Views: 39 Author: Site Editor Publish Time: 2022-07-08 Origin: Site
Aluminum is an electronegative metal, its electrode potential is -0.5~-3.0V, and the potential of 99.99% aluminum to calomel reference electrode in 5.3%NaCl+0.3%H2O2 is (-0.87+0.01)V. From a thermodynamic point of view, aluminum alloy is one of the most reactive industrial raw metals. However, in many oxidizing media, water, atmosphere, some neutral solutions and many weakly acidic media and strong oxidizing media, aluminum has quite high stability. This is because in the above-mentioned medium, aluminum can form a dense continuous oxide film on its surface, its molar volume ratio is about 30% of aluminum, this oxide film or anodizing film is under the action of positive pressure, when it is destroyed It will be generated immediately and can play a very good protective role.
Generally, the oxide film is stable in the solution with pH value of 4.0~9.0, and also stable in concentrated nitric acid (pH value of 1) and concentrated sodium hydroxide (pH value of 13).The electrode potential of aluminum is largely determined by the insulating properties of the oxide film. Therefore, all factors that can improve the density of the oxide film, increase the thickness of the oxide film, and improve the insulating performance of the oxide film will contribute to the improvement of the corrosion resistance. Conversely, any factor that reduces the effective protective ability of the oxide film, whether mechanical or chemical, will drastically reduce the corrosion resistance of aluminum.
Generally speaking, the basic types of aluminum and aluminum alloy corrosion are: pitting corrosion, galvanic corrosion, crevice corrosion, intergranular corrosion, stress corrosion, exfoliation corrosion, fatigue corrosion, filiform corrosion, etc. Here, only a few common corrosion phenomena in the production and use of architectural aluminum extrusions are introduced.
Pitting corrosion, also known as hole corrosion, is a very localized corrosion form that produces pinpoint, point, and hole shapes on metals. Pitting corrosion is a unique form of anodic reaction and is an autocatalytic process. Aluminum suffers from pitting corrosion in the atmosphere, fresh water, seawater and neutral aqueous solution, and can even lead to perforation in severe cases, but the development of corrosion holes may eventually stop, and the amount of corrosion remains at a limit value. The limit degree of pitting corrosion is related to the medium and alloy, such as the relationship between the corrosion degree and time of 6063 alloy and 6351 alloy extrusion material under different atmospheric conditions.
Tests show that there must be anions that destroy local passivation, such as chloride ions and fluoride ions, in the medium of aluminum alloy pitting corrosion. In addition, there must be substances that promote the cathodic reaction, such as dissolved oxygen and copper ions in the aqueous solution. From the point of view of aluminum alloy series, high-purity aluminum is generally difficult to cause pitting corrosion, and the pitting corrosion of aluminum alloys containing copper is the most obvious, while aluminum-manganese series and aluminum-magnesium series alloys have better pitting corrosion resistance.
Pitting corrosion for aluminium alloy
Galvanic corrosion is also a characteristic corrosion form of aluminum. The natural potential of aluminum is very negative. When aluminum is in contact (or electrical contact) with other metals, aluminum is always in the anode to accelerate its corrosion. Galvanic corrosion is also known as bimetallic corrosion, and its severity is determined by the relative positions of the two metals in the potential sequence. The greater the potential difference, the more serious the galvanic corrosion, and almost all aluminum alloys cannot avoid galvanic corrosion.
Anodizing for alumium extrusion parts
There is a gap between the two surfaces of aluminum itself or aluminum and other materials. 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. Crevice corrosion has little to do with alloy type, and crevice corrosion can occur even in very corrosion-resistant alloys. In recent years, the mechanism of crevice corrosion has been further studied, and the acidic environment at the top of the crevice is the driving force of corrosion. Corrosion under deposit (scale) is a form of crevice corrosion, and corrosion under mortar on the surface of 6063 aluminum alloy extruded profile is an example of corrosion under scale.
Aluminum anodizing line
Pure aluminum does not undergo intergranular corrosion, while aluminum-copper, aluminum-copper-magnesium and aluminum-zinc-magnesium alloys are susceptible to intergranular corrosion. The cause of intergranular corrosion is generally related to improper heat treatment. Alloying elements or intermetallic compounds are precipitated along the grain boundaries, and the anode constitutes a corrosion cell relative to the grains, causing the grain boundary corrosion to accelerate.
Filamentous corrosion is a kind of sub-film corrosion, which develops under the film in the form of worms. This film can be a paint film or other coatings, and generally does not occur under the anodized film. Filiform corrosion was first discovered under the coating of aircraft, and was related to alloy composition, pretreatment before coating and environmental factors, such as humidity, temperature, chloride and so on.
We will publish below two articles about anodizing for aluminum profiles later.
Aluminum extrusion profile
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