Magnesium alloy anti-corrosion technology
1, chemical conversion treatment
Magnesium alloy chemical conversion membrane can be divided into: chromate system, organic acid system, phosphate system, KMnO4 system, rare earth element system, and stannate system.
The structure of the traditional chromate film with Cr as skeleton is very dense, Cr with structure water has very good self-repairing function, and the corrosion resistance is very strong. However, Cr has high toxicity and high cost of wastewater treatment. It is imperative to develop chromium-free conversion treatment. Magnesium alloys can be processed in KMnO4 solution to obtain amorphous structure chemical conversion film, and the corrosion resistance is equivalent to chromate film. The chemical conversion treatment of alkaline stannate can be used as a pretreatment of electroless nickel plating on magnesium alloys to replace the traditional process that contains harmful ions such as Cr, F or CN. The porous structure of the chemical conversion film exhibits good adsorption in the activation before plating, and can change the adhesion and corrosion resistance of the nickel layer.
The conversion film obtained by the organic acid treatment can simultaneously possess the comprehensive properties of corrosion protection, optics and electronics, and plays an important role in the new development of chemical conversion treatment.
The chemical conversion film is thin, soft, and has weak protection capabilities. It is generally used only as a middle layer of a decorative or protective layer.
2. Anodizing
Anodizing provides better wear-resistant and corrosion-resistant coating base coat than chemical conversion, and has good bonding power, electrical insulation and thermal shock resistance, and is one of the commonly used surface treatment technologies for magnesium alloys. .
The anodic oxidation of traditional magnesium alloy electrolytes generally contains chromium, fluorine, phosphorus and other elements, not only pollute the environment, but also harm human health. In recent years, research and development of environmentally friendly processes obtained oxide film corrosion resistance and other properties than the classic process Dow17 and HAE have a greater degree of improvement. The excellent corrosion resistance comes from the uniform distribution of Al, Si and other elements on the surface after anodization, which results in the formation of an oxide film with good compactness and integrity.
It is generally believed that the presence of pores in the oxide film is a major factor affecting the corrosion resistance of the magnesium alloy. It was found that by adding an appropriate amount of silicon-aluminum sol to the anodizing solution, the thickness, density, and porosity of the oxide film can be improved to some extent. Moreover, the sol component will cause the filming speed to increase rapidly and gradually, but it does not affect the X-ray diffraction phase structure of the film layer.
However, the anodic oxide film is relatively brittle and porous, and it is difficult to obtain a uniform oxide film layer on a complicated workpiece.
3, metal coating
Magnesium and magnesium alloys are the hardest metals to plate for the following reasons:
(1) Magnesium oxide easily formed on the surface of magnesium alloy is not easy to remove, and it seriously affects the bond strength 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 combined plating after loosening is very loose;
(3) The second phase (such as rare earth phase, γ equal) has different electrochemical characteristics, which may lead to uneven deposition;
(4) The standard potential of the plating layer is much higher than that of the magnesium alloy substrate. Any one via hole will increase the corrosion current and cause serious electrochemical corrosion. The electrode potential of magnesium is very negative. It is difficult to avoid the hydrogen evolution of the pinhole 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 coating porosity.
Therefore, the chemical conversion film method is generally used to first dip zinc or manganese, and then copper plating, and then other electroplating or electroless plating, in order to increase the binding force of the coating. Magnesium alloy electroplating layers include Zn, Ni, Cu-Ni-Cr, and Zn-Ni coatings, and the electroless plating is mainly Ni-P, Ni-WP, and the like.
A single electroless nickel plating layer is sometimes insufficient to protect magnesium alloys well. It has been studied that by combining an electroless Ni plating layer with an alkaline electroplated Zn-Ni plating layer, a plating layer of about 35 μm in thickness can withstand neutral salt spray corrosion of 800-1000 h after passivation. Some people also used electroless nickel as the bottom layer, and then electroplated with 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 denser.
Plating or electroless plating is a surface treatment method that simultaneously achieves superior corrosion resistance and electrical, electromagnetic and decorative properties. The disadvantage is that the Cr, F, and plating solutions in the pre-treatment process have serious environmental pollution; most of the plating contains heavy metal elements, which increases the difficulty and cost of recovery. Due to the nature of the magnesium matrix, the binding force also needs to be improved.
4, laser processing
Laser processing mainly includes laser surface heat treatment and laser surface alloying.
Laser surface heat treatment, also known as laser annealing, is actually a surface rapid solidification treatment. Laser surface alloying is a new technology based on laser surface heat treatment. Laser surface alloying can obtain alloy layers with different hardness and have a metallurgical bonding interface. The single-layer and multi-layer alloyed layer can also be produced on the high-purity magnesium alloy by using the laser irradiation source cladding.
When a Cu-Zr-Al alloy cladding coating is prepared on a magnesium alloy surface using a broadband laser, the alloy coating has high hardness, elastic modulus, and wear resistance due to the enhancement of various intermetallic compounds formed in the coating. Sex and corrosion resistance. Due to the presence of the rare earth element Nd, the multilayered laser coating obtained after the laser rapid fused treatment can significantly refine the crystal grains and improve the compactness and integrity of the cladding layer.
Laser processing can handle the surface of complex geometries, but magnesium alloys are prone to oxidation, evaporation, vaporization, porosity, and thermal stress during laser processing, and designing the correct processing process is critical.
5, other surface treatment technology
Ion implantation is a method of injecting accelerated energetic ions (Al, Cr, Cu, etc.) at a high speed on the surface to be treated under the action of an electrostatic field with a voltage of ten to several hundred kV in a high vacuum state. The injected ions are neutralized and left in the vacant or interstitial sites of the sample solid solution, forming an unbalanced surface layer.
It has been considered that the increase in corrosion resistance results from the densification of natural oxides, the radiation of implanted ions, and the formation of nitrides of magnesium. The performance of the modified layer is related to the amount of ions implanted and the thickness of the modified layer, and the MgO on the surface of the substrate also has a certain role in promoting the improvement of the corrosion resistance of the modified layer.
Vapor deposition refers to evaporation deposition of coatings, both physical vapor deposition (PVD) and chemical vapor deposition (CVD). It is the use of magnesium alloy can make Fe, Mo, Ni and other impurities significantly reduced, while using the coating to cover the various defects of the substrate, to avoid the formation of local corrosion of the battery, so as to achieve the purpose of improving corrosion resistance.
Compared with other surface treatment technologies for magnesium alloys, organic coating protection technologies have the advantages of variety and color variety, wide adaptability, low cost, and simple process. Currently widely used are solvent-based organic coatings. Because of their solvent-free, low-contamination, uniform thickness, and better corrosion resistance, powder-type organic coatings have gained popularity in recent years in applications such as automotive and computer housings.
1, chemical conversion treatment
Magnesium alloy chemical conversion membrane can be divided into: chromate system, organic acid system, phosphate system, KMnO4 system, rare earth element system, and stannate system.
The structure of the traditional chromate film with Cr as skeleton is very dense, Cr with structure water has very good self-repairing function, and the corrosion resistance is very strong. However, Cr has high toxicity and high cost of wastewater treatment. It is imperative to develop chromium-free conversion treatment. Magnesium alloys can be processed in KMnO4 solution to obtain amorphous structure chemical conversion film, and the corrosion resistance is equivalent to chromate film. The chemical conversion treatment of alkaline stannate can be used as a pretreatment of electroless nickel plating on magnesium alloys to replace the traditional process that contains harmful ions such as Cr, F or CN. The porous structure of the chemical conversion film exhibits good adsorption in the activation before plating, and can change the adhesion and corrosion resistance of the nickel layer.
The conversion film obtained by the organic acid treatment can simultaneously possess the comprehensive properties of corrosion protection, optics and electronics, and plays an important role in the new development of chemical conversion treatment.
The chemical conversion film is thin, soft, and has weak protection capabilities. It is generally used only as a middle layer of a decorative or protective layer.
2. Anodizing
Anodizing provides better wear-resistant and corrosion-resistant coating base coat than chemical conversion, and has good bonding power, electrical insulation and thermal shock resistance, and is one of the commonly used surface treatment technologies for magnesium alloys. .
The anodic oxidation of traditional magnesium alloy electrolytes generally contains chromium, fluorine, phosphorus and other elements, not only pollute the environment, but also harm human health. In recent years, research and development of environmentally friendly processes obtained oxide film corrosion resistance and other properties than the classic process Dow17 and HAE have a greater degree of improvement. The excellent corrosion resistance comes from the uniform distribution of Al, Si and other elements on the surface after anodization, which results in the formation of an oxide film with good compactness and integrity.
It is generally believed that the presence of pores in the oxide film is a major factor affecting the corrosion resistance of the magnesium alloy. It was found that by adding an appropriate amount of silicon-aluminum sol to the anodizing solution, the thickness, density, and porosity of the oxide film can be improved to some extent. Moreover, the sol component will cause the filming speed to increase rapidly and gradually, but it does not affect the X-ray diffraction phase structure of the film layer.
However, the anodic oxide film is relatively brittle and porous, and it is difficult to obtain a uniform oxide film layer on a complicated workpiece.
3, metal coating
Magnesium and magnesium alloys are the hardest metals to plate for the following reasons:
(1) Magnesium oxide easily formed on the surface of magnesium alloy is not easy to remove, and it seriously affects the bond strength 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 combined plating after loosening is very loose;
(3) The second phase (such as rare earth phase, γ equal) has different electrochemical characteristics, which may lead to uneven deposition;
(4) The standard potential of the plating layer is much higher than that of the magnesium alloy substrate. Any one via hole will increase the corrosion current and cause serious electrochemical corrosion. The electrode potential of magnesium is very negative. It is difficult to avoid the hydrogen evolution of the pinhole 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 coating porosity.
Therefore, the chemical conversion film method is generally used to first dip zinc or manganese, and then copper plating, and then other electroplating or electroless plating, in order to increase the binding force of the coating. Magnesium alloy electroplating layers include Zn, Ni, Cu-Ni-Cr, and Zn-Ni coatings, and the electroless plating is mainly Ni-P, Ni-WP, and the like.
A single electroless nickel plating layer is sometimes insufficient to protect magnesium alloys well. It has been studied that by combining an electroless Ni plating layer with an alkaline electroplated Zn-Ni plating layer, a plating layer of about 35 μm in thickness can withstand neutral salt spray corrosion of 800-1000 h after passivation. Some people also used electroless nickel as the bottom layer, and then electroplated with 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 denser.
Plating or electroless plating is a surface treatment method that simultaneously achieves superior corrosion resistance and electrical, electromagnetic and decorative properties. The disadvantage is that the Cr, F, and plating solutions in the pre-treatment process have serious environmental pollution; most of the plating contains heavy metal elements, which increases the difficulty and cost of recovery. Due to the nature of the magnesium matrix, the binding force also needs to be improved.
4, laser processing
Laser processing mainly includes laser surface heat treatment and laser surface alloying.
Laser surface heat treatment, also known as laser annealing, is actually a surface rapid solidification treatment. Laser surface alloying is a new technology based on laser surface heat treatment. Laser surface alloying can obtain alloy layers with different hardness and have a metallurgical bonding interface. The single-layer and multi-layer alloyed layer can also be produced on the high-purity magnesium alloy by using the laser irradiation source cladding.
When a Cu-Zr-Al alloy cladding coating is prepared on a magnesium alloy surface using a broadband laser, the alloy coating has high hardness, elastic modulus, and wear resistance due to the enhancement of various intermetallic compounds formed in the coating. Sex and corrosion resistance. Due to the presence of the rare earth element Nd, the multilayered laser coating obtained after the laser rapid fused treatment can significantly refine the crystal grains and improve the compactness and integrity of the cladding layer.
Laser processing can handle the surface of complex geometries, but magnesium alloys are prone to oxidation, evaporation, vaporization, porosity, and thermal stress during laser processing, and designing the correct processing process is critical.
5, other surface treatment technology
Ion implantation is a method of injecting accelerated energetic ions (Al, Cr, Cu, etc.) at a high speed on the surface to be treated under the action of an electrostatic field with a voltage of ten to several hundred kV in a high vacuum state. The injected ions are neutralized and left in the vacant or interstitial sites of the sample solid solution, forming an unbalanced surface layer.
It has been considered that the increase in corrosion resistance results from the densification of natural oxides, the radiation of implanted ions, and the formation of nitrides of magnesium. The performance of the modified layer is related to the amount of ions implanted and the thickness of the modified layer, and the MgO on the surface of the substrate also has a certain role in promoting the improvement of the corrosion resistance of the modified layer.
Vapor deposition refers to evaporation deposition of coatings, both physical vapor deposition (PVD) and chemical vapor deposition (CVD). It is the use of magnesium alloy can make Fe, Mo, Ni and other impurities significantly reduced, while using the coating to cover the various defects of the substrate, to avoid the formation of local corrosion of the battery, so as to achieve the purpose of improving corrosion resistance.
Compared with other surface treatment technologies for magnesium alloys, organic coating protection technologies have the advantages of variety and color variety, wide adaptability, low cost, and simple process. Currently widely used are solvent-based organic coatings. Because of their solvent-free, low-contamination, uniform thickness, and better corrosion resistance, powder-type organic coatings have gained popularity in recent years in applications such as automotive and computer housings.
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