is a chemical element whose importance alone justifies the existence of Metalpedia. While any metallurgist, chemist, or engineer immediately recognizes its relevance, the general public knows it only fragmentarily. For many, its most immediate reference is the intense blue glass—popular for centuries—while for others, it is its medical use, particularly the radioactive isotope Co⁶⁰, an emitter of high-energy gamma rays, used in radiotherapy against various types of cancer. This use is not merely a technical detail: in my own experience, this treatment saved my mother's life, and that sole circumstance makes it, for me, a metal of inestimable value. However, its importance is not limited to the ornamental or therapeutic: cobalt is the third member of the "ferrous trio," alongside iron (Fe) and nickel (Ni), with which it shares magnetic properties and a wide range of industrial and strategic applications. Before titanium became widely implanted in prosthetics and reconstructive surgery, cobalt-chromium alloys were the preferred choice for bone implants due to their corrosion resistance and biocompatibility. Similarly, cobalt-based superalloys are essential in aviation engines, where mechanical stability at extreme temperatures is a vital requirement.

Its scientific history begins in the modern era, when alchemy still enjoyed academic recognition. At that time, only seven metals were known: five transition metals (iron, copper, silver, gold, and mercury) and two post-transition metals (tin and lead). Other elements like arsenic and antimony, though isolated, were confused with tin or lead due to the similarity of their minerals. Zinc, known in India, was not identified as an independent element until decades later, and bismuth (Bi), although used since ancient times, was officially recognized as such in 1753 thanks to Claude François Geoffroy. In this context, cobalt became the first transition metal discovered and officially recognized since Antiquity. Its isolation is attributed to the Swedish chemist Georg Brandt (Riddarhyttan, June 26, 1694 – Stockholm, April 29, 1768), who, between 1732 and 1735 (a still debated date), identified it as an independent element and demonstrated that the intense blue color of glass came from its compounds. Brandt was a pioneer in a full sense: he was the first to isolate a new element by himself and to determine its characteristic properties. His disciple Axel Fredrik Cronstedt, following the path laid out, would discover nickel in 1751, thus completing the identification of the other metal that, along with cobalt and iron, forms the core of ferromagnetic metals.

Cobalt salts have been prized for more than a millennium, particularly in ancient China, where their blue pigment was applied to ceramics, porcelains, and glass. This "cobalt blue" is a cobalt aluminate, obtained naturally or deliberately synthesized, whose color, of unparalleled intensity and purity, has even been imitated in textile fashion, where "cobalt" designates a specific shade. Even though more economical synthetic pigments exist, true cobalt blue retains prestige and demand for its chromatic stability and resistance to fading.

In its metallic state, cobalt is rarely found native, except in exceptional conditions such as those of certain meteorites. In the Earth's crust, it appears in small concentrations, associated with iron, copper, and especially nickel, which is why it is often obtained as a byproduct of the extraction of these metals. Its deposits are distributed worldwide, but the most important reserves are in the Democratic Republic of Congo, which concentrates a significant fraction of global production. Other producing countries include Canada, Russia, Australia, China, Uruguay, southern Brazil, and Cuba, which has large reserves.

The name "cobalt" comes from the Old Germanic term kobold, meaning "goblin" or "gnome," figures from mining folklore to whom the mischief of "stealing" the copper sought by miners and replacing it with a useless metal, difficult to melt with the techniques of the time, was attributed. Similarly, nickel received its name from the spirit Nickel, with whom it shared the same deceptive reputation. This coincidence is not accidental: even today, cobalt, nickel, and copper are frequently found associated in the same minerals, which explains both their joint discovery and their combined exploitation.

Today, cobalt has transitioned from being a "slag metal" for ancient mining to a first-order strategic resource, with its own market and critical applications for modern technology. Steel transformed human history, but for certain functions—from alloys resistant to extreme temperatures to high-performance lithium-ion batteries—cobalt is irreplaceable. Understanding its characteristics, sources, and applications not only allows us to appreciate its current importance but also to anticipate its role in the industry and society of the future.

Cobalt (Co) is a transition metal that is part of the select group of ferromagnetic elements along with iron (Fe) and nickel (Ni), with which it shares multiple physical and chemical similarities, in addition to presenting some parallel behavior with manganese (Mn). From an astrophysical point of view, cobalt is one of the metals produced by the r-process in massive stars that, at the end of their cycle, release their matter during supernova explosions, dispersing it through space along with other heavy elements. In its pure state, free of corrosion, it presents a silver-white color with a characteristic metallic luster, differing from the slightly bluish hue of iron and the pale golden tone of nickel. However, its surface reacts easily to prolonged exposure to the elements, developing thin layers of corrosion that can give it yellowish, pinkish, or bluish reflections, depending on the exact composition of these oxidation products.

Crystallographically, pure cobalt presents two structural modifications (polymorphism) depending on the temperature. At room temperature, the most stable phase is hexagonal close-packed (ε-Co), characterized by its high rigidity and low ductility. Upon reaching approximately 400 °C, part of this structure transforms into the face-centered cubic phase, known as austenite (γ-Co), which is metastable and, unlike steel austenite, can coexist with the hexagonal phase even after cooling. Austenite is more ductile than the hexagonal phase, although cobalt, in any of its pure forms, cannot be considered a malleable metal. To process it into products such as wires or sheets, alloys are used, generally with nickel in proportions close to 20% by mass, which significantly increases its ductility. Its hardness, 5 on the Mohs scale, positions it as a robust material, resistant to pulverization, but brittle under severe impacts, especially in its alpha phase (ε).

When melted under vacuum, cobalt solidifies into spheroidal nodules, similar to those observed in manganese, though more resistant to fracture. It possesses notable crushing resistance, surpassing even metals like chromium (Cr), manganese, and antimony (Sb), although its lack of malleability limits certain uses. Its ferromagnetism is maintained up to a Curie temperature of 1115 °C, at which point it loses its magnetic properties. Alloys with manganese or nickel improve both ductility and malleability, while combinations with chromium generate surprisingly tough and highly corrosion-resistant materials. The addition of molybdenum (Mo) enhances these qualities, increasing toughness and reinforcing protection against aggressive environments, which is why Co-Cr-Mo alloys are indispensable in high-demand industrial applications, such as prosthetics, turbines, and cutting tools.

In terms of its metallurgical compatibility, cobalt forms alloys with most transition metals, although with notable exceptions: it does not amalgamate with mercury (Hg) and shows mutual repulsion with silver (Ag) in a liquid state. With copper (Cu) and gold (Au), it is miscible only in limited proportions, although there is a stable alloy of 75 parts gold and 25 parts cobalt, prized in jewelry for its warm tone and resistance. With refractory metals such as rhenium (Re) or tungsten (W), alloys of great thermal resistance are obtained, widely used in the aerospace and energy industries. In contrast, combinations with scandium (Sc), yttrium (Y), and lanthanides are uncommon and tend to behave more like intermetallic compounds than metallic solid solutions. Among the latter, the samarium-cobalt (SmCo) alloy stands out, forming the basis of high-power and thermally stable permanent magnets, although less intense than neodymium (Nd₂Fe₁₄B) magnets and considerably more expensive. With p-block metals and alkali metals, alloys are practically impossible, although, unlike iron, cobalt can dissolve small quantities of aluminum (Al), which expands its possibilities in the design of functional materials.

Cobalt-chromium alloys constitute the most relevant group among those that use cobalt as a base metal, as they are distinguished by their exceptional resistance to high temperatures, mechanical wear, and chemical corrosion. These qualities, derived from the remarkable affinity of cobalt and chromium for oxygen, generate a protective passive layer that persists even in severe oxidizing environments, making these alloys materials of choice for applications where thermal expansion, loss of hardness, or structural degradation under extreme heat are unacceptable. Within this group, the most recognized is Stellite®, developed by Elwood Haynes at the beginning of the 20th century and registered by the Kennametal Stellite Company. Originally conceived to exploit the chemical resistance of cobalt and chromium, this superalloy evolved with the addition of elements such as molybdenum and tungsten, which improve its hardness, thermal stability, and anticorrosive capacity, consolidating it as an indispensable material in extreme conditions, including the aerospace industry, the manufacture of cutting tools, and medical implantology.

With a mirror-white color comparable to rhodium or platinum, Stellite® retains its luster for years without showing signs of oxidation, although its inherent hardness and brittleness make conventional machining difficult. For this reason, casting in a controlled atmosphere or vacuum is preferred to avoid the incorporation of impurities. Its high cost, considerably greater than that of any stainless steel, is justified by its extraordinary chemical and mechanical stability, especially when alloyed with up to 10% by mass of molybdenum, which also confers biocompatibility. Thanks to this property, cobalt-chromium alloys have acquired a prominent role in the manufacture of orthopedic and dental prosthetics, directly competing with titanium alloys and advanced ceramics like zirconia (ZrO₂). Although titanium offers lower density and, therefore, greater lightness, cobalt-chromium exhibits superior wear resistance and ease of molding that simplifies complex production processes.

The addition of tungsten introduces tungsten carbide (WC) into the microstructure, a compound of high hardness and thermal stability that reinforces wear resistance even under mechanical loads and extreme temperatures. Unlike carbides formed by molybdenum or those generated by chromium, WC possesses a more stable crystalline structure and a considerably higher melting point, which prolongs the useful life of industrial tools. Similarly, molybdenum improves resistance to reducing environments, elevating the performance of these alloys above 316L stainless steel in marine and chemical environments. This improvement is more notable in versions with low carbon content, since the formation of intergranular carbides, while increasing hardness, can reduce corrosion resistance in aggressive solutions.

Thermally, cobalt-chromium alloys maintain their hardness and microstructural stability even when operating at temperatures that degrade high-speed steels and other iron-based superalloys. With carbon contents reaching up to 5% by mass, they are optimized to resist friction and heat without significant loss of mechanical properties, as the chromium, molybdenum, or tungsten carbides present in the cobalt matrix do not dissociate at high temperatures. This phenomenon, coupled with a favorable brittle-ductile transition with increasing temperature, allows them to maintain a balance between hardness and toughness, essential for turbine components, cutting tools, and industrial mechanisms that operate continuously.

In its pure state, cobalt shows corrosion resistance intermediate between iron and nickel, being stable against dry or humid air, fresh water, and alkalis at room temperature, although its acid resistance is limited: nitric acid aggressively attacks it at any concentration, while concentrated sulfuric acid passivates it, and hydrochloric acid corrodes it slowly. True excellence in chemical resistance is obtained by alloying it with chromium, molybdenum, or tungsten, which, by forming stable carbides, reinforce the integrity of the metallic matrix and allow its use in environments where the combination of corrosion and high temperatures would destroy other materials. Additionally, small additions of vanadium, titanium, or tantalum act as dopants that optimize the distribution and stability of the carbides, increasing performance in high-precision applications.

In the medical and jewelry fields, the low biological reactivity of these alloys makes them ideal for prolonged contact with the body without inducing adverse reactions, while in engineering and manufacturing, their resistance to wear and friction makes them a justified investment when durability is a priority. Their unique balance of hardness, chemical resistance, and thermal stability, along with the ability to maintain mechanical properties under extreme conditions, ensures that cobalt-chromium alloys continue to occupy a prominent place in contemporary advanced metallurgy.

Compared to iron and nickel, cobalt is used for fewer applications; however, in those where it is used, it is practically irreplaceable.

As a base metal, it has three main uses:               

Superalloys

Similar to nickel, cobalt is used to manufacture so-called "superalloys" of extreme resistance to heat and corrosion at high temperatures, as they retain their toughness and hardness. Even elite high-speed steels (e.g., AISI M50), initially developed for high-responsibility engines and bearings in the aeronautical industry, are inferior in this aspect to cobalt superalloys. It is alloyed with Chromium, Molybdenum, Titanium, Rhenium, and Ruthenium (in the case of particularly special alloys) for the manufacture of aerospace engines, missile warheads, rotors, blades, and turbines. The chemical composition of these superalloys is very complex, and the manufacturing process is extremely costly. Suffice it to say that many of them are only manufactured in certain strategic locations in the United States and the European Union. In Eurasia, Russia and China have their own versions. There are cobalt-based superalloys, others are nickel-based, and others where both metals are present in similar proportions.

Nickel-based superalloys are more corrosion-resistant and can operate at sub-zero temperatures; however, they are less hard (wear resistance) than cobalt-based ones. They are the most popular (e.g., Hastelloy, Inconel, René) and accessible.

Cobalt-based superalloys better withstand abrupt temperature changes. This is an important detail, as the temperature increase is not always gradual. They also operate better at extreme temperature levels (they are designed to retain their hardness up to 1000 Cº or even higher). They are used in the military, cruise aircraft, aerospace and aeronautical industry, high-performance or competition cars (including F1 single-seaters), medicine, etc. They do not perform well at sub-zero temperatures as they are not austenitic.

Cobalt/Nickel-based superalloys exist but are not widely used. It should be noted that both nickel-based superalloys contain some cobalt, and vice versa (unless the part in question must be exposed to radiation). Both families of superalloys are very expensive, and only a few companies produce them. Ask yourself why an airplane costs so many millions...

Hard Alloys

Cobalt has been used since the early 20th century for the manufacture of hard and corrosion-resistant alloys. Few people know that these alloys preceded stainless steel (which is cheaper) at a time when table cutlery was still made of silver, alpaca, or even pewter. The Cobalt-Chromium combination is the best known and has been marketed under the name Stellite for over forty years. There are other variations, such as Vitallium, but they are very similar. They are extremely hard, tough, and corrosion-resistant alloys. The price is expensive compared to the best steels, but the performance is superior.

Other metals that usually accompany chromium are Molybdenum and Tungsten (note that all three metals are part of the same family), although it is not uncommon to find small percentages of Silicon, Manganese, Iron, and Nickel. The carbon content is very high (up to 6% in special cases) because cobalt does not form carbides; instead, it acts as a matrix ("glue") in the intergranular formation of carbides produced by the aforementioned metals that combine with carbon at the time of melting, resulting in an extraordinarily hard yet tough metallic compound. These alloys become sufficiently ductile when hot, which is why they do not fracture even when the part becomes "red hot."

 Cobalt vs Steel. Explanation

Iron-based alloys known as high-speed steels (HSS) are metallic alloys where iron is the base metal. They are alloyed with Vanadium, Chromium, Molybdenum, and Tungsten. Other more expensive variants use Niobium, Tantalum, etc., but essentially, they use the first four metals for carbide formation. When high-speed steel is melted and these metals are added, the high carbon content (never less than 0.95%) combines with these metals to form microscopic intergranular carbides that give the material its high hardness. However, if the steel had an excess of carbon (e.g., 2%), it would go from being a robust material to a "crystalline" compound that would violently shatter on slight impact. This happens because iron is also sensitive to carbon; that is, part of the carbon added to the alloy during melting will be dissolved by the iron, forming cementite, which is very hard and brittle. Because iron forms its own carbide, and this is very brittle, the carbon content should not exceed 1.5%. I repeat: the reason is that iron forms its own carbide.

Cobalt is different. It can become contaminated with carbon and indeed reacts with it, but it does not form carbides, as in the case of iron. Instead, it dissolves the carbides of other metals, specifically the same metals added to typical high-speed steel. Since cobalt does not absorb free carbon (in its graphite form), the amount of carbon can reach up to 4 or 5% without becoming too brittle to be used safely, as it will be dissolved by the high content of Chromium, Molybdenum, and/or Tungsten. Cobalt only acts as a "base" for other metals that do form carbides, which is why its performance is superior. Another detail to consider is that, no matter how much the temperature rises, the alloy will remain stable: even in quality high-speed steel, the tempering it normally undergoes to increase its hardness is lost once a sufficient temperature is reached. Curiously, some modern high-speed steels contain cobalt to increase the carbon percentage. With this, I hope to have cleared up any doubts.

Cermets

In the manufacture of cermets (materials that combine properties of metals and ceramics), cobalt is by far the most popular base metal. It is only substituted by nickel if corrosion resistance must be high. Between 8 and 20% of cobalt is used to give the desired shape to carbide powder (normally tungsten carbide), thus achieving a wide range of objects very useful to humankind.

The famous tungsten carbide, called "Widia" in Spain and some other countries, is a cermet of this compound agglomerated with a metallic base, almost always cobalt. It is used for drill bits, saws, and sandpaper: it is very hard.

The content of the base metal (Cobalt or Nickel) is adjusted according to the demand to be met. A higher amount of Cobalt/Nickel means more toughness but less hardness, and vice versa.

In tungsten carbide jewelry, cobalt is alloyed with chromium to increase corrosion resistance or is not used directly (nickel is the typical replacement) because it is less corrosion-resistant. However, the use of nickel in mechanical applications makes no sense, as it is too soft a metal compared to cobalt. The price of both versions is usually similar.

 Jewelry

With copper, the so-called "Sun Bronze" (Sonnenbronze in German) is manufactured, which also contains aluminum to increase corrosion resistance. I myself was interested in making a ring with this alloy, but no jeweler agreed to try it due to unfamiliarity with cobalt and its high melting point, which translates into a greater amount of fuel and wax.

With gold, it forms two types of coloration, depending on the quantity and manufacturing process carried out. "Blue Gold" is traditional yellow gold with a bluish or even purple "touch." It is achieved by dissolving pure cobalt in liquid gold or by melting cobalt in a vacuum furnace and then adding the gold. It has a rather strange color, is not very popular, although I personally find it interesting. High-purity iron can also be used for this purpose. They are usually not ferromagnetic.

By mixing cobalt with chromium in a master alloy that is then added to gold, a similar tint to the one described previously is achieved, which is then heated red-hot with a flame to deliberately oxidize it to achieve a matte black metallic finish. This is called "Black Gold," not to be confused with petroleum, which is also called that due to its great economic value.

With platinum, alloys are much more popular. In Asia (especially South Korea and Japan), jewelers often alloy platinum with cobalt to harden it. This procedure is also carried out in Germany. It is expensive, not because of cobalt itself, but because of platinum and its high melting point. These are hard alloys, more expensive than the finest 18k gold, only accessible to a few.

Cobalt, or rather, a cobalt-based alloy, was recently popularized by Scott Kay, an American entrepreneur who has sold it state-to-state as a superior alternative to titanium and tungsten carbide in the modern era of alternative jewelry. His alloy, BioBlu27, is very corrosion-resistant, tough, and hard. It resembles Stellite but has less carbon. Its high chromium content gives it an appearance similar to rhodium-plated white gold or even platinum, although the alloy itself does not need any plating as, in its pure state, it presents a perfect white with incredible luster. It is more expensive—for now—than Grade 2 and 5 titanium alloys (used in titanium jewelry) and tungsten carbide.

In Magnets

Cobalt, a ferromagnetic metal, is used to manufacture powerful magnets as an alternative to ferrite doped with Barium and Strontium (Speaker Magnets). The most famous are the now obsolete AlNiCo magnets, which take their name from their chemical composition (Aluminum + Nickel + Cobalt) with an iron base. Another combination that dominated the market for approximately two decades was Cobalt-Samarium, which was very fragile and expensive. Both have fallen out of use in favor of Neodymium magnets, which are much more powerful and cheaper to produce.

Platinum alloys with cobalt percentages as low as 10% exhibit ferromagnetism: they are attracted by magnets. An alloy of 76.80% Platinum and 22.20% Cobalt shows magnetic permeability superior to some steel combinations. This percentage corresponds to the chemical composition PtCo, meaning one cobalt atom for each platinum atom. Due to the affinity of both, a crystalline structure is created that can respond as a pure ferromagnetic metal would. You will not find details like those I have just provided in any Spanish-language book. I can guarantee it.

 As a Pigment

Cobalt aluminate with the chemical formula CoAl₂O₄ is used, which is easily prepared, although it requires a lot of fuel. It is prepared by burning alumina (Aluminum Trioxide – Al₂O₃) together with cobalt monoxide (black in color – formula: CoO). The reaction takes place at about 1200 Cº, producing the pigment.

It is used to paint ceramics, imparting a color known as royal blue, a dark shade, though not as dark as navy blue; likewise, the color is less intense than those achieved with Prussian Blue (made with Iron) and also Ultramarine (similar to natural Lapis Lazuli).

In its pure state, it can be used to tint soda-lime glass (common quartz-based glass). It is also used as an enamel.

 Use in Medicine. Cobalt-60

Cobalt is a monoisotopic element very sensitive to nuclear exposure, especially to fast protons and neutrons. The natural isotope, Co-59, is deliberately bombarded with fast neutrons to convert it into Co-60, a high-energy gamma ray emitter that is doped in low doses into plasmas used for the treatment of many types of cancer. It is usually administered to the patient intravenously. It has saved thousands of lives worldwide and continues to do so. It is the cornerstone of so-called chemotherapy.

  The Mechanism is Easy to Understand

Despite being radioactive, the isotope is still a cobalt atom, so chemically it can form organometallic bonds with the patient's body (for example, for the synthesis of Vitamin B12). The body cannot distinguish between a neutral isotope (Co-59) and a radioactive one (Co-60) because both have the same number of protons (27), so they act identically with respect to their chemical properties (the number of electrons is conserved). The biological system uses both isotopes in the same way; however, due to the instability of Co-60, it decays into Ni-60 (a nickel atom), releasing two high-energy gamma rays that penetrate and destroy cancer cells. The process is uncontrollable and destroys both diseased and healthy tissue equally, which is why it is so aggressive. Once the atom has transmuted, that is, it has changed from a cobalt atom to a nickel atom, it is naturally excreted by the body because nickel does not form organometallic compounds essential for human life, at least, as far as is known. Thanks to this chemical element, my mother overcame a very aggressive skin cancer (melanoma). As an amateur scientist and a lover of chemistry, and therefore of all metals, cobalt holds a special place in my heart. It is one of my favorite metals.

Cobalt plays a crucial role as a trace element, being indispensable for life. At the center of Vitamin B12, known as cobalamin, this metal acts as a nucleus in a complex organometallic molecule, similar to hemoglobin. In both cases, a metallic atom—cobalt in cobalamin and iron in hemoglobin—is surrounded by a chain of carbon, hydrogen, oxygen, and nitrogen, essential for the functioning of the human body. The relevance of cobalt is not limited to this vitamin, as it participates in the formation of other vital organic compounds. However, both excess and deficiency of cobalt can cause health problems. Cobalt poisoning, though rare, occurs from excessive exposure, while deficiency, associated with diets poor in nutrients containing the metal, can lead to equally serious disorders.

In the market, cobalt is positioned as one of the most expensive base metals, surpassing iron and copper, and fluctuating in price against nickel depending on demand. Although historically cheaper than titanium, titanium prices have decreased, sometimes equating with cobalt. Its commercialization is closely linked to nickel-rich regions, as both metals are often found associated in nature. In emerging powers like China, Saudi Arabia, India, and Malaysia, cobalt has gained relevance due to its use in superalloys for the aeronautical industry, where aircraft manufacturers consume large quantities for components requiring high-temperature and corrosion resistance. Although many countries have cobalt mines, few base their economy on this resource, which, like all metals, is non-renewable, underscoring the importance of its sustainable management.