Strictly speaking, magnets don't have the concept of "expiration" like food does, but their magnetism may weaken over time under the influence of various factors.
Natural Attenuation
• The magnetic domains inside magnets will maintain a relatively stable arrangement under ideal conditions, thus maintaining their magnetism. However, in the natural environment, as time passes, the arrangement of magnetic domains may gradually become disordered. This is an extremely slow process, and for ordinary magnets, the weakening of magnetism caused by this natural attenuation is usually not obvious in a short period of time (such as several years or even decades). For example, common ferrite magnets can maintain their magnetism for a long time under normal storage conditions.
Influence of Environmental Factors
• Temperature: Temperature has a significant impact on the magnetism of magnets. When the temperature rises, the magnetic domains inside the magnets will become more active due to thermal motion, resulting in the disruption of the arrangement of magnetic domains. Different types of magnets have different Curie temperatures. When the temperature exceeds this value, the magnetism of the magnets will decline sharply or even disappear. For example, the Curie temperature of Nd - Fe - B magnets is generally around 300 - 400 °C. In a high-temperature environment, their magnetism will be quickly lost. In a relatively low-temperature environment, although the magnetism will not disappear completely like in a high-temperature environment, it may also be weakened.
• Humidity and Corrosion: If magnets are exposed to a humid environment for a long time without proper protective measures, corrosion may occur. For some magnets with coating protection, such as nickel-plated Nd - Fe - B magnets, if the coating is damaged, substances such as moisture and oxygen in the air will erode the internal metal components of the magnets, thereby affecting their magnetism. For example, if ferrite magnets get rusty due due to moisture, their magnetism will also be affected.
• External Magnetic Field Interference: When magnets are in an environment with a strong external magnetic field, the external magnetic field may change the arrangement direction of the magnetic domains inside the magnets. If the external magnetic field is strong enough and acts for a long time, the original magnetism of the magnets may be changed or even reversed. For example, near some large motors or transformers where the magnetic field strength is relatively high, if small magnets are placed nearby, their magnetism may be interfered with.
Influence of Mechanical Factors
• Intense vibration or impact may lead to changes in the arrangement of magnetic domains inside magnets. Especially for some magnets with strong magnetism but brittle texture, such as Nd - Fe - B magnets, after being subjected to severe mechanical shocks, the disordered arrangement of magnetic domains may occur, thus weakening their magnetism. For example, during transportation, if Nd - Fe - B magnets are not properly packaged and shock-absorbed, their magnetism may decline after bumps.
