According to the usual estimation method, the direct economic loss caused by corrosion accounts for about 3% of GDP (gross national product) in China every year, and the consumption of steel due to corrosion is about 1/3 of the annual output, about 1/10 of the total output is not recyclable. Aluminum and aluminum alloys are much more resistant to corrosion than steel, and the corrosion losses are much smaller, but no matter what the metal material, no matter how high the corrosion resistance, there will always be more or less corrosion losses in use. The annual corrosion loss of aluminum is about 0.5% of the annual aluminum production. In 2020, China produced 37.3 million tons of primary aluminum, according to this estimate, aluminum corrosion loss of about 186,500 tons. The corrosion of aluminum and aluminum alloy mainly includes pitting corrosion, intergranular corrosion, stress corrosion cracking and layered corrosion. The corrosion resistance of 6000 series alloy is not as good as that of 1000,3000 and 5000 series alloy, but it is much higher than that of 2000 and 7000 series alloy. The intergranular corrosion susceptibility of 6000 series aluminum alloy with important structure should be evaluated.

First, the classification of aluminum corrosion from the appearance of corrosion, aluminum corrosion can be divided into full corrosion and local corrosion, the former also known as uniform corrosion, also known as the overall corrosion, is in contact with the environment of the material surface uniform corrosion and wear. The corrosion of aluminum in alkaline solution is typical of uniform corrosion, such as alkaline wash, the corrosion results in aluminum surface thinning at approximately the same rate, the mass reduction. However, it should be pointed out that absolute uniform corrosion does not exist and thickness reduction varies from place to place. Local corrosion refers to the corrosion occurred in specific areas or parts of the structure, can be divided into the following categories: 01. Pitting occurs in a very localized area or location on the surface of a metal, causing holes or pits to expand and even perforate. Pitting occurs when the crater diameter is less than the pit depth, and pitting occurs when the crater diameter is greater than the pit depth. In fact, there is no strict boundary between pitting and pitting.

Pitting corrosion of aluminum in chloride-containing aqueous solutions is typical. In the corrosion of aluminum, pitting corrosion is the most common, which is caused by the difference of potential between a certain area of aluminum and the substrate, or by the presence of impurities with different potential between aluminum and the substrate.

02. Intergranular corrosion is a kind of corrosion that occurs at the grain boundary of a metal or an alloy without significant erosion of the grain or the crystal itself. This corrosion can severely degrade the mechanical properties of the Selective leaching, resulting in structural damage or accidents.

The reason for intergranular corrosion is that under certain conditions the grain boundary is very active, such as there are impurities at the grain boundary, or an alloy element increases or decreases at the grain boundary, that is to say, there must be a thin region on the grain boundary which is electronegative to the rest of the aluminum, it preferentially corrodes. Al-cu, AI-Mg-Si, Al-Mg, Al-Zn-Mg alloys are sensitive to intergranular corrosion.

03. GALVANIC corrosion galvanic corrosion is also a characteristic corrosion form of aluminum. When a less reactive metal and a more reactive metal such as aluminum (anode) are in contact in the same environment or when a conductor is connected, a couple is formed and a current flow is induced, resulting in galvanic corrosion. GALVANIC corrosion is also called bimetal corrosion or contact corrosion. Aluminum’s natural potential is very negative, when aluminum contact with other metals, aluminum always anodic, corrosion accelerated, almost all aluminum and aluminum alloys can not avoid galvanic corrosion. The greater the potential difference between the two metals, the more serious the galvanic corrosion. It should be noted that in galvanic corrosion, the area factor is extremely important, large cathode and small anode is the most unfavorable combination. 04. Crevice corrosion occurs when the same or different metals are in contact with each other, or when metals are in contact with non-metals. Crevice corrosion occurs at or near the crevice. There is no corrosion outside the crevice due to the lack of oxygen in the crevice, because a concentration cell is formed. Crevice corrosion is almost independent of the alloy type and can occur even in very corrosion-resistant alloys. The acidic environment at the top of the gap is the driving force of corrosion, which is a kind of corrosion under sediment (scale) , and the corrosion under mortar on the surface of 6063 alloy aluminum profile is a very common corrosion under the gap. Flange connection surface, nut fastening surface, LAP surface, weld porosity, rust layer and sediment layer in the metal surface silt, scale, impurities, etc. will cause crevice corrosion. 05. Stress Corrosion cracking (SCC) SCC is the corrosion cracking caused by the coexistence of tensile stress and specific corrosion media. The stresses may be external or residual stresses in the metal, which may result from deformation during processing or from drastic temperature changes during quenching, it may also be caused by a change in volume due to a change in internal structure.

The stresses caused by riveting, bolt fastening, pressing and cold shrinking are also residual stresses. Stress Corrosion cracking occurs when the tensile stress on the metal surface reaches the yield strength RP0.2, the residual stresses of 2000 and 7000 series thick aluminum alloy plates can be produced during quenching, which should be eliminated by pre-stretching before aging treatment to avoid deformation or even being carried into the aircraft parts. 06. Layered corrosion, also known as exfoliation, exfoliation and layered corrosion, can be abbreviated as exfoliation. It is a special corrosion form of 2000 series, 5000 series, 6000 series and 7000 series alloys, once that happens, it can peel off layer by layer like MICA.

07. Filamentous corrosion is a kind of corrosion that can develop worm-like under aluminum paint film or other coating, but it is not found under anodic oxide film. Filiform corrosion is related to material composition, pretreatment before coating and environmental factors. Environmental factors are temperature, humidity, chloride, etc. .

The intergranular corrosion of the second and 6000 series alloys is widely used in wrought aluminum alloys nowadays. The 6000 series alloys which can be strengthened by heat treatment are a kind of Al-Mg-Si and Al-Mg-Si-cu alloys. In 2018, there were 706 common and non-common alloys registered with The Aluminum Association (Inc.) , of which 6000 series alloys are The most, accounting for 126, or 18% , and have been widely used in construction industry, structure and transportation equipment, because they have good processing formability, moderate strength and excellent corrosion resistance. But if the alloy composition ratio is not appropriate, or heat treatment parameters are not suitable, or processing is not appropriate, then in the environment containing chlorine intergranular corrosion will occur. In most cases, intergranular corrosion occurs in alloys with a small amount of copper and high Si/Mg content. In general, the copper content of most copper-containing alloys is less than 0.4% . Only the copper contents of the four alloys such as 6013,6113,6056 and 6156 are as high as 1.1% , adding copper to Al-Mg-Si alloy is to improve the mechanical properties of the alloy. It is found that cu-rich segregation layer and Cathodic q-phase precipitates are often found in alloys with intergranular corrosion sensitivity by high-resolution scanning electron microscopy. The Q phase is a quaternary intermetallic phase with molecular formula Cu2Mg8Si5Al4, which precipitates along the grain boundary and causes the anodic dissolution of the adjacent solid solution to form a spancipate free zone.

3. intergranular corrosion sensitivity test there are two common test methods to determine the intergranular corrosion sensitivity of aluminum alloys: Field test and accelerated immersion test. In order to accelerate the corrosion process, the solution of potassium chloride containing hydrochloric acid (ISO 11846method b) or the solution of potassium chloride with hydrogen peroxide (ASTM G110) are often used. After the test, the Cross section of the specimen was observed by metallography or the loss of mechanical properties was measured. The results of the ISO 11846 accelerated test are in good agreement with those of the Field Test in marine atmosphere, serious Corrosion (uniform intergranular corrosion) occurs at nearly all grain boundaries near the specimen surface, while in-situ corrosion (local corrosion) occurs only in a limited area. Nevertheless, accelerated testing is still the standard method to accurately determine whether materials have grain boundary corrosion. The automobile industry often judges whether the 6000 series aluminum alloy has intergranular corrosion according to ISO 11846method B. In accordance with this standard, a small sample (surface area < 20cm2) is immersed in an acidic sodium chloride solution (PH = 1) at room temperature for 24 hours, and then metallographic examination is carried out to determine the corrosion type (pitting or intergranular corrosion) , it is also necessary to determine the corrosion damage surface percentage and the maximum corrosion depth.

Recent studies have shown that allowing major changes in test conditions does not have a significant impact on the reproducibility of test results. In particular, the ratio of electrolyte volume to sample surface area should not be less than 5mL/cm2, otherwise the intergranular corrosion rate will be greatly affected. The condition of corrosion on the surface of the sample is that there must be cathodic reaction (hydrogen, oxygen reduction) , and the Ph value of the test solution increases with time, which makes the electrolyte corrosion decrease. Among the 8 series wrought aluminum alloys, 6000 series is a kind of Al-Mg-Si (Cu, Zn) alloy, which is one of the most susceptible alloys to intergranular corrosion. In order to test the intergranular corrosion tendency of 6000 series alloy, one of the most effective methods is to carry out Alkali etching and decontamination treatment after ISO 11846 standard test, and use concentrated nitric acid solution for decontamination treatment. However, the experimental results will be influenced by ETCHING FOR 2 ~ 5 min in NaOH solution at 50 ~ 60 °C and 5% ~ 10% by mass fraction. An effective alternative to alkaline etching is the removal of aluminum from iron-rich primary particles on the surface by nitric acid/hydrofluoric acid solution. Aluminum particles are known to accelerate the corrosion of aluminum alloys in chloride solutions because they are local microcathodes and they are the source of intergranular corrosion. The corrosion of the alloy in nitric acid/fluoride solution was slower than that in alkaline solution. The 6000 series alloy is not only a kind of deformable aluminum alloy with wide application, large output and many varieties (grades) , but also one of the deformable alloys with high susceptibility to intergranular corrosion, the structure design is reasonable, the manufacture is fine, this kind of corrosion may avoid completely. The intergranular corrosion sensitivity of 6000 series aluminum alloy structure and components is also closely related to its working environment.

 Source: Caitong