Passivation of NdFeB magnet

Passivation treatment – simple, low-cost, strong corrosion resistance

Neodymium-iron-boron(NdFeB) magnets, with their excellent magnetic properties, have become an indispensable key material in the field of contemporary high technology, and are widely used in many fields such as electric vehicles, wind power generation, aerospace, precision instruments, and medical equipment.

However, due to its high content of active rare earth elements and iron, neodymium magnets are highly susceptible to corrosion from environmental factors such as moisture and oxygen in the atmosphere, leading to surface erosion and subsequently affecting their magnetic properties and lifespan. High temperatures and humidity, as well as acidic and alkaline environments, can accelerate the corrosion process of magnets, especially in environments with chloride ions. Iron oxide may react with chlorides to form more soluble iron chloride, accelerating the corrosion of magnets.

The surface of neodymium-iron-boron magnets may have a reddish-brown iron oxide Fe2O3 after corrosion, while neodymium may also form neodymium oxide Nd2O3 or neodymium hydroxide Nd(OH)2. The formation of these oxides can reduce the magnetic properties of neodymium magnets, ultimately leading to the inability of equipment (permanent magnet motors, nuclear magnetic resonance machines) to function properly.

Passivation
Passivated neodymium magnet

In order to improve the service life of products, in addition to using metal plating and spraying epoxy coating, some customers will require the use of surface passivation to improve the corrosion resistance of neodymium magnets. Unlike the other two methods, passivation treatment does not directly cover the magnet surface with protective layers of other materials (such as metal nickel, metal zinc, and epoxy resin), but instead improves its corrosion resistance by changing the chemical properties of the magnet surface material.

Principle of surface passivation

Passivation of NdFeB magnet 11

We know that iron can dissolve quickly in dilute nitric acid. However, after a period of reaction in concentrated nitric acid, the dissolution phenomenon almost completely stops. This is because concentrated nitric acid can quickly react with iron and form a dense protective film of iron oxide (FeO3·nH2O) on the surface, which prevents further reaction.

However, due to its weaker oxidizing properties, dilute nitric acid cannot quickly form such a protective film, so it continues to penetrate into the interior of the iron for dissolution.

Passivation is the active use of strong oxidants or electrochemical methods to oxidize magnets, forming a dense passivation film on their surface, transforming the chemical properties of the surface into an inactive state, thereby improving their corrosion resistance.

Neodymium Magnet Passivator

A neodymium magnet passivator is a specialized chemical mixture designed to protect the surface of neodymium magnets and prevent rusting. Its main components and their functions are as follows:

1. Metal Passivators:

Function: These components are the core of the passivator, functioning by chemically reacting with the metal surface to form an extremely thin yet dense protective layer. This layer effectively isolates the metal from direct contact with corrosive media such as air and moisture, significantly preventing oxidation and corrosion.

2. Surfactants:

Function: Surfactants possess properties that lower surface tension, enhancing the solution’s wetting ability on the metal surface. They assist in distributing the passivator and other components more evenly across the metal surface, improving the effectiveness of the passivation process. Additionally, they help remove oil and impurities from the metal surface, providing a clean substrate for the formation of the passivation layer.

3. Buffer Agents:

Function: During chemical treatment, buffer agents maintain the pH of the solution within a stable range, preventing drastic fluctuations in acidity or alkalinity that could affect the passivation reaction and the quality of the passivation film. A stable pH environment is crucial for ensuring the effective operation of the passivator and the formation of a high-quality passivation layer.

4. Complexing Agents:

Function: The complexing agent assists in the dissolution of metal ions from the surface of the neodymium magnet and their re-deposition to form a protective film. It can also adjust the dissolution rate of metal ions, thereby controlling the growth speed and thickness of the passivation film, ensuring that the film layer is even and dense. The complexed film layer formed is more stable than a simple metal oxide film layer, able to better resist corrosion factors in the environment, and is less likely to fall off.

Together, these components create a comprehensive protection system that not only effectively prevents rusting of neodymium-iron-boron magnets but also enhances their surface quality and long-term stability, extending their service life.

Process flow of neodymium magnet passivation

Degreasing → Water Wash → Ultrasonic Water Wash → Acid Wash → Water Wash → Ultrasonic Water Wash → Deionized Water Wash → Deionized Water Wash → Passivation Treatment → Deionized Water Wash → Deionized Water Wash → Dehydration → Drying.

1.Degreasing:

Degreasing only removes oils, dust, sweat marks, and attached metal chips from the surface of NdFeB magnets; it does not remove rust. Surface oils on NdFeB magnets come from the material processing process, such as anti-rust oils used for corrosion prevention during storage and transportation, cutting fluids encountered when machining parts, etc.

To ensure the quality of the surface treatment of NdFeB magnets, surface oils must be thoroughly removed. Due to the variety of oil types and varying degrees of contamination, the degreasing process is relatively complex. During the degreasing process, to prevent corrosion and subsequent residue, it is recommended to use a degreaser with low free alkalinity and low total alkalinity.

2.Acid Wash:

The purpose of acid wash is to remove residual dust and rust on the surface of NdFeB magnets. The acid wash solution usually uses 2% to 4% nitric acid, with a time of 0.5 to 2.0 minutes. High acid concentration and excessively long acid wash time are both detrimental to the magnet.

3.Passivation:

Place the magnet in a container containing the passivation solution for immersion or spray for a period of time, or passivate the magnet by using it as an anode and passing electricity through it, thereby forming a passivation film on the surface of the magnet.

4.Water Washes and Drying:

In the passivation process flow for neodymium magnets, multiple water wash steps, including regular water wash, ultrasonic water wash, and deionized water wash, each play different roles and work together to ensure the quality and performance of the final product.

(1) Water Wash after Degreasing:

Removes residual degreaser and loose oil. Prevents re-deposition of oil in subsequent processes, ensuring effective results for acid wash and passivation treatment.

(2) Ultrasonic Water Wash:

Utilizes the cavitation effect of ultrasound to more thoroughly remove impurities from the surface and micro-pores. This is very effective for cleaning residual substances in surface pores, ensuring absolute surface cleanliness.

(3) Water Wash after Acid Wash:

Removes residual acid wash solution and reaction products, preventing the remaining acid from corroding the neodymium magnet.

(4) Deionized Water Wash:

Uses deionized water or high-purity water for the final rinse, removing all residual ions and impurities. Avoids the formation of precipitates on the surface of neodymium magnets due to ions present in ordinary water (such as calcium, magnesium, etc.), affecting the adhesion of the passivation layer and the corrosion resistance properties.

(5) Dehydration and Drying:

Timely removal of all water from the surface to prevent water marks and corrosion.

Development History of Neodymium Magnet Passivation

Due to its low cost, ease of use, and excellent ability to enhance metal corrosion resistance, chromate has been used as a passivating agent for various metals and alloys since its first application on magnesium in 1924, and it has continued to be used up to the present day. In the early development of neodymium-iron-boron passivation technology, chromate passivators were primarily used.

The mixed-metal passivation layer formed by this chromate treatment mainly consists of trivalent chromium and hexavalent chromium, where trivalent chromium forms the framework, while hexavalent chromium (chromate ions) easily leaches out from the passivation film to act as a corrosion inhibitor, offering good rust prevention and wear resistance.

However, over the past decade, people have gained a deeper understanding of chromates, recognizing their high toxicity and carcinogenic potential. For example,1-2g of chromic acid or 6-8g of potassium dichromate can lead to kidney failure, liver damage, blood disorders, and death. Long-term exposure to chromates can cause rashes, blisters, and ulcers, and excessive inhalation of chromates may result in liver cancer.

As a result, government regulations strictly limit the levels of chromates in the workplace, and workers using chromates must be informed about the associated health risks. According to the Occupational Safety and Health Administration (OSHA) in the United States, the air content of insoluble chromates should be less than 1 mg/m³ in workplaces where employees work 8 hours a day and 40 hours a week.

After 2007, in accordance with the EU’s Restriction of Hazardous Substances (RoHS) requirements, the traditional highly polluting hexavalent chromium passivation process for neodymium magnets was phased out, replaced by a new, less polluting trivalent chromium passivation process. However, this method inevitably reduced the corrosion resistance of the passivation layer.

In recent years, purchasers of neodymium magnets have increasingly demanded higher corrosion resistance after passivation, making standalone passivation techniques difficult to meet technical standards. A commonly used process now is the composite conversion film technique, which involves phosphating followed by passivation. By filling the pores in the phosphating film, the corrosion resistance of the composite conversion film can be significantly improved.

Improvements in magnet passivation technology mainly revolve around two aspects. One aspect is utilizing safer production processes to reduce the impact of hazardous substances on people and the environment during production. The other aspect is finding low-toxicity or non-toxic alternatives to chromates, with organic-inorganic hybrid technologies based on silane materials being a hot topic of international research.

Currently, some chromate-free magnet passivators have appeared on the market. However, whether these alternatives will truly become mainstream still requires more time for verification.

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