Anticorrosion technology of magnesium alloy

1. Chemical conversion treatment
The chemical conversion coating of magnesium alloy can be divided into chromate series, organic acid series, phosphate series, KMnO4 series, rare earth element series and stannate series according to the solution.

The traditional chromate film has a dense structure with Cr as the framework, while Cr containing structured water has a good self-repair function and strong corrosion resistance. However, Cr is highly toxic and the cost of wastewater treatment is high. Therefore, it is imperative to develop chromium-free conversion treatment. The magnesium alloy is treated in KMnO4 solution to obtain a chemical conversion coating of amorphous structure, and its corrosion resistance is equivalent to that of chromate coating.

The chemical conversion treatment of alkaline stannate can be used as a pretreatment for electroless nickel plating of magnesium alloys, replacing traditional processes containing harmful ions such as Cr, F, or CN. The porous structure of the chemical conversion coating shows good adsorption during the activation before plating, and can change the bonding force and corrosion resistance of the nickel plating layer.

The conversion coating obtained by organic acid treatment can simultaneously possess comprehensive properties such as corrosion protection, optics and electronics, and it occupies a very important position in the new development of chemical conversion treatment.

The chemical conversion film is thin, soft, and weak in protection. It is generally only used as a decorative or intermediate layer of protection.

2 . Anodizing

Anodizing can obtain a better wear-resistant and corrosion-resistant coating base coating than chemical conversion, and has good bonding force, electrical insulation and thermal shock resistance. It is one of the commonly used surface treatment technologies for magnesium alloys. .

The electrolyte of traditional magnesium alloy anodic oxidation generally contains chromium, fluorine, phosphorus and other elements, which not only pollutes the environment, but also harms human health. In recent years, the corrosion resistance of the oxide film obtained by the environmentally-friendly process researched and developed is greatly improved compared to the classic process Dow17 and HAE. The excellent corrosion resistance comes from the uniform distribution of Al, Si and other elements on the surface after anodic oxidation, so that the formed oxide film has good compactness and integrity.

It is generally believed that the pores in the oxide film are the main factor affecting the corrosion resistance of magnesium alloys. Studies have found that adding an appropriate amount of silicon-aluminum sol to the anodic oxidation solution can improve the thickness and density of the oxide film to a certain extent, and reduce the porosity. In addition, the sol component will cause the film formation rate to increase rapidly and slowly in stages, but basically does not affect the X-ray diffraction phase structure of the film.

However, the anodic oxide film is brittle and porous, and it is difficult to obtain a uniform oxide film on complex workpieces.

3. Metal coating

Magnesium and magnesium alloys are the most difficult metals to plate. The reasons are as follows:

(1) Magnesium oxide, which is easily formed on the surface of magnesium alloy, is not easy to clean, and seriously affects the adhesion of the coating;

(2) The electrochemical activity of magnesium is too high. All acidic plating solutions will cause rapid corrosion of the magnesium matrix, or the substitution reaction with other metal ions is very strong, and the coating bonding after substitution is very loose;

(3) The second phase (such as rare earth phase, γ equal) has different electrochemical characteristics, which may cause uneven deposition;

(4) The standard potential of the coating is much higher than that of the magnesium alloy substrate. Any through hole will increase the corrosion current and cause serious electrochemical corrosion. However, the electrode potential of magnesium is very negative. It is difficult to avoid hydrogen evolution caused by pinholes during plating. ;

(5) The compactness of magnesium alloy castings is not very high, and there are impurities on the surface, which may become the source of pores in the coating.

Therefore, the chemical conversion coating method is generally used to first dip zinc or manganese, then plate copper, and then perform other electroplating or electroless plating treatments to increase the bonding force of the coating. Magnesium alloy electroplating layer has coatings such as Zn, Ni, Cu-Ni-Cr, Zn-Ni, etc. The electroless plating layer is mainly Ni-P, Ni-W-P and other coatings.

A single electroless nickel layer is sometimes insufficient to protect magnesium alloys. It has been studied that by combining the electroless Ni layer and the alkaline electroplating Zn-Ni coating, the coating with a thickness of about 35μm can withstand 800-1000h neutral salt spray corrosion after passivation. Some people also use electroless nickel plating as the bottom layer, and then use DC electroplating nickel to obtain a microcrystalline nickel coating. The average crystal grain size is 40nm. Due to the refinement of the crystal grains, the porosity of the coating is greatly reduced and the structure is more compact.

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Electroplating or electroless plating is a surface treatment method that simultaneously obtains superior corrosion resistance and electrical, electromagnetic and decorative properties. The disadvantage is that the Cr, F and the plating solution in the pretreatment cause serious environmental pollution; most of the plating layer contains heavy metal elements, which increases the difficulty and cost of recovery. Due to the characteristics of the magnesium matrix, the binding force needs to be improved.

4. Laser processing

Laser treatment mainly includes laser surface heat treatment and laser surface alloying.

Laser surface heat treatment is also called laser annealing, which is actually a rapid surface solidification treatment method. The laser surface alloying is a new technology based on laser surface heat treatment. Laser surface alloying can obtain alloy layers with different hardnesses and have metallurgical bonding interfaces. Single-layer and multi-layer alloying layers can also be prepared on high-purity magnesium alloy by using the cladding action of laser radiation source.

When a broadband laser is used to prepare Cu-Zr-Al alloy cladding coating on the surface of magnesium alloy, the alloy coating has high hardness, elastic modulus and wear resistance due to the enhancement effect of various intermetallic compounds formed in the coating Resistance and corrosion resistance. However, due to the presence of the rare earth element Nd, the laser multi-layer coating obtained after the laser rapid melting treatment can significantly refine the crystal grains, which can improve the compactness and integrity of the cladding layer.

Laser processing can process surfaces with complex geometries, but magnesium alloys are prone to oxidation, evaporation, vaporization, pores, and thermal stress during laser processing. It is important to design the correct processing technology.

5. Other surface treatment technologies

Ion implantation is a method in which accelerated high-energy ions (Al, Cr, Cu, etc.) impact the surface to be processed at a high speed under the action of an electrostatic field with a voltage of ten to hundreds of KV in a high vacuum state and are injected into the sample. The injected ions are neutralized and left in the vacancies or gaps of the sample solid solution, forming an unbalanced surface layer.

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Some studies believe that the improvement of corrosion resistance is due to the densification of natural oxides, the radiation of implanted ions, and the formation of magnesium nitrides. The performance of the modified layer obtained is related to the amount of implanted ions and the thickness of the modified layer, and the MgO on the surface of the substrate also has a certain promotion effect on the improvement of the corrosion resistance of the modified layer.

Vapor deposition is the vapor deposition of coatings. There are physical vapor deposition (PVD) and chemical vapor deposition (CVD). It is used to greatly reduce the content of impurities such as Fe, Mo, and Ni in the magnesium alloy, and at the same time, the coating is used to cover various defects of the substrate to avoid the formation of local corrosion cells, thereby achieving the purpose of improving the corrosion resistance.