ALUMINIUM

Name: Aluminum (from "Alum")

Symbol: Al

Group: 13

Period: 3

Block: p

Category: Post-transition metals

Atomic number: 13

Atomic mass: 26.98 u

Electrons per shell: 2, 8, 13

Electronegativity: 1.61

Density: 2.7 g/cc

Melting point: 660.32ºC

Boiling point: 2519ºC

Thermal conductivity: 235 W (m·K)

Electrical conductivity: 3.8 × 10^7 S/m

Magnetic order: Paramagnetic

Ordinary state: Solid

Oxidation states: +3

Mohs hardness: 2.75

Vickers Hardness: 167 MPa

Brinell Hardness: No data

Most stable isotopes: Al-27 (100% - monoisotopic element)

Discoverer: Friedrich Wöhler, German (1825)

Aluminum (Al), a post-transition metal with atomic number 13, is the most abundant metal in the Earth's crust. It is formed in stars capable of burning silicon, where it is an intermediate product. The theoretical process implies that Mg-26 nuclei are bombarded with a proton (H1), resulting in the stable isotope Al-27.

Despite its abundance, the history of its isolation and large-scale production is relatively recent. It is extracted primarily from bauxite, a rock composed of aluminum hydroxides such as gibbsite (Al(OH)3), boehmite (AlO(OH)), and diaspore (AlO(OH)). Discovering the pure metal was a significant challenge due to its enormous affinity for oxygen.

Contrary to popular belief, aluminum is not the lightest metal; that title belongs to magnesium or even lithium if the latter is considered a "metal" in the structural sense, which is not always the case. The confusion stems from the industrial and technological fame of aluminum compared to magnesium, better known for its role as a biological nutrient than as a structural metal. Both share low density, ease of generating passive oxide layers, and relatively low melting points, but they differ in that magnesium plays a key role in organic chemistry and living organisms, while aluminum has little biological relevance.

A lithophile element par excellence, aluminum always appears combined in more or less complex oxides and shows affinity for alkaline and alkaline earth metals, although not as much for carbon, sulfur, halogens, or refractory metals. It is an essential part of the structural rocks of the Earth's crust and is present in more than 250 known minerals, mostly silicates. However, in most of these minerals, its concentration is not high enough to justify its extraction, so, despite its actual abundance, a large part of its atoms are dispersed in the rock matrix. The mineral from which it is predominantly obtained is bauxite, a rock composed primarily of three minerals: boehmite and diaspore, both with formula AlO(OH)₃ but with distinct crystalline structures, and gibbsite Al(OH)₃. Although these hydroxides can occur independently, their combination forms the most important aluminum ore. Interestingly, precious stones such as sapphire and ruby ​​are crystalline forms of corundum, Al₂O₃, i.e., hydrogen-free aluminum trioxide, although they are not considered commercial ores.

Its extraction is complex due to its enormous affinity for oxygen, which explains why its discovery came relatively late. The partial isolation of aluminum was first achieved in 1824 by Hans Christian Ørsted, although his samples were impure, being an alloy rich in potassium. A year later, in 1825, Friedrich Wöhler reproduced the experiment and obtained small, purer samples, sufficient for study. However, industrial-scale production would not arrive until decades later, when Henri Deville presented the metal in 1854 and began production in 1856, although it was still expensive.

The real revolution came in 1886, when Frenchman Paul Héroult and American Charles Hall developed and patented the Hall-Héroult process, which remains the basis for today's production. Shortly after, in 1889, Austrian Carl Bayer patented the Bayer process, which allows high-purity alumina to be obtained from bauxite. Both processes are complementary: the Bayer method refines bauxite to obtain Al₂O₃, and the Hall-Héroult process electrolytically reduces it to produce the metal.

From then on, aluminum ceased to be an exotic material, reserved for prestigious applications, and became a massive industrial resource. Its price fell dramatically, and its properties—low density, good mechanical strength in alloys, and corrosion resistance—made it the second most widely used metal in the world, after steel. Although pure, it lacks the rigidity to compete with the latter, it surpasses steel in lightness and natural resistance to oxidation. Today, it is also the second cheapest metal, after iron.

High-purity aluminum stands out for its silvery-white appearance, similar to that of uncorroded silver, with a high reflectivity index that makes it visually attractive. With a density of 2.70 g/cm³ in its purest form, it is one of the lightest metals used in industry, combining this lightness with excellent mechanical properties. Malleable and ductile, aluminum resists wear from mechanical stress under moderate conditions, with or without exposure to heat or moderately aggressive atmospheres. It can be hammer-worked, and some of its alloys can be hardened through traditional heat treatments, similar to those applied to steel with a minimum carbon content of 0.45% by mass, which expands its usefulness in structural applications.

Unlike other metals in its group, aluminum is an exceptional conductor of heat and electricity. In terms of weight, it is superior to copper and silver as an alternative for wire fencing, as one kilogram of aluminum occupies a greater volume, allowing for the manufacture of longer or thicker cables, which compensates for its lower intrinsic conductivity compared to these metals. However, its hardness is relatively low, at 2.75 on the Mohs scale, and its toughness is limited, being more malleable than ductile. Although it is possible to produce aluminum wires of reduced thickness, its mechanical properties are significantly improved by alloying it with specific elements, often metals, which enhance its strength and versatility for industrial and technological applications.

Aluminum is a metal widely valued in metallurgy for its remarkable ability to resist corrosion, thanks to the formation of a passive oxide layer that protects it from subsequent chemical and environmental attacks. This layer, chemically identical to massive alumina (Al₂O₃), is generated naturally when aluminum comes into contact with ambient oxygen. This aluminum trioxide acts as a protective barrier, giving the metal outstanding resistance under normal conditions, both in dry and humid air. Unlike metals such as magnesium, which have limitations in aqueous environments, aluminum maintains good stability in fresh and salt water. However, its resistance is compromised in the presence of chlorides, such as those found in seawater, where it can experience localized corrosion, especially in the form of pitting. Compared to alloys specifically designed for marine environments, such as copper, nickel-copper, superalloys, or highly alloyed stainless steels, aluminum offers adequate, but not exceptional, resistance, and is therefore generally used in applications where extreme corrosion protection is not required.

The passive aluminum oxide layer relies on the presence of oxygen to regenerate in the event of damage or wear. For this reason, the use of aluminum in components designed for prolonged submersion, such as offshore oil platform pillars, is uncommon. Although aluminum surpasses carbon steel in terms of corrosion tolerance in aqueous environments, its mechanical properties are inferior, making it less suitable for highly demanding structural applications. In these cases, materials such as stainless steels or nickel alloys are preferable, as they combine greater mechanical strength with superior corrosion protection.

In its pure state, aluminum exhibits moderate resistance to acids, but is particularly vulnerable to alkalis, where it suffers significant degradation. Fortunately, most organic substances do not affect aluminum, making it an ideal material for applications in the food industry. Common examples include beverage cans such as soda or beer, the well-known "aluminum foil" (actually thin sheets of hammered aluminum), the inner lining of thermoses for preserving hot beverages such as coffee or tea, as well as inexpensive plates, trays, and cutlery. Furthermore, aluminum is a non-toxic, non-hazardous material and generally does not cause allergic reactions. Although rare cases of metal allergies do occur, these are extremely rare, and aluminum is considered safe for the vast majority of users. In terms of chemical resistance, aluminum shares certain similarities with tin, exhibiting comparable behavior in specific chemical environments.

A critical aspect to consider is that aluminum, like other metals such as titanium, chromium, and magnesium, loses some of its corrosion resistance when alloyed. The purity of aluminum is critical to the formation and stability of its protective oxide layer. When alloying elements are introduced to improve mechanical properties, such as strength or ductility, some of this purity is sacrificed, which can reduce the material's ability to withstand corrosive environments. For example, aluminum alloys designed for structural applications are often less corrosion resistant than pure aluminum, requiring careful analysis when selecting the appropriate material for each application.

Aluminum, a lightweight and versatile metal, stands out as one of the most widely used materials in industry, surpassed only by steel. Unlike iron, its main competitor, aluminum finds applications both in its pure form (with purities of 99.9% or higher) and in alloys, making it a valuable resource for a wide range of sectors. Its combination of lightness, corrosion resistance, and malleability makes it ideal for applications where weight, durability, and aesthetics are key factors. However, the mechanical properties of pure aluminum, such as its stiffness and hardness, are limited, which directs its use toward applications that take advantage of its natural resistance to corrosion thanks to the immediate formation of a protective layer of aluminum oxide (Al₂O₃). On the other hand, aluminum alloys, designed to improve mechanical strength, expand its range of applications, although often at the cost of lower corrosion resistance. The following explores the applications of pure aluminum and its alloys, highlighting their importance in various industrial sectors. In its pure form, aluminum is not known for its resistance to mechanical stress or abrasive wear, as it is a relatively soft and brittle material. However, its ductility and, above all, its high malleability make it an ideal candidate for applications where these properties are essential. The passive oxide layer that naturally forms on its surface gives it superior corrosion resistance compared to many alloys, making it especially suitable for environments where exposure to corrosive agents is a concern. It is important to note that in metallurgy, material design involves a trade-off between properties: improving one aspect, such as mechanical strength, often means sacrificing another, such as corrosion resistance. Only so-called superalloys, such as those based on nickel, manage to combine exceptional performance across multiple parameters, but their high cost limits their use to specialized applications.

One of the most notable uses of pure aluminum is in the food industry. Thanks to its non-toxicity and resistance to corrosion by organic substances, it is widely used in the manufacture of containers, cans, packaging, and coatings. Everyday examples include beverage cans, such as soda and beer, as well as aluminum foil, which is used to wrap food. It is also found inside thermoses to maintain the temperature of hot drinks, such as coffee or tea, and in inexpensive kitchenware, such as trays and plates. This versatility is due to the fact that aluminum does not react with most organic compounds, ensuring food safety. Another area where pure aluminum shines is in applications that take advantage of its high reflectivity. Although the highest-quality mirrors are made from silver, aluminum is used in common mirrors and light-sensitive optical components, such as telescopes. Its ability to reflect visible light and other wavelengths makes it a valuable material for scientific instruments and industrial applications where reflectivity is crucial.

In the electrical industry, pure aluminum plays a fundamental role, especially in the manufacture of conductive cables. Although its electrical conductivity is approximately 60% that of copper and even lower than that of silver, its lower density and cost make it an attractive alternative. With the rising price of copper and the growing demand for conductive materials in emerging economies such as China, India, Brazil, and the Middle East, aluminum has gained ground as a substitute in electrical applications. In the electronics field, techniques are being explored to coat aluminum with copper, improving its conductivity without losing the advantages of its lightness and low cost, although the results still do not match the performance of pure copper. This approach reflects the efforts of modern engineering to develop economical solutions in the face of the rising costs of traditional materials.

In the chemical industry, high-purity aluminum is used as a catalyst in reactions where it acts as an anode, sometimes alloyed with zinc to optimize its performance. This application takes advantage of aluminum's electrochemical properties, which make it suitable for specific processes where a strong, reactive material is required under controlled conditions.

Aluminum (Al) was considered a precious metal during the 19th century, a status that made it more valuable even than gold and silver. The most famous anecdote from this period features the French emperor Napoleon III.

At the 1855 Paris World's Fair, aluminum was first presented to the public, dazzling everyone with its lightness and brilliance. At the time, the process for producing it, devised by Henri Sainte-Claire Deville, was extremely expensive. Impressed by its unique properties, Napoleon III had a complete aluminum dinner service made for his personal use and to entertain his most distinguished guests at banquets. While ordinary diners had to settle for gold and silver cutlery and plates, the emperor's honored guests ate on aluminum tableware, demonstrating the exclusivity and high value of the metal at the time. This anecdote reflects how aluminum went from being a luxurious and exotic commodity to becoming, in just a few decades, a common industrial material, thanks to the invention of more efficient production methods such as the Hall-Héroult process and the Bayer method.