RHENIUM

Name: Rhenium (in allusion to the German Rhine River)

Symbol: Re

Group: 7

Period: 6

Block: d

Category: Transition metals

Atomic number: 75

Atomic mass: 186.207 u

Electrons per shell: 2, 8, 18, 32, 13, 2

Electronegativity: 1.9

Density: 21.02 g/cc

Melting point: 3186ºC

Boiling point: 5596ºC

Thermal conductivity: 48 W (m·K)

Electrical conductivity: 5.6 × 10^6 S/m

Magnetic order: Paramagnetic

Ordinary state: Solid

Oxidation states: +4

Mohs Hardness: 7

Vickers Hardness: 2450 MPa

Brinell Hardness: 1320 MPa

Most stable isotopes: Re-185 (37.4%), Re-187 (62.6%)

Discoverer: Ida and Walter Noddack, couple, collaboration with Otto Berg, Germans (1925)

Rhenium is a steel-gray metal that, when polished, exhibits an intense metallic luster, although it tends to darken over time due to surface oxidation. Similar to tungsten in appearance, it surpasses it in ductility and malleability, although it is slightly heavier and less hard. With a density that positions it as the fourth heaviest metal after osmium, iridium, and platinum, rhenium is also one of the scarcest elements in the Earth's crust, with an abundance of barely one part per billion. This rarity makes it the scarce and most expensive non-precious metal in the world, exceeding in price, gram for gram, metals like pure silver.

Rhenium is primarily obtained as a byproduct of mining minerals such as iron, nickel, copper, and especially molybdenum, with which it shares a strong chemical affinity. Although it can be found in its native state, this occurs under exceptional geological conditions. Classified as a refractory metal, it possesses the second highest melting point among metals (3,180 °C, only surpassed by tungsten) and the highest boiling point of all elements (5,596 °C). These properties make it ideal for high-temperature environments. Commercially, it is presented in the form of fine powder or sintered bars, although its use remains little known outside specialized industrial circles.

In industry, rhenium stands out for its incorporation into nickel and cobalt superalloys, essential for high-performance applications in aeronautics, such as aircraft turbines, rocket engines, and missile components. Its resistance to extreme temperatures and corrosion makes it a critical material for military parts, gas turbines, and high-performance engines used by agencies like NASA or in defense projects of global powers. Furthermore, rhenium is used in high-strength filaments, electrical contacts, and thermocouples, where its durability is key. Although considered semi-noble or semi-precious, it is not used in jewelry, unlike other metals like platinum.

As a refractory and siderophile metal, rhenium shows a notable affinity for ferrous metals such as iron, cobalt, and nickel, dissolving easily in them when they are in a liquid state. It forms alloys with other refractory metals, such as molybdenum and niobium, as well as with those of the platinum group, but not with copper, silver, gold, or the zinc group metals. Curiously, mercury can affect it, although it does not form stable alloys with it or with p-block metals. Despite its proximity to osmium in the periodic table and its similarities to platinum group metals, rhenium does not belong to this group, although some experts classify it as a noble metal due to its corrosion resistance. It is non-toxic, which expands its versatility in industrial applications.

Rhenium does not form carbides, but it does form stable compounds with boron, such as borides, which reinforce its usefulness in high-strength applications. Its high cost, driven by its scarcity and the complexity of its extraction, limits its use to cutting-edge sectors, but its unique combination of properties makes it irreplaceable in critical applications where performance under extreme conditions is essential.

Rhenium, a refractory metal from period 6 of the periodic table, exhibits extraordinary corrosion resistance, a characteristic expected in elements of its group. This property is due to the formation of a protective surface oxide layer, known as passivation, which acts as a barrier against the most common corrosive agents, including acids and bases. In environments such as water (whether fresh or salt), dry or humid air, rhenium remains chemically inert, showing remarkable stability. It does not form sulfates or react with organic acids, hydrochloric acid (regardless of its concentration), or seawater, making it ideal for applications in aggressive environments, such as the marine or chemical industry.

However, certain chemical agents can affect rhenium under specific conditions. Oxidizing agents slowly corrode it, while in the presence of halogens or concentrated sulfuric acid, rhenium reacts at elevated temperatures. Nitric acid, especially in high concentrations, causes a rapid reaction, dissolving the metal more easily. Surprisingly, rhenium is insoluble in aqua regia, a mixture of acids capable of dissolving noble metals such as gold or platinum, highlighting its unique chemical resistance. When finely divided, such as in powder form, rhenium can react violently with the aforementioned agents, increasing its reactivity due to the larger exposed surface area.

At elevated temperatures, the stability of rhenium is compromised. Above 400 °C, the passive oxide layer decomposes, leading to the formation of rhenium tetraoxide (Re₂O₇), a highly volatile compound that evaporates rapidly. This reaction means that rhenium cannot be melted in the presence of oxygen, as violent oxidation would interfere with the process. Likewise, welding pure rhenium presents challenges due to its affinity for oxygen, requiring controlled atmospheres, such as vacuum or inert gases, to prevent the formation of undesirable oxides. These properties make it a demanding but invaluable material in applications where corrosion resistance and extreme conditions are critical, such as in the manufacture of components for turbines, chemical reactors, or aerospace equipment.

Rhenium, a refractory metal with exceptional properties, finds its main application in the aeronautical and aerospace industry, where it is predominantly used in the form of alloys. In its pure state, rhenium is remarkably hard, but when integrated into alloys, its primary purpose is to enhance corrosion resistance and improve mechanical toughness. In this sense, it shares similarities with molybdenum, although it is distinguished by not forming carbides, which makes it unique in applications where this characteristic is crucial. Its ability to withstand extreme conditions, such as high temperatures and corrosive environments, makes it a strategic material for high-performance components.

One of the most notable applications of rhenium is its combination with pure tungsten, where it acts as an agent that increases the ductility and malleability of the material. Up to 20% rhenium can be dissolved in the tungsten matrix without compromising the integrity of the alloy; however, exceeding this limit causes the separation of the elements, resulting in a more brittle alloy. These rhenium-tungsten alloys are commonly marketed as wires, which can be coiled for diverse applications. The electronics industry is one of the main consumers of these wires, utilizing them in components such as filaments, electrical contacts, and high-precision sensors, surpassed only by the aerospace industry, which uses these alloys in turbines, combustion chambers, and other elements subjected to extreme conditions.

In steel production, the use of rhenium is limited, as its benefits can be replicated with more abundant and economical metals, such as molybdenum or tungsten. However, in nickel and cobalt superalloys, rhenium plays a critical role, being incorporated in proportions of up to 6%. In these alloys, it acts as a hardener, preserving toughness even at extremely high temperatures and improving corrosion resistance. These superalloys, with complex chemical compositions and high cost, are essential in the manufacture of components for aircraft turbines, rocket engines, missiles, and other military and aerospace applications, where performance under rigorous conditions is indispensable. For example, turbine blades of fighter jets or space rockets depend on these superalloys to withstand temperatures close to 1,500 °C and corrosive environments.

Both pure rhenium and its alloys are mainly produced by powder metallurgy, a process that allows obtaining materials with homogeneous structures and optimized properties. This method is particularly suitable for rhenium due to its high melting point (3,180 °C) and its reactivity with oxygen, which makes conventional casting processes difficult. The versatility of rhenium in these applications, combined with its scarcity and high cost, positions it as an elite material, reserved for sectors where durability, thermal resistance, and chemical stability are essential to guarantee exceptional performance.

Tungsten (W) and rhenium (Re), refractory metals from period 6, are competitors in demanding industrial sectors such as aeronautics, aerospace, and electronics, due to their outstanding physical and chemical properties. Their combination of high density, hardness, and thermal resistance makes them ideal for applications where durability under extreme conditions is essential, although their differences determine their suitability for specific uses.

Both metals are remarkably dense, with rhenium (21.02 g/cm³) surpassing tungsten (19.25 g/cm³) by 8.42%. In hardness, tungsten reaches 7.5 on the Mohs scale, slightly above rhenium's 7, making it more scratch-resistant. Their melting points, among the highest of all metals, are 3,422 °C for tungsten and 3,186 °C for rhenium, while rhenium boasts the highest boiling point of all elements (5,596 °C). These thermal properties make them key materials for components exposed to extreme temperatures, such as aircraft turbines, high-strength filaments, crucibles for metal casting, and combustion chambers in space rockets.

From a mechanical point of view, rhenium stands out for its greater ductility, which facilitates its shaping compared to tungsten, which is more rigid, although both present comparable malleability. This advantage makes rhenium ideal for processes requiring plastic deformation, such as the manufacture of fine wires. Rhenium-tungsten alloys, with up to 20% rhenium dissolved in the tungsten matrix, combine tungsten's hardness with rhenium's workability. These alloys, marketed as coilable wires, are essential in the electronics industry for filaments, electrical contacts, and high-precision thermocouples, as well as in the aerospace industry for turbine components and rocket engines. However, if the rhenium content exceeds 20%, the elements separate, making the alloy more brittle.

Chemically, both metals exhibit notable corrosion resistance, but with particularities. Rhenium, thanks to a passivating oxide layer (Re₂O₇), better resists alkalis and aqua regia, a mixture of acids (HNO₃ + HCl) capable of dissolving noble metals such as gold or platinum. In contrast, tungsten shows greater stability against concentrated nitric acid (HNO₃) and sulfuric acid (H₂SO₄), although it reacts slowly with them. Neither can be melted in the presence of oxygen due to their high reactivity at elevated temperatures, requiring controlled atmospheres (vacuum or inert gases, such as argon) during processing. Regarding compounds, tungsten forms highly valued carbides, such as tungsten carbide (WC), known as Widia, widely used in cutting, drilling, and mining tools for its extreme hardness. Rhenium, for its part, does not form carbides, but produces borides, such as rhenium diboride (ReB₂), a material of exceptional hardness comparable to superabrasives, although its high cost and manufacturing difficulty limit it to specialized applications, such as coatings for high-strength tools or components in extreme environments.

Historically, rhenium was used in Nazi Germany during World War II as a substitute for tungsten, when the high demand for the latter drove up its price. However, rhenium loses mechanical strength above 500 °C, while tungsten and its alloys maintain their rigidity up to approximately 1,200 °C. This difference makes tungsten preferred in applications requiring stability at higher temperatures, such as industrial furnaces, melting crucibles, or aerospace engine components. On the other hand, rhenium shines in nickel (Ni) and cobalt (Co) superalloys, where it is incorporated in proportions of up to 6% to improve toughness and corrosion resistance at moderate temperatures. These superalloys, with complex chemical compositions and high cost, are fundamental in the manufacture of turbine blades for fighter jets, rocket engines, and missiles, withstanding temperatures close to 1,500 °C in corrosive environments.

The scarcity of rhenium, with an abundance of barely 1 ppb in the Earth's crust, makes it a significantly more expensive material than tungsten, limiting its use to high-value applications. While tungsten dominates in cutting tools and wear parts thanks to its carbides, rhenium is reserved for cutting-edge sectors, such as aeronautics and the space industry, where its ductility, corrosion resistance, and thermal stability are irreplaceable. Both metals are frequently produced by powder metallurgy, a process that guarantees homogeneous structures and optimizes their properties, especially in the case of rhenium, whose high reactivity with oxygen makes conventional casting methods difficult. The choice between tungsten and rhenium depends on the specific demands of each application, balancing cost, workability, and performance under extreme conditions.

The discovery of rhenium (Re), like that of tungsten, is shrouded in historical controversy regarding its authorship and date. The international scientific community officially attributes the discovery to the German married couple Walter and Ida Noddack, who identified it between 1922 and 1923, although the exact date remains in dispute. However, Japan claims that Masataka Ogawa discovered it in 1908, generating a debate that reflects tensions similar to those observed in the discoveries of tungsten and vanadium.

The core of the conflict lies in the definition of "discovery": should it be attributed to the one who first isolates an element or to the one who theorizes its existence, even if they fail to isolate it? In 1908, Ogawa was looking for the eka-manganese predicted by Dmitri Mendeleev, which would correspond to element 43 (technetium, Tc). Believing he had found it, he named it nipponium (from the Latin Nippon, for Japan, similar to francium for France). However, later spectral analyses of his samples, preserved since 1908, revealed that it was not element 43, but element 75, rhenium. Thus, Ogawa isolated rhenium in a relatively pure state without being aware of it, confusing it with another element due to the chemical similarities between rhenium and technetium, both belonging to Group 7 of the periodic table, headed by manganese (Mn). This chemical confusion is explained by the shared properties of these metals, such as their ability to form oxides and their behavior in compounds.

For their part, the Noddacks not only isolated rhenium but also correctly identified it as a new element, naming it in honor of the Rhine River (Rhenus in Latin), solidifying their official recognition. Their work included a detailed analysis of its chemical and physical properties, including its high density (21.02 g/cm³) and its melting point (3,186 °C), which positioned it as a refractory metal of great interest. Despite this, Ogawa's achievement, though fortuitous, raises a strong argument: he was the first to isolate the element, a significant technical milestone in the chemistry of the time.

This debate transcends simple attribution, as it reflects the challenges of early 20th-century chemistry, when analytical techniques, such as spectroscopy, were under development and the precise identification of new elements was complex. The scarcity of rhenium in the Earth's crust (1 ppb) further hindered its study, making its discovery, whether by Ogawa or the Noddacks, require significant advances in metallurgy and analytical chemistry. Today, rhenium is valued in cutting-edge applications, such as superalloys for aerospace turbines (with Ni or Co) and electronic filaments, but its discovery story remains a fascinating point of discussion in the scientific community.

The German married couple Ida Tacke and Walter Noddack marked a milestone in the history of chemistry by discovering rhenium (Re) in 1922 or 1923, on the banks of the Rhine River (Rhenus in Latin), from which the element takes its name. Unlike other controversial cases, the Noddacks not only theorized the existence of rhenium but also managed to isolate and identify it correctly, an achievement that grants them official recognition as its discoverers. This advance, supported by advanced analytical techniques for the time, such as X-ray spectroscopy, allowed them to characterize its physical and chemical properties, including its density (21.02 g/cm³), melting point (3,186 °C), and corrosion resistance, which positioned it as a refractory metal of great value.

The discovery of rhenium is considered a significant milestone, as it was the last chemically stable element discovered in the periodic table, just one year after hafnium (Hf) in 1922. The stability of rhenium, with 75 protons, distinguishes it from heavier elements that tend to nuclear instability, such as technetium (Tc). The Noddacks, aware of the element's uniqueness, named it in honor of the Rhine River, reflecting a cultural link to their homeland. Their work not only expanded the knowledge of refractory metals but also opened the door to cutting-edge industrial applications, such as its use in nickel (Ni) and cobalt (Co) superalloys for aerospace turbines, electronic filaments, and high-precision thermocouples, where its resistance to extreme temperatures (up to 1,500 °C in superalloys) and corrosion is crucial.

In addition to their contribution to the discovery of rhenium, the Noddacks are known for their pioneering research in nuclear chemistry. They proposed the possibility of nuclear fission in 1934, an idea initially controversial but which proved fundamental for later developments in nuclear physics. The scarcity of rhenium in the Earth's crust (1 ppb) and its high cost have limited its use to specialized sectors, but the Noddacks' legacy endures in modern metallurgy, where rhenium is an essential material for high-performance components in the aeronautical, electronic, and defense industries. Their meticulous scientific work consolidated rhenium as a key element in the periodic table and in technological industry.