What are the properties and application characteristics of aluminum and aluminum alloys

       The density of aluminum is smaller (the density of aluminum with a purity of 99.75% is 2.703g/cm3), which is about 1/3 of that of zinc or steel, so that a coating of the same thickness can be obtained.

    Aluminum is a very active metal. In the atmosphere, a thin and dense oxide film can be formed on the surface of aluminum, which makes aluminum have good corrosion resistance in the atmosphere. At the same time, compared with iron, aluminum has a low electrochemical potential and has an electrochemical protective effect. Secondly, it also has anti-corrosion effect in water with a temperature above 60 degrees Celsius. However, aluminum is not resistant to corrosion by strong acids or alkalis, and pitting corrosion is prone to occur in the presence of halide ions. Therefore, similar to zinc, aluminum is widely used for anti-corrosion spraying of steel structures.

    The aluminum materials usually used for thermal spraying are mostly pure aluminum, and the purity of aluminum wire is generally required to be more than 99.7%. Considering the refinement of particles and the uniformity of wire feeding, hard aluminum wire should be used.

    1) Aluminum magnesium alloy

    The aluminum-magnesium (AI-Mg) alloy used for thermal spraying anti-corrosion construction is mainly an aluminum-magnesium binary alloy containing Mg5%. The alloy is usually a single-phase solid solution structure under production conditions. The electrochemical performance of the aluminum-magnesium alloy coating is more active than that of the pure aluminum coating, and the cathodic protection effect on the steel substrate is stronger. During the spraying process, magnesium is more likely to evaporate and oxidize, forming spinel oxides. In corrosive medium, the aluminum-magnesium alloy coating has better self-sealing ability to block pores, and can well protect the steel matrix. In the event of mechanical damage to the coating, the aluminum-magnesium alloy coating will provide effective corrosion protection to the steel substrate.

    In the anti-corrosion field of large-scale steel structure parallel-frame facilities in offshore oil exploration sites, the application ratio of aluminum-magnesium alloy coatings is much larger than that of conventional Zn or AI coatings. The aluminum-magnesium alloy spray coating can be obtained by flame spraying or arc spraying. Among them, arc spraying has high heat source temperature, high coating bonding strength, and high productivity. Therefore, in anti-corrosion construction, arc spraying technology has replaced flame spraying technology and is in a dominant position.

    2) Rare earth aluminum alloy

    The rare earth aluminum alloy materials used in anti-corrosion thermal spraying mainly include pure aluminum and rare earth element alloys and aluminum magnesium rare earth alloys.

    The atomic radius of rare earth elements ranges from 0.174 μm to 0.204 μm, which is larger than the atomic radius of aluminum (0.143 μm). Rare earth elements are relatively active. With the increase of rare earth elements, the strength and plasticity of aluminum alloys are improved. The addition of rare earth elements can change the crack initiation position and propagation path during the fracture process of the alloy, which is beneficial to the toughening of the alloy. At the same time, with the increase of rare earth content in the aluminum alloy, both the tensile strength and hardness increase. The addition of rare earth elements can change the crack initiation position and the entire path of crack propagation during the fracture process of the alloy, which is beneficial to the toughening of the alloy. At the same time, with the increase of rare earth content in the aluminum alloy, both the tensile strength and hardness increase. It can be seen that with the addition of rare earth, the mechanical properties of the alloy are greatly improved. At the same time, rare earth alloys can form complex composition compounds with high thermal hardness in aluminum alloys, which can improve the high temperature performance of the alloys.

    Compared with pure aluminum, the corrosion resistance of rare earth aluminum alloys is significantly improved. The addition of rare earth changes the structure of the oxide film on the surface of the alloy and increases the thickness of the oxide film, so that the surface of the alloy is bright and beautiful, and the corrosion resistance of the product is improved.

    3) Aluminum silicon alloy

     The typical composition of aluminum-silicon alloy is 94.4% aluminum and 6.0% silicon. The melting point is 621 degrees Celsius. The aluminum-silicon alloy coating has higher hardness, but the corrosion resistance is lower than that of the pure aluminum coating, so the coating is usually not used for anti-corrosion purposes, and is mostly used to repair the wear and repair of aluminum castings and aluminum workpieces. The aluminum alloy coating can also be sprayed on glass as a heat reflective or light reflective coating.