The service life of neodymium magnets is one of the issues that many motor manufacturers focus on. The length of its service life is directly related to the durability of permanent magnet motors and wind turbines. We can judge the service life of neodymium magnets according to the amount of magnetic flux loss generated by neodymium magnets over time.
Firstly, the conclusion is that if neodymium magnets are stored in an environment with appropriate temperature, humidity, and without strong external magnetic fields or other factors affecting their magnetic properties, their magnetic properties can be almost permanently maintained. This is also the reason why neodymium magnets are called permanent magnets.
The following is the experimental data on the service life of neodymium magnets：
| The flux records of 8 sintered NdFeB magnet samples that were directly exposed to room temperature air during a 12-year period 【Hcj=18kOe(1440kA/m)，Pc= -2】 | |||||||||||
| Test date | Days | Temperature /℃ | Sample number | Average value of flux loss /% | |||||||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | ||||
| 2003-06-06 | 0 | 一 | 142.5 | 141.1 | 141.5 | 140.9 | 142.4 | 142.8 | 142.9 | 140.4 | 0.00 |
| 2004-06-18 | 377 | 一 | 142.6 | 141.3 | 141.6 | 140.9 | 142.8 | 142.6 | 142.0 | 140.5 | 0.084 |
| 2005-06-10 | 734 | 28 | 142.2 | 140.8 | 141.3 | 140.7 | 142.4 | 142.5 | 142.9 | 140.4 | -0.0112 |
| 2005-11-25 | 901 | 22 | 142.7 | 141.4 | 141.9 | 141.3 | 143.0 | 143.0 | 143.3 | 140.4 | -0.217 |
| 2006-06-20 | 1109 | 24 | 142.8 | 141.5 | 141.9 | 141.2 | 142.8 | 143.0 | 143.5 | 140.7 | -0.002 |
| 2007-06-21 | 1475 | 23 | 142.6 | 141.2 | 141.7 | 141.0 | 142.8 | 142.8 | 142.0 | 140.5 | -0.250 |
| 2008-06-17 | 1836 | 24 | 142.6 | 141.2 | 141.6 | 141.0 | 142.6 | 142.9 | 142.0 | 140.3 | -0.195 |
| 2009-06-24 | 2208 | 23.5 | 142.7 | 141.3 | 141.8 | 141.0 | 142.8 | 142.9 | 142.0 | 140.3 | -0.187 |
| 2010-06-28 | 2577 | 23.5 | 142.0 | 141.0 | 141.4 | 140.7 | 142.4 | 142.5 | 142.7 | 140.1 | -0.452 |
| 2011-06-17 | 2931 | 22.8 | 142.6 | 141.0 | 141.6 | 140.9 | 142.6 | 142.7 | 142.9 | 140.2 | -0.365 |
| 2012-06-29 | 3308 | 22.7 | 142.5 | 141.1 | 141.6 | 140.9 | 142.6 | 142.8 | 142.9 | 140.2 | -0.365 |
| 2013-07-16 | 3690 | 24.5 | 142.2 | 140.6 | 141.6 | 140.0 | 142.0 | 142.2 | 142.4 | 139.7 | -0.526 |
| 2015-08-07 | 4441 | 27.5 | 141.8 | 140.5 | 141.0 | 140.0 | 141.8 | 142.0 | 142.2 | 139.6 | -0.448 |
| Note: When calculating the average value of flux loss, convert the flux data to 23℃ according to the temperature coefficient of -0.09%/℃. | |||||||||||
Researchers conducted observations and measurements on eight sintered neodymium iron boron magnet samples for as long as 12 years. The intrinsic coercivity (Hcj) of these samples is 18kOe (1440kA/m), and the permeability coefficient (Pc) is -2. Without plating, they are directly exposed to the atmospheric environment with a room temperature of 22°C – 28°C.
Experiments show that the relative magnetic flux loss measured in the first six years is very small, but an inflection point appears around 2208 days (about 6 years). Obvious rust spots can be seen on the surface of the magnet after 6 years of placement, which means that the surface and interior of the magnet have begun to oxidize and corrode. As time goes by, its oxidation or corrosion range will continue to expand, and the rate of performance attenuation will also accelerate significantly.
According to the current experimental data, the expected magnetic flux loss in 30 years does not exceed 1%, the magnetic flux loss in 50 years is about 1.3%, and the corresponding time for a 2% loss is about 150 years. This result shows that in non-extreme environments, if the service life of the magnet is defined as the time corresponding to a magnetic flux loss of less than 5%, even if there is no corrosion-resistant coating on the surface, sintered neodymium iron boron magnets still have a very long service life.
Of course, neodymium magnets may also work in some extreme environments with high temperature and high humidity. Motor manufacturers can use low weight loss magnets with coatings and high coercivity to manufacture products. After composition optimization and surface protection treatment, the oxidation resistance and corrosion resistance of this kind of neodymium magnet have been greatly improved. For sintered neodymium magnets with high enough coercivity, the service life can completely exceed 30 to 50 years.
Learn more about low weight loss magnets：What Is Low Weight Loss Magnet? | HQ Magnet (hq-magnet.com)
What affects the service life of neodymium magnets?
The ideally perfect working or storage environment is almost non-existent. We need to understand the reasons for demagnetization of neodymium magnets in order to extend its service life as much as possible.
1. Mechanical shock
Neodymium magnets are relatively brittle in texture and are very easily broken due to bumps and drops. Strong mechanical shock can cause microscopic defects such as cracks and dislocations inside neodymium magnets. These physical damages will not only affect the appearance of the magnet but also seriously affect its magnetic properties.
The magnetism of neodymium magnets mainly comes from the orderly arrangement of magnetic domains inside. Mechanical shock also generates stress, causing the boundaries of magnetic domains to move and distort, and making the arrangement of magnetic domains chaotic. Just like an originally neatly arranged team becomes disorderly after being subjected to a violent impact. The directions of magnetic moments are no longer consistent, further reducing the overall magnetism of neodymium magnets.
During use and storage, it is necessary to pay attention to avoiding strong mechanical shocks or high-frequency vibrations to prevent demagnetization.
2. Corrosion and oxidation
Neodymium magnets themselves are very prone to oxidation or corrosion. In the absence of a coating and in a harsh environment, ordinary neodymium magnets will soon show signs of rust.
Neodymium is the main raw material of neodymium iron boron magnets. As a lanthanide element, it has typical active metal properties (standard potential E0(Nd³+/Nd)=-2.431V). It is easily oxidized in air and can react rapidly with water to produce hydrogen. On the other hand, neodymium iron boron alloy is a multiphase structure, and the electrochemical potentials between the phases vary greatly, which is easy to cause electrochemical corrosion.
During the sintering process of neodymium magnets, defects such as micropores, roughness, and loose structure are prone to appear inside and on the surface of the magnet. In the processing processes such as pickling and electroplating of neodymium magnets, some acidic substances or impurities may remain in these microstructures. These residues and defects provide convenient conditions for magnet corrosion in harsh environments.
Impurity elements such as O, H, Cl and their compounds will be doped into neodymium magnets during the manufacturing process. The elements that have the greatest impact on corrosion are O and Cl. Magnets are oxidized and corroded by O, while Cl and its compounds will accelerate the oxidation process of the magnet.
In general, the working environment, raw materials and manufacturing process will all affect the corrosion rate of neodymium magnets. Neodymium magnets should be kept away from high temperature and high humidity as much as possible. They can be sealed or placed in a dry and constant temperature environment. For some special application scenarios, protective coatings (nickel, zinc or epoxy) must be used.
The manufacturing process can be further improved. By reducing the contact between raw materials and oxygen and using ultrasonic cleaning to remove acidic substance residues, the possibility of internal corrosion of magnets can be minimized to the greatest extent.
3. High temperature environment.
When the temperature rises, the atomic thermal motion inside the neodymium iron boron magnet intensifies, making the thermal disturbance on the magnetic moment stronger. This thermal disturbance will destroy the orderly arrangement of magnetic moments and make the direction of magnetic moments randomized, thereby reducing the magnetization intensity of the magnet.
When the temperature exceeds the Curie temperature, the magnetism of the magnet will completely disappear. The Curie temperature refers to the temperature at which the spontaneous magnetization intensity in a magnetic material drops to zero. According to different temperature resistance levels of neodymium magnets, its Curie temperature is around 80°C to 210°C. In a high-temperature environment, if the temperature exceeds the Curie temperature for a period of time, irreversible demagnetization will occur in neodymium iron boron magnets.
Even if the temperature does not exceed the Curie temperature, frequent thermal demagnetization phenomena will also accelerate the aging of neodymium magnets and reduce their service life.
4. Strong alternating magnetic field
An alternating magnetic field is a magnetic field whose magnitude and direction change continuously with time. In an alternating magnetic field, the value of the magnetic field intensity is not constant but changes periodically according to a certain rule. For example, the magnetic field intensity can change back and forth between positive and negative maximum values, just like a sine wave. At the same time, the direction of the magnetic field will also continuously change with time.
When a neodymium magnet is exposed to a strong alternating magnetic field, the magnetic domains must continuously adjust their alignment direction to adapt to the change in the direction of the magnetic field.
After multiple magnetic field reversals, some magnetic domains cannot return to their original directions, resulting in a decrease in the overall magnetism of the neodymium magnet. If the intensity of the alternating magnetic field exceeds the coercivity of the neodymium magnet, significant demagnetization of the magnet will occur.
Generally, during recycling or reprocessing, a demagnetizer that can generate a strong alternating magnetic field is used to eliminate the magnetism of neodymium magnets first. Once a motor containing neodymium magnets is exposed to a strong alternating magnetic field, it basically means the scrapping of the motor.
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