Anodic oxidation passivation method of neodymium iron boron permanent magnet zinc coating

Abstract

The invention provides an anodic oxidation passivation method of a neodymium-iron-boron permanent magnet zinc coating, which comprises the following steps of: performing electrochemical anodic oxidation by taking a neodymium-iron-boron permanent magnet with a zinc coating on the surface as an anode, a Pt sheet as a cathode and a passivation solution as an electrolyte, forming a passivation layer on the surface of the neodymium-iron-boron permanent magnet with the zinc coating on the surface, and cleaning and drying to obtain the neodymium-iron-boron permanent magnet with the zinc coating on the surface. A chromium-free passivation film is formed on the neodymium-iron-boron permanent magnet zinc coating, and passivation of the neodymium-iron-boron permanent magnet zinc coating is completed; the passivation solution is prepared from 10 g / L to 20 g / L of molybdate, 10 g / L to 15 g / L of ammonium fluozirconate, 1 g / L to 10 g / L of etidronic acid, 1 g / L to 10 g / L of sodium dihydrogen phosphate, 1 g / L to 10 g / L of nitric acid, 1 g / L to 10 g / L of phosphoric acid and the balance distilled water. According to the method disclosed by the invention, a complete, uniform and compact passivation film is formed on the surface of the zinc coating after passivation, the corrosion potential is improved while the corrosion current is effectively reduced, and the time for generating a rust spot in a salt spray test exceeds 72 hours.

Description

Technical Field

[0001] The invention belongs to the technical field of metal surface protection, and in particular relates to an anodic oxidation passivation method for a zinc coating of a neodymium iron boron permanent magnet. Background Art

[0002] Sintered Nd-Fe-B, a third-generation permanent magnet material, is known as the "King of Magnets." In recent years, Nd-Fe-B permanent magnets have been widely used in energy conservation and emission reduction, new energy, industrial robotics, and low-altitude spacecraft. This has created significant opportunities for the development of NdFeB materials. However, due to its three-phase structure and the large potential difference between the phases, sintered Nd-Fe-B is susceptible to electrochemical corrosion, which has limited its development. Therefore, improving the corrosion resistance of sintered Nd-Fe-B has become a key focus in this field. Metal coatings, due to their low cost and good corrosion resistance, are widely used in the protection of sintered Nd-Fe-B. Among them, electroplated Zn coatings are widely used in industrial production due to their advantages of low cost, simple processing, and excellent corrosion resistance. However, the Zn coating itself has poor protective capabilities and must be passivated after the electroplating process. In the past, hexavalent chromium or trivalent chromium passivation was usually used for surface treatment of electroplated Zn coatings. However, with the deepening of the concept of green industrial development, toxic hexavalent chromium has been gradually eliminated, and trivalent chromium also faces the problem of being easily oxidized to hexavalent chromium. Chromium-free passivation technology for zinc plating is a trend of green development. The passivation layer needs to have good corrosion resistance, and at the same time needs to be dense, flat, with low corrosion current and higher self-corrosion potential.

[0003] Therefore, it is necessary to develop a chromium-free passivation technology for sintered Nd-Fe-B electroplated coatings. Summary of the Invention

[0004] To address the shortcomings of existing technologies, particularly the low corrosion potential and high corrosion current of existing passivation methods, the present invention provides a method for anodic oxidation passivation of the zinc coating on NdFeB permanent magnets. This method incorporates zirconate into a molybdate composite chromium-free passivation solution and employs an anodic oxidation passivation method. This method improves the low corrosion potential of molybdate chromium-free passivation and produces a dense passivation film.

[0005] The technical solutions of the present invention are as follows: A method for anodizing and passivating a zinc coating of a NdFeB permanent magnet comprises the following steps: The NdFeB permanent magnet with a zinc-plated surface is used as an anode, the Pt sheet is used as a cathode, and the passivation solution is used as an electrolyte to perform electrochemical anodic oxidation to form a passivation layer on the surface of the NdFeB permanent magnet with a zinc-plated surface. After cleaning and drying, a chromium-free passivation film is formed on the zinc-plated layer of the NdFeB permanent magnet, thereby completing the passivation of the zinc-plated layer of the NdFeB permanent magnet. The passivation solution comprises the following components: 10-20 g / L molybdate, 10-15 g / L ammonium fluorozirconate (F6H8N2Zr), 1-10 g / L hydroxyethylidene diphosphonic acid (C2H8O7P2, organic phosphonic acid HEDP), 1-10 g / L sodium dihydrogen phosphate (NaH2PO4), 1-10 g / L nitric acid (HNO3), 1-10 g / L phosphoric acid (H3PO4), and the balance is distilled water.

[0006] According to the present invention, the method of electroplating a zinc layer on the surface of a NdFeB permanent magnet is a prior art in the art; the present invention does not limit the method of electroplating a zinc layer, and any method of electroplating a Zn layer on the surface of a NdFeB permanent magnet can be used.

[0007] According to the preferred embodiment of the present invention, the molybdate in the passivation solution is one or more of sodium molybdate, ammonium molybdate, potassium molybdate and magnesium molybdate.

[0008] Preferably, according to the present invention, the passivation solution is prepared according to the following method: adding nitric acid solution, phosphoric acid solution, hydroxyethylidene diphosphonic acid and sodium dihydrogen phosphate to distilled water, stirring and ultrasonicating at 30-50°C to obtain an acid solution; adding molybdate and ammonium fluorozirconate to distilled water, stirring and ultrasonicating at 60-80°C to obtain a salt solution; mixing the acid solution and the salt solution, stirring and ultrasonicating at 30-50°C; then cooling to room temperature, adding distilled water to make up the volume, and obtaining a passivation solution.

[0009] According to the preferred embodiment of the present invention, the volume of distilled water used in preparing the acid solution and the distilled water used in preparing the salt solution is not specifically limited, as long as it can dissolve the raw materials, and can be excessive, such as 20-45% of the volume of the passivation solution.

[0010] Preferably, according to the present invention, the mass concentration of the nitric acid solution is 50-70wt%, and the mass concentration of the phosphoric acid solution is 85wt%.

[0011] Preferably, according to the present invention, the electrochemical anodization method is constant voltage anodization; the voltage of the constant voltage anodization is 8-12V, and the time of the constant voltage anodization is 50-70s; the electrochemical anodization process is carried out at room temperature.

[0012] According to the preferred embodiment of the present invention, the cleaning is rinsing with distilled water, and the drying is drying at 100-120° C. for 10-20 min.

[0013] According to the present invention, the average thickness of the obtained passivation film is 10-100 nm, and the passivation film layer not only effectively reduces the corrosion current but also increases the corrosion potential.

[0014] Room temperature in the present invention has a well-known meaning, which refers to 25±5°C.

[0015] The technical features and beneficial effects of the present invention are as follows: 1. The present invention compounds zirconate into the molybdate composite chromium-free passivation solution. Zirconate has strong coupling and hydrolysis effects. After hydrolysis, it easily reacts with metals such as Al and Zn to form Zr hydroxide and its complex, which can be dehydrated to form oxide after drying. Zr oxide has the properties of ceramics and has a low corrosion tendency, which can improve the problem of low corrosion potential of molybdate chromium-free passivation. The present invention compounds ammonium fluorozirconate into the molybdate composite passivation solution and uses an anodic oxidation process to form a complete, uniform and dense passivation film on the surface of the zinc coating. The chromium-free passivation film layer of the present invention not only effectively reduces the corrosion current but also increases the corrosion potential, thereby improving the corrosion resistance of the NdFeB permanent magnet. The time for rust spots to appear in the salt spray test exceeds 72 hours, and the magnet has excellent corrosion resistance.

[0016] 2. The present invention takes into account both the requirements of anti-corrosion performance and the concept of green development, and develops a new chromium-free passivation technology; the optimization effect of the anodizing method adopted in the present invention on the coating surface provides a feasible idea for developing a new passivation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of the passivation process of the present invention.

[0018] Figure 2 Neutral salt spray test results of the coatings of the present invention: (a) the passivation coating in Example 1; (b) the passivation coating in Comparative Example 5. DETAILED DESCRIPTION

[0019] The technical solution of the present invention is further illustrated and described below through specific implementation methods, but the present invention is not limited thereto; anything not fully described in the present invention is the existing technology in the field.

[0020] Example 1 A method for anodizing and passivating a zinc coating of a NdFeB permanent magnet comprises the following steps: (1) Chamfering, degreasing, and rust-removing several pieces of NdFeB permanent magnets with a size of 20 mm × 15 mm × 2 mm, and then galvanizing them. The residual plating solution on the surface of the magnets after galvanizing is washed with water, and then dried to obtain NdFeB permanent magnets with a surface galvanized layer. (2) Preparation of passivation solution To 400 mL of distilled water, 7.69 g of a 65 wt% nitric acid solution, 5.88 g of an 85 wt% phosphoric acid solution, 5 g of hydroxyethylidene diphosphonic acid, and 5 g of sodium dihydrogen phosphate were added, and the mixture was stirred and ultrasonicated at 40° C. for 10 min to obtain an acid solution. To 400 mL of distilled water, 15 g of sodium molybdate and 15 g of ammonium fluorozirconate were added, and the mixture was stirred and ultrasonicated at 70° C. for 30 min to obtain a salt solution. The acid solution and the salt solution were mixed, and the mixture was stirred and ultrasonicated at 40° C. for 10 min. The mixture was then cooled to room temperature, and distilled water was added to make the volume 1 L to obtain a passivation solution.

[0021] The composition of the obtained passivation solution is: 15g / L sodium molybdate (Na2MoO4), 15g / L ammonium fluorozirconate (F6H8N2Zr), 5g / L hydroxyethylidene diphosphonic acid (C2H8O7P2, organic phosphonic acid HEDP), 5g / L sodium dihydrogen phosphate (NaH2PO4), 5g / L nitric acid (HNO3), 5g / L phosphoric acid (H3PO4), and the balance is distilled water; (3) Passivation The NdFeB permanent magnet with the zinc-plated surface obtained in step (1) is used as the anode, the Pt sheet is used as the cathode, and the passivation solution obtained in step (2) is used as the electrolyte to perform constant voltage anodic oxidation at a voltage of 10 V for 60 seconds to form a passivation layer on the surface of the NdFeB permanent magnet with the zinc-plated surface. The magnet is then taken out, rinsed twice with distilled water, and dried in an oven at 120°C for 10 minutes to form a chromium-free passivation film on the zinc-plated layer of the NdFeB permanent magnet, thereby completing the passivation of the zinc-plated layer of the NdFeB permanent magnet. The process diagram is shown in FIG. Figure 1 As shown, the anode is connected to the positive pole of the power supply, and the cathode is connected to the negative pole of the power supply. Only some ions in the electrolyte are marked in the figure.

[0022] Example 2 A method for anodizing and passivating the zinc coating of a NdFeB permanent magnet is as described in Example 1, except that in step (3), the voltage is 8 V, and the remaining steps remain unchanged.

[0023] Comparative Example 1 A method for anodizing and passivating the zinc coating of a NdFeB permanent magnet is as described in Example 1, except that in step (2), the passivation solution does not contain sodium molybdate, and the remaining steps remain unchanged.

[0024] Comparative Example 2 A method for anodizing and passivating the zinc coating of a NdFeB permanent magnet is as described in Example 1, except that in step (3), the voltage is 20 V, and the remaining steps remain unchanged.

[0025] Comparative Example 3 A method for anodizing and passivating the zinc coating of a NdFeB permanent magnet is as described in Example 1, except that in step (3), the anodizing time is 30 seconds, and the remaining steps remain unchanged.

[0026] Comparative Example 4 A method for anodizing and passivating the zinc coating of a NdFeB permanent magnet is as described in Example 1, except that in step (3), the anodizing time is 90 seconds, and the remaining steps remain unchanged.

[0027] Comparative Example 5 A method for anodizing and passivating the zinc coating of a NdFeB permanent magnet is as described in Example 1, except that in step (2), the content of ammonium fluorozirconate in the passivation solution is 5 g / L, and the remaining steps remain unchanged.

[0028] Comparative Example 6 A method for anodizing and passivating the zinc coating of a NdFeB permanent magnet is as described in Example 1, except that in step (2), the content of ammonium fluorozirconate in the passivation solution is 20 g / L, and the remaining steps remain unchanged.

[0029] Comparative Example 7 A method for anodizing and passivating the zinc coating of a NdFeB permanent magnet is as described in Example 1, except that in step (3), a DC constant current anodizing method is adopted, the current is set to 1 A / cm, the anodizing time is 30 s, and the remaining steps remain unchanged.

[0030] Comparative Example 8 A method for anodizing and passivating the zinc coating of a NdFeB permanent magnet is as described in Example 1, except that in step (3), the passivation is performed by immersion, the passivation time is 80 seconds, and stirring is performed during the immersion process (the speed is 100 r / min).

[0031] Test Example 1 The passivated NdFeB permanent magnets obtained in the examples and comparative examples were subjected to potentiodynamic polarization and salt spray resistance tests.

[0032] (1) The electrolytic cell medium in the potentiodynamic polarization test was a sodium chloride aqueous solution with a mass concentration of 3.5 wt%. A Pt sheet was placed opposite the sample surface as a counter electrode. A calomel electrode was connected in the solution as an auxiliary electrode. A passivated NdFeB permanent magnet was used as a working electrode. The potentiodynamic polarization test results of the embodiments and comparative examples are shown in Table 1.

[0033] As shown in Table 1, Example 1 exhibits the lowest corrosion current and the highest corrosion potential. In other comparative examples, the voltage, anodizing time, and amount of ammonium fluorozirconate are changed, and the resulting films have higher corrosion currents and lower corrosion potentials.

[0034] (2) Neutral salt spray test The passivated NdFeB permanent magnets of Example 1 and Comparative Example 5 were subjected to a neutral salt spray test, and were compared with unpassivated NdFeB permanent magnets.

[0035] A sodium chloride aqueous solution with a mass concentration of 3.5wt% was used as the spray source and the spray temperature was 35℃. The results are as follows Figure 2 As shown. Figure 2 It can be seen that the time for pitting corrosion to occur in the salt spray resistance test of Example 1 is extended to 72 hours.

[0036] Therefore, based on the above data results, it can be seen that the zinc-plated passivation layer of the NdFeB permanent magnet of the present invention can provide better corrosion protection for the NdFeB magnet.

[0037] As described above, the basic principles, main features, and advantages of the present invention are shown and described. Those skilled in the art should understand that the present invention is not limited to the above-described embodiments. The above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. In other words, equivalent changes and modifications made within the scope of the present invention and the contents of the specification should still fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for anodizing and passivating the zinc coating of a NdFeB permanent magnet, characterized in that: The steps are as follows: The NdFeB permanent magnet with a zinc-plated surface is used as an anode, the Pt sheet is used as a cathode, and the passivation solution is used as an electrolyte to perform electrochemical anodic oxidation to form a passivation layer on the surface of the NdFeB permanent magnet with a zinc-plated surface. After cleaning and drying, a chromium-free passivation film is formed on the zinc-plated layer of the NdFeB permanent magnet, thereby completing the passivation of the zinc-plated layer of the NdFeB permanent magnet. The passivation solution comprises the following components: 10-20 g / L molybdate, 10-15 g / L ammonium fluorozirconate, 1-10 g / L hydroxyethylidene diphosphonic acid, 1-10 g / L sodium dihydrogen phosphate, 1-10 g / L nitric acid, 1-10 g / L phosphoric acid, and the balance is distilled water.

2. The anodic oxidation passivation method for the zinc coating of the NdFeB permanent magnet according to claim 1, characterized in that: The molybdate in the passivation solution is one or more of sodium molybdate, ammonium molybdate, potassium molybdate and magnesium molybdate.

3. The anodic oxidation passivation method for the zinc coating of the NdFeB permanent magnet according to claim 1, characterized in that: The passivation solution is prepared according to the following method: adding nitric acid solution, phosphoric acid solution, hydroxyethylidene diphosphonic acid and sodium dihydrogen phosphate to distilled water, stirring and ultrasonicating at 30-50°C to obtain an acid solution; adding molybdate and ammonium fluorozirconate to distilled water, stirring and ultrasonicating at 60-80°C to obtain a salt solution; mixing the acid solution and the salt solution, stirring and ultrasonicating at 30-50°C; then cooling to room temperature, adding distilled water to adjust the volume to obtain the passivation solution.

4. The anodic oxidation passivation method for the zinc coating of the NdFeB permanent magnet according to claim 1, characterized in that: The mass concentration of the nitric acid solution is 50-70 wt %, and the mass concentration of the phosphoric acid solution is 85 wt %.

5. The anodic oxidation passivation method for the zinc coating of the NdFeB permanent magnet according to claim 1, characterized in that: The electrochemical anodizing method is constant voltage anodizing; the voltage of the constant voltage anodizing is 8-12V, and the time of the constant voltage anodizing is 50-70s; the electrochemical anodizing process is carried out at room temperature.

6. The anodic oxidation passivation method for the zinc coating of a NdFeB permanent magnet according to claim 1, characterized in that: The cleaning is performed by rinsing with distilled water, and the drying is performed by drying at 100-120° C. for 10-20 minutes.

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