Corrosion and Protection of Magnesium Alloys

1 Summary summarizes the corrosion behavior and protection status of magnesium alloys, including the corrosion mechanism, corrosion morphology, negative education effect, as well as the protection of irritants and hair potentials.

Existential questions and development trends.

1 Corrosion of Magnesium Alloys The corrosion behavior of magnesium alloys The reason for the corrosion of magnesium alloys is that the introduction of alloying elements and impurity elements results in the appearance of a magnesium alloy phase. In corrosive ytterbium medium, the highly chemically active iron matrix is ​​very sensitive to the formation of corrosive cells with alloying elements and impurities, and induces galvanic corrosion; in addition, the magnesium alloy's natural corrosion products are loose and porous, and the protective ability of axe leads to magnesium alloys. The corrosive reaction town has continued to develop.

The corrosion reaction for magnesium alloys has not been systematically described so far.

However, in neutral and alkaline media, the corrosion reaction of magnesium alloys is similar to that of pure magnesium, and the following four reactions mainly occur.

The total corrosion reaction Mg 10 2H20 step response 8 = 2, times = ratio + Song Guangling et al studied the corrosion behavior of the He 17 alloy in 1 solution, found that magnesium is the main element of the melt into the solution, also part of the sum, 1 heart 1. This proves that 4 reactions can be used to erode magnesium alloys Huo Weiwei, male, born in 1974, Ph.D. student, 1 primary response of Hawthorn 21, also can rule out the corruption of the liberty of the child. Cambodian direction.

Magnesium alloys have to be eroded in the form of corrosion.

1 galvanic corrosion of magnesium alloys is very easy to galvanic corrosion, 41 out of a small, the only part of the body adjacent to the Ministry of corrosion are usually issued ten Yan, can contact other metal magnesium alloy, also 1 We can collect +40% of gold within 1 gold.

There are many factors that affect the galvanic corrosion. In addition to being electrically conductive, the potential difference between the anode and the cathode, the large polarizability, the low ratio of the surface to the germanium, and the near distance can all lead to accelerated galvanic corrosion and vice versa.

In the containing non-oxidizing medium, the corrosion potential of the gold-based gold crucible is prone to pitting; in neutral or alkaline, the corrosion morphology of the gold is usually pitting corrosion.

3 stress corrosion cracking 30, in poultry, a solution of 1 shake off distilled water. All the alloys of tantalum, tons are corrosion cracking of the tilt; in alkaline medium only. At 12 o'clock, magnesium alloy ash shows good resistance to stress corrosion cracking; in fluoride or fluorine-containing solutions. Magnesium alloys also have good resistance to stress corrosion cracking.

4 Intergranular corrosion and filiform corrosion Magnesium alloys are not sensitive to intergranular corrosion. Corrosion cannot develop along grain boundaries. Filament corrosion occurs when the protective coating or anodic oxidation film moves due to corrosion of the battery on the magnesium alloy surface. .

12 magnesium alloy mask. The study of magnesium alloy face masks should be focused on both film structure and corrosion resistance. It is generally considered that in most environments, the main component of the magnesium alloy mask is MgOH2, which exists in a crystalline form. Marakmi and Sat0L9 think that the mask of magnesium alloy is similar to MgOH2, but it is different from MgOH2. It is an insulator. Because MgOH2 is easy to deliquescence, and the structure is loose, it is easy to exfoliate, so this view can well explain the poor corrosion resistance of most magnesium alloys, but it can not explain the good corrosion resistance of 2910 alloy surface.

For magnesium alloys containing elements such as AlMnZn, when exposed to the atmosphere, it was found that the mask was enriched by the first component lul. The analysis of Mg, the matrix metal, and the magnesium-rich oxides became thinner as the content of AlMnZn increased. . This phenomenon may be related to the strong tendency to form a stable passivation film, but experimental evidence of this point of view is sufficient, and the method of ex-situ analysis of corrosion products is used, and the error is large.

Song Guangling et al. studied the corrosion behavior of 2125501 and 291 alloys in aqueous solution. It was found that the 221 alloy, the phase surface and the 2501 alloy, had different corrosion product film structures. In the 221 alloy surface, the film structure is MgOH2MgOAUO, the matrix metal, there is no obvious interface between the layers; in the AZ501 alloy surface, the film structure is Al, Mg, OH, 8,0, the matrix gold, while the 291 alloy has both It also has 3 phases, so its mask has both a film structure of 221 alloy and 2501 alloy. Although this layered method can explain some corrosion behavior of MgAlZn alloy in NaCl solution, this layered method has a mechanical mechanism. 4 can explain the mask formation and growth process.

Only 3 alternatives, 3 et al.12 studied the microstructure of the 2910 alloy when the corrosion behavior of the ring, found that the 29th alloy alloy cast face mask for the MgOH2 a small amount of P phase MgAl oxide and trace MgH2, and for the die casting 2910 alloy found The mask is a very amorphous structure, which remains to be studied.

1.3 Magnesium alloy's negative differential effect The corrosion of magnesium alloy has a special electrochemical phenomenon. With the increase of external potential or the increase of the applied current density, the anodic dissolution reaction speeds up, and the cathode hydrogen evolution reaction speed also accelerates, namely magnesium alloy. There is a negative difference effect 013, and the polarization curves of the cathode and the cathode conform to the Ding law. At the corrosion potential, the reaction rate of the cathode and anode is equal. When the electrode potential rises to a more positive value. At 1 o'clock, the anodic reaction rate increased to 8 along the line, while the cathodic reaction rate decreased from 1 line to 1. However, in experiment 1, it was found that the magnesium alloy anode reaction rate and cathodic reaction rate were large at the same time as the applied voltage was raised. , Magnesium electrode anodic polarization hydrogen evolution reaction rate increases along the line of the old polarization potential of the actual hydrogen evolution reaction rate is much greater than the theoretical polarization curve expected at the same time the anodic dissolution rate of magnesium, change, for a given The actual dissolution rate of the corrosion potential is significantly greater than that expected from the theoretical polarization curve.

The cause of the negative difference effect + can be explained by a simple dynamic process. The corrosive field has proposed the following mechanisms: First, several mechanisms for the formation of metastable monovalent magnesium ions, and the formation of one or more flavours have their own limits, and they have not yet been established. This study has important theoretical significance.

2 The status quo of magnesium alloy protection To do a good job of magnesium alloy protection work and improve the service life of magnesium alloy materials. So far there have been many meaningful research efforts. Focus on the following two aspects.

21 Control of metallurgy Insolation from magnesium alloys Corrosion resistance Metallurgical factors include alloy groups. Hey. Impurity element phase composition and microstructure. Different elements have different effects on the corrosion performance of magnesium alloys. Can be divided into categories is the content of small foot 51, the impact on the corrosion rate is not significant, such as 1 MnNaSiZrCePrY and other element cable; the other is to make the corrosion rate slightly increased, such as 28; the first is to make the corrosion rate extreme To increase the effect, the method of raising the purity of the magnesium alloy can be adopted to reduce the content of the 0, etc. chord cable below the critical value 2. The corrosion resistance of the high-purity magnesium produced by the electrolytic method is greatly improved. For example, the corrosion resistance of the 291 alloy in the salt spray test is about 100 times that of 29, which exceeds that of the cast aluminum alloy six.

The phase composition has a lot of colorful effect on the corrosion performance of magnesium alloys. The processing technology of the alloy is different, and the phase composition and containment are different, resulting in different color ring mechanisms. Rapid solidification can improve the phase composition and microstructure of the material, make the matrix more homogeneous, reduce defects, and suppress local corrosion; at the same time, the solid solubility of the alloy increases the range of the new phase formation and alloy composition, and the original harmful elemental rope can exist. Ding Youyi or less harmful parts, slow down corrosion. The literature, 22 shows that the corrosion resistance of rapidly solidified magnesium alloys is significantly better than that of ordinary cast magnesium alloys. Document 23 shows that the corrosion behavior of the phases in the Alloy 629 is dependent on their aluminum content and local current density. When the current density is high, the eutectic with high aluminum content will preferentially erode at the grain boundary, and if the current density is low, the primary and internal phases within the grain will preferentially erode. In addition, the three-phase effect, in the dissolution process of magnesium alloys, can play a dual role in preventing the dissolution of magnesium alloys and as a galvanic corrosion cathode; if the 0 phase has a small discontinuous distribution of the body æž³ fraction, it is 0 for the phase. The phase is the cathodic phase and is the primary site for cathodic hydrogen evolution reactions. Right, the corrosion of the phase matrix accelerates, whereas if it is continuously distributed in a large mesh, it acts as a hindrance to the corrosion reaction.

2.2 surface treatment technology section, because of its simple process. low cost. However, it has been widely used in the world and the following methods of surface treatment of magnesium alloys are as follows.

Chemical conversion treatment of magnesium alloys 2. At present, chemical treatment of aqueous solutions mainly composed of phosphate and dichromate is mainly used, and the process is relatively mature.

The anodized anodic oxide film of magnesium alloy is an electrochemical conversion film, which is thicker than the chemical conversion film and has high hardness, high abrasion resistance and corrosion resistance, and is an ideal base layer for long-term anti-corrosion coating; however, it has a porous, anode. After the oxidation must be closed.

5 Formation of chromium or chromium nitride coating 28 on magnesium alloy surfaces by physical vapor deposition techniques, machine coating, such as polyethylene polypyrrole coating 291. Recently, phosphate permanganate and fluozirconate have also been studied. Salt is used as an oxidizing solution for the chromium-free environmental protection coating 3, and a dilute conversion film coating formed by dipping the magnesium alloy into a rare earth salt solution.

3 Outlook In the past few decades, the corrosion mechanism and protective measures for magnesium alloys have been studied. Although the chromate treatment in oxidation has a good process stability and stability, it contains toxic, 6+, and can not meet the environmental cleanliness and no Contamination requirements; anodic oxidation is difficult to form a complete and dense protective film on the magnesium alloy surface, must be after appropriate treatment in order to be applied in practice. In order to meet the requirements for the industrialization of magnesium alloys, the development of low-chromium and non-chromatized environmentally-friendly chemical oxidation techniques, micro-arc plasma anodization technology, magnetron sputtering surface nanotechnology, laser surface alloying technology, etc. are the current magnesium alloy surface treatment technology. Direction of development. It is of great practical significance and far-reaching historical significance to deeply develop and implement the research on corrosion and protection technology of magnesium alloys in order to promote the industrialization of magnesium alloys in China and to implement the strategy for the development of the western region and meet the challenges of entering the evaluation.

Zeng Xiaoqin, Wang Qudong, Lu Yizhen, et al. Foundry, 1998, 39 Dai Changsong, Wu Yiyong, Wang Dianlong, et al. Ordnance Materials Science and Engineering, Burks, Meyer 0 Introduction to High Temperature Oxidation. Beijing Metallurgical Industry produced 13 Cao Chunan. Electrochemical corrosion principle. Beijing Chemical Industry Publishing, 1989 20 Zhu, Fang. Corrosion resistance of non-ferrous metals and their applications. Beijing Chemical Industry Press, 1995 24 Zeng Aiping, Xue Ying, Qian Yufeng, et al. Corrosion Science and Protection Technology, 2000, editor Zhang Hanmin