Corrosion of aluminum and intergranular corrosion of 6000 series aluminum alloys
According to the conventional estimation method, the direct economic loss caused by corrosion in China accounts for about 3% of GDP (gross national product) every year, and the steel consumed by corrosion accounts for about 1/3 of the annual output, of which about 1/10 of the total output is non recyclable. The corrosion resistance of aluminum and aluminum alloy is much higher than that of steel, and the corrosion loss is much smaller than that of steel. However, no matter what metal material or how high its corrosion resistance is, it will always produce corrosion loss more or less in use. The annual corrosion loss of aluminum is about 0.5% of the aluminum output of that year. In 2020, China’s primary aluminum output will be 37.3 million tons. According to this estimation, the corrosion loss of aluminum will be about 186500 tons.
The corrosion of aluminum and aluminum alloys mainly includes pitting corrosion, intergranular corrosion, stress corrosion cracking, layered corrosion and so on. 6000 series alloy has the largest output among wrought aluminum alloys. Although its corrosion resistance is not as good as 1000 series, 3000 series and 5000 series aluminum alloys, it is much higher than 2000 series and 7000 series aluminum alloys. The intergranular tendency of 6000 series alloy is also relatively large. The intergranular corrosion sensitivity of 6000 series aluminum alloy materials for important structures should be evaluated.
Classification of aluminum corrosion
From the appearance of corrosion, aluminum corrosion can be divided into comprehensive corrosion and local corrosion. The former is also called uniform corrosion, also known as overall corrosion, which refers to the loss caused by uniform corrosion on the surface of materials in contact with the environment. The corrosion of aluminum in alkaline solution is a typical uniform corrosion, such as alkali washing. The corrosion result is that the aluminum surface becomes thinner at approximately the same rate and the mass is reduced. However, it should be pointed out that absolute uniform corrosion does not exist, and the thinning of thickness is different everywhere. Local corrosion refers to that the occurrence of corrosion is limited to specific areas or parts of the structure, which can be divided into the following categories:
Pitting corrosion occurs in very local areas or parts of the metal surface, resulting in caves or pits and expanding to the inside, or even perforation. If the pit diameter is less than the pit depth, it is called pitting corrosion; If the diameter of the pit mouth is greater than the depth of the pit, it can be called pit erosion. In fact, there is no strict boundary between pitting and pitting.
The typical pitting corrosion of aluminum in aqueous solution containing chloride. In the corrosion of aluminum, pitting corrosion is the most common, which is caused by the difference between the potential of a certain area of aluminum and the potential of aluminum matrix, or by the existence of impurities with different potential from that of aluminum matrix.
This kind of corrosion is a kind of selective corrosion at the grain boundary of metal or alloy without obvious corrosion of grain or crystal itself, which will sharply reduce the mechanical properties of materials, resulting in structural damage or accidents.
The reason for intergranular corrosion is that under some conditions, the grain boundary is very active, such as impurities at the grain boundary, or an alloy element at the grain boundary increases or decreases, that is, there must be a thin layer of electronegative area on the grain boundary, which gives priority to corrosion. This kind of corrosion can occur in high-purity aluminum in hydrochloric acid and high-temperature water. AI Cu, AI mg Si, Al Mg and Al Zn Mg alloys are sensitive to intergranular corrosion.
Galvanic corrosion is also a characteristic corrosion form of aluminum. When a less active metal and a more active metal such as aluminum (anode) are in contact in the same environment or a conductor is connected, galvanic couple is formed and current flow is caused, resulting in galvanic corrosion. Galvanic corrosion is also called bimetallic corrosion or contact corrosion. The natural potential of aluminum is very negative. When aluminum is in contact with other metals, aluminum is always anode, and the corrosion is accelerated. Almost all aluminum and aluminum alloys cannot avoid galvanic corrosion. The greater the potential difference between the two metals in contact, the more serious the galvanic corrosion is. It should be noted that in galvanic corrosion, the area factor is very important, and large cathode and small anode are the most unfavorable combination.
When the same or different metals are in contact, or the metal is in contact with non-metal, a gap will be formed, and corrosion will occur at or near the gap. There is no corrosion outside the gap, which is caused by the lack of oxygen in the gap, because a concentration cell is formed at this time. Crevice corrosion is almost independent of the type of alloy, even very corrosion-resistant alloys can occur. The acidic environment at the top of the gap is the driving force of corrosion and a kind of corrosion under sediment (scale). The mortar corrosion on the surface of 6063 alloy building aluminum profile is a very common crack corrosion under scale. The joint corrosion will be caused by the sludge, scale and impurities on the metal surface of flange connection surface, nut fastening surface, lap surface, weld pore, under rust layer and sink layer.
Stress corrosion cracking
Stress corrosion cracking is the corrosion cracking caused by the coexistence of tensile stress and specific corrosion medium. The stress can be external or residual stress inside the metal. The latter may be caused by deformation during processing and manufacturing, severe temperature change during quenching, or volume change caused by internal structure change.
The stress caused by riveting, bolt fastening, press fit and cold shrinkage fit is also residual stress. When the tensile stress on the metal surface reaches the yield strength Rp0.2, stress corrosion cracking will occur. Residual stress will be generated during quenching of 2000 series and 7000 series aluminum alloy thick plates, which shall be eliminated by pre stretching before time effect treatment, In order to avoid deformation or even bring it into the parts when machining aircraft parts.
Also known as delamination, spalling and layered corrosion, which can be referred to as denudation for short. It is a special corrosion form of 2000 series, 5000 series, 6000 series and 7000 series aluminum alloys. It is mostly seen in extruded materials. Once it occurs, it can be stripped down layer by layer like mica.
It is a kind of wormlike corrosion that can develop under the aluminum paint film or other coatings, but this corrosion under the anodic oxide film is not found. It generally occurs under the coatings of aircraft aluminum structural parts and building or structural aluminum parts. Filiform corrosion is related to material composition, pretreatment before coating and environmental factors. Environmental factors refer to temperature, humidity, chloride, etc.
Intergranular corrosion of 6000 Series Alloys
Among the wrought aluminum alloys used today, 6000 series alloys that can be strengthened by heat treatment are the most widely used. They are a kind of Al Mg Si and Al Mg Si Cu alloys. In 2018, 706 common and non common alloys were registered in the Aluminum Association, Inc., of which the 6000 series alloys were the most, 126, accounting for 18%. They have been widely used in the construction industry, structural field and transportation equipment because they have good processing formability, moderate strength and excellent corrosion resistance. However, if the alloy composition ratio is inappropriate, or the selection of heat treatment parameters is inappropriate, or the processing and forming is improper, intergranular corrosion will occur in the environment containing chlorine.
In most cases, intergranular corrosion occurs in alloys containing a small amount of copper and high Si / mg. Generally, the copper content of most copper containing alloys is no more than 0.4%, and the copper content of only four alloys such as 6013, 6113, 6056 and 6156 is 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 when the alloys with intergranular corrosion sensitivity are observed by high-resolution scanning transmission electron microscope, copper rich segregation layer and cathodic q-phase precipitate are often found. Q phase is a quaternary intermetallic phase with molecular formula of Cu2Mg8Si5Al4. It precipitates along the grain boundary, causing anodic dissolution of adjacent solid solutions to form a spancipitate free zone.
Intergranular corrosion sensitivity test
There are two common inspection methods to determine the intergranular corrosion sensitivity of aluminum alloy: field test and accelerated immersion test. In the accelerated test, in order to accelerate the corrosion, potassium chloride solution containing hydrochloric acid (ISO 11846 method B) or potassium chloride solution with hydrogen peroxide (ASTM G110) is often used. After the test, the cross section of the sample shall be observed metallographically or the loss of mechanical properties shall be measured.
The accelerated test results of ISO 11846 are highly consistent with the field test results of marine atmosphere. However, during the accelerated test, almost all grain boundaries near the sample surface are severely corroded (uniform intergranular corrosion), while the surface of the field test sample is corroded only in limited areas (local corrosion). Nevertheless, accelerated test is still a standard method to accurately judge whether the material has grain boundary corrosion.
The automotive industry often judges whether 6000 series aluminum alloy has intergranular corrosion according to ISO 11846 method B standard. When testing according to this standard, first immerse the small sample (surface area < 20cm2) in room temperature acidic sodium chloride solution (pH = 1) for 24 hours, and then conduct metallographic inspection to determine the corrosion type (pitting corrosion or intergranular corrosion). In addition, it is also necessary to determine the percentage of corrosion damaged surface and the maximum corrosion depth.
Recent research shows that allowing some large changes to the test conditions will not have a great impact on the reproducibility of the test results. In particular, the standard stipulates that the ratio of electrolyte volume to sample surface area shall not be less than 5ml/cm2, otherwise it will have a great impact on the rate of intergranular corrosion.
The condition of corrosion on the sample surface is that there must be cathodic reaction (hydrogen precipitation and oxygen reduction), and the pH value of the test solution increases with time, so that the electrolyte corrosion decreases.
Among the 8 series of wrought aluminum alloys, 6000 series alloy is a kind of Al Mg Si (Cu, Zn) series alloy, which is one of the alloys most prone to intergranular corrosion, that is to say, this series alloy has quite strong intergranular corrosion sensitivity.
In order to test the intergranular corrosion tendency of 6000 series alloys, one of the most effective methods is to carry out alkaline etching after the ISO 11846 standard test, and then carry out decontamination treatment. Concentrated nitric acid solution is used for decontamination treatment. However, the test results will be affected by etching in NaOH solution with temperature of 50 ~ 60 ℃ and mass fraction of 5% ~ 10% for 2 ~ 5min.
An effective alternative to alkali etching is to use nitric acid / hydrofluoric acid solution, which can effectively remove aluminum from iron rich protoplast points on the surface. As we all know, aluminum particles can accelerate the corrosion of aluminum alloy in chloride solution because they are local micro cathodes, and these particles are also the source of intergranular corrosion. The corrosion of the alloy in nitric acid / fluoride solution is slower than that in alkali solution.
6000 series alloy is not only a kind of deformed aluminum alloy with wide application, large output and many varieties (brands), but also one of the deformed alloys with high sensitivity to intergranular corrosion. However, as long as the process specifications, especially the heat treatment process, reasonable structural design and excellent manufacturing are strictly observed in production, this corrosion can be avoided. The intergranular corrosion sensitivity of 6000 series aluminum alloy structures and parts is also closely related to their working environment. Full attention should be paid to the design of structures.
Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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