Name: Molybdenum

Symbol: Mo

Group: 6

Period: 5

Block: d

Category: Transition metals

Atomic number: 42

Atomic mass: 95.95 u

Electrons per shell: 2, 8, 18, 13, 1

Electronegativity: 2.16

Density: 10.28 g/cc

Melting point: 2623 ºC

Boiling point: 4639 ºC

Thermal conductivity: 139 W (m·K)

Electrical conductivity: 2 × 10^7 S/m

Magnetic order: Paramagnetic

Ordinary state: Solid

Oxidation states: -4, -2, -1, 0, 1, 2, 3, 4, 5, 6

Mohs hardness: 5.5

Vickers hardness: 1530 MPa

Brinell hardness: 1500 MPa

Most stable isotopes: Mo-92 (14.65%), Mo-94 (9.19%), Mo-95 (15.87%), Mo-96 (16.67%), Mo-97 (9.58%), Mo-98 (24.29%), and Mo-100 (9.74%)

Discoverer: Petter Hjelm (1781)

Molybdenum is named after the mineral from which it was first mined, molybdenite. This mineral, a molybdenum sulfide (MoS₂) mineral, is the mineral from which the transition metal was first mined. Dark and lustrous, molybdenite bears a striking resemblance to galena (lead sulfide) and pure graphite, which led to significant historical confusion. In English, the term "lead" is used for both lead and, colloquially, for the graphite used in pencils, even though graphite (a form of carbon) and lead are chemically unrelated except for belonging to the same group on the periodic table, with carbon at the top of the family that includes lead. This visual similarity between molybdenite, galena, and graphite fueled the mistaken belief that molybdenite contained lead or was a variant of graphite. The name "molybdenum" derives from the Greek term "molybdos," meaning "lead," reflecting this initial confusion, as the ancients associated the mineral with lead due to its appearance.

In 1754, Swedish chemist Bengt Qvist investigated molybdenite in an attempt to extract lead, but concluded that the mineral did not contain the element, marking a first step toward clarifying its nature. Years later, in 1778, the renowned German-Swedish chemist Carl Wilhelm Scheele further studied molybdenite and determined that it not only lacked lead, but was also not a form of graphite, as had been speculated. Scheele proposed that the mineral contained an entirely new chemical element, previously unknown to science. In 1781, his compatriot Peter Jacob Hjelm managed to isolate the pure metal through a reduction process, confirming the existence of molybdenum as a distinct element. Although the name "molybdenum" has an erroneous etymological origin, linked to ancient confusion with lead, and is a difficult term to pronounce, it remained the definitive name for the element, a testament to its fascinating history of discovery.

This identification process not only clarified the nature of molybdenum but also highlighted its importance in modern chemistry and metallurgy. Molybdenite, in addition to being the main source of molybdenum, remains valued in industry, not only for the metal itself but also for its applications as a high-temperature lubricant thanks to molybdenum disulfide, which rivals graphite in low-friction properties. The history of molybdenum, from its confusion with other minerals to its recognition as a key element in stainless steels, superalloys, and catalysts, reflects the advancement of chemical science in the 18th century and its impact on current technologies, cementing molybdenum as an indispensable material in modern industry.

It is a dark gray transition metal with a moderate luster. It stands out in metallurgy for its exceptional combination of hardness, with a value of 5.5 on the Mohs scale, and ductility that distinguishes it from other elements in its group, such as chromium and tungsten. This metal exhibits remarkable corrosion resistance, a property that makes it an essential component in the manufacture of stainless steels and high-performance superalloys. Even at concentrations as low as 2%, molybdenum acts as a powerful anticorrosive agent, protecting the steel from aggressive environments, such as those exposed to chemicals or salt water. In the highest quality stainless steels, its content can increase to 8%, while in advanced superalloys, especially those based on nickel or cobalt for aeronautical applications and high-temperature turbines, molybdenum can represent between 20% and 22% of the composition, providing outstanding mechanical and thermal resistance. Molybdenum plays two main roles in the metallurgical industry. First, its ability to form stable carbides makes it ideal for increasing hardness and wear resistance in maraging steels and high-speed steels, widely used in the manufacture of cutting tools, drill bits, and industrial components subjected to high mechanical stress. Second, its role as an alloying element significantly improves corrosion resistance and toughness in stainless steels, making them essential in sectors such as construction, the chemical industry, and marine engineering. Molybdenum is highly compatible with metals such as nickel, cobalt, and titanium, integrating perfectly into superalloys that withstand extreme conditions, such as those found in aircraft engines or nuclear reactors. However, it does not form alloys with p-block metals or those in the copper family, such as copper, silver, or gold, due to differences in their chemical structures. In the case of titanium, although fusion is possible, molybdenum is primarily used as a hardener, improving mechanical properties without compromising the material's lightness.

Despite its relative scarcity in the Earth's crust and its high cost, molybdenum is an indispensable material in modern metallurgy due to its versatility and unique properties. Its use is not limited to steels and superalloys; it also finds applications in chemical catalysts, such as in fertilizer production, and in high-temperature lubricants, where compounds such as molybdenum disulfide offer exceptional performance. The combination of these characteristics makes molybdenum a strategic resource in industries that require materials with high durability, strength, and stability, consolidating it as a fundamental pillar in advanced materials engineering.

It exhibits distinctive chemical behavior that makes it essential in advanced metallurgy, especially in the manufacture of stainless steels and superalloys. Unlike chromium, molybdenum is vulnerable to oxidizing acids, such as nitric acid, which vigorously attack its structure, compromising its integrity and forming oxides or soluble compounds that dissolve the metal. This sensitivity to strong oxidants, such as concentrated nitric acid, contrasts with its extraordinary resistance to reducing acids, such as hydrochloric acid (HCl) or solutions containing free chlorine ions. This property makes molybdenum a key component in protecting stainless steels against corrosion in aggressive environments, such as those found in the chemical, petrochemical, and marine industries, where exposure to chlorides and sulfur compounds is common. 

Molybdenum's contribution to pitting corrosion resistance is particularly notable, as its effect on the PREN (Pitting Resistance Equivalent Number) index is 3.3 times greater than that of chromium, making it a critical ally in improving the durability of steels under extreme conditions. This characteristic marks the difference between standard stainless steels, such as AISI 304, and higher-strength steels, such as AISI 316, which incorporates between 2% and 3% molybdenum in its composition. The presence of molybdenum in AISI 316, although slightly increasing its cost, passivates the steel against corrosive attack from seawater and other saline environments, protecting it from the formation of chloride-induced pitting and cracking. Furthermore, molybdenum improves the steel's resistance to sulfur compounds, such as sulfuric acid or sulfurous gases, making it ideal for applications in refineries, chemical processing plants, and industrial environments exposed to corrosive agents. 

Beyond its role in stainless steels, molybdenum is valued in superalloys for high-temperature applications, such as aircraft turbines and nuclear reactors, where its chemical and mechanical stability under extreme conditions is crucial. Its ability to resist corrosion in reducing media, combined with its ability to form carbides that increase hardness, makes it a versatile material in materials engineering. Although its susceptibility to oxidizing acids requires careful design in certain applications, molybdenum remains an indispensable element in modern industry, providing a unique combination of strength, durability, and corrosion protection that few metals can match.

Molybdenum is a key element in modern metallurgy due to its extraordinary corrosion resistance, especially against chlorides and chlorine compounds, as well as its ability to increase hardness and mechanical strength when alloyed with steel. This combination of properties makes it an essential component in the manufacture of stainless steels and superalloys designed for demanding environments, such as the marine, chemical, and petrochemical industries. In stainless steels, molybdenum is incorporated in varying proportions depending on the steel family and the specific purpose of the alloy. 

In ferritic and martensitic steels, it is added in small amounts, typically between 0.5% and 2%, to improve edge retention and hardness—critical properties in cutting tools, blades, and industrial components subject to intense wear. This additional hardness, which would not be achieved with chromium alone, results from the formation of molybdenum carbides, which strengthen the steel matrix and improve its resistance to mechanical wear. In austenitic and duplex steels, molybdenum plays a primarily anticorrosive role, significantly increasing resistance to pitting and crevice corrosion in aggressive environments, such as seawater or chloride-rich solutions. Surprisingly, a molybdenum content as low as 2% can radically transform an alloy's properties, increasing its PREN (Pitting Resistance Equivalent Number) index up to 3.3 times that of chromium, ensuring greater protection against localized corrosion. 

For example, the difference between AISI 304 and AISI 316 stainless steel lies in the addition of 2% to 3% molybdenum in the latter, making it ideal for applications in marine environments, desalination plants, and chemical processing equipment where exposure to chlorides is constant. In addition to its contribution to corrosion resistance, molybdenum improves the performance of steels under high-temperature conditions, a crucial attribute in applications such as aviation turbines, nuclear reactors, and industrial furnaces. Unlike other metals, molybdenum allows steels to maintain their mechanical strength and anti-corrosion properties even under extreme temperatures, thanks to the formation of a surface oxide layer that is more stable and protective than that generated by chromium. 

This oxide layer, composed primarily of molybdenum oxides, acts as a robust barrier against corrosive agents, including sulfur compounds and aggressive gases, extending the life of components in harsh environments. Molybdenum's versatility also extends to its use in nickel and cobalt superalloys, where its ability to stabilize the metal's crystalline structure at high temperatures makes it indispensable in the aerospace and energy industries. In short, molybdenum not only strengthens stainless steels but also expands their applicability in sectors where durability, heat resistance, and corrosion protection are critical.

Molybdenum is rarely used alone in steel alloys without the presence of chromium, except in specific cases such as high-speed steels and tool steels, where its ability to form carbides significantly improves hardness and wear resistance. In most cases, molybdenum acts as an ideal complement to chromium, enhancing the alloys' anti-corrosion properties, especially in aggressive environments dominated by halogens such as chlorine. This synergy between molybdenum and chromium is essential in stainless steels designed to operate in extreme conditions, such as seawater, chlorinated swimming pool water, or industrial environments where chloride-rich solutions are handled, such as desalination plants, petrochemical facilities, and chemical processing systems. The addition of molybdenum, even in proportions as low as 2% to 3%, provides superior resistance to chloride-induced pitting and crevice corrosion, a critical advantage that chromium alone cannot offer under such conditions.

Molybdenum contributes to the formation of a surface oxide layer on stainless steel alloys, which coexists with the chromium oxide layer and is noticeably more stable, particularly against chlorides. While the chromium oxide layer, composed primarily of Cr₂O₃, offers general corrosion protection in many environments, its effectiveness decreases in the presence of chloride ions, which can penetrate it and cause localized corrosion. In contrast, the molybdenum oxide layer, composed of oxides such as MoO₂ and MoO₃, acts as a more robust and resistant barrier to these corrosive agents, ensuring greater material durability. This superior stability allows steels such as AISI 316, which contains molybdenum, to outperform AISI 304 in marine applications and environments with high chloride concentrations, such as swimming pools, coastal pipeline systems, or equipment exposed to brine. Furthermore, the combination of molybdenum and chromium improves the steel's resistance to other corrosive compounds, such as sulfur derivatives, expanding its applicability in industries where materials must withstand extreme chemical and thermal conditions.

The complementarity of molybdenum with chromium not only optimizes corrosion protection but also strengthens the alloys' mechanical properties, such as high-temperature strength and toughness, making them ideal for applications in turbines, nuclear reactors, and aerospace components. In high-speed steels, molybdenum is used without much dependence on chromium, as its ability to form hard carbides improves the performance of cutting and drilling tools subject to intense wear. This versatility makes molybdenum an indispensable element in materials engineering, where its combination with chromium allows for the development of stainless steels and superalloys capable of withstanding the most hostile environments, from the ocean floor to the extreme conditions of the energy industry.

Molybdenum is a carbide former and has a stronger affinity for carbon than iron. It is superior to chromium in this respect, because molybdenum carbides are stable at higher temperatures, tougher, and more resistant to corrosion. It is found in large quantities in so-called high-speed steels (HSS) as a substitute for (or in association with) tungsten. In martensitic stainless steels such as AISI 440C, the amount is close to 0.6% by mass. These steels are very hard, tough, and corrosion-resistant.

Although it can be found in martensitic and ferritic steels, the metal is used in these cases for its increased hardness, rather than for its corrosion resistance. Molybdenum is added to austenitic, duplex, and super-duplex stainless steels primarily for its added value in resisting pitting corrosion caused by chlorine ions and their compounds. 316L steel, the only one used in jewelry, typically contains 2% molybdenum, a sufficient amount to protect the alloy from the corrosive effects caused by halogens (particularly chlorine). Other grades, also used in jewelry but less well-known, such as AISI 904L, contain between 4 and 5% molybdenum by mass.

Molybdenum is a powerful alpha-array agent and promotes the formation of ferrite in stainless steels. Therefore, the amounts of nickel required to fix austenite often reach 20% or 30% in the case of superaustenitic steels (e.g., AISI 904L and 254SMO, respectively) if the chromium content is also high (>20%). See the Schaeffler diagram for more information.