Neodymium magnets, known as the strongest permanent magnets, are highly susceptible to corrosion and will rust rapidly if they are not protected. The chemical composition that gives them their strength is also the source of their weakness against environmental degradation. Exposure to moisture and oxygen can quickly start the process of oxidation on an unprotected magnet. This means that virtually all commercial neodymium magnets must be manufactured with a protective barrier to ensure their longevity and performance.
The Inherent Susceptibility of Neodymium
The magnetic field produced by neodymium magnets is due to their composition, an alloy of neodymium, iron, and boron, abbreviated as NdFeB. Iron (Fe) makes up a significant portion of the alloy and is the primary element responsible for the magnet’s susceptibility to rust. Rust is the oxidation of iron, where the metal reacts with oxygen and water to form iron oxide.
The rare-earth metal neodymium (Nd) itself is also highly reactive, contributing to the alloy’s instability when exposed to the environment. Neodymium readily gives up electrons in the presence of moisture and elevated temperatures. This chemical reactivity makes the magnet’s surface prone to degradation, even in moderately humid conditions.
During the manufacturing process, the components are sintered, resulting in a microstructure of grains. Corrosion tends to penetrate and propagate along the boundaries between these grains, compromising the magnet’s structure from the inside out. This internal degradation can lead to a complete breakdown of the material if left unchecked. The porous nature of the sintered material accelerates the oxidation process by allowing moisture to penetrate the core.
How Protective Coatings Work and Why They Fail
To counteract corrosion, manufacturers apply a protective layer that acts as a physical barrier against moisture and oxygen. The industry standard coating is often a triple-layer plating of Nickel-Copper-Nickel (Ni-Cu-Ni), which provides good corrosion resistance for many indoor applications. Other common coatings include zinc, which is economical and forms a protective oxide layer when it corrodes, and epoxy, which provides a robust, waterproof seal.
The function of these coatings is straightforward: they encapsulate the NdFeB core, preventing the contact necessary for oxidation to occur. The copper layer in Ni-Cu-Ni plating helps level the rough surface of the sintered magnet, while the nickel layers offer a hard, protective exterior. More advanced options like Parylene offer an ultrathin chemical barrier often used in specialized industries.
However, these coatings are thin, typically measuring only 15 to 35 microns, and their protection is dependent on their integrity. A scratch, chip, or physical impact can breach the plating, exposing the core to the environment. Even microscopic flaws, such as pinholes from the plating process or moisture trapped beneath the coating during manufacturing, can allow corrosion to start. Once the barrier is compromised, moisture accesses the reactive material, and oxidation begins at the point of failure.
Effects of Corrosion on Magnetic Performance
Once corrosion penetrates the protective coating, the oxidation process begins to destroy the magnet’s structure. The resulting iron oxide is an expansive substance that occupies a greater volume than the original material. This expansion creates internal pressure, causing the magnet material to flake and crumble away into a powder.
The structural degradation is not cosmetic; it directly impacts the magnet’s performance by disrupting the magnetic domains within the alloy. As the material degrades, there is a loss of magnetic field strength, known as demagnetization. Chemical reactions caused by the corrosion also alter the internal composition, leading to a decline in the magnet’s overall effectiveness. This process shortens the operational life of the component, leading to premature failure in the device or system where the magnet is used.