Magnesium is the lightest metal structural material # Pure magnesium has poor mechanical properties. However, magnesium alloys have attracted much attention because of their small volume, high specific strength, good processability, good electromagnetic shielding, good vibration reduction, electrical and thermal conductivity. Magnesium alloys have been widely used in aerospace industry from the early days to the present in automotive materials, optical instruments, electronic telecommunications, military industry and other aspects. However, the chemical stability of magnesium is low, the electrode potential is very negative, and the wear resistance, hardness and high temperature resistance of magnesium alloy are also poor. To some extent, it restricts the wide application of magnesium alloy materials. Therefore, how to improve the comprehensive properties of magnesium alloy, such as strength, hardness, wear resistance, heat resistance and corrosion resistance, and carry out appropriate surface strengthening has become an important topic in the development of materials today.
Magnesium alloy is one of the lightest metal structural materials, and its density is only 1.3g/cm3~1.9g/cm3, which is about 2/3 of Al and 1/4 of Fe. Magnesium alloy has the advantages of high specific strength, high specific stiffness, good shock absorption, electrical conductivity, thermal conductivity, electromagnetic shielding and dimensional stability, and easy recycling. Known as the most promising green material in the 21st century for its light weight and excellent comprehensive performance, it is widely used in aerospace, automobile manufacturing, electronic communication and other fields. However, the high chemical and electrochemical activity of magnesium alloy seriously restricts its application. Proper surface treatment can improve the corrosion resistance of magnesium alloy.
1. Micro-arc oxidation treatment
Micro-arc oxidation technology, also known as micro-plasma oxidation or anodic spark deposition, is essentially a high-pressure anodic oxidation and a new metal surface treatment technology. This process is a process in which micro-area plasma arc discharge is generated on the anode surface under appropriate pulse electrical parameters and electrolyte conditions, and the original oxide on the anode melts instantly, and at the same time, it is cooled by electrolyte, and then a ceramic oxide film grows on the metal surface in situ. Compared with the common anodic oxidation film, the porosity of this film is greatly reduced, which greatly improves the corrosion resistance and wear resistance. At present, micro-arc oxidation technology is mainly used in the surface treatment of non-ferrous metals such as Al, Mg and Ti or their alloys. The oxide film formed by micro-arc oxidation technology of magnesium alloy is mainly composed of MgO and MgAl2O4 spinel phases, with a total thickness of more than 100Lm, and has an obvious three-layer structure: an outer loose layer, a middle dense layer and an inner bonding layer. The dense layer finally accounts for 90% of the total film thickness and forms a micro-metallurgical bond with the matrix. There are many holes and other defects in the loose layer, and their physical and chemical properties are related to the selection of electrical parameters, the formula of electrolyte and the characteristics of the sample itself during micro-arc oxidation treatment. Compared with the common anodic oxidation film, the micro-arc oxidation film has the advantages of small void, low void ratio, close combination with the substrate, and great improvement in corrosion resistance and wear resistance. The film formed by micro-arc oxidation technology has excellent comprehensive performance, firm combination with matrix, simple process and little environmental pollution. At present, its growth law, growth mechanism and influencing factors have been deeply studied, and it has been applied in industry to some extent. It is a surface treatment technology of magnesium alloy with development potential.
2. Chemical transformation
Chemical conversion is to form oxide or metal compound passivation film on metal surface in chemical treatment solution. The chemical conversion film is thin and weak in adhesion, which can only slow down the corrosion rate, but can not effectively prevent corrosion, and needs further coating. There are many studies on chemical transformation of magnesium alloys, the most mature one is chromate transformation, but Cr6+ is toxic, which harms human health and pollutes the environment. In recent years, a series of new conversion processes that are harmless to the environment and health have been developed. These conversion processes can be roughly divided into two types: organic compound solution and inorganic salt solution conversion treatment. The former includes phytic acid conversion [1,2], silane derivative conversion [3] and acid salt conversion [4], while the latter includes phosphating [5-7], stannate conversion [8], fluoride conversion [9,10], phosphate-permanganate conversion [11-13] and rare earth conversion film [14-16].
2.1 phytic acid conversion phytic acid (C6H18O24P6) treatment
Phytic acid to phytic acid is a rare metal multi-tooth chelating agent, which has a unique structure and is a brand-new non-toxic and environmentally friendly metal surface treatment agent. Phytic acid is chemically adsorbed on the metal surface, forming a dense monomolecular organic protective film, which can effectively prevent aggressive anions from entering the metal surface and inhibit metal corrosion. At present, the research on phytic acid transformation in magnesium alloys is still relatively few, and only a few domestic scholars have conducted preliminary research.
Zheng Runfen et al. [1] studied the composition and corrosion resistance of phytic acid conversion film on AZ91D magnesium alloy. It is found that the main components of the film are magnesium salt and aluminum salt of phytic acid, and the film has a fiber network crack structure, which is firmly combined with the matrix, and the corrosion resistance is obviously improved. The corrosion potential is 0.4V higher than that of chromate conversion, and the corrosion current density is 5 orders of magnitude lower than that of chromate conversion. Liu et al. [2] also used phytic acid conversion solution to form a protective film on magnesium alloy, and the corrosion resistance of the film is equivalent to that of chromate conversion film. The monomolecular organic film formed by phytic acid conversion has similar chemical properties to organic coatings, and the adhesion with organic coatings is enhanced, which can further improve the corrosion resistance of magnesium alloys.
2.2 Phosphating treatment of magnesium phosphide
The research on phosphating treatment of magnesium phosphide was carried out earlier. The phosphating film formed has a microporous structure, which is firmly combined with the matrix and has good adsorption, and can be used as the bottom layer of magnesium alloy before painting. Kouisni et al. [5,6] developed a phosphating process for magnesium alloy. The phosphating solution is mainly composed of Na2HPO4, H3PO4, Zn(NO3)2, and the phosphating film formed is mainly composed of Zn3 (PO4) 2 4H2O. The formation mechanism of phosphating film, the influence of various components of phosphating solution and the corrosion behavior of the phosphating film in boric acid buffer solution were discussed. The results showed that the self-corrosion potential increased by about 700mV after phosphating, and the corrosion resistance reached 15h, and its corrosion resistance was expected to be further improved. Li et al. [7] prepared a uniform and fine zinc-based composite phosphating film on AZ91D magnesium alloy by adding sodium molybdate and corrosion inhibitor to the phosphating solution. The phosphating film is mainly composed of Zn3 (PO4) 2 4H2O and elemental Zn particles. The results show that adding sodium molybdate into phosphating solution can make the phosphating film structure more detailed, improve the adhesion between substrate and organic coating and its corrosion resistance, and increase the self-corrosion potential by about 500mV. Other chromium-free conversion treatments also improve the corrosion resistance of magnesium alloys. Among them, the corrosion resistance of phosphate-permanganate conversion film is equivalent to that of chromate conversion film, which can replace chromate conversion film. The composite film obtained by phosphoric acid-manganese carbonate-manganese nitrate conversion has good adhesion, uniformity and continuity, and the corrosion resistance is better than that of chromate conversion film.
3. Self-assembled monolayer
Self-assembled monolayers (SAMs) are ordered monolayers with regular orientation and tight arrangement formed by immersing metals or metal oxides in dilute solution containing active molecules and adsorbing them on the substrate through chemical bonds. The preparation method is simple and has high stability. At present, self-assembled monolayers have been successfully prepared on metals such as Fe, Cu, Al, etc. However, it is difficult to prepare self-assembled monolayers on Mg and its alloys because Mg is easily oxidized, but there are still some domestic scholars who have tried to study it.
Yong Zhiyi et al. [18] prepared the oriented monolayer with -NH- as the head group and -CH3 as the tail group on the surface of AZ91D magnesium alloy for the first time by using imidazoline oleate aqueous solution. The corrosion inhibition of the self-assembled film on magnesium alloy was studied, and the formation process and protection efficiency of the self-assembled film were studied by contact angle, FT-IR, EIS and linear polarization. Under the optimum process conditions, the protection efficiency (PE) reached 98.1%. Liu et al. [19] prepared a dense self-assembled monolayer on AZ91D magnesium alloy by using alcohol solution of carboxylate, and the PE value was as high as 98.5%. The results show that the longer the alkyl chain, the longer the assembly time and the better the corrosion resistance of the self-assembled film.
4. Anodizing
Anodizing is to form a thick and relatively stable oxide film on the metal surface through electrochemical oxidation. The anodic oxide film of Mg is thicker than that of chemical conversion, with higher strength, higher hardness and better corrosion resistance. The anodic oxide film of magnesium alloy has a double-layer structure: a thin dense inner layer and a thick porous outer layer. The pores of the outer layer do not penetrate the inner layer, and the corrosion resistance of the pores of the outer layer is further improved after painting, dyeing, sealing or passivation.
4.1 Ordinary Anodizing
Typical anodizing processes of magnesium alloys are HAE process developed by Evangelides in the United States and DOW 17 process developed by Dow Chemical Company [20]. In the early days, toxic compounds containing Cr were used in anodic oxidation treatment. At present, anodic oxidation processes of soluble silicate, hydroxide and metaaluminate have been developed. In the process of anodic oxidation of magnesium alloy, the composition of treatment solution strongly affects the structure and composition of anodic oxidation film, and anodic oxidation films with different properties can be obtained with different oxidation solutions. Cao Fahe et al. [21] evaluated the microstructure and corrosion resistance of anodic oxide films obtained by different oxidation solutions, and considered that the applied voltage and the composition of oxidation solution had a crucial influence on the microstructure and properties of the oxide films. In alkaline solution, under the synergistic effect of NaAlO2 and Na2SiO3, the oxide film obtained has excellent corrosion resistance, the self-corrosion current density is 1.87×10-7A/cm2, and the neutral salt spray corrosion resistance is more than 500h (the oxide film is not sealed). Zhou Lingling and others [22] studied an environmentally-friendly anode process, and the microhardness of the obtained film was as high as 558.4HV, and its corrosion resistance was far better than that of the protective film prepared by the traditional process containing CrDOW17.
4.2 plasma oxidation plasma oxidation
As an environment-friendly treatment technology, a surface treatment technology developed in recent years was first used to improve the wear resistance and corrosion resistance of aluminum alloys. It uses high voltage discharge to generate thermal plasma, and uses the instantaneous high temperature in the plasma area to directly grow ceramic membrane on the metal surface in situ. The coating obtained by plasma oxidation has excellent comprehensive properties, firm combination with matrix, simple process and little environmental pollution, which is an important development direction of magnesium alloy surface treatment. Plasma oxidation can be divided into two types: one is plasma chemical action in aqueous solution; The other is to replace the aqueous solution with oxygen plasma. The latter is a more advanced and environmentally friendly process, and plasma oxidation can also be combined with physical vapor deposition (PVD) to obtain a wear-resistant and corrosion-resistant film [23, 24]. Timoshenko et al. [25] used NaOH and Na3PO4 electrolyte to oxidize magnesium alloy, with a film thickness of 60μm and porosity.
5, electroplating and electroless plating
The electrochemical activity of magnesium alloy is very high, and the plating solution will corrode the magnesium alloy matrix, and Mg and cations in the plating solution will be replaced, resulting in a loose and porous coating with poor adhesion. Therefore, the magnesium alloy must be properly pretreated. The traditional pretreatment includes zinc dipping and direct electroless plating, and then electroless plating or electroplating is carried out after a protective film is formed. At present, there are many studies on electroless nickel plating on magnesium alloys [29 ~ 31]. The research shows that a reasonable pretreatment process plays a vital role in the implementation of the whole electroless plating technology, the quality of the coating and the bonding force between the coating and the substrate. U.S. Patent [32] puts magnesium alloy into CuSO4 solution, and Mg replaces Cu in ultrasonic wave, thus forming a dense Cu film on magnesium alloy, and then plating Ni/Ti/Mn/Al/Fe/Co/Zr/Mo/Nb/W by electroless plating/electroplating/brush plating or their combination, and the inner Cu film provides cathodic protection for Mg and its alloy. After directly electroless nickel plating on AZ91D magnesium alloy, Gu et al. [33] deposited Ni nano-coating by direct current. The particle size of the coating is about 40nm, the structure is fine, the porosity is low, the surface of the coating is dense, and the hardness is much higher than that of the matrix. Ni nano-coating has high corrosion resistance and strength, which is expected to promote the application of magnesium alloys. Zhu Liqun et al. [34] obtained a composite coating on the surface of AZ91D magnesium alloy by electrodeposition and low-temperature heat treatment. After plating a layer of zinc, tin was plated to obtain a Zn-Sn composite coating with good adhesion and a uniform surface. After heat treatment at 190 10℃ for 12 hours, the composite coating formed a three-layer structure due to the diffusion of Sn: the inner layer was dense, consisting of Sn and Mg2Sn; The middle layer consists of Zn and ZnO; The outer layer is loose, and the main component is Sn. The results show that the corrosion resistance of this three-layer coating is better than that of Zn-Sn coating. Because the coating has a more positive potential than the magnesium alloy matrix, it is prone to galvanic corrosion compared with the magnesium alloy cathode. To achieve the corrosion protection requirements of magnesium alloy, it mainly depends on whether the coating is uniform, non-porous and has a certain thickness. Nano-composite plating will be a new direction of magnesium alloy surface protection.
6. Liquid deposition and sol-gel coating
The research of obtaining inorganic, organic and inorganic-organic hybrid films on magnesium alloy surface by liquid deposition (LPD) and sol-gel method is still in the trial stage. Some researchers have obtained nano-oxide films by these methods, which is a new direction of magnesium alloy surface treatment.
6.1 Liquid Deposition (LPD)
Liquid deposition is a method to generate oxide films from aqueous solution of metal fluoride. By adding water, boric acid or metal Al, metal fluoride is slowly hydrolyzed into metal oxide and deposited on the surface of the substrate. Hu Junhua et al. [35] prepared anatase TiO2 _ 2 thin films on AZ31 magnesium alloy by LPD method for the first time, with an average particle size of 100nm. The surface of the thin films is composed of particles ranging from 150 nm to 200 nm, and the thickness of the thin films is about 7 μ m.. It is found that lower hydrolysis temperature and shorter deposition time are beneficial to improve the corrosion resistance of the film.
6.2 sol-gel coating
In recent years, the research on the preparation of organic-inorganic hybrid materials and inorganic composites by sol-gel method is very active. Sol-gel coating can improve the corrosion resistance of metals, but it is difficult to directly coat magnesium alloys, because magnesium alloys react with some components in the sol, resulting in poor bonding. There are three ways to improve the adhesion of film: organic-inorganic hybrid, inorganic composite film and multilayer composite film. Khramov et al. [36] modified the silica sol with siloxane containing phosphate groups, and prepared an organic-inorganic hybrid film on AZ31B magnesium alloy. The components in the film can react chemically with the magnesium alloy matrix to form P-O-Mg bonds, which greatly improves the adhesion and corrosion resistance of the film. Phani et al. [37] prepared SiO _ 2-Al _ 2O _ 3-CEO _ 2 composite film on magnesium alloy by sol-gel technology. The nano-composite Al _ 2O _ 3-CEO _ 2 was dispersed in SiO _ 2 matrix, and after annealing at 180℃ and 140℃ respectively, the hardness and elastic modulus reached 4.5GPa and 98GPa, respectively, and the salt spray corrosion resistance test reached 96h. It is considered that CeO2 improves the corrosion resistance of the film, and Al2O3 improves the adhesion and wear resistance.
The combination of anodic oxidation and sol-gel method can greatly improve the adhesion of sol-gel film by using the porous characteristics of anodic oxidation film. Tan et al. [38] sprayed the prepared sol on the surface of AZ91D magnesium alloy after anodic oxidation. After repeated spraying, the film thickness can reach 57μm and the self-corrosion potential can be increased to-0.8V..
7. Vapor deposition
7.1 Physical Vapor Deposition (PVD)
Physical vapor deposition (PVD) is a process of transporting gas atoms, molecules and ions (gaseous state, plasma state) generated by high-temperature evaporation, sputtering, electron beam, plasma, ion beam, laser beam, arc and other energy forms from solid (liquid) plating materials, depositing and condensing on the solid surface and generating solid thin films. PVD deposition is fast and pollution-free, but the disadvantage is that the adhesion and uniformity of the film are poor, so it must be properly treated before and after deposition, and Ti ion implantation is an effective surface modification method [39].
According to different protection requirements, there are many studies on depositing metal nitride films on magnesium alloys by PVD process [40 ~ 42]. Initially, it was to meet the strength and wear resistance of magnesium alloys, and now it is attached importance to the application as protective films. Wu et al. [43] deposited ceramic/metal double coating on AZ31 magnesium alloy by multi-target magnetron sputtering technology. The Al2O3/Al film greatly improved the corrosion resistance of the alloy, and the Al2O3/Ti film improved the mechanical properties of the alloy surface. Hikmet et al. [44] deposited multilayer AlN and AlN/TiN films on AZ91 magnesium alloy by DC electromagnetic sputtering PVD method, in which the former has better corrosion resistance.
7.2 Plasma enhanced chemical vapor deposition (PECVD).
PECVD relies on the kinetic energy of electrons in cold plasma to activate the gas phase chemical reaction, which has the advantages of low deposition temperature and high deposition rate, and is especially suitable for magnesium alloys. Voulgaris et al. [45] used radio frequency (RF)PECVD to deposit SiOxCyHz thin films from tetraethylorthosilicate (TEOS) on the surface of magnesium alloy, with good film coverage, smoothness and improved corrosion resistance. Diamond-like carbon (DLC) films prepared by PECVD can significantly improve the hardness and wear resistance of magnesium alloys, effectively reduce the friction coefficient and improve the corrosion resistance [46 ~ 48].
8, spraying
8.1 Thermal Spraying Thermal Spraying Technology
Using gas (liquid) fuel or arc, plasma arc, laser as heat source, the spraying material is heated to a molten or semi-molten state, atomized by high-speed airflow, and then sprayed and deposited, thus forming a coating with firm adhesion. In recent years, thermal spraying technology has a good application prospect in magnesium alloy surface modification, which is a good long-term protection method, but it will cause strong oxidation of magnesium matrix during spraying. Chiu et Al. [49] arc sprayed aluminum on the surface of AZ31 magnesium alloy, and the Al coating formed was then heat treated and anodized to form a layer of Al2O3, which greatly improved the corrosion resistance. The dense WC-12Co coating [50] was deposited on magnesium alloy by high-speed flame spraying (HVOF) technology. The high kinetic energy of WC-Co will produce self-roughness effect, and it has good adhesion with the matrix. However, the WC-Co coating without sealing treatment can not protect the matrix, but will accelerate the corrosion. If a layer of Al is sprayed in advance, the corrosion resistance of magnesium alloy will be greatly improved. In addition, it is also an effective anti-corrosion method to seal with organic paint after spraying WC-Co.
8.2 Cold spraying and cold spraying technology
Cold spraying and cold spraying technology is a new spraying technology in recent years. It uses electric energy to heat a high-pressure gas stream (protective gas such as N2 or He) to a certain temperature, and the gas stream is accelerated through a Laval tube to generate a supersonic beam, which is used to accelerate powder particles, impact the surface of the substrate at supersonic speed, and form a coating through plastic deformation of the solid. The cold spray coating is a deformed structure, and after being treated under special conditions, the nanostructure structure can be obtained. Cold spraying on magnesium alloy surface can prevent the oxidation of magnesium alloy surface during spraying. Domestic scholars [51] studied the cold spraying of rapidly solidified Zn-Al alloy powder on the surface of AK63 magnesium alloy for the first time, and obtained a dense coating. There was no sintering and melting phenomenon at the interface between the sprayed coating and the matrix, and the coating had strong adhesion with the matrix of magnesium alloy, which greatly improved the hardness of magnesium alloy.
9. Laser cladding alloy coating
Some scholars at home and abroad have studied the materials and properties of magnesium alloy laser cladding, which shows that laser cladding can refine the surface structure of magnesium alloy and change the structure of magnesium alloy. It is an effective method to improve the surface properties of magnesium alloy and has a good prospect. Yue et al. [52] laser cladding 1.5mm thick Zr65Al7.5Ni10Cu17.5 amorphous alloy on pure Mg substrate. The results show that the microhardness of cladding alloy layer is increased to HV550~600, and the corrosion potential of cladding layer is 1120mV higher than that of standard sample. Gao et al. [53] prepared Al-Si alloy on the surface of AZ91HP magnesium alloy by broadband laser cladding technology. The cladding layer contains Mg2Si, β-Mg17Al12 and Mg2Al3 metal compounds and α-Mg. It was found that the microhardness increased by 340%, the wear resistance increased by 90%, and the corrosion resistance of the cladding layer was greatly improved due to grain refinement and redistribution of Mg intermetallic compounds.
Concluding remarks
As a new structural material, magnesium alloy will be more and more widely used, and its corresponding surface treatment methods will also be developed rapidly. Chromium treatment of magnesium alloys pollutes the environment and harms human health in production. Many researchers are looking for new methods to replace the existing treatment technology. Phosphating treatment is a promising method in chromium-free treatment of magnesium alloys and has the trend of replacing chromium treatment. Micro-arc oxidation treatment technology has the characteristics of simple process and wide material adaptability, and the obtained film is uniform and hard, which will be a development direction of magnesium alloy anodizing. Organic coating can play a long-term protective role, but the combination between coating and substrate is not very close, which is also an important factor restricting its development. Developing new coating materials and coating technology is a good way to improve the performance of organic coating. Therefore, it is of great practical significance and economic benefits to strengthen the development of magnesium alloy surface treatment technology, deeply study the formation mechanism of protective film, and further improve the performance of surface protective film to improve the corrosion resistance of magnesium alloy.
Authors: School of Materials Science and Engineering, Jiangsu University
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