The corrosion of aluminum and aluminum alloys mainly includes pitting, intergranular corrosion, stress corrosion cracking, and exfoliation corrosion. Although aluminum has fairly high corrosion resistance, no matter what metal material it is or how high its corrosion resistance is, some degree of corrosion loss will always occur during use. The annual corrosion loss of aluminum is about 0.5% of the annual aluminum production. Among deformed aluminum alloys, the 6000 series has the highest production. Although its corrosion resistance is not as good as that of the 1000, 3000, or 5000 series aluminum alloys, it is much higher than that of the 2000 and 7000 series aluminum alloys. The 6000 series alloys also have a greater tendency for intergranular corrosion, so a sensitivity assessment for intergranular corrosion should be conducted for 6000 series aluminum alloy materials used in important structures.
Classification of Aluminum Corrosion
From the perspective of corrosion morphology, aluminum corrosion can be divided into general corrosion and localized corrosion. The former, also known as uniform corrosion or overall corrosion, refers to the uniform deterioration of the material surface in contact with the environment. Corrosion of aluminum in alkaline solutions is a typical example of uniform corrosion, such as in alkaline cleaning, where the result is that the aluminum surface thins at approximately the same rate, leading to a reduction in mass. However, it should be noted that absolute uniform corrosion does not exist, as the thinning of thickness varies in different areas. Localized corrosion refers to corrosion confined to specific regions or parts of a structure and can be further divided into the following categories:
1. Pitting Corrosion
Pitting corrosion occurs in extremely localized areas or spots on the metal surface, causing cavities or pits that extend inward, and can even result in perforation. When the diameter of the pit opening is smaller than the pit depth, it is called pitting corrosion; when the diameter of the pit opening is larger than the pit depth, it can be called crevice corrosion. In fact, there is no strict boundary between pitting corrosion and crevice corrosion. The corrosion that occurs on aluminum in aqueous solutions containing chlorides is a typical example of pitting corrosion. In aluminum corrosion, pitting corrosion is the most common and is caused by a certain area of aluminum having a different potential from the base metal potential, or by the presence of impurities with a potential different from the aluminum base metal potential.
2. Intergranular Corrosion
This type of corrosion occurs at the grain boundaries of metals or alloys without significant attack on the grains or crystals themselves. It is a form of selective corrosion that can drastically reduce the mechanical properties of the material, potentially causing structural damage or accidents. The cause of intergranular corrosion is that under certain conditions, grain boundaries are very active, such as the presence of impurities at the grain boundaries, or an increase or decrease of a specific alloying element at the grain boundaries. In other words, there must be a thin region at the grain boundary that is electronegative relative to the rest of the aluminum, which corrodes preferentially. High-purity aluminum can experience this type of corrosion in hydrochloric acid and high-temperature water. Alloys such as Al-Cu, Al-Mg-Si, Al-Mg, and Al-Zn-Mg are all sensitive to intergranular corrosion.
3. Galvanic Corrosion
Galvanic corrosion is also a characteristic form of aluminum corrosion. When a less reactive metal and a more reactive metal, such as aluminum, come into contact in the same environment or are connected by a conductor, a galvanic couple is formed, causing the flow of current and resulting in galvanic corrosion. Galvanic corrosion is also called bimetallic corrosion or contact corrosion. Aluminum has a very negative natural potential, and when it comes into contact with other metals, it always acts as the anode, accelerating corrosion. Almost all aluminum and aluminum alloys cannot avoid galvanic corrosion. The greater the potential difference between the two metals in contact, the more severe the galvanic corrosion. It should be noted that in galvanic corrosion, the area ratio is extremely important; a large cathode paired with a small anode is the most unfavorable combination.
4. Crevice Corrosion
When metals of the same or different kinds, or metals with non-metals, come into contact, crevices are formed, leading to corrosion at the crevice or adjacent areas, while areas outside the crevice remain uncorroded. This is caused by a lack of oxygen within the crevice, resulting in the formation of a concentration cell. Crevice corrosion is almost independent of the alloy type, and even highly corrosion-resistant alloys can experience it. The acidic environment at the top of the crevice is the driving force for corrosion and is a form of under-deposit corrosion. Corrosion beneath mortar on 6063 alloy architectural aluminum profiles is a very common type of deposit-induced crevice corrosion. Flange connections, nut fastening surfaces, lap joints, weld pores, areas under rust layers, and deposits such as sludge, scale, and impurities on the metal surface can all trigger crevice corrosion.
5. Stress Corrosion Cracking
Stress corrosion cracking is corrosion cracking caused by the coexistence of tensile stress and specific corrosive media. The stress can be external or residual stress within the metal, the latter of which may be generated by deformation during processing and manufacturing, by intense temperature changes during quenching, or by volume changes due to internal structural changes. Stresses caused by riveting, bolt fastening, press fits, or shrink fits are also residual stresses. Stress corrosion cracking occurs when the tensile stress on the metal surface reaches the yield strength Rpo.2. Residual stress is generated in thick plates of 2000 and 7000 series aluminum alloys during quenching and should be eliminated by pre-stretching before aging treatment to prevent deformation when machining aircraft parts or even transferring to the parts.
6. Intergranular Corrosion
This type of corrosion is also called exfoliation, flaking, or lamellar corrosion, and can be simply referred to as exfoliation. It is a special form of corrosion in 2000, 5000, 6000, and 7000 series alloys, commonly seen in extrusions. Once it occurs, it can peel layer by layer like mica.
7. Filiform Corrosion
This is a type of corrosion that can develop in a worm-like pattern under the paint film or other coatings of aluminum materials, but this corrosion has not been found under anodized films. It generally occurs beneath the coatings of aircraft aluminum structural components and aluminum parts in buildings or structures. Filiform corrosion is related to the material composition, pre-treatment before coating, and environmental factors. Environmental factors refer to temperature, humidity, chlorides, and so on.
Intergranular Corrosion of 6000 Series Alloys
In most cases, intergranular corrosion occurs in alloys containing small amounts of copper and high levels of Si/Mg. Usually, the copper content in most copper-containing alloys does not exceed 0.4%, with only four alloys-6013, 6113, 6056, and 6156-having copper content as high as 1.1%. Copper is added to Al-Mg-Si alloys to improve the mechanical properties of the alloy. Studies have found that in alloys susceptible to intergranular corrosion, high-resolution scanning transmission electron microscopy often reveals copper-rich segregated layers and cathodic Q-phase precipitates. The Q phase is a quaternary intermetallic phase with the molecular formula Cu2Mg8Si5Al4, precipitating along grain boundaries and causing anodic dissolution of the adjacent solid solution, resulting in precipitation-free zones.
Intergranular Corrosion Susceptibility Test
When determining the intergranular corrosion susceptibility of aluminum alloys, two common test methods are used: on-site testing and accelerated immersion testing. In accelerated tests, to speed up corrosion, solutions such as potassium chloride containing hydrochloric acid (ISO 11846 Method B) or potassium chloride with added hydrogen peroxide (ASTM G110) are often used. After testing, the specimen's cross-section is examined metallographically or its mechanical property loss is measured. The results of ISO 11846 accelerated tests are highly consistent with on-site tests in marine atmospheres. However, in accelerated tests, aluminum materials susceptible to intergranular corrosion show severe corrosion at nearly all grain boundaries near the specimen surface (uniform intergranular corrosion), whereas in on-site tests, the specimen surface only shows corrosion in limited areas (localized corrosion). Nonetheless, accelerated testing remains the standard method to accurately determine whether a material is prone to grain boundary corrosion.
