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Effect of sol-gel composite treatment on corrosion resistance of magnesium alloy based on computer simulation.

1. Introduction

In nature, the storage of magnesium is very rich, the crust is only less than aluminum, iron, is the third highest content of structural metal. China Magnesite amount has reached 53 billion tons, Qinghai Salt Lake also contains large amounts of magnesium, is China's important magnesium resources (Crespo, 2015; Baojun, 2016). The existing reserves of magnesium resources in China have been the first in the world. Magnesium is a silver white light metal, high strength; its various structures are applicable to become the lightest quality of metal materials (Junying, 2009; Lopez, 2010). Magnesium alloy has many excellent properties, easy processing, low processing cost, good heat dissipation, thermal conductivity, and good electrical conductivity; shock absorption, anti-electromagnetic interference of excellent; density is small, plastic good, good superconductivity, and storage rich. These advantages make the application of magnesium alloy is more and more extensive. Magnesium alloy is known as "green environmental protection material for twenty-first Century", so it is of great significance to the development and utilization of magnesium and magnesium alloys (Mengguo, 2014; Fen, 2015).

Magnesium alloy density is small, compared with other metal materials products, the same volume; the weight of magnesium alloy products will reduce a lot (Shaohua, 2012; Pereira, 2015). Advocate of saving energy consumption in, the magnesium alloy applied to the automobile industry, can effectively reduce vehicle weight, scientific analysis, used in automotive fuel is 60% in their car, so the application of magnesium alloy with low density making cars can effectively reduce the fuel cost, along with the amount of CO emission will be reduced, can effectively improve the greenhouse effect, which for the realization of the sustainable development has important significance (Shiyan, 2009). At present, magnesium alloy die-casting parts for automotive products have a lot of, such as body, wheel, driving wheel, gear box, cylinder cover, brake pedal frame, etc.. Nowadays, the electronic industry is developing very fast. The magnesium alloy components are widely used in computer and communication equipment with the advantages of high thermal stability, good thermal conductivity, light weight, high stiffness and so on. And the application of magnesium alloy in the electronic industry is growing at a rate of 25% per year. Because the magnesium alloy has excellent electromagnetic shielding properties, the mobile phone has replaced the original PC/ABS plastic with magnesium alloy, which improves the quality of the call. In recent years, magnesium alloy is more and more popular in the electronic industry.

Magnesium Corrosion in essence is the occurrence of magnesium electrochemical and chemical reactions, was oxidized to Magnesium Oxide. C[O.sub.2] in the presence of Mg [(OH).sub.2] and [H.sub.2]C[O.sub.3] will react to produce magnesium carbonate. Similarly, if there is a S[O.sub.2], and Mg [(OH).sub.2] response, resulting in Magnesium Sulfate. But these films are water soluble, relatively loose, the ability to protect the substrate is very poor. In addition, due to the negative electrode potential of magnesium balance, -2.37V, compared to all other metals, magnesium is almost all anode polarity. So magnesium and its alloys are easily corroded in wet conditions, acidic and neutral media. In solution, the corrosion process is as follows:

Mg+2[H.sub.2]O [right arrow] Mg[(OH).sub.2]+[H.sub.2][up arrow]

The common corrosion types are mainly galvanic corrosion, pitting corrosion, filiform corrosion and stress corrosion of these.

* Galvanic corrosion: Compared with other metals, the standard electrode potential of magnesium is low, when the magnesium in contact with other metals, usually as the anode react cathode is direct contact and magnesium metal, or other metal magnesium alloy contains an internal, magnesium alloy on galvanic corrosion sensitivity than strong, so for the magnesium alloy the main forms of corrosion is galvanic corrosion.

* Pitting corrosion: Magnesium, as self passivating metal, when faced with chloride ion of non-oxidizing substances will corrosion potential and pitting corrosion occurred in, when the salt solution appeared to be alkaline or neutral, magnesium alloy is extremely susceptible to pitting and when the solution containing heavy metal pollutants will accelerate the pitting corrosion process.

* Filamentous corrosion: Filiform corrosion may occur in the layer below with anodic oxide film or protective coating of magnesium alloy and pure magnesium metal generally do not occur filiform corrosion, but not after treatment of AZ91D alloy is filiform corrosion may occur. On the surface of the alloy to form a protective oxide film, oxide film covering filiform corrosion, but hydrogen will destroy the protective oxide film.

* Stress corrosion: Stress corrosion of the metal in some tensile stress and corrosion of the common role of the media will be brittle fracture, aluminum containing magnesium alloy stress corrosion (SCC) is very sensitive. When the pH value is greater than a certain value, the magnesium alloy exhibits a better stress corrosion resistance; but in distilled water or in the neutral aqueous solution containing Cl-, magnesium alloys is prone to stress corrosion; in fluorinated magnesium alloy will be exhibited better resistance to stress corrosion performance.

2. Corrosion resistance of magnesium alloys

It is necessary to carry out a certain surface protection treatment before using magnesium and magnesium alloy products. At present, in order to improve the performance of magnesium alloy to use time became long, applications try to expand the scope, has many techniques are applied to the magnesium alloy surface treatment, the main treatment methods commonly used in a chemical oxidation, metal coating, anodic oxidation, organic coating, micro arc oxidation, some methods can be matched with organic coating, through treatment, relatively dense protective film formed on the surface of magnesium alloy, or coating so as to improve the corrosion resistance of magnesium alloy.

2.1. Chemical oxidation treatment

The chemical oxidation of the magnesium alloy is a chemical reaction between the substrate and the solution, and a protective passivation film is formed on the surface of the magnesium alloy. The corrosion prevention effect is better than that of the natural oxide film, because the film is uniform, and the process is simple, and can be applied to various shapes of parts. And the thin film can provide a better coating of the substrate. But on the other hand, because of its thin film, corrosion resistance, wear resistance will be limited, only to a certain extent, the speed of corrosion can be slowed down, cannot be used as a long-term anti-corrosion or wear protective layer to use. There are a lot of methods of magnesium alloy chemical conversion film processing, according to the different treatment liquid ingredients can usually be divided into chromate, organic acids, potassium permanganate and zirconium fluoride acid salt system, are often used, technology is the most mature processing solution is chromate based chemical treatment solution. The hybrid film formed on the surface of magnesium alloy has good bonding strength with the substrate, strong self-repairing ability and stable performance. The corrosion resistance of chromate film in a large extent is determined by the content of chromium in the film, with the increase of hexavalent chromium ion content, the corrosion resistance of the film will increase, but C6 is toxic, need high cost of wastewater treatment, environmental regulations have limited its emissions. Therefore, people have begun to study effective chromium free conversion technology. In addition, people began to pay attention to basic research, study the microstructure and growth mechanism of chemical conversion film, and hope that this basic research can promote the development of new chemical conversion film.

2.2. Anodic oxidation treatment

Anodic oxidation technique is a kind of widely used traditional technology and its working principle is using electrochemical method in the electrolyte and must process conditions, let magnesium or magnesium alloy as anode, let the iron, stainless steel or nickel as cathode, connected to the external current, on the surface of magnesium or magnesium alloy will have a layer of thick and relatively stable like porcelain hard film anode oxidation method with respect to the natural oxidation film speed has a great advantage in fibroblasts.

Anodic oxidation of the resulting oxide film thickness, with a certain degree of strength and hardness, the corrosion of the magnesium substrate has a certain degree of protection, wear resistance and corrosion resistance than the chemical conversion film. The film layer is a double-layer structure, the inner layer is a dense layer, and the outer layer is a porous layer. Because of its porous structure, it can be further used as a base for further coating of paint and coating. This can make the magnesium alloy sample is beautiful, corrosion resistance and strong. Ethylene resin, epoxy resin or epoxy resin and other organic compounds are usually used for filling and sealing. The structure, properties and color of the oxide film formed by anodic oxidation method are affected by the composition, concentration, temperature, electrolysis condition and other factors of the electrolytic solution. Among them, the electrolyte is very important factor, magnesium alloy anodic oxidation can be done in two kinds of alkaline or acidic electrolyte, the composition of the treatment liquid seriously affect the structure and composition of the anodic film. Acid solution to Dowl7 as the representative, the film generally contains the Mg[F.sub.2], [mg.sub.3](P[O.sub.3]) 2 and other components is not clear chromium compounds, formation of film porosity is higher, the film has good heat resistance, four or five days is maintained even under very high temperature does not affect the binding force of the coating performance and coating with the substrate. The corrosion resistance of pure magnesium and AZ91D magnesium alloy has been greatly improved compared with that of chemical conversion film after Dowl7 process. As the representative of the alkaline solution to hae, NaOH is the solution of the fundamental group, in a solution containing NaOH magnesium alloy easily anodic oxidation film, the main component is Mg [(OH).sub.2], and film general porous, protective performance is relatively poor. In order to improve the structure and performance of the membrane, it is necessary to add other ingredients in the electrolyte.


2.3. Sol-gel technology

Since 1970, with the development of electronic technology and other high-tech fields, sol gel technology, as a new high-tech manufacturing technology, has developed very quickly. The sol-gel method can be used to obtain the coating with special mechanical properties and chemical protection. Sol gel technology has been widely used in the preparation of thin films, fibers, powders and composite materials. Sol gel coating method is based on the principle of colloid chemistry, with a suitable salt as the initial raw materials through hydrolysis and condensation made transparent and stable sol, after dip coated on the surface of the workpiece, after drying, calcination and sintering to obtain surface film method. The general use of inorganic salts in the industry, the initial material with halide or hydroxide, the purity is not high. The sol--gel method is generally used for the solution of organic alcohol solution, with the metal alcohol salt as the precursor, after two steps of reaction, it will be transparent, and there is a certain viscosity of the sol solution.

Sol gel method, homogeneous sol coated on substrate method has a lot of, common with Czochralski method, spin coating or spraying method, after pulling, dry and repeats the process, thicker films are formed. Finally high temperature curing processing can form compact film. Surface modification of materials by Sol--gel method is a new method. Compared with other methods, the sol-gel method has the following advantages:

1. Alkoxide material mostly soluble in alcohol, easily purified, and the synthesis of materials with high purity, and is in the state of low viscosity liquid mixing raw materials, chemical reactions in solution to the ceramic obtained uniform, fine and dense, can get highly uniform group divided into oxide coating, is conducive to the preparation of composite oxide.

2. The required elements can be added to improve the film properties, because the solution is easy to disperse evenly, and the fusion of the molecules is good;

3. Equipment requirements are relatively simple, there is no strict requirements on the shape of the matrix, and the size of the particle size distribution of the coating is narrow, particle size.

4. The temperature of heat treatment is low, and the mobility is good.

5. Some new inorganic materials or organic inorganic composite materials can not be prepared by other methods, can be used in sol-gel method, such as the Zr[O.sub.2] content of glass prepared by sol gel method up to 48%.

6. In the process of operation, there are many kinds of methods to change the surface structure and performance of the film. Sealing of magnesium alloy micro arc oxidation film by sol gel method, can improve the film thickness and the compactness, by micro arc oxidation film holes, protect the substrate, to further improve the corrosion resistance of the coating corrosion medium barrier.

Sol--gel method has many advantages, but it cannot be ignored. First, sol--gel method used raw materials more expensive, and aging, heat treatment time is relatively long; secondly because in the wet gel contains large amounts of water and solvent, in dry heat treatment process in general will be accompanied by the shrinkage, easily lead to film cracking, the film also is relatively thin. Therefore, to make better sol gel films need to reaction mechanism, the operation process in detail.


3. Characterization and properties of magnesium alloy composite films

3.1. Surface morphology of composite membranes

Figure 3 for magnesium alloy micro arc oxidation coating by sol gel sealing treatment after the formation of composite membrane surface topography, the acceleration voltage of 500 kV, which figure 3 (a) is a film of the macroscopic surface morphology, figure 3 (b) to (d) is gradually enlarge the morphology of film. From the enlarged figure (c) and (d) can see the micro arc oxidation coating on the surface by dip coating method to seal the whole, hole basically is sealed, some holes with bubble like protrusions. This is mainly due to the sol coating in the drying process of internal moisture and solvent evaporation induced. Sol gel coated evenly covered outside the micro arc oxidation film; but due to the film in the process of drying, evaporation of solvent and water at the same time the volume have great contraction, resulting in the film is part of the crack, obviously, after sealing treatment, film appearance more smooth and compact.

As shown in Figure 4 for the magnesium alloy surface after the composite treatment of the energy spectrum, table 3 for the content of the various elements of the spectrum. From Figure 1 and table 4 we can see that the main elements of the composite film measured by O, Mg, C, Si and Ti, the percentage of its mass were: O 33.36%, Mg 22.2%, C 21.88%, Si 8.47%, Ti 5.07%. As can be seen from the table, spectrum contained in the micro arc oxidation film elements. This is due to the sol--gel sheet mask layer is too thin, film is too thin, in the detection of electron penetration sol membrane to Magnesium Alloy Microarc oxidation film.



3.2. Infrared spectrum of dry gel powder

In order to analyze the bond property and the product of titanium sol, the infrared spectrum of TiO2 sol was detected, and the infrared spectrum of the test was shown in Figure 5. According to various functional groups of absorption peak position analysis of the sol in the infrared spectrum absorption peak corresponding to the functional groups can be identified the following characteristic absorption peaks: in the wavenumber for 3200 ~ 3500 [cm.sup.-1] absorption peaks corresponding is hydroxyl (O-H) stretching vibration absorption peaks can be seen in the vicinity of the 3367 [cm.sup.-1] is (O-H) absorption peak; powder samples in 1635 [cm.sup.-1] has an absorption peak, is caused by the oh bending vibration frequency of absorption peak. The absorption peaks at 2866 [cm.sup.-1] and 1396 [cm.sup.-1] at the wave number are respectively -C[H.sub.2]- bending vibration and deformation vibration peak, and the wave number is at 1541 [cm.sup.-1]. At 1076 [cm.sup.-1] vibration peaks of Ti-O-Ti, that has formed Ti[O.sub.2] particles in solution. The telescopic peak at 530 Ti-O was [cm.sup.-1], which showed that Ti (OC4H9) 4 did not completely hydrolyzed.


3.3. Thin film X-ray photoelectron spectroscopy spectra

Composite thin film XPS map as shown in figure 6. We can see that the surface of the sample is mainly composed of five elements: Mg, O, Si, Ti and C. The sample surface has the C element is the adsorption C, because the sample is easy to adsorb the C atom in the air, generally in the XPS sample analysis test can find the C element peak.

Figure 7 is a narrow area of S12p, O1s and Mg2p scanning. The fitting results can be obtained: the surface of the prepared composite film contains the element composition of the micro arc oxidation film. The Mg element is present in Si, and the [Mg.sup.2+] element is present in the form of the complex of the oxide and the oxide, and the Ti element exists in the composite state of the oxide and the oxide. In Figure 7(b) the spectrum of T12p is composed of four peaks: 455.20 eV, 458.23 eV, 462 eV and 464.50 eV. Which, consistent 455.20eV spectral peak and XPS database tio1.5 T12p3/2 standard spectral peak 455.20 EV; peak 458.23 EV and 464.50 EV peak respectively with XPS database of Ti[O.sub.2] in T12P standard peaks in 458.30 EV and T12pi/2 standard 464.50 EV close to, said Ming Ti bound for Ti[O.sub.2]; 462 EV spectral peak and XPS database T12[O.sub.3] T12pi/2 standard spectral peak 462 EV is consistent. XPS results show that the sol gel film contains low valence titanium compounds. (c) in the Ois spectrum, the peak at 531.20 eV corresponds to the Ti[O.sub.2], which is in agreement with the results of T12p, and the peak at 533.10 eV is caused by (-C[H.sub.2]O-) n group. The polymerization state of titanium in sol gel film is also described. The other two peaks in the spectrum are 531.75 eV and 532.28 eV, respectively, corresponding to [Mg.sub.2]Si[O.sub.4] and Si[O.sub.2].



4. Evaluation of corrosion resistance

4.1. Polarization curve analysis

Fig. 8 is polarization curves of magnesium alloy composite films under different pH values of 3.5%NaCl in water solution. The corresponding corrosion potential and corrosion current density of each sample under different pH are listed in table 2. As can be seen from the table, composite film in neutral salt solution corrosion current density is the smallest, and acid or alkali concentration increase that the corrosion current density of the film increased, indicating that acidic and alkaline media of oxide film have damaging effects, and with the increase of the concentration of acid or alkali, destructive also increases. In the film elements analysis showed that the composite film contains not only MgO and mgsio4, the two substances, also containing TiO2 and Ti complexes, effective sealing of micro arc oxidation film, thereby inhibiting invasion of corrosive particles [Cl.sup.-], decreases the self-corrosion current density, and the corrosion rate decreases and increases the density of thin films and corrosion resistance, from the table and ceramic film corrosion current density contrast will found after combined treatment, the corrosion current density of the film reduces one order of magnitude. The corrosion resistance of magnesium alloy treated by micro arc oxidation and sol gel composite was obviously improved.


4.2. AC impedance analysis

Figure 9 is magnesium alloy composite membranes at different pH values under 3.5% NaCl aqueous solution of AC impedance spectroscopy, which figure 9 (a) for AC impedance Nyquist plot, from the Nyquist diagram can see, impedance shape are basically the same, from high frequency capacitance arc diameter size can determine the oxide film layer corrosion resistance strength and film capacitor size, which samples in solution pH impedance spectrum maximum radius, in acidic and alkaline solutions measured film impedance values decreased, and with the increase of concentration of acid and alkali, corrosion resistance is weak, from the figure can also see a film in acidic media more easily destroyed. But the ceramic membrane is only partially destroyed, the coating on the magnesium alloy substrate still plays a protective role. And comparison of micro arc oxidation film in different pH values of 3.5% NaCl aqueous solution of AC impedance spectra can be seen, in the same conditions, the composite films in high frequency capacitance arc diameter to capacitance arc diameter that is larger than the micro arc oxidation film.


Figure 9 (b), AC impedance Bode diagram, from the impedance of the Bode diagram can be seen at the low frequency end, with the increase of pH impedance value decreases, and in acidic medium, the impedance values decreasing. The results show that the composite membrane in acidic medium is more vulnerable to corrosion. It can be seen from the phase Bode diagram 9 (c), under different pH, Magnesium Alloy Microarc oxidation film samples were a time constant, the phase angle of the maximum value at 100 degrees, with increase of pH, phase angle decreased. The AC impedance spectra of micro arc oxidation films under different pH values of 3.5%NaCl aqueous solution can be seen. Under the same conditions, the phase angle of the composite film is larger than that of the micro arc oxidation film.

5. Conclusion

Magnesium alloy has many superior properties, such as high specific strength and high specific stiffness, good thermal and electrical conductivity, electromagnetic shielding capability is strong, have strong advantages in the field of electronics field, the field of automobile manufacturing and missiles, satellites and other, known as the "green engineering materials". But the electrode potential of magnesium low (2.34 V) and film is loose, therefore, the magnesium alloy in most media corrosion resistance is poor, implications and limitations of the application of magnesium alloy products although microarc oxidation technology in surface treatment of titanium, aluminum and other metals and alloys have been very successful, but is on magnesium alloy surface treatment process is flawed. Single micro arc oxidation ceramic film belongs to the porous structure, even some of the pores from the coating surface and extends to the protected substrate surface and corrosive medium into magnesium alloy matrix provides the channel, which to micro arc oxidation coating anti corrosive hidden, it is difficult to adapt to complex environment on the corrosion resistance of the material requirements. Therefore, it is very important to deal with the pore sealing of the micro arc oxidation layer. Sol gel method is a promising method for the preparation of environmentally friendly coatings, which has the advantages of low cost, simple process equipment, low process temperature.

The sol gel method for magnesium alloy surface processing, to observe the morphology of the films by sol, after combined treatment, the micropores on micro arc oxidation film were effective sealing; thin films with fine crack, but overall film appears to be flat and smooth and dense. By energy spectrum, the element composition of the composite film and the element content of C was 21.88%, O was 33.36%, Na was 0.97%, Mg was 29.2%, Al was 1.05%, Si was 8.47%, Ti was 5.07%. By infrared detection of dry gel, the functional groups of O-H, -C[H.sub.2]-, Ti-O and Ti-O-Ti in the dried gel were detected. XRD detection, in addition to the composite film containing micro arc oxidation of the material in the film, mainly contains Ti[O.sub.2], MgTi[O.sub.3], Mg[Ti.sub.2][O.sub.5] and Mg[Ti.sub.2][O.sub.4]. The main composition of the film was detected by XPS, and the XRD was detected. By electrochemical test, the corrosion resistance of the composite film in acidic, neutral and alkaline 3.5% NaCl aqueous solution was better than that of the micro arc oxidation film under the same conditions.

Recebido/Submission: 16/04/2016

Aceitacao/Acceptance: 30/06/2016


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Yufeng Wang

Shanghai Institute of Technology, Shanghai 201418, China
Table 1--Magnesium alloy composite thin film energy
spectrum of each element content

Element     Weight %   Atomic %   Net Int.   Net Int.

C K         22.76      31.25      34.79      0.05
O K         32.17      36.58      152.34     0.02
Na K        0.95       0.69       11.56      0.13
Mg K        27.6       20.32      659.17     0.02
Al K        1.14       0.6        19.42      0.09
Si K        8.52       5.26       189.13     0.03
Ti K        5.11       1.75       74.35      0.05

Table 2--Corrosion potential and corrosion current density

Corrosive    [E.sub.corr]/V   [J.sub.corr]/A * [cm.sup.-2]

pH=3             -1.472                2.192x10-7
pH=5             -1.461                6.347x10-7
pH=7             -1.433                9.466x10-7
pH=9             -1.441                7.532x10-7
pH=ll            -1.475                4.149x10-7
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Author:Wang, Yufeng
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Date:Aug 1, 2016
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