Aluminum corrosion is the gradual decay of aluminum molecules into their oxides, thereby reducing their physical and chemical properties. Aluminum is essentially a reactive metal, but it is also a passive metal. This means that while the nascent aluminum reacts with oxygen and water in the environment, the resulting compounds form a layer on the surface that protects the underlying material from further corrosion.
This non-reactive oxide layer adheres well to the surface and resists peeling, similar to stainless steel. Unlike deliberate processes such as laser etching, aluminum anodizing or brightening, corrosion is a slow process that can take place over months or years. Aluminum is unique in that there are many different types of corrosion pathways. Understanding these different corrosion phenomena is the basis for implementing control measures to reduce or completely prevent their occurrence.
The corrosion of aluminum and aluminum alloys mainly includes pitting corrosion, intergranular corrosion, stress corrosion cracking, lamellar corrosion, etc. The 6000 series alloy is the most popular among deformed aluminum alloys. Although its corrosion resistance is not as good as that of the 1000 series, 3000 series, and 5000 series aluminum alloys, it is much higher than that of the 2000 series and 7000 series aluminum alloys. 5000 series aluminum are the well known marine aluminum alloys.
The 6000 series alloy also has a relatively large intergranular tendency. The 6000 series aluminum alloy materials like 6082t6 grade aluminium used in important structures should be evaluated for intergranular corrosion sensitivity.
Judging from the corrosion morphology, the corrosion of aluminum can be divided into comprehensive corrosion and local corrosion. The former is also called uniform corrosion or overall corrosion, which refers to the uniform corrosion and loss of the material surface in contact with the environment. The corrosion of aluminum in alkaline solution is a typical uniform corrosion, such as alkali cleaning. The corrosion result is that the aluminum surface becomes thinner at approximately the same rate and the weight is reduced.
Localized corrosion means that the occurrence of corrosion is limited to specific areas or parts of the structure, and can be divided into the following categories:
Pitting corrosion occurs in very localized areas or parts of the metal surface, causing holes or pits that expand inward and even cause perforations. Aluminum in chloride-containing aqueous solutions is a typical pitting corrosion. In the corrosion of aluminum, pitting corrosion is common and is caused by the potential of a certain area of aluminum being different from the potential of the matrix, or by the presence of impurities with potentials that are different from the potential of the aluminum matrix.
This kind of corrosion is a selective corrosion that occurs at the grain boundaries of metals or alloys when the grains or crystals themselves are not significantly eroded. It will drastically reduce the mechanical properties of the material, causing structural damage or accidents.
The reason for intergranular corrosion is that the grain boundaries are very active under certain conditions. For example, there are impurities at the grain boundaries, or an alloy element increases or decreases at the grain boundaries. This means that there must be a thin region on the grain boundary that is electronegative to the rest of the aluminum, which corrodes preferentially. This kind of corrosion can occur in high-purity aluminum in hydrochloric acid and high-temperature water. AICu, AI-Mg-Si, Al-Mg, and Al-Zn-Mg alloys are all sensitive to intergranular corrosion.
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