Strontium salt and calcium salt composite chemical conversion liquid for magnesium alloy, preparation method of strontium salt and calcium salt composite chemical conversion liquid and preparation method of conversion film

Abstract

The invention relates to the technical field of magnesium alloy surface treatment, in particular to strontium salt and calcium salt composite chemical conversion liquid for magnesium alloy, a preparation method and a preparation method of a conversion film. The method comprises the following steps: firstly, preparing a saturated calcium bicarbonate solution and a saturated strontium hydroxide solution; sequentially adding a saturated strontium hydroxide solution and hydrogen peroxide into water to obtain a strontium hydroxide preparation solution; preparing a sodium bicarbonate solution; mixing the saturated calcium bicarbonate solution, the strontium hydroxide preparation solution and the sodium bicarbonate solution to obtain a strontium salt and calcium salt composite chemical conversion solution; and the magnesium alloy part to be treated is placed in the strontium salt and calcium salt composite chemical conversion liquid, and after no bubble emerges, the magnesium alloy part subjected to surface treatment is taken out, washed with water and blow-dried. The method is low in cost, low in energy consumption and environmentally friendly, and the prepared composite conversion film composed of SrCO3, CaCO3, MgCO3, MgO, Al2O3 and other phases is uniform, fine, smooth and flat and has excellent corrosion resistance.

Description

Technical Field

[0001] This invention relates to the field of magnesium alloy surface treatment technology, specifically to a strontium salt-calcium salt composite chemical conversion solution for magnesium alloys, its preparation method, and its method for preparing a conversion film. Background Technology

[0002] Magnesium alloys are commonly used engineering materials, prized for their lightweight, superior mechanical properties, and excellent thermal and electrical conductivity. They also offer advantages such as shock and impact resistance, electromagnetic interference protection, non-toxicity, and easy recyclability, making them widely applicable in fields like electronics, aerospace, and lighting equipment. However, magnesium alloys are highly chemically reactive and prone to corrosion without protective coatings, which somewhat limits their application.

[0003] While traditional chromate conversion coatings can meet corrosion resistance requirements, their use is limited due to carcinogenicity and environmental concerns. To address the environmental issues of chromates, researchers have developed various chromium-free chemical conversion coating technologies. US Patent 9228263 discloses a chromium-free self-healing conversion coating technology using a composite system of magnesium nitrate, aluminum nitrate, and vanadium compounds. This system provides long-term corrosion protection through the self-healing mechanism of vanadium compounds, achieving hundreds of hours of salt spray testing. Chinese Patent CN112323055A describes an antimonate conversion coating system, preparing the coating through multi-step surface treatment and antimonate conversion solution immersion. US Patent Application US20020174915A1 proposes a phosphate-permanganate composite system for chemical conversion treatment of magnesium alloys within a pH range of 1.5-7.

[0004] Although existing chromium-free conversion coating technologies have improved in terms of environmental friendliness, the following problems still exist: (1) Single-component conversion coatings often cannot simultaneously meet the requirements of density and corrosion resistance; (2) Although existing vanadium-based and antimony-based systems are chromium-free, they still pose certain environmental and health risks; (3) Phosphate systems require strict control of pH conditions, resulting in high process complexity. Therefore, developing a green, environmentally friendly, and easy-to-operate method for preparing corrosion-resistant magnesium alloy chemical conversion coatings is of great significance for broadening the application range of magnesium alloys. Summary of the Invention

[0005] To address the problems of existing technologies, this invention proposes a strontium-calcium salt composite chemical conversion solution for magnesium alloys, its preparation method, and a method for preparing the conversion film. The specific solutions are as follows:

[0006] A strontium-calcium salt composite chemical conversion solution for magnesium alloys is prepared by mixing calcium bicarbonate, strontium hydroxide, hydrogen peroxide, and sodium bicarbonate. The theoretical concentrations of calcium bicarbonate, strontium hydroxide, hydrogen peroxide, and sodium bicarbonate according to the amount added are 115-120 g / L, 0.20-0.82 g / L, 45-141 g / L, and 3-10 g / L, respectively.

[0007] A method for preparing a strontium-calcium salt composite chemical conversion solution for magnesium alloys includes the following steps:

[0008] Step 1: Prepare a saturated calcium bicarbonate solution and a saturated strontium hydroxide solution;

[0009] Step 2: Add saturated strontium hydroxide solution and 30% hydrogen peroxide to water in sequence. The volume ratio of saturated strontium hydroxide solution: 30% hydrogen peroxide: water is 3-8: 5-10: 10. Maintain the pH at 6±0.2 to obtain the strontium hydroxide solution.

[0010] Step 3: Prepare a sodium bicarbonate solution with a concentration of 42.86–142.86 g / L;

[0011] Step 4: Mix the saturated calcium bicarbonate solution, strontium hydroxide solution, and sodium bicarbonate solution in a volume ratio of 70:23:7 to obtain a strontium salt-calcium salt composite chemical conversion solution.

[0012] Further, in step 2, 30-80 mL of saturated strontium hydroxide solution is added to 100 mL of water, and then 50-100 mL of 30% hydrogen peroxide is added to obtain a strontium hydroxide solution; in step 3, 3-10 g of sodium bicarbonate is dissolved in 70 mL of water to obtain a sodium bicarbonate solution.

[0013] A method for preparing a strontium salt-calcium salt composite chemical conversion film for magnesium alloys involves using the strontium salt-calcium salt composite chemical conversion solution prepared by the above method, placing the magnesium alloy part to be treated in the strontium salt-calcium salt composite chemical conversion solution at 20-60°C, and after no more bubbles emerge, removing the surface-treated magnesium alloy part, washing it with water, and drying it.

[0014] Furthermore, after drying, the surface-treated magnesium alloy parts are baked at 70–90°C for 20–40 minutes, preferably at 80°C for 30 minutes.

[0015] The conversion film prepared using the conversion solution of this invention is composed of phases such as SrCO3, CaCO3, MgCO3, MgO, and Al2O3. Utilizing the difference in atomic radii between alkaline earth metals such as calcium and strontium salts, the voids in the film layer are filled, forming a dense chemical conversion film with excellent corrosion resistance. Furthermore, this method is low-cost, low-energy-consumption, environmentally friendly, and produces a uniform, fine, smooth, and flat conversion film with excellent corrosion resistance. Attached Figure Description

[0016] The embodiments of the present invention will be further described below with reference to the accompanying drawings, wherein:

[0017] Figure 1 The following is a SEM image of Example 1 during the chemical conversion membrane conversion process;

[0018] Figure 2 The attached Figure 1 Element mapping diagram;

[0019] Figure 3 The image shown is a SEM image of Example 1 after the chemical conversion film was formed;

[0020] Figure 4 The attached Figure 3 Element mapping diagram;

[0021] Figure 5 A macroscopic photograph of Example 1 after a 96-hour neutral salt spray test is shown. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0023] A method for preparing a strontium-calcium salt composite chemical conversion film for magnesium alloys includes the following steps:

[0024] Step 1: Prepare a saturated calcium bicarbonate solution and a saturated strontium hydroxide solution;

[0025] Step 2: Add saturated strontium hydroxide solution and 30% hydrogen peroxide to water in sequence. The volume ratio of saturated strontium hydroxide solution: 30% hydrogen peroxide: water is 3-8: 5-10: 10 to maintain the pH at 6±0.2, thus obtaining the strontium hydroxide solution. Specifically, 30-80 mL of saturated strontium hydroxide solution can be added to 100 mL of water, followed by 50-100 mL of 30% hydrogen peroxide to obtain the strontium hydroxide solution.

[0026] Step 3: Prepare a sodium bicarbonate solution with a concentration of 42.86–142.86 g / L; specifically, dissolve 3–10 g of sodium bicarbonate in 70 mL of water to obtain a sodium bicarbonate solution.

[0027] Step 4: Mix the saturated calcium bicarbonate solution, strontium hydroxide solution, and sodium bicarbonate solution in a volume ratio of 70:23:7 to obtain a strontium salt-calcium salt composite chemical conversion solution.

[0028] Step 5: Place the magnesium alloy part to be treated in a strontium-calcium salt composite chemical conversion solution at 20-60℃. Upon immersion, numerous bubbles will be generated on the surface. As time progresses, the number of bubbles will gradually decrease. Once no more bubbles emerge, remove the surface-treated magnesium alloy part, wash it with water, and dry it. After drying, bake the surface-treated magnesium alloy part at 70-90℃ for 20-40 minutes, preferably at 80℃ for 30 minutes. The chemical conversion process is as follows:

[0029] Mg + H₂O₂ → Mg(OH)₂ (1)

[0030] Al+H2O2——→Al(OH)3+H2O (2)

[0031] HCO3 - ——→CO3 2- +H + (3)

[0032] Ca 2+ +CO3 2- —→CaCO3↓ (4)

[0033] Sr 2+ +CO3 2- —→SrCO3↓ (5)

[0034] Mg 2+ +CO3 2- ——→MgCO3↓ (6)

[0035] Example 1

[0036] A method for preparing a strontium-calcium salt composite chemical conversion film for magnesium alloys includes the following steps:

[0037] Step 1: Prepare a saturated calcium bicarbonate solution and a saturated strontium hydroxide solution;

[0038] Step 2: Take 30 mL of saturated strontium hydroxide solution and add it to 100 mL of water, then add 100 mL of 30% hydrogen peroxide to maintain the pH at 6 ± 0.2, thus obtaining the strontium hydroxide solution.

[0039] Step 3: Dissolve 5g of sodium bicarbonate in 70mL of water to obtain a sodium bicarbonate solution.

[0040] Step 4: Take 700 mL of saturated calcium bicarbonate solution, 230 mL of strontium hydroxide solution, and 70 mL of sodium bicarbonate solution and mix them evenly to obtain a strontium salt-calcium salt composite chemical conversion solution.

[0041] Step 5: Suspend the polished AZ80 magnesium alloy rolled plate (50*50*3mm) in the above conversion solution at 20℃. A large number of bubbles will be generated on the surface upon initial placement. As time progresses, the number of bubbles gradually decreases. After approximately 48 hours, when no more bubbles emerge, remove the magnesium alloy plate, wash it with water, and dry it with high-pressure air. Then, bake it at 80℃ for 30 minutes. (See attached image) Figure 1 , 2 The image shows the surface morphology of the chemical conversion film during the conversion process. It can be seen that a strontium salt and calcium salt film gradually forms on the magnesium alloy surface. (See attached image.) Figure 3 , 4 As shown, a dense strontium-calcium salt film can be seen forming on the surface of the magnesium alloy; as attached... Figure 5 As shown, the treated magnesium alloy rolled plate was subjected to a neutral salt spray test, and no obvious corrosion traces were observed after 96 hours.

[0042] Example 2

[0043] A method for preparing a strontium-calcium salt composite chemical conversion film for magnesium alloys includes the following steps:

[0044] Step 1: Prepare a saturated calcium bicarbonate solution and a saturated strontium hydroxide solution;

[0045] Step 2: Take 50 mL of saturated strontium hydroxide solution and add it to 100 mL of water, then add 80 mL of 30% hydrogen peroxide to maintain the pH at 6 ± 0.2, and obtain the strontium hydroxide solution.

[0046] Step 3: Dissolve 8g of sodium bicarbonate in 70mL of water to obtain a sodium bicarbonate solution.

[0047] Step 4: Take 700 mL of saturated calcium bicarbonate solution, 230 mL of strontium hydroxide solution, and 70 mL of sodium bicarbonate solution and mix them evenly to obtain a strontium salt-calcium salt composite chemical conversion solution.

[0048] Step 5: Suspend the polished AM60 magnesium alloy rolled plate (50*50*3mm) in the above-mentioned conversion solution at 60℃. Numerous bubbles will initially appear on the surface, but these will gradually decrease over time. After approximately 6 hours, when no more bubbles emerge, remove the magnesium alloy plate, wash it with water, and dry it with high-pressure air. Then, bake it at 80℃ for 30 minutes. Perform a neutral salt spray test on the treated magnesium alloy rolled plate; no obvious corrosion marks were observed after 96 hours.

[0049] Example 3

[0050] A method for preparing a strontium-calcium salt composite chemical conversion film for magnesium alloys includes the following steps:

[0051] Step 1: Prepare a saturated calcium bicarbonate solution and a saturated strontium hydroxide solution;

[0052] Step 2: Take 80 mL of saturated strontium hydroxide solution and add it to 100 mL of water, then add 50 mL of 30% hydrogen peroxide to maintain the pH at 6 ± 0.2 to obtain the strontium hydroxide solution.

[0053] Step 3: Dissolve 10g of sodium bicarbonate in 70mL of water to obtain a sodium bicarbonate solution.

[0054] Step 4: Take 700 mL of saturated calcium bicarbonate solution, 230 mL of strontium hydroxide solution, and 70 mL of sodium bicarbonate solution and mix them evenly to obtain a strontium salt-calcium salt composite chemical conversion solution.

[0055] Step 5: Suspend the polished AM60 magnesium alloy rolled plate (50*50*3mm) in the above-mentioned conversion solution at 40℃. Upon immersion, numerous bubbles are generated on the surface. As time progresses, the number of bubbles gradually decreases. After approximately 10 hours, when no more bubbles emerge, remove the magnesium alloy plate, wash it with water, and dry it with high-pressure air. Then, bake it at 80℃ for 30 minutes. Perform a neutral salt spray test on the treated magnesium alloy rolled plate; after 96 hours, no obvious corrosion marks are observed.

[0056] The foregoing descriptions have outlined some exemplary embodiments of the present invention. It is understood that these embodiments are merely illustrative and do not constitute a limitation on the scope of protection of the present invention. Features in these embodiments can be rearranged in suitable ways, and the resulting solutions remain within the scope of protection claimed by the present invention. All other embodiments obtained by those skilled in the art based on the foregoing embodiments without inventive effort, i.e., all modifications, equivalent substitutions, and improvements made within the spirit and principles of this application, fall within the scope of protection claimed by the present invention.

Claims

1. A strontium-calcium salt composite chemical conversion solution for magnesium alloys, characterized in that, It is prepared by mixing calcium bicarbonate, strontium hydroxide, hydrogen peroxide and sodium bicarbonate, with theoretical concentrations of calcium bicarbonate, strontium hydroxide, hydrogen peroxide and sodium bicarbonate of 115-120 g / L, 0.20-0.82 g / L, 45-141 g / L and 3-10 g / L respectively according to the amount added.

2. A method for preparing a strontium salt-calcium salt composite chemical conversion solution for magnesium alloys according to claim 1, characterized in that, Includes the following steps: Step 1: Prepare a saturated calcium bicarbonate solution and a saturated strontium hydroxide solution; Step 2: Add saturated strontium hydroxide solution and 30% hydrogen peroxide to water in sequence. The volume ratio of saturated strontium hydroxide solution: 30% hydrogen peroxide: water is 3-8: 5-10:

10. Maintain the pH at 6±0.2 to obtain the strontium hydroxide solution. Step 3: Prepare a sodium bicarbonate solution with a concentration of 42.86–142.86 g / L; Step 4: Mix the saturated calcium bicarbonate solution, strontium hydroxide solution, and sodium bicarbonate solution in a volume ratio of 70:23:7 to obtain a strontium salt-calcium salt composite chemical conversion solution.

3. The method for preparing the chemical conversion solution according to claim 2, characterized in that, In step 2, 30-80 mL of saturated strontium hydroxide solution is added to 100 mL of water, and then 50-100 mL of 30% hydrogen peroxide is added to obtain a strontium hydroxide solution; in step 3, 3-10 g of sodium bicarbonate is dissolved in 70 mL of water to obtain a sodium bicarbonate solution.

4. A method for preparing a strontium-calcium salt composite chemical conversion film for magnesium alloys, using a strontium-calcium salt composite chemical conversion solution for magnesium alloys prepared according to claim 2 or 3, characterized in that, The magnesium alloy parts to be treated are placed in a strontium-calcium salt composite chemical conversion solution at 20-60°C. After no more bubbles emerge, the surface-treated magnesium alloy parts are removed, washed with water, and dried.

5. The method for preparing the chemical conversion membrane according to claim 4, characterized in that, After drying, the surface-treated magnesium alloy parts are baked at 70–90°C for 20–40 minutes.

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