Pitting corrosion. Pitting corrosion, also known as pore corrosion, is a very local corrosion form that produces pinpoints, spots, and holes on metals. Pitting corrosion is a unique form of anodic reaction and an autocatalytic process. Aluminum will undergo pitting corrosion in the atmosphere, fresh water, or neutral aqueous solution, and in severe cases, it can lead to perforation, but the corrosion holes may eventually stop developing, and the amount of corrosion maintains a limit value. The limit degree of pitting corrosion is related to the medium alloy, such as Chloride ions, fluoride ions, etc. There must also be substances that promote the cathodic reaction, such as dissolved oxygen in the aqueous solution, copper ions, etc. High-purity aluminum alloy profiles are generally difficult to pit corrosion, and copper-containing aluminum alloys are the most obvious pitting corrosion.
Galvanic corrosion. It is also a characteristic corrosion form of aluminum. The natural potential of aluminum is very negative. When aluminum is in contact with other metals, aluminum is always in the anode to accelerate corrosion. Galvanic corrosion is also called bimetallic corrosion. The severity is determined by the relative position of the two metal potential sequences. The greater the potential difference between them, the more serious the galvanic corrosion, and almost all aluminum alloy profiles cannot avoid galvanic corrosion.
Crevice corrosion, there is a gap when the aluminum alloy profile itself or the surface of aluminum and other materials is in contact. Due to the effect of the differential gas-filled battery, the corrosion in the crevice is accelerated, but there is no effect outside the crevice. The crevice corrosion has little to do with the type of alloy, even if it is very corrosion-resistant. Alloys can also produce crevice corrosion. In recent years, the mechanism of crevice corrosion has been studied more deeply. The acidic environment at the top of the crevice is the driving force of corrosion. Corrosion under sediment is a form of crevice corrosion.
Intergranular corrosion. The cause of intergranular corrosion is related to improper heat treatment. Alloying elements or intermetallic compounds are precipitated along the grain boundaries. Compared with the grains, the anode constitutes a corrosion battery, causing the acceleration of intergranular corrosion.
Filamentous corrosion, filamentous corrosion is a kind of corrosion under the film, which develops under the film in the form of worms. This film can make the paint film or other layers, and generally does not occur under the anodic oxide film. Filamentous corrosion and Alloy composition, pretreatment before coating of aluminum alloy profiles are related to environmental factors, such as humidity, temperature, chloride, etc.
