- 3 December 2024
Aluminum has the ability to actively interact with oxygen. The resulting A1 2 O 3 aluminum oxide covers the product surface with a strong and dense film. Oxidation of aluminum at normal temperature after reaching the maximum film thickness practically ceases. The maximum thickness of the film after exposure of aluminum to air at 20 ° C is established after 7-14 days and reaches 5-10 nm.
Oxidation of aluminum can be explained by the good protective properties of the oxide layer. This is confirmed by the well-known principle that a dense film with protective properties is formed if the ratio of the volume of the oxide to the volume of the oxidized metal is greater than one, and for aluminum this ratio is 1.24, while for magnesium it is 0.79. Due to the poor protective properties of the magnesium oxide layer, its oxidation, unlike that of aluminum, occurs continuously, and the thickness of the layer increases linearly over time.
Scientists were quick to appreciate the oxidation of aluminum. This is because the reaction makes the metal's surface virtually corrosion-resistant. As long as it is not damaged in the form of scratching or bending, it is the oxidation of aluminum that ensures its safety. The reaction is possible because of the characteristics of aluminum oxide itself, which is its ability to adsorb gases, especially water vapor. The latter is retained by the oxide layer up to the melting point of the metal. Distinguished by its considerable mechanical strength (20 MPa at a thickness of 10 -5 cm), the aluminum oxide layer, despite its higher density than aluminum (2.85-3.95), is easily held on the metal surface by surface tension forces. The coefficient of thermal expansion of the oxide layer is almost 6 times smaller than the coefficient of expansion of aluminum, so cracks form in the oxide layer when the metal is heated.
If the oxidized aluminum contains alloying additives, the composition of the oxide layer may change. In the composition of the oxide layer of alloys containing silicon or magnesium, the presence of sillimanite (Al 2 O 3 -SiO) and magnesium spinel (MgO-Al 2 O 3 ) is detected, respectively. If the oxidized aluminum contains impurities with alkali and alkaline earth elements, the oxide layer is enriched with their oxides. Such a complex oxide layer is looser, more hygroscopic and less protective of the metal against gas diffusion.
Oxidation of aluminum complicates the welding process. Thanks to its high melting point (2050°C), the oxide layer does not melt during the welding process and covers the metal with a durable coating. During welding, measures must be taken to destroy and remove the layer and protect the metal from reoxidation. Due to the high chemical strength of the joint, it is almost impossible to recover aluminum from the oxide under welding conditions. It is also impossible to bind Al 2 O 3 into a strong compound in the reaction acid + base = salt. Therefore, the action in welding aluminum is based on the processes of dissolution and washing away the dispersed oxide layer.
Fluxes and electrode coatings for welding aluminum and its alloys are similarly constructed. The agent for oxidation of aluminum is low-melting mixtures of chloride salts of alkali and alkaline earth elements, to which a small amount of fluoride compounds are added to activate the action of the flux.
In aluminum alloys, the concentration of alloying elements is generally small and rarely exceeds 5-10%. If we take into account the extremely high activity of aluminum toward oxygen and its ability to reduce many metals from their oxides, we can not expect significant oxidation losses of elements such as Cu, Mn, Fe, Si, Zn, which occurs in low concentrations in alloys.
The exception may be magnesium, which has a much higher affinity for oxygen than aluminum. Approximate calculations show that in an aluminum alloy the greatest oxidation of magnesium is observed when its content in the alloy is a few tenths of a percent. The presence of a strong oxide layer on the metal surface affects the nature of the transfer of metal droplets. When welding in an oxidizing environment, the size of the droplets exiting the electrode reaches a large value, and arc burning is unstable. By reducing the oxidizing effect of the atmosphere and using coatings on the electrodes, the size of the transferred droplets can be reduced.
