What type of magnet is not easy to rust in water?

Magnets are sometimes exposed to water in certain applications, either briefly or continuously. If magnets do not have good corrosion resistance, they are prone to rusting and failure. Today, we will introduce several types of magnets that are resistant to rust.

Recommendation 1: Samarium cobalt magnets

Samarium-cobalt magnets are primarily composed of the rare earth elements samarium (Sm) and cobalt (Co), with only trace amounts of other metals (such as iron, copper, zirconium, etc.). Cobalt itself has excellent corrosion resistance, and the resulting alloy structure exhibits strong resistance to water, moisture, and even many chemical substances. They typically do not require surface coatings for use in aquatic or harsh environments. They are commonly used in high-performance motors, sensors, medical devices, aerospace, and marine applications.

The accompanying image shows a 6x2mm circular samarium cobalt magnets.

Recommendation 2: AlNiCo

Aluminum-nickel-cobalt alloys are primarily composed of aluminum (Al), nickel (Ni), cobalt (Co), and iron (Fe). Despite containing iron, their unique alloy structure confers excellent corrosion resistance. The surface of aluminum-nickel-cobalt alloy easily forms a dense, stable aluminum oxide (Al₂O₃) protective layer. This layer effectively isolates the internal iron from contact with water and oxygen, preventing further rusting, similar to the rust-proofing principle of stainless steel. The addition of nickel and cobalt further enhances the overall corrosion resistance.

Therefore, aluminum-nickel-cobalt alloys also exhibit excellent corrosion resistance and typically do not require surface coatings. They are commonly used in old-style instruments, sensors, electroacoustic devices, motors, and applications requiring long-term exposure to humid or aquatic environments (e.g., shipboard instruments, water meters).

Note: Its magnetic properties are lower than those of samarium-cobalt and neodymium-iron-boron.

Recommendation 3: Ceramic magnets / ferrite magnets:

Ferrite magnets are mainly composed of iron oxide (Fe₂O₃) and other metal oxides (such as strontium oxide SrO or barium oxide BaO) sintered together. They are inherently very stable oxide ceramics. The iron elements inside are already in an oxidized state (Fe³⁺) and are difficult to further oxidize (rust) under normal temperature and pressure conditions. Additionally, ferrite is a poor electrical conductor, significantly reducing the likelihood of electrochemical corrosion.

Recommendation 4: Plastic coated NdFeB magnet

Neodymium iron boron magnets are highly susceptible to rust, but encapsulation with epoxy resin, plastic, PTFE (Teflon), etc. can effectively prevent water penetration and rust. However, once the coating is damaged, there is still a risk of rusting inside the magnet.

When selecting magnets for use in water, in addition to corrosion resistance, it is necessary to comprehensively consider multiple factors such as magnetic performance, temperature stability, and cost.

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