A method for improving corrosion resistance of magnesium alloy

Abstract

The application relates to the field of magnesium alloy corrosion prevention technology and discloses a method for improving the corrosion resistance of magnesium alloy, which comprises the following steps: uniformly mixing carbon dioxide and oxygen with a mass fraction of 99.999% in proportion to obtain a mixed atmosphere; polishing and polishing the magnesium alloy, performing ultrasonic cleaning with ethanol, and then performing thermal oxidation treatment in a heating device into which the mixed atmosphere is introduced, and keeping warm, so that an outer layer film of MgCO3 and CaCO3 and an inner layer film of a composite protective film of CaO, MgO and Al2O3 are generated on the surface of the magnesium alloy; the method provided by the application makes the outer layer film on the surface of the magnesium and the magnesium alloy have a thickness of 95-125 nm, the inner layer film has a thickness of 30-40 nm, the composite film is flat and dense, has good adhesion, and significantly improves the corrosion resistance of the magnesium and the magnesium alloy, the electrochemical impedance is increased by 10 times, the corrosion current density is correspondingly reduced by 10 times, and the salt spray test shows that the corrosion resistance time is increased from 24 h to 144 h.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of magnesium alloy corrosion protection, and in particular to a method for improving the corrosion resistance of magnesium alloy. BACKGROUND

[0002] Magnesium (Mg) and its alloys have been widely used in the fields of automotive engineering, aerospace, transportation, electronic industry and biomedical engineering due to their high specific strength, good machinability, strong shock absorption, good biocompatibility and good electrical and thermal conductivity. In recent years, the rapid development of economic, environmental friendliness and mobility brought by structural lightweight has made lightweight materials occupy a pivotal position. Magnesium (Mg) is one of the primary choices for lightweight materials and is known as the "green engineering material of the 21st century". However, the low equilibrium potential of magnesium leads to the magnesium matrix acting as the anode of most secondary phases, which easily causes galvanic corrosion. In addition, the protective film on the surface of Mg is loose and porous, which easily falls off and cannot provide good protection for the magnesium matrix, resulting in poor corrosion resistance of magnesium alloy and a very fast corrosion rate, which seriously limits the use of magnesium and its alloys and is the primary factor hindering their development.

[0003] Currently, the research and development work to solve the corrosion problem of magnesium alloy mainly focuses on the following two aspects: 1) changing the composition of magnesium alloy, reducing harmful impurities, and applying certain processing methods to change the organization and phase composition of magnesium alloy through alloying, thermal oxidation treatment, and mechanical deformation; 2) surface treatment methods that isolate the metal from the atmosphere to prevent corrosion. Among them, the surface treatment techniques of magnesium alloy include chemical conversion film, anodic oxidation, micro-arc oxidation, gas deposition and laser surface treatment. In the chemical conversion film method, the hexavalent chromium in the chromate solution is carcinogenic and has great harm to the human body and the environment, and the recovery cost is high. In the anodic oxidation method, the thickness, chemical composition and microstructure of the coating are greatly affected by the current and are not easy to control. In the micro-arc oxidation method, the micro-arc oxidation area is easily affected by the surface roughness and is prone to breakdown at the groove position. In the gas deposition method and laser surface treatment, the requirements for the pre-treatment of the workpiece are high and the energy consumption is large, which is not suitable for large-scale surface processing. In addition, the protective film formed on the Mg matrix is generally MgO, and the PBR of MgO is <1, which cannot isolate the Cl - environment. CaCO3 and MgCO3 not only have the property of existing in Cl - environment for a long time, but also have the excellent property of PBR≈2, which have not been paid attention to. In summary, most of the widely used surface treatment techniques have the disadvantages of environmental pollution, complex process and limited performance of the film layer, which restrict their development.

[0004] Therefore, the above technical solution has the following deficiencies in actual use: the existing method for improving the corrosion resistance of magnesium alloy cannot meet the requirements of large-scale production, environmental protection and low energy consumption. Therefore, a green and environmentally friendly method for improving the corrosion resistance of magnesium alloy with low energy consumption, protective film containing carbonate and meeting the requirements of large-scale production is urgently needed. SUMMARY

[0005] The purpose of the present application is to provide a method for improving the corrosion resistance of magnesium alloy by thermal oxidation treatment in a mixed atmosphere, to solve the problem of poor corrosion resistance of magnesium alloy and the extremely fast corrosion rate which seriously limits the use of magnesium and its alloys. The present application provides a simple and low-cost method for improving the corrosion resistance of magnesium alloy without involving strong acids and bases, which is green and environmentally friendly. This method can effectively improve the corrosion resistance of magnesium and magnesium alloy, reduce the process risk and economic decline caused by corrosion of magnesium and magnesium alloy, and lay the foundation for the expansion of the use of magnesium and magnesium alloy.

[0006] To achieve the above purpose, the present application adopts the following technical solutions:

[0007] A method for improving the corrosion resistance of magnesium alloy, comprising the following steps:

[0008] Step 1: uniformly mix carbon dioxide and oxygen with a mass fraction of 99.999% in proportion to obtain a mixed atmosphere;

[0009] Step 2: polish and polish the magnesium alloy, and then ultrasonically clean it with ethanol, and then perform thermal oxidation treatment in a heating device with the mixed atmosphere, and keep warm, so that the outer layer film of the magnesium alloy surface is MgCO3 and CaCO3, and the inner layer film is a composite protective film of CaO, MgO and Al2O3.

[0010] Preferably, the ratio of carbon dioxide and oxygen in step 1 is (1-3):1.

[0011] Preferably, the flow rate of the mixed atmosphere formed by carbon dioxide and oxygen into the heating device in step 1 is 150ml / min-250ml / min.

[0012] Preferably, the magnesium alloy is polished to 3000-7000 mesh in step 2.

[0013] Preferably, the ultrasonic cleaning time with ethanol in step 2 is 1min-5min.

[0014] Preferably, the thermal oxidation treatment temperature in step 2 is 350℃-450℃.

[0015] Preferably, the holding time in step 2 is 900 min-1440 min.

[0016] Preferably, the outer layer film of the composite protective film in step 2 is located at the interface between the magnesium alloy surface oxide film and the mixed atmosphere; and the inner layer film of the composite protective film is located at the interface between the magnesium alloy substrate and the magnesium alloy surface oxide film.

[0017] Preferably, the thickness of the outer layer film of the composite protective film in step 2 is 95-125 nm, and the thickness of the inner layer film is 30-40 nm.

[0018] The present application has the following beneficial effects:

[0019] The method for improving the corrosion resistance of magnesium alloy by heat oxidation treatment under mixed atmosphere provided by the present application generates a composite film with an outer layer film thickness of 95-125 nm and an inner layer film thickness of 30-40 nm on the surface of magnesium and magnesium alloy by heat oxidation treatment of magnesium and magnesium alloy under a mixed atmosphere of carbon dioxide and oxygen. The composite film is flat and dense, has good adhesion, and significantly improves the corrosion resistance of magnesium and magnesium alloy, with an increase of 10 times in electrochemical impedance and a decrease of 10 times in corrosion current density. The salt spray test shows that the corrosion resistance time is increased from 24 h to 144 h. In addition, the method has low energy consumption, low equipment cost, and simple operation process, and the production process does not use strong acid and alkali, which meets the green environmental protection requirements. The use of the method will significantly improve the use rate, utilization rate and market share of magnesium and magnesium alloy, reduce the industrial risk and economic loss caused by the easy corrosion of magnesium and magnesium alloy, and lay a foundation for the promotion and development of modern industry. BRIEF DESCRIPTION OF DRAWINGS

[0020] Fig. 1 The surface scanning electron microscopic morphology of AM60+2.0Ca magnesium alloy without heat oxidation treatment in Example 1 after corrosion in 5wt.% NaCl salt spray for 24 h, 48 h and 72 h, and the surface scanning electron microscopic morphology of AM60+2.0Ca magnesium alloy after heat oxidation treatment after corrosion in 5wt.% NaCl salt spray for 24 h, 72 h and 144 h;

[0021] Fig. 2 The cross-sectional scanning electron microscopic morphology of AM60+2.0Ca magnesium alloy without heat oxidation treatment in Example 1 after corrosion in 5wt.% NaCl salt spray for 24 h, 48 h and 72 h, and the cross-sectional scanning electron microscopic morphology of AM60+2.0Ca magnesium alloy after heat oxidation treatment after corrosion in 5wt.% NaCl salt spray for 24 h, 72 h and 144 h;

[0022] Fig. 3The weight loss graph of the AM60+2.0Ca magnesium alloy without heat oxidation treatment in Example 1 after corrosion in 5wt.% NaCl salt spray for 24h, 48h and 72h and the AM60+2.0Ca magnesium alloy after heat oxidation treatment after corrosion in 5wt.% NaCl salt spray for 24h, 72h and 144h;

[0023] Fig. 4 The Nyquist graph of the AM60+2.0Ca magnesium alloy without heat oxidation treatment and after heat oxidation treatment in Example 1;

[0024] Fig. 5 The polarization curve graph of the AM60+2.0Ca magnesium alloy without heat oxidation treatment and after heat oxidation treatment in Example 1;

[0025] Fig. 6 The TEM test result schematic diagram of the composite protective film on the surface of the magnesium alloy after heat oxidation treatment. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments of the present application.

[0027] The method for improving the corrosion resistance of the magnesium alloy provided by the embodiments of the present application comprises the following steps:

[0028] (1) uniformly mixing carbon dioxide and oxygen with a mass fraction of 99.999% according to a proportion;

[0029] (2) polishing and polishing the magnesium alloy, and then performing ultrasonic cleaning with ethanol, and then performing heat oxidation treatment in a tubular furnace with the mixed atmosphere, and then keeping warm for a certain time, so that the outer layer film of the magnesium alloy is MgCO3 and CaCO3, and the inner layer film is a composite film of CaO, MgO and Al2O3, and the corrosion resistance of the magnesium and the magnesium alloy is improved.

[0030] As another preferred embodiment of the present application, the mixed atmosphere is a mixed gas of CO2:O2=1:1-1:3. If the CO2 content is too high, the protective film cannot be generated and the protective film is generated. The protective film is broken and the base metal is directly oxidized.

[0031] As another preferred embodiment of the present application, the polishing of the magnesium alloy needs to be polished to 3000-7000 meshes. If the mesh number is less than 3000, the metal surface scratch is obvious, and the corrosion is preferentially performed at the scratch.

[0032] As another preferred embodiment of the present application, the ultrasonic cleaning with ethanol is performed for 1min-5min.

[0033] As another preferred embodiment of the present application, the thermal oxidation treatment temperature is 400-450℃. If the thermal oxidation treatment temperature is too high, the alloy will be preferentially oxidized, and if it is too low, the thickness of the outer layer MgCO3 and CaCO3 film formed will be insufficient.

[0034] As another preferred embodiment of the present application, the thermal oxidation treatment holding time is 600-1200 min. If the holding time is too short, the thickness of the outer layer MgCO3 and CaCO3 film formed will be insufficient, and if it is too long, energy will be wasted.

[0035] As another preferred embodiment of the present application, the flow rate of the mixed atmosphere of CO2 and O2 during the thermal oxidation treatment is 150-250 ml / min.

[0036] Example 1:

[0037] The method for improving the corrosion resistance of a magnesium alloy by thermal oxidation treatment under a mixed atmosphere provided by the embodiments of the present application includes the following steps:

[0038] (1) uniformly mix carbon dioxide and oxygen with a mass fraction of 99.999% in a ratio of 1:3 to form a mixed atmosphere of carbon dioxide and oxygen in a ratio of 1:3;

[0039] (2) polish the AM60+2.0Ca magnesium alloy to 5000 mesh. Then polish with diamond polishing paste, and place the polished sample in ethanol for ultrasonic cleaning for 3 min, and then take out the sample and dry it with cold air;

[0040] (3) place the sample dried with cold air in a tube furnace with the mixed atmosphere introduced into it for thermal oxidation treatment, with a thermal oxidation treatment temperature of 440℃, a thermal oxidation treatment holding time of 900 min, and a flow rate of the mixed atmosphere of CO2 and O2 during the thermal oxidation treatment of 200 ml / min;

[0041] (4) cool to room temperature with the furnace, and obtain a corrosion-resistant magnesium product with an outer layer film of a composite film of MgCO3 and CaCO3, and an inner layer film of a composite film of CaO, MgO, and Al2O3.

[0042] Example 2:

[0043] The method for improving the corrosion resistance of a magnesium alloy by thermal oxidation treatment under a mixed atmosphere provided by the embodiments of the present application includes the following steps:

[0044] (2) polish the AZ80+1.5Ca magnesium alloy to 5000 mesh, and then polish with diamond polishing paste, and place the polished sample in ethanol for ultrasonic cleaning for 3 min, and then take out the sample and dry it with cold air;

[0045] (1), (3) and (4) are the same as example 1.

[0046] Example 3:

[0047] The method for improving the corrosion resistance of magnesium alloy by thermal oxidation treatment under mixed atmosphere provided by the embodiment of the application comprises the following steps:

[0048] (1) uniformly mixing carbon dioxide and oxygen with a mass fraction of 99.999% in proportion to form a mixed atmosphere with a carbon dioxide to oxygen mixing ratio of 1:1.5;

[0049] (2)-(4) are the same as example 1

[0050] Example 4:

[0051] The method for improving the corrosion resistance of magnesium alloy by thermal oxidation treatment under mixed atmosphere provided by the embodiment of the application comprises the following steps:

[0052] (2) polishing the AM60+2.0Ca magnesium alloy to 7000 meshes, then polishing the polished sample with diamond polishing paste, and placing the polished sample in ethanol for ultrasonic cleaning for 3 min, and taking out the sample and blowing it dry with cold air;

[0053] (1), (3) and (4) are the same as example 1.

[0054] Example 5:

[0055] The method for improving the corrosion resistance of magnesium alloy by thermal oxidation treatment under mixed atmosphere provided by the embodiment of the application comprises the following steps:

[0056] (3) placing the sample dried with cold air in a tubular furnace with mixed atmosphere introduced for thermal oxidation treatment, the thermal oxidation treatment temperature is 400 DEG C, the thermal oxidation treatment holding time is 900 min, and the flow rate of carbon dioxide and oxygen forming the mixed atmosphere during the thermal oxidation treatment is 200 ml / min;

[0057] (1), (2) and (4) are the same as example 1.

[0058] Example 6:

[0059] The method for improving the corrosion resistance of magnesium alloy by thermal oxidation treatment under mixed atmosphere provided by the embodiment of the application comprises the following steps:

[0060] (3) placing the sample dried with cold air in a tubular furnace with mixed atmosphere introduced for thermal oxidation treatment, the thermal oxidation treatment temperature is 440 DEG C, the thermal oxidation treatment holding time is 1440 min, and the flow rate of carbon dioxide and oxygen forming the mixed atmosphere during the thermal oxidation treatment is 200 ml / min;

[0061] (1), (2) and (4) are the same as example 1.

[0062] Example 7:

[0063] The method for improving the corrosion resistance of magnesium alloy by thermal oxidation treatment under mixed atmosphere provided by the embodiment of the application comprises the following steps:

[0064] (1) uniformly mix carbon dioxide and oxygen with a mass fraction of 99.999% in proportion to form a mixed atmosphere with a carbon dioxide to oxygen mixing ratio of 1:1.5;

[0065] (2)-(4) are the same as example 2

[0066] Example 8:

[0067] The method for improving the corrosion resistance of magnesium alloy by thermal oxidation treatment under mixed atmosphere provided by the embodiment of the application comprises the following steps:

[0068] (2) polish the AZ80+1.5Ca magnesium alloy to 7000 meshes. Then polish the sample with diamond polishing paste, and place the polished sample in ethanol for ultrasonic cleaning for 3 min, and take out the sample and blow dry with cold air;

[0069] (1), (3) and (4) are the same as example 2.

[0070] Example 9:

[0071] The method for improving the corrosion resistance of magnesium alloy by thermal oxidation treatment under mixed atmosphere provided by the embodiment of the application comprises the following steps:

[0072] (3) place the sample dried with cold air in a tubular furnace with mixed atmosphere introduced for thermal oxidation treatment, the thermal oxidation treatment temperature is 400 DEG C, the thermal oxidation treatment holding time is 900 min, and the flow rate of carbon dioxide and oxygen forming the mixed atmosphere during the thermal oxidation treatment is 200 ml / min;

[0073] (1), (2) and (4) are the same as example 2.

[0074] Example 10:

[0075] The method for improving the corrosion resistance of magnesium alloy by thermal oxidation treatment under mixed atmosphere provided by the embodiment of the application comprises the following steps:

[0076] (3) place the sample dried with cold air in a tubular furnace with mixed atmosphere introduced for thermal oxidation treatment, the thermal oxidation treatment temperature is 440 DEG C, the thermal oxidation treatment holding time is 1440 min, and the flow rate of carbon dioxide and oxygen forming the mixed atmosphere during the thermal oxidation treatment is 200 ml / min;

[0077] (1), (2) and (4) are the same as Example 2.

[0078] Based on the above-mentioned examples, the following conclusions are drawn:

[0079] Referring to Figs. 1-3 Fig. 6 shows the SEM images and weight loss rate of AM60+2.0Ca without heat oxidation treatment after 24h, 48h and 72h corrosion in 3.5wt% NaCl salt spray and AM60+2.0Ca with mixed atmosphere heat oxidation treatment after 24h, 72h and 144h corrosion in 3.5wt% NaCl salt spray. For AM60+2.0Ca without heat oxidation treatment, the surface has already been seriously corroded after 24h. With the increase of corrosion time, the corrosion pits on the surface of the alloy gradually grow, and almost occupy the entire alloy surface at 72h. For AM60+2.0Ca with heat oxidation treatment, the surface only has slight corrosion at the matrix position near the grain boundary after 24h, and the corrosion gradually expands along the matrix at 72h, but until 144h, part of the surface of AM60+2.0Ca with heat oxidation treatment is still not corroded. The corrosion pit depth of AM60+2.0Ca without heat oxidation treatment reaches 65μm after 24h, and reaches 120μm when the corrosion time reaches 72h. The corrosion pit depth of AM60+2.0Ca with heat oxidation treatment is very small after 24h, and only increases to 20μm when the corrosion time extends to 144h (which is obviously smaller than 65μm of AM60+2.0Ca without heat oxidation treatment after 24h). The corresponding energy spectrum shows that the corrosion pits first appear near the grain boundary, which is related to the occurrence of micro galvanic corrosion. The salt spray test shows that the corrosion resistance time is increased from 24h to 144h. And the weight loss rate of AM60+2.0Ca with heat oxidation treatment at 144h is one tenth of that of AM60+2.0Ca without heat oxidation treatment at 72h. This all shows that the outer film of MgCO3 and CaCO3 and the inner film of CaO, MgO and Al2O3 composite protective film can slow down the corrosion rate of magnesium alloy.

[0080] Referring to Figs. 4-5 Fig. 7 shows the Nyquist plots and polarization curves of AM60+2.0Ca without heat oxidation treatment and AM60+2.0Ca with mixed atmosphere heat oxidation treatment. For AM60+2.0Ca without heat oxidation treatment, the Nyquist plot shows a capacitive arc in the high frequency region and an inductive arc in the low frequency region (only the charge transfer resistance is 296.9Ωcm 2 ), and the corrosion potential is lower (-1.56V) and the corrosion current is larger (112.8μA cm -2), which indicates that the corrosion process of AM60+2.0Ca alloy is relatively rapid. The Nyquist plot of AM60+2.0Ca alloy after mixed atmosphere thermal oxidation treatment shows two high-frequency capacitance arcs and one low-frequency inductance arc, and the radius of the capacitance arc is larger (the total charge transfer resistance and surface film resistance is 3634.2 Ωcm 2 ), the corrosion potential is higher (-1.48 V), and the corrosion current is smaller (14.26 μA cm -2 ), which indicates that the outer film is MgCO3 and CaCO3, and the inner film is a composite protective film of CaO, MgO and Al2O3, which can improve the corrosion resistance of magnesium alloy.

[0081] Fig. 6 TEM test results of AM60+2.0Ca (CO2:O2 is 3:1 alloy after thermal oxidation treatment are shown. Fig. 6 (a)-(f) give the STEM images of the cross-section protective layer of AM60+2.0Ca alloy after thermal oxidation treatment and the corresponding energy spectrum of Mg, Al, Ca, O and C. Fig. 6 The cross-section STEM images and the corresponding energy spectrum in (a)-(f) show that the signals of Mg, Ca, O and C can be detected in the whole surface protective layer, and the signal of C is mainly distributed in the outer layer. The signal of Al can only be measured in the area close to the substrate. Therefore, the MgCO3 and CaCO3 protective film formed by mixed atmosphere thermal oxidation treatment is the main reason for improving the corrosion resistance of AM60+2.0Ca alloy.

[0082] The above is only the preferred specific embodiment of the present application, but the protection scope of the present application is not limited to this. Any skilled person in the art can make equivalent replacement or change according to the technical solution and the inventive concept of the present application within the technical range disclosed by the present application, which should be covered in the protection scope of the present application.

Claims

1. A method for improving corrosion resistance of a magnesium alloy, characterized by, The method comprises the following steps: Step 1: mixing carbon dioxide and oxygen with a mass fraction of 99.999% in proportion to obtain a mixed atmosphere; Step 2: polishing and polishing the magnesium alloy, ultrasonic cleaning with ethanol, and then performing thermal oxidation treatment in a heating device with the mixed atmosphere, the thermal oxidation treatment temperature is 350-450 DEG C, and the magnesium alloy surface is generated with an outer film of MgCO3 and CaCO3, and an inner film of CaO, MgO and Al2O3.

2. The method for improving the corrosion resistance of a magnesium alloy according to claim 1, characterized by, The proportion of carbon dioxide and oxygen in step 1 is (1-3):

1.

3. The method for improving corrosion resistance of magnesium alloy according to claim 1, characterized in that, The flow rate of the mixed atmosphere formed by carbon dioxide and oxygen in step 1 into the heating device is 150-250 ml / min.

4. The method for improving corrosion resistance of magnesium alloy according to claim 1, characterized in that, The magnesium alloy is polished to 3000-7000 mesh in step 2.

5. The method for improving corrosion resistance of magnesium alloy according to claim 1, characterized in that, The ultrasonic cleaning time with ethanol in step 2 is 1-5 min.

6. The method for improving corrosion resistance of magnesium alloy according to claim 1, characterized in that, The holding time in step 2 is 900-1440 min.

7. The method for improving corrosion resistance of magnesium alloy according to claim 1, characterized in that, The outer film of the composite protective film in step 2 is located at the interface between the magnesium alloy surface oxidation film and the mixed atmosphere; the inner film of the composite protective film is the interface between the magnesium alloy matrix and the magnesium alloy surface oxidation film.

8. The method for improving corrosion resistance of magnesium alloy according to claim 1, characterized in that, The outer film thickness of the composite protective film in step 2 is 95-125 nm, and the inner film thickness is 30-40 nm.

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