Sintered neodymium magnets are the strongest permanent magnets around — and some of the most rust-prone. Leave one unprotected in a humid workshop and it starts to pit within days. In a laboratory test, a bare 1 cm³ magnet kept in air at 150 °C was completely corroded in about 51 days. Moisture is the enemy — knowing why helps you protect them.
Why NdFeB rusts so fast
Three things make it vulnerable. First, the manufacturing process leaves the material full of microscopic pores that trap moisture. Second, the neodymium-rich parts of the material react eagerly with oxygen and water. Third, salt and chlorides speed everything up: chloride ions slip into the pores and attack the metal underneath, turning a protected surface into an actively corroding one.
Once started, it accelerates: pits grow, the surface flakes and powders, and the magnet shrinks — losing the material that carries its field.
Does corrosion weaken the magnet?
Yes — and permanently. Corrosion destroys the magnetic phase, so the magnet's strength drops as the rust spreads. It is not a temporary demagnetization you can fix; the material is gone for good. That is why coating is essential: nickel, zinc or epoxy plating seals the surface and keeps moisture out.
How to keep magnets alive
- Always coat them — nickel for visible parts, epoxy for harsh environments (see coatings compared).
- Store dry. Keep unused magnets in sealed bags with desiccant, away from humidity.
- Beware of condensation. Moving magnets from a cold warehouse into a warm room makes them sweat — dry them before sealing.
- Check the salt-spray rating of the coating if the magnet will face outdoor or marine air.
The bottom line
NdFeB corrosion is fast, permanent and preventable. A few microns of the right coating turn a rust-prone powder block into the dependable magnet in your phone, motor or speaker.
Read on: magnet coatings compared → · demagnetization: a different kind of failure →