The answer to whether a magnet can rust is a qualified yes, but it depends entirely on the magnet’s composition. Rust is a specific form of corrosion, known chemically as iron oxide, which occurs when iron reacts with oxygen and water. Since not all magnets contain significant amounts of iron, their susceptibility to this degradation varies widely. The material used determines its vulnerability, ranging from highly susceptible to virtually immune to true rust.
Understanding Rust and Magnetic Materials
True rust is the visible, reddish-brown result of an electrochemical reaction called oxidation, which specifically requires metallic iron, moisture, and oxygen. Iron atoms lose electrons to oxygen atoms, forming hydrated iron(III) oxide, the substance recognized as rust. Therefore, a magnet’s alloy must contain a substantial percentage of iron to be vulnerable to this type of decay.
Even magnets without iron can still degrade through general corrosion, which is the deterioration of a material due to a reaction with its environment. For example, some magnets can be chemically attacked by acids or bases, but this damage is chemically distinct from the iron-based breakdown of rust. Magnetic materials are often alloys, and the stability of these elements dictates their resistance to environmental degradation. The presence or absence of iron is the most important factor in determining if a magnet will develop true rust.
Which Magnet Types Are Vulnerable to Degradation
The susceptibility to rust or corrosion is directly linked to the magnet’s base materials, dividing them into categories of vulnerability.
Highly Vulnerable (True Rust)
Neodymium magnets (NdFeB), composed of Neodymium, Iron, and Boron, are the most powerful commercial magnets and are highly vulnerable. Iron makes up a significant portion of their alloy (often around 64–68% by weight), and this component rapidly oxidizes when exposed to moisture. This oxidation starts quickly, causing the magnet to crumble and flake, which leads to a loss of magnetic strength. The manufacturing process can also leave the magnet with a porous structure, allowing moisture to penetrate the surface and accelerate corrosion.
Low Vulnerability (Corrosion/Degradation)
Samarium Cobalt (SmCo) magnets offer high resistance to corrosion because they are made primarily of samarium and cobalt, with cobalt replacing most of the iron content. Their inherent stability means they usually do not require protective coatings for basic corrosion resistance, even in moist environments.
Alnico magnets, made from Aluminum, Nickel, and Cobalt, also exhibit good corrosion resistance, despite containing iron as a base element. Although Alnico can show surface corrosion when exposed to water for extended periods, it remains stable against common solvents and is often used without a protective finish.
Rust Resistant
Ferrite, or ceramic, magnets are resistant to rust and general corrosion, making them ideal for outdoor and marine applications. These magnets are already composed of iron oxide (specifically strontium or barium ferrite), which is a stable, pre-rusted compound. Since the iron is already in a chemically stable, oxidized state, it cannot oxidize further to form new rust. A coating may occasionally be applied for aesthetic reasons or to prevent the shedding of fine magnetic powder in clean environments.
Protective Coatings and Handling
For magnets with high iron content, like Neodymium, protective coatings create a barrier against moisture and oxygen. The most common defense is a triple-layer plating of nickel-copper-nickel (Ni-Cu-Ni), which provides a durable metallic shell. Zinc plating is a cost-effective alternative that protects the magnet by sacrificing itself, oxidizing to form a white layer that shields the underlying material.
For maximum protection in harsh or wet environments, such as marine applications, a plastic or epoxy coating completely encapsulates the magnet. This polymer layer creates a robust, waterproof seal resistant to chemical attack. Keeping all vulnerable types in a dry, low-humidity environment is the best practice to prevent the initial onset of corrosion.