NdFeB magnets rust. The neodymium-rich grain boundary phase that makes the material strong also makes it electrochemically vulnerable — in humid air it oxidizes, and once corrosion starts at the surface it works inward along the grain boundaries until the magnet powders. Coating is not a cosmetic decision; it is the difference between a magnet that lasts the product's life and one that fails inside a sealed enclosure nobody can open.
The Main Options
| Coating | Typical thickness | Protection profile | Best fit |
|---|---|---|---|
| Ni-Cu-Ni (nickel) | 10–20 µm | Good general barrier; bright finish; the industry default | Indoor, dry, consumer and industrial general use |
| Zinc | 7–15 µm | Sacrificial protection; matte finish; softer surface | Cost-sensitive, mild environments |
| Epoxy (black) | 15–30 µm | Excellent humidity and salt resistance; electrically insulating | Humid, marine, outdoor; motors |
| Parylene C | 5–25 µm | Pinhole-free conformal film; chemically inert; vacuum-safe, no outgassing | Medical, vacuum, electronics, thin-gap assemblies |
| Phosphating / passivation | 1–3 µm | Temporary protection; excellent adhesive bonding base | Magnets that will be glued or overmolded |
| Specialty (Au, Ag, Sn, Ni+epoxy) | varies | Solderable, biocompatible-facing, or double-barrier combinations | Soldered connections, medical-adjacent, severe duty |
What Datasheets Don't Tell You
Salt-spray hours are not a spec you can copy-paste
Neutral salt spray (NSS) results for the same coating type vary widely with thickness, surface preparation and edge geometry — published figures range from 24 to 96+ hours for Ni-Cu-Ni, more for epoxy. Use them for ranking, not for guarantees. If corrosion life is contractual, define the test standard (e.g. ASTM B117), duration and acceptance criteria with your supplier.
Edges and holes are the weak points
Electroplated coatings thin at sharp edges and inside small holes — exactly where corrosion starts. If your design has tight inside corners or small through-holes and a humid environment, say so. The answer may be thicker epoxy, Parylene, or a geometry tweak rather than "more nickel."
Coating thickness eats your tolerance budget
15–30 µm of epoxy on every face changes press-fit dimensions and magnetic air gaps. Dimensions on drawings should state whether they apply before or after coating — a detail that prevents an entire category of assembly disputes.
Bonding changes the answer
Adhesive bonds to bare nickel poorly compared to phosphated or epoxy surfaces. If the magnet will be glued into a rotor or housing, the coating decision belongs to the assembly engineer, not just the corrosion engineer. Ni-Cu-Ni plus the wrong adhesive is a retention failure waiting for thermal cycling.
Selection by Real Exposure Profile
- Indoor, dry, room temperature → standard Ni-Cu-Ni. Don't overbuy.
- Humidity, condensation, marine air → epoxy, or Ni-Cu-Ni with epoxy overcoat for severe duty.
- Medical, vacuum, or thin precision gaps → Parylene C — thin, conformal, pinhole-free, no outgassing.
- Soldered or conductive interfaces → tin or silver topcoats.
- Adhesive-bonded into an assembly → phosphated or epoxy surface, chosen with the adhesive system.
Our default question: "Where will this magnet live?" Answer with the environment, not the coating name — we'll recommend the coating, state it on the drawing, and verify it with coating reports on shipment.