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    Home > Coatings News > Paints and Coatings Market > Analysis of corrosion behavior and protection of magnesium aluminum alloy

    Analysis of corrosion behavior and protection of magnesium aluminum alloy

    • Last Update: 2020-11-18
    • Source: Internet
    • Author: User
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    1 Magnesium corrosion
    Magnesium is the most chemically active metal of all industrial alloys, with a standard electrode potance of -2.3V, about 2V lower than the standard electrode level of iron and about 0.7V lower than the standard electrode poton of aluminum. In a dry atmosphere, the surface of magnesium can form an oxidizing film, which can resist the corrosion of magnesium substrates from the outside world to a certain extent.
    However, due to the low density of this layer of oxidizing film and the large number of gaps, its corrosion resistance is very limited, while also showing significant chemical and electrochemical activity, and corrosion will further accelerate
    C.
    in air with high humidity and the presence of Cl
    - ions. The electrochemical corrosion of magnesium is mainly manifested in the process of hydrogen ions, including pitting and total corrosion, and dissolves very quickly. In a thin solution rich in Cl
    - ions, there is a large difference between the corrosion potentiance of magnesium and its standard potentior, and its corrosion potentior is about -1.7V, which may be related to the protection of magnesium surface oxide film. When the pH value is high, the magnesium surface can form a thick layer of Mg (OH)
    2precipitation film, which can play a good protective role on the magnesium substrate.
    2 Magnesium aluminum alloy corrosion behavior under various conditions
    2.1 atmospheric corrosion
    In atmospheric environment, magnesium corrosion process is mainly oxygen depolarization process, the specific degree of corrosion is mainly affected by atmospheric humidity and various pollutant content. Generally speaking, the air humidity size has no obvious effect on the corrosion degree of magnesium aluminum alloy, only when other corrosive pollutants exist in the atmosphere, the air humidity size on the corrosion effect of magnesium aluminum alloy is reflected in the painting. If there is basically no corrosive substance in the atmosphere, the corrosion behavior of magnesium aluminum alloy will be very weak no matter how high the air humidity is. When there are a lot of corrosive pollutants containing oil in the atmosphere, the corrosion rate of magnesium aluminum alloy surface will increase rapidly. Magnesium aluminum alloy as a material, if working in the industrial atmosphere and marine atmosphere, its corrosion resistance is poor. In the dry and clean air environment, the maintenance film of magnesium surface can form a strong protective effect, so that the corrosion of magnesium aluminum alloy is very weak.
    2.2 Corrosion in a variety of media
    In most organic acids, inorganic acids, and neutral media solutions, magnesium aluminum alloy corrosion resistance is poor, and when the removal of magnesium aluminum alloy surface oxide film, even distilled water can lead to faster corrosion of magnesium aluminum alloy. However, when magnesium aluminum alloy is in chromic acid solution, its surface will form a certain passivation film, so that it has better stability and corrosion resistance. If it is easy to contain Cl-ions and SO42-ions, the corrosion of the surface of magnesium aluminum alloy will accelerate, and if it is in a solution containing SiO32-, CrO42-, Cr2O72-plasma, the surface of magnesium aluminum alloy and oxygen atoms in various ions chemical reaction on its surface to form a layer of oxide film, which has a high corrosion resistance.
    2.3 Capacitive corrosion and total corrosion
    Because the electrode poton of magnesium aluminum alloy is lower than that of most metals, electrode corrosion occurs when magnesium aluminum alloys come into contact with these metals. For Fe, Ni, Cu and other metals with low hydrogen over potential, if present as impurities in the content of magnesium aluminum alloy, it will be corroded with magnesium as a battery, resulting in serious electronegation of magnesium aluminum alloy. For Zn, Cd and other metals with higher hydrogen over-potons, the electrolytic corrosion of magnesium aluminum alloy is not obvious.
    2.4 Pitting and Silk Corrosion
    Magnesium aluminum alloys will have pitting when exposed to non-oxidizing media containing Cl-ions, and the pitting of magnesium aluminum alloy surfaces will be further aggravated in neutral or alkaline solutions. If the magnesium aluminum alloy is immersed in the NaCl solution, after some induction, the surface of the magnesium aluminum alloy will appear pitting phenomenon.
    3 Magnesium aluminum alloy surface corrosion protection and treatment
    Magnesium aluminum alloy corrosion resistance is not enough to make it limited in a number of applications. To solve the problem of corrosion protection of magnesium aluminum alloy, we can improve the purity of magnesium aluminum alloy from its internal impurity control, develop new corrosion-resistant alloy ion injection magnesium aluminum alloy or laser surface change, etc. to achieve, for large-scale industrial production, can be achieved by means of protective film and layer treatment, with high economy. At present, the methods of anti-corrosion treatment of magnesium aluminum alloy surface mainly include chemical treatment, anode oxidation and metal plating.
    3.1 Chemical treatment
    through chemical treatment can make magnesium aluminum alloy surface form a layer of oxidation protection film, the protective film can form a close combination with magnesium aluminum alloy, blocking the external corrosion media directly erode the magnesium aluminum alloy surface substrate, improve the corrosion resistance of magnesium aluminum alloy. In industrial production, the commonly used chemical treatment method of magnesium aluminum alloy is to use sodium chromate and magnesium fluoride to form a protective film on the surface of magnesium aluminum alloy to protect, the protective film after erosion can also slowly self-repair, very good protection effect. The addition of rare earth elements can also form a protective film on the surface of magnesium aluminum alloy, according to relevant research, the protective film in the pH of 8.5 buffer solution can make Alcoa alloy corrosion rate significantly reduced, but in the pH of 8.5 of the aggressive solution, its protection performance will be reduced.
    3.2 anode oxidation
    magnesium aluminum alloy anode oxidation energy storage compared to chemical treatment methods, can make magnesium aluminum alloy corrosion resistance has been greatly improved. Magnesium aluminum alloy anode oxidation of the oxide film pore rate is high, and the shape of the hole is irregular, if these gaps are not treated, it is easy to lead to corrosion media more easily through the oxide film directly erode the magnesium aluminum alloy surface. The anode oxidation treatment of magnesium aluminum alloy needs to reduce the gap density of the oxidation film. Microarc oxidation technology is a kind of anode oxidation technology developed in recent years, by placing magnesium aluminum alloy in electrolyte aqueous solution, and connecting high voltage to make the micropores in the oxide film of magnesium aluminum alloy surface produce spark discharge spots, improve the protection capacity of oxidation film. Therefore, for ordinary anode oxidation methods, the technology can make magnesium aluminum alloy corrosion resistance can be greatly improved.
    3.3 Metal plating
    magnesium aluminum alloy anti-corrosion metal plating usually chooses chemical nickel plating, magnesium aluminum alloy after acid wash activation, in the magnesium collective can be deposited directly to obtain the chemical nickel plating layer, but because of the strong chemical activity of magnesium itself, in the process there will be a large number of hydrogen desemination, resulting in a decrease in the binding force of the nickel coating and magnesium substrate. The addition of a zinc immersion process in the middle can significantly increase the binding force between the nickel-plated layer and the magnesium substrate, while also increasing the surface brightness of the magnesium aluminum alloy. In addition, fluoride can also be used as an active agent, without zinc immersion process, through chemical nickel plating, passivation and heat treatment, nickel plating layer can obtain good electrical properties, corrosion resistance, weldability and environmental stability, and can make the device's small hole interior is also covered by nickel plating layer, this process is very suitable for spacecraft components on the united States aluminum alloy stringent corrosion requirements.
    Summary
    With the expansion of the application range of magnesium alloy in various industries, its characteristics are increasingly being paid attention to, at present, the relevant industry will focus its research on plastic improvement and corrosion resistance improvement. Magnesium aluminum alloy, as the most widely used magnesium alloy material, is of great significance to the study of its corrosion behavior.
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