A neodymium-iron-boron passivation solution and a method for its preparation and use

Abstract

This invention relates to a passivation solution for NdFeB materials and its preparation and application methods, belonging to the field of inorganic surface treatment. It solves one of the problems of poor corrosion resistance of NdFeB materials and the environmentally unfriendly nature of phosphorus-containing protective coatings in existing technologies. This invention discloses a passivation solution for treating NdFeB materials. The components of the passivation solution, by concentration, are: lanthanum nitrate 3-10 g / L, sodium citrate 3-10 g / L, sodium molybdate 1-6 g / L, sodium tetraborate 0.2-1 g / L, hydrogen peroxide 1-5 ml / L, and deionized water as the solvent. After treating NdFeB materials with the passivation solution disclosed in this invention according to the accompanying method, the corrosion resistance of the materials is significantly improved without causing pollution or damage to the environment.

Description

Technical Field

[0001] This invention relates to the field of inorganic surface treatment technology, and in particular to a neodymium iron boron passivation solution and its preparation and application methods. Background Technology

[0002] Neodymium iron boron (NdFeB) magnets are a type of rare-earth permanent magnet material composed of alloying elements such as neodymium, iron, and boron. Due to their excellent magnetic properties, they are widely used in high-tech fields such as electronic communications, medical equipment, new energy vehicles, and aerospace. NdFeB magnets are powder alloys containing chemically reactive neodymium, which has poor corrosion resistance. When exposed to the surrounding environment, they are easily attacked by corrosive media, which limits their further applications.

[0003] Currently, surface coatings are generally used to improve the corrosion resistance of NdFeB magnets. Common surface protective coatings include electroplating, electrophoresis, phosphating, and spraying. Literature review shows that phosphating is currently the most prevalent chemical conversion film on NdFeB magnets. Other types of chemical conversion films are rarely mentioned. Only CN103060803B invention patent mentions obtaining a yttrium conversion film on the surface of NdFeB permanent magnets, where it exists as a transition layer and cannot be used as a standalone film. CN102400125B mentions a method for double-layer protection of NdFeB magnet materials using a titanium / zirconium conversion film and an organic coating; the titanium / zirconium conversion film also exists as a transition layer.

[0004] However, phosphating solutions / films contain a large amount of phosphorus, which causes serious environmental pollution, and therefore their use is being gradually restricted. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a NdFeB passivation solution and its preparation and application method to solve one of the problems of poor corrosion resistance of existing NdFeB materials and serious environmental pollution of existing phosphating film treatment technology.

[0006] The invention discloses a passivation solution for treating neodymium iron boron materials. The components of the passivation solution, in terms of concentration, are: lanthanum nitrate 3-10 g / L, sodium citrate 3-10 g / L, sodium molybdate 1-6 g / L, sodium tetraborate 0.2-1 g / L, hydrogen peroxide 1-5 ml / L, and deionized water as the solvent.

[0007] Specifically, the passivation solution also contains a pH adjuster, making the pH value of the passivation solution 2.0 to 3.5.

[0008] Specifically, the pH adjuster is one or more of nitric acid, sulfuric acid, and hydrochloric acid.

[0009] The present invention also discloses a method for preparing the passivation solution, characterized in that:

[0010] S1: Measure out 80% of the volume of deionized water used to prepare the solution;

[0011] S2: Add lanthanum nitrate, sodium citrate, sodium molybdate, sodium tetraborate, and hydrogen peroxide, and stir until dissolved;

[0012] S3: Make up to volume with deionized water.

[0013] Specifically, step S2 includes: adding lanthanum nitrate, sodium citrate, sodium molybdate, sodium tetraborate, and hydrogen peroxide in sequence. After adding one raw material, stir thoroughly until it is completely dissolved before adding the next raw material.

[0014] Specifically, step S3 involves adjusting the pH value to 2.0–3.5 by adding a pH adjuster after making up the volume with deionized water.

[0015] The present invention also discloses a method for applying the passivation solution, characterized in that:

[0016] S21: Rinse the degreased NdFeB parts with tap water to remove any residual degreaser from the surface;

[0017] S22: Clean the NdFeB parts after S21 treatment;

[0018] S23: Immerse the iron-boron parts obtained in S22 into the passivation solution, leave them for a period of time, take them out, rinse and dry them to obtain NdFeB parts with a gray-brown rare earth chemical passivation film on the surface.

[0019] Specifically, step S22 includes: immersing the NdFeB components obtained in step S21 in a 3% to 5% nitric acid solution for pickling, leaving them at room temperature for a total of 40 to 60 seconds, during which time they are repeatedly rinsed with tap water and ultrasonically cleaned until the black oxide film on the NdFeB surface is removed, exposing the silver-gray NdFeB substrate.

[0020] Specifically, step S23 involves immersing the neodymium iron boron components obtained in step S22 into a passivation solution, holding them at 30-40°C for 30-40 minutes, rinsing them with tap water, and finally drying them at 80°C.

[0021] Specifically, the chemical passivation film is a mixture composed of lanthanum oxide, lanthanum hydroxide, iron oxide, and iron hydroxide.

[0022] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0023] 1. Passivating iron-neodymium boron materials with the passivation solution provided by this invention can form a passivation film on the surface of the iron-neodymium boron material, consisting of a mixture of lanthanum oxide, lanthanum hydroxide, iron oxide, and iron hydroxide. In this passivation film, lanthanum oxide, lanthanum hydroxide, iron oxide, and iron hydroxide are all insoluble substances with stable chemical properties. The formed insoluble passivation film isolates the substrate from external corrosive media, thereby protecting it from corrosion. It exhibits excellent corrosion resistance in special working environments with high temperature and high humidity, especially superior to the currently widely used phosphating film process.

[0024] 2. The passivation solution provided by this invention does not contain phosphorus or other chemical elements that easily pollute the environment, and no other ingredients or additives that easily cause environmental pollution need to be introduced during preparation and application. The entire process, from ingredient preparation to application and the final product (passivation film), is environmentally friendly and contributes to the construction of green industry.

[0025] 3. The passivation solution preparation method and application process provided by the present invention are simple and easy to implement, the raw materials are readily available, the process conditions are mild, most operations can be carried out at room temperature, the cost is controllable, the safety factor is high, the passivation film quality is stable, and it is suitable for corrosion resistance treatment of large-scale parts.

[0026] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0027] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0028] Figure 1 Flowchart of passivation process for NdFeB parts;

[0029] Figure 2 Microscopic morphology photograph of the passivation film in Example 1. Detailed Implementation

[0030] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0031] The invention discloses a passivation solution for treating neodymium iron boron materials. The components of the passivation solution, by concentration, are: lanthanum nitrate 3-10 g / L, sodium citrate 3-10 g / L, sodium molybdate 1-6 g / L, sodium tetraborate 0.2-1 g / L, hydrogen peroxide 1-5 ml / L, and deionized water as the solvent.

[0032] The roles and concentrations of each component are selected based on the following criteria:

[0033] Lanthanum nitrate: Lanthanum nitrate is the main film-forming agent in the passivation solution. With the synergistic effect of film-forming aids such as sodium citrate, sodium molybdate, sodium tetraborate, and hydrogen peroxide, it forms a passivation film composed of a mixture of lanthanum oxide, lanthanum hydroxide, iron oxide, and iron hydroxide. When the concentration of lanthanum nitrate is below 3 g / L, the passivation film is not dense enough; when the concentration is above 10 g / L, the density of the passivation film does not improve significantly further.

[0034] Sodium citrate: Sodium citrate is a complexing agent that complexes lanthanum ions, resulting in a denser passivation film. When the sodium citrate concentration is below 3 g / L, the lanthanum ions are not sufficiently complexed, leading to a decrease in the density of the passivation film. At the same time, too much sodium citrate should not be added. If the concentration exceeds 10 g / L, the complexed lanthanum ions become too stable, affecting the passivation film formation rate and thickness. A passivation film that is too thin will affect its corrosion resistance.

[0035] Sodium molybdate: Sodium molybdate acts as an oxidizing agent, increasing its oxidizing power. This leads to a higher lanthanum oxide content in the rare earth passivation film, resulting in better corrosion resistance. Therefore, when the sodium molybdate concentration is below 1 g / L, the corrosion resistance of the rare earth passivation film is insufficient; when the sodium molybdate concentration is above 6 g / L, further increasing the concentration does not further enhance its effect.

[0036] Hydrogen peroxide: Hydrogen peroxide acts as an oxidant, further oxidizing the rare earth passivation film and increasing the ratio of lanthanum oxide and iron oxide, thus improving its corrosion resistance. Therefore, when the hydrogen peroxide concentration is below 1 ml / L, the corrosion resistance of the rare earth passivation film is insufficient. However, too much hydrogen peroxide is also undesirable. When the concentration exceeds 5 ml / L, excessive hydrogen peroxide has an escape effect. As the hydrogen peroxide rapidly escapes in bubbles, it leads to a loose rare earth passivation film, which in turn reduces its corrosion resistance.

[0037] Sodium tetraborate: Sodium tetraborate is a buffer solution that helps to buffer rapid pH changes, which can affect the compactness of rare earth passivation films. When the sodium tetraborate concentration is below 0.2 g / L, the buffering effect is not significant; when the sodium tetraborate concentration is above 1 g / L, it is difficult to dissolve due to its poor solubility.

[0038] The specific film formation process and synergistic mechanism are as follows: In the acidic passivation solution, the surface of NdFeB first dissolves. As the dissolution proceeds, the local pH of the NdFeB surface increases, causing lanthanum ions to precipitate as lanthanum oxide and lanthanum hydroxide. At the same time, the dissolved iron ions also precipitate as iron oxide and iron hydroxide. During the reaction, the presence of oxidant can promote the production of more lanthanum oxide and iron oxide, resulting in a denser passivation film formed by the precipitation, which plays a role in corrosion resistance.

[0039] Specifically, the passivation solution also contains a pH adjuster, making the pH value of the passivation solution 2.0–3.5. The pH adjuster is one or more acidic pH adjusters such as nitric acid, sulfuric acid, and hydrochloric acid, and is preferably free of other anions, with nitric acid being the preferred choice.

[0040] pH adjusters primarily function to regulate pH. The pH of the passivation solution significantly impacts passivation film formation. Too low a pH hinders passivation film formation, while too high a pH slows it down considerably. Below pH 2.0, the acidity is too strong, causing NdFeB to dissolve too quickly, resulting in a rapidly dissolving passivation film and hindering its formation. Similarly, above pH 3.5, the passivation film formation process slows down, making it difficult to form a sufficiently thick chemical passivation film.

[0041] Specifically, the passivation solution preferably comprises, by concentration: 4-6 g / L lanthanum nitrate, 4-6 g / L sodium citrate, 1.5-3 g / L sodium molybdate, 0.2-0.3 g / L sodium tetraborate, and 1-2 ml / L hydrogen peroxide.

[0042] More preferably, the passivation solution comprises, by concentration: 5 g / L lanthanum nitrate, 5 g / L sodium citrate, 1.5 g / L sodium molybdate, 0.25 g / L sodium tetraborate, 1.5 ml / L hydrogen peroxide, and deionized water as the solvent. A pH adjuster is added to adjust the pH of the solution to 2.0–3.5. This passivation solution has been verified to have the best passivation effect.

[0043] The present invention also discloses a method for preparing the passivation solution, characterized in that:

[0044] S1: Measure out 80% of the volume of deionized water used to prepare the solution;

[0045] S2: Add lanthanum nitrate, sodium citrate, sodium molybdate, sodium tetraborate, and hydrogen peroxide, and stir to dissolve.

[0046] S3: After adjusting the volume with deionized water.

[0047] Specifically, step S2 involves adding lanthanum nitrate, sodium citrate, sodium molybdate, sodium tetraborate, and hydrogen peroxide in sequence. After adding one chemical, stir thoroughly until it is completely dissolved before adding the next chemical.

[0048] Specifically, step S3 involves adjusting the pH value to 2.0–3.5 by adding a pH adjuster after bringing the solution to volume with deionized water. The pH of the passivation solution has a significant impact on the formation of the passivation film; if the pH is too low, the passivation film is difficult to form, and if the pH is too high, the passivation film forms too slowly.

[0049] The present invention also discloses a method for applying the passivation solution, the steps of which are as follows:

[0050] S21: Rinse the degreased NdFeB parts with tap water to remove any residual degreaser from the surface;

[0051] S22: Thoroughly clean the NdFeB parts after S21 treatment;

[0052] S23: Immerse the iron-boron parts obtained in S22 in the passivation solution, leave them for a period of time, take them out, rinse and dry them to obtain NdFeB parts with a gray-brown rare earth chemical passivation film on the surface.

[0053] The pretreatment operation of S21 is mainly to remove residual oil from the surface of the parts. The presence of oil will affect the effect of subsequent oxide film removal and passivation film formation, resulting in a decrease in the corrosion resistance of the parts.

[0054] Specifically, step S22 involves immersing the NdFeB components obtained in step S1 in a 3%–5% nitric acid solution for pickling, leaving them at room temperature for a total of 40–60 seconds, during which time they are repeatedly rinsed with tap water and ultrasonically cleaned until the black oxide film on the NdFeB surface is completely removed, exposing the silver-gray NdFeB substrate.

[0055] Specifically, step S23 involves immersing the NdFeB components obtained in step S22 into the passivation solution. Larger components do not need to be turned over; immersion is sufficient. Smaller components require slight turning to ensure the passivation solution penetrates into fine structures or gaps. The solution is held at 30–40°C for 30–40 minutes, then rinsed with tap water and finally dried at 80°C. The passivation film thickness is closely related to the holding time. Experiments have shown that a holding time of 30–40 minutes results in a suitable passivation film thickness. Too short a holding time leads to insufficient passivation film coverage and thickness; too long a holding time does not significantly improve the passivation effect.

[0056] Specifically, the chemical passivation film is a mixture of lanthanum oxide, lanthanum hydroxide, iron oxide, and iron hydroxide. The detailed elemental composition of the passivation film is shown in Table 1.

[0057] Table 1. Elemental content of passivation film in Example 1 (energy dispersive spectroscopy)

[0058] element mass percentage Atomic weight percentage C 15.98 39.16 O 18.22 33.53 Al 0.35 0.38 Fe 40.96 21.59 Mo 2.86 0.88 La 1.57 0.33 Pr 4.75 0.99 Nd 15.31 3.12 total 100.00 100.00

[0059] Based on the above ingredient requirements, preparation method and application method, the following examples and comparative examples are provided: (The reagents used in the following examples and comparative examples are all commercially available analytical grade conventional reagents).

[0060] Example 1

[0061] At room temperature, weigh / measure 5g of lanthanum nitrate, 5g of sodium citrate, 1.5g of sodium molybdate, 0.25g of sodium tetraborate, and 1.5ml of hydrogen peroxide. Add them to an appropriate amount of deionized water, stir until completely dissolved, and dilute to 1L. Add an appropriate amount of nitric acid solution to adjust the pH to 2.8 for later use.

[0062] The NdFeB parts to be treated were pickled and derusted in a 3% nitric acid solution for 1 minute, then rinsed with water, and then immersed in a 30°C chemical passivation solution for 30 minutes. During the process, the parts were slowly turned over to ensure that each product formed a uniform film. After the chemical passivation treatment, the products were rinsed with water and dried.

[0063] Example 2

[0064] At room temperature, weigh / measure 4g of lanthanum nitrate, 6g of sodium citrate, 3g of sodium molybdate, 0.25g of sodium tetraborate, and 1.5ml of hydrogen peroxide. Add them to an appropriate amount of deionized water, stir until completely dissolved, and dilute to 1L. Add an appropriate amount of nitric acid solution to adjust the pH to 2.0 for later use.

[0065] The NdFeB parts to be treated were pickled and derusted in a 3% nitric acid solution for 1 minute, then rinsed with water, and then immersed in a 30°C chemical passivation solution for 30 minutes. During the process, the parts were slowly turned over to ensure that each product formed a uniform film. After the chemical passivation treatment, the products were rinsed with water and dried.

[0066] Example 3

[0067] At room temperature, weigh / measure 8g of lanthanum nitrate, 8g of sodium citrate, 5g of sodium molybdate, 0.75g of sodium tetraborate, and 3ml of hydrogen peroxide. Add them to an appropriate amount of deionized water, stir until completely dissolved, and dilute to 1L. Add an appropriate amount of nitric acid solution to adjust the pH to 2.8 for later use.

[0068] The NdFeB parts to be treated were pickled and derusted in a 3% nitric acid solution for 1 minute, then rinsed with water, and then immersed in a 30°C chemical passivation solution for 30 minutes. During the process, the parts were slowly turned over to ensure that each product formed a uniform film. After the chemical passivation treatment, the products were rinsed with water and dried.

[0069] Example 4

[0070] At room temperature, weigh / measure 5g of lanthanum nitrate, 5g of sodium citrate, 1.5g of sodium molybdate, 0.25g of sodium tetraborate, and 1.5ml of hydrogen peroxide. Add them to an appropriate amount of deionized water, stir until completely dissolved, and dilute to 1L. Add an appropriate amount of nitric acid solution to adjust the pH to 2.8 for later use.

[0071] The obtained NdFeB products were pickled and derusted in a 3% nitric acid solution for 1 minute, then washed with water, and then immersed in a 20°C chemical passivation solution for 20 minutes. During the process, the products were slowly turned over to ensure that each product formed a uniform film. After the chemical passivation treatment, the products were washed with water and dried.

[0072] Comparative Example 1

[0073] At room temperature, weigh / measure 5g sodium citrate, 1.5g sodium molybdate, 0.25g sodium tetraborate, and 1.5ml hydrogen peroxide, add them to an appropriate amount of deionized water, stir until completely dissolved, and dilute to 1L. Add an appropriate amount of nitric acid solution to adjust the pH to 2.8 for later use.

[0074] After pickling and removing rust in a 3% nitric acid solution for 1 minute, the neodymium iron boron products are washed with water and then immersed in a 30°C chemical passivation solution for 30 minutes. During the process, the products are slowly turned over to ensure that each product forms a uniform film. After the chemical passivation treatment, the products are washed with water and dried.

[0075] Comparative Example 2

[0076] At room temperature, weigh / measure 2g of lanthanum nitrate, 5g of sodium citrate, 1.5g of sodium molybdate, 0.25g of sodium tetraborate, and 1.5ml of hydrogen peroxide. Add them to an appropriate amount of deionized water, stir until completely dissolved, and dilute to 1L. Add an appropriate amount of nitric acid solution to adjust the pH to 2.8 for later use.

[0077] The obtained NdFeB products were pickled and derusted in a 3% nitric acid solution for 1 minute, then washed with water, and then immersed in a 30°C chemical passivation solution for 30 minutes. During the process, the products were slowly turned over to ensure that each product formed a uniform film. After the chemical passivation treatment, the products were washed with water and dried.

[0078] Comparative Example 3

[0079] At room temperature, weigh / measure 5g of lanthanum nitrate, 1.5g of sodium molybdate, 0.25g of sodium tetraborate, and 1.5ml of hydrogen peroxide. Add them to an appropriate amount of deionized water, stir until completely dissolved, and dilute to 1L. Add an appropriate amount of nitric acid solution to adjust the pH to 2.8 for later use.

[0080] The obtained NdFeB products were pickled and derusted in a 3% nitric acid solution for 1 minute, then washed with water, and then immersed in a 30°C chemical passivation solution for 30 minutes. During the process, the products were slowly turned over to ensure that each product formed a uniform film. After the chemical passivation treatment, the products were washed with water and dried.

[0081] Comparative Example 4

[0082] At room temperature, weigh / measure 5g of lanthanum nitrate, 15g of sodium citrate, 1.5g of sodium molybdate, 0.25g of sodium tetraborate, and 1.5ml of hydrogen peroxide. Add them to an appropriate amount of deionized water, stir until completely dissolved, and dilute to 1L. Add an appropriate amount of nitric acid solution to adjust the pH to 2.8 for later use.

[0083] The obtained NdFeB products were pickled and derusted in a 3% nitric acid solution for 1 minute, then washed with water, and then immersed in a 30°C chemical passivation solution for 30 minutes. During the process, the products were slowly turned over to ensure that each product formed a uniform film. After the chemical passivation treatment, the products were washed with water and dried.

[0084] Comparative Example 5

[0085] At room temperature, weigh / measure 5g of lanthanum nitrate, 5g of sodium citrate, 0.25g of sodium tetraborate, and 1.5ml of hydrogen peroxide. Add them to an appropriate amount of deionized water, stir until completely dissolved, and dilute to 1L. Add an appropriate amount of nitric acid solution to adjust the pH to 2.8 for later use.

[0086] The obtained NdFeB products were pickled and derusted in a 3% nitric acid solution for 1 minute, then washed with water, and then immersed in a 30°C chemical passivation solution for 30 minutes. During the process, the products were slowly turned over to ensure that each product formed a uniform film. After the chemical passivation treatment, the products were washed with water and dried.

[0087] Comparative Example 6

[0088] At room temperature, weigh / measure 5g of lanthanum nitrate, 5g of sodium citrate, 1.5g of sodium molybdate, 0.25g of sodium tetraborate, and 7ml of hydrogen peroxide. Add them to an appropriate amount of deionized water, stir until completely dissolved, and dilute to 1L. Add an appropriate amount of nitric acid solution to adjust the pH to 2.8 for later use.

[0089] The obtained NdFeB products were pickled and derusted in a 3% nitric acid solution for 1 minute, then washed with water, and then immersed in a 30°C chemical passivation solution for 30 minutes. During the process, the products were slowly turned over to ensure that each product formed a uniform film. After the chemical passivation treatment, the products were washed with water and dried.

[0090] Comparative Example 7

[0091] At room temperature, weigh / measure 5g of lanthanum nitrate, 5g of sodium citrate, 1.5g of sodium molybdate, and 0.25g of sodium tetraborate, and add them sequentially to an appropriate amount of deionized water. Stir until completely dissolved, and dilute to 1L. Add an appropriate amount of nitric acid solution to adjust the pH to 2.8 for later use.

[0092] The obtained NdFeB products were pickled and derusted in a 3% nitric acid solution for 1 minute, then washed with water, and then immersed in a 30°C chemical passivation solution for 30 minutes. During the process, the products were slowly turned over to ensure that each product formed a uniform film. After the chemical passivation treatment, the products were washed with water and dried.

[0093] Comparative Example 8

[0094] At room temperature, weigh / measure 5g of lanthanum nitrate, 5g of sodium citrate, 1.5g of sodium molybdate, 0.25g of sodium tetraborate, and 1.5ml of hydrogen peroxide. Add them to an appropriate amount of deionized water, stir until completely dissolved, and dilute to 1L. Add an appropriate amount of nitric acid solution to adjust the pH to 1.5 for later use.

[0095] The obtained NdFeB products were pickled and derusted in a 3% nitric acid solution for 1 minute, then washed with water, and then immersed in a 30°C chemical passivation solution for 30 minutes. During the process, the products were slowly turned over to ensure that each product formed a uniform film. After the chemical passivation treatment, the products were washed with water and dried.

[0096] Table 2 shows a comparison of the results of high-temperature and high-humidity tests (relative humidity 90% RH, temperature 60℃) on the NdFeB chemical passivation film products of the examples and comparative examples.

[0097] Table 2 Results of high temperature and high humidity experiments in the examples and comparative examples

[0098]

[0099]

[0100] Comparative analysis reveals the following:

[0101] In Comparative Example 1, no lanthanum nitrate was added to the passivation solution. Since there was no main film-forming agent in the passivation solution, only a very thin iron oxide and hydroxide film could be formed, which had poor corrosion resistance. Rust appeared after 6 hours of high temperature and high humidity test.

[0102] In Comparative Example 2, a small amount of lanthanum nitrate was added to the passivation solution. Due to the low content of lanthanum nitrate as the main film-forming agent in the passivation solution, only a thin passivation film could be formed, resulting in poor corrosion resistance. Rust appeared after 12 hours of high temperature and high humidity testing.

[0103] In Comparative Example 3, no sodium citrate was added to the passivation solution. Since sodium citrate is also a complexing agent in the passivation solution, it affects the density of the passivation film. Therefore, although a chemical passivation film is formed, the density of the passivation film is insufficient, the corrosion resistance is average, and rust appears after 10 hours of high temperature and high humidity test.

[0104] In Comparative Example 4, an excess of sodium citrate was added to the passivation solution. Since sodium citrate is a complexing agent in the passivation solution, the excess sodium citrate made the lanthanum ions too stable. As a result, the chemical passivation film formed was thin and had poor corrosion resistance. Rust appeared after 16 hours of high temperature and high humidity testing.

[0105] In Comparative Example 5, no sodium molybdate was added to the passivation solution. Sodium molybdate exists as an oxidant, which affects the ratio of lanthanum oxide to lanthanum hydroxide and iron oxide to iron hydroxide in the passivation film. Obviously, a higher ratio of lanthanum oxide and iron oxide results in a denser passivation film. After adding sodium molybdate, the corrosion resistance of the passivation film is significantly improved, and rust appears after 18 hours of high temperature and high humidity testing.

[0106] In Comparative Example Six, no hydrogen peroxide was added to the passivation solution. Hydrogen peroxide also exists as an oxidant, increasing the ratio of lanthanum oxide to lanthanum hydroxide and iron oxide to iron hydroxide in the passivation film. Although hydrogen peroxide has good oxidizing properties, it has an escape effect. Too much hydrogen peroxide will produce a large number of bubbles, causing the passivation film to become loose. Therefore, it is not advisable to add too much. A small amount of hydrogen peroxide can improve the corrosion resistance of the passivation film. Rust appeared after 16 hours of high temperature and high humidity test.

[0107] In Comparative Example 7, excessive hydrogen peroxide was added to the passivation solution. Hydrogen peroxide exhibits an escape effect, and too much hydrogen peroxide will generate a large number of bubbles, causing the passivation film to become loose and streaky red rust to form on the surface of the passivation film. The appearance does not meet the requirements, and there is no value in further corrosion testing.

[0108] In Comparative Example 8, the passivation solution was adjusted to pH 1.5, resulting in excessively rapid substrate corrosion and the appearance of numerous bubbles. This was due to hydrogen evolution from the acid corrosion of the NdFeB substrate. The excessively low pH caused severe corrosion and significant hydrogen evolution, leading to a noticeably loose passivation film with poor corrosion resistance. Rust appeared after 8 hours of high-temperature and high-humidity testing.

[0109] In Example 4, the decrease in temperature and passivation time led to a thinner passivation film, which reduced the corrosion resistance of the passivation film, and rust appeared after 18 hours of high temperature and high humidity testing.

[0110] In Examples 1 to 3, the passivation solutions contained all the necessary components at concentrations within the specified ranges, and the passivation operation parameters were within the optimal ranges. The passivation solutions produced a grayish-brown passivation film on the NdFeB product, which was dense and had few defects. The passivation film showed no corrosion after 24 hours of high-temperature and high-humidity testing.

[0111] Therefore, in the passivation solution of the present invention, lanthanum nitrate mainly exists as a film-forming agent, sodium molybdate and hydrogen peroxide mainly exist as oxidants, and sodium citrate and sodium tetraborate mainly exist as film-forming aids.

[0112] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A passivation solution for treating neodymium iron boron rare earth permanent magnet materials, characterized in that: The passivation solution comprises, by concentration: lanthanum nitrate 3-10 g / L, sodium citrate 3-10 g / L, sodium molybdate 1-6 g / L, sodium tetraborate 0.2-1 g / L, hydrogen peroxide 1-3 ml / L, deionized water as the solvent, and one or more of nitric acid, sulfuric acid, and hydrochloric acid as pH adjusters to make the pH of the passivation solution 2.0-3.

5. Lanthanum nitrate is the main film-forming agent, sodium citrate is the complexing agent used to complex lanthanum ions, sodium molybdate and hydrogen peroxide constitute a composite oxidant used to increase the proportion of lanthanum oxide and / or iron oxide in the passivation film, and sodium tetraborate is a pH buffer solution. The synergistic effect of lanthanum nitrate, sodium citrate, sodium molybdate, sodium tetraborate, and hydrogen peroxide forms a grayish-brown composite passivation film on the surface of the neodymium iron boron permanent magnet material. The composite passivation film is a mixture composed of lanthanum oxide, lanthanum hydroxide, iron oxide, and iron hydroxide. The neodymium iron boron rare earth permanent magnet material is immersed in the passivation solution, treated at 30-40°C for 30-40 minutes, then taken out, rinsed with tap water, and dried at 80°C to obtain a composite passivation film. The composite passivation film rusts for more than 24 hours in a high temperature and high humidity environment of 90%RH and 60°C.

2. A method for preparing the passivation solution according to claim 1, characterized in that: S1: Measure out 80% of the volume of deionized water used to prepare the solution; S2: Add lanthanum nitrate, sodium citrate, sodium molybdate, sodium tetraborate, and hydrogen peroxide, and stir until dissolved; S3: Make up to volume with deionized water.

3. The preparation method according to claim 2, characterized in that: Step S2 includes: adding lanthanum nitrate, sodium citrate, sodium molybdate, sodium tetraborate, and hydrogen peroxide in sequence. After adding one raw material, stir thoroughly until it is completely dissolved before adding the next raw material.

4. The preparation method according to claim 2, characterized in that: Step S3 involves adjusting the pH value to 2.0–3.5 by adding a pH adjuster after bringing the volume to a final volume with deionized water.

5. A method for applying the passivation solution according to claim 1, characterized in that: S21: Rinse the degreased NdFeB parts with tap water to remove any residual degreaser from the surface; S22: Clean the NdFeB parts after S21 treatment; S23: Immerse the iron-boron parts obtained in S22 into the passivation solution, leave them for a period of time, take them out, rinse and dry them to obtain NdFeB parts with a gray-brown rare earth chemical passivation film on the surface.

6. The application method according to claim 5, characterized in that: Step S22 includes: immersing the NdFeB components obtained in step S21 in a 3% to 5% nitric acid solution for pickling, leaving it at room temperature for a total of 40 to 60 seconds, during which time it is repeatedly rinsed with tap water and ultrasonically cleaned until the black oxide film on the NdFeB surface is removed, exposing the silver-gray NdFeB substrate.

7. The application method according to claim 5, characterized in that: Step S23 involves immersing the neodymium iron boron components obtained in step S22 into a passivation solution, holding them at 30-40°C for 30-40 minutes, rinsing them with tap water, and finally drying them at 80°C.

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