Aluminum

Chemical and physical properties of aluminum

A non-noble, silvery-gray, unstable, malleable metal, electrically highly conductive. At temperatures below 1.18 K, it is superconductive. In nature, it predominantly occurs in compound form, with the most well-known ore being bauxite Al2O3 . 2 H2O (aluminum oxide dihydrate).

It forms compounds in oxidation states +I to +III, with the most common and stable being aluminum compounds. In acidic environments, it forms an aluminum cation in aqueous solutions, whereas in alkaline environments, it forms an aluminate anion [AlO2]–. In its pure state, aluminum is highly reactive, quickly covering itself with a thin layer of Al2O3 oxide in air, protecting the metal from further oxidation.

Aluminum is very soluble in dilute acids. Concentrated nitric or sulfuric acid, however, like atmospheric oxygen, covers it with a passive layer of oxide. Alkali metal hydroxides also easily dissolve aluminum metal, forming aluminates (AlO2)–.

Aluminum and aluminum alloys are highly weldable by almost all welding methods, with the exception of the alloy duralumin, which is difficult to weld.

Aluminum was isolated in metallic form in 1825 by Danish physicist Hans Christian Ørsted.

Aluminum and its occurrence in nature

Due to the high reactivity of aluminum, we practically only encounter its compounds in nature.

Aluminum is the third most abundant element in the Earth's crust. According to the latest available data, aluminum constitutes 7.5–8.3% of the Earth's crust. Its concentration in seawater is very low, only 0.01 mg Al/l, and in the universe, there are approximately half a million hydrogen atoms for every aluminum atom.

The most common aluminum-based rock is bauxite, Al2O3 · 2 H2O. It is usually accompanied by other impurities based on silicon, titanium, iron oxides, and others.

Another significant mineral is cryolite, sodium hexafluoroaluminate Na3AlF6, primarily used as a flux to lower the melting temperature of aluminum oxide in the electrolytic production of aluminum.

Minerals based on aluminum oxide Al2O3 are very significant and valuable. Corundum is ranked at 9 on the Mohs hardness scale. Technical aluminum oxide is also called aloxite and is widely used in the production of sandpaper.

Precious stones whose basic material is aluminum oxide differ by impurities causing their characteristic color. The red ruby is colored by chromium oxide impurities, while blue sapphire contains mainly trace amounts of titanium and iron oxides.

Both mentioned forms of corundum are among the most valuable gemstones in the world, but they also have significant technical applications. Sapphire tips excel in hardness and durability and are used in high-end scientific measuring instruments. Ruby is known as the material used to construct the world’s first laser.

Manufacture of aluminum

Although aluminum is one of the most abundant elements in the Earth's crust, its industrial production was, until fairly recently, a highly challenging process. This is mainly because elemental aluminum cannot be easily metallurgically extracted from its ore like iron from coke in a blast furnace. Only the mastery of industrial electrolysis of molten metal ores allowed for the current annual production of hundreds of thousands of tons of pure aluminum.
During electrolysis, from a molten mixture of pre-purified bauxite and cryolite at about 950 °C, elemental aluminum is deposited at the cathode, and oxygen is formed at the graphite anode, which immediately reacts with the electrode material, producing the toxic gaseous carbon monoxide, CO.
In the former Czechoslovakia, the manufacture of aluminum sheets began in 1933, and later in 1954, the production of consumer goods from aluminum foils also started in Břidličná. The company continues production. Furthermore, aluminum production began in 1953 in Slovak Žiar nad Hronom, with most bauxite imported from Hungary. Production using the Söderberg technology was ceased here in 1998.

Aluminum alloys

However, the most important application of aluminum is in alloy form, the most well-known without a doubt being the alloy with magnesium, copper, and manganese, known as duralumin. This material has significantly greater strength and hardness compared to pure aluminum while maintaining a very low density. At the same time, they are also highly resistant to corrosion. All these properties make duralumin an ideal material for the aviation and automotive industries, but it is also used in the production of elevators, bicycles, lightweight ladders, and similar applications.

Very well-known aluminum alloys are the 6061 and 7005 categories. But what's the difference between them? The 7005 series alloy (composed of 93.35% Al, 0.13% Cr, 1.4% Mg, 0.45% Mn, 0.03% Ti, 4.5% Zn, 0.14% Zr) is often replaced by the Alloy 6061, which is composed of 98.1% Al, 0.020% Cr, 1% Mg, 0.28% Mn, 0.6% Si. The material Al 6061 has slightly lower mechanical properties but is easier to weld because, unlike 7005, it does not contain zinc, which can cause cracks during welding. Compared to, for example, cycling frames made from the 7005 material, which can harden on their own, Al 6061 frames must always be heat-treated. The disadvantage of the 6061 alloy is the fact that they have twenty percent lower strength than the 7005 series. On the other hand, the 6061 material has one advantage: due to its greater softness, it is easier to shape, and at the same tube thickness compared to 7005, it will certainly be lighter.