Why do neodymium magnets have a coating?
Short answer: neodymium magnets have a coating because the material (NdFeB) consists of about 65% iron and also contains neodymium, which oxidises quickly. Unprotected, a neodymium magnet will rust and crumble in humid air within days to weeks. The standard coating is nickel-copper-nickel (NiCuNi) of 10 – 20 µm. For damp places or outdoors, choose epoxy, plastic or rubber.
Coatings compared
| Coating | Layer thickness (approx.) | Salt spray test (indicative) | Suitable for |
|---|---|---|---|
| NiCuNi (nickel-copper-nickel) | 10 – 20 µm | 24 – 72 hours | dry indoor spaces; the standard |
| Zinc | 8 – 15 µm | 12 – 24 hours | dry, as a base for glue |
| Epoxy (usually black) | 15 – 30 µm | 72 – 200 hours | damp rooms, sheltered outdoors |
| NiCuNi + epoxy | 25 – 40 µm | 200+ hours | harsher conditions |
| Gold / rose / gunmetal (on NiCuNi) | thin top layer | as NiCuNi | visible, decorative applications |
| Plastic casing (ABS/PVC) | 0.5 – 1 mm | fully sealed | moisture, outdoors, education, hygiene |
| Rubber casing | 0.5 – 1.5 mm | fully sealed | paintwork, cars, grip, scratch-free |
The salt spray test (neutral, according to ISO 9227) is an accelerated corrosion test: the number of hours until the first rust. It is a comparative measure, not a lifespan in years.
How is a NiCuNi coating built up?
Three layers are applied by electroplating: a thin nickel layer for adhesion, a copper layer that seals the pores, and a hard nickel top layer for shine and wear resistance. Together they give good protection indoors. A scratch or chipped edge does open the coating, and that is where rust starts.
Does a coating affect the strength?
A thin metal coating (NiCuNi, gold) hardly at all. A thick plastic or rubber casing does: it acts as a gap between the magnet and the steel. A rubber or plastic-coated magnet therefore has less holding force than a bare magnet of the same size. Rubber does give more grip, so it is less likely to slide away.
Which coating should you choose?
- Indoors, dry: NiCuNi
- Damp, bathroom, cellar, outdoors under a canopy: epoxy
- Outdoors, wet, or for children and education: plastic-coated
- On paintwork, cars and delicate surfaces: rubber
An extensive selection guide is in which coating or finish should you choose?
Frequently asked questions
Is the nickel coating a problem with a nickel allergy?
With brief contact usually not, but with prolonged skin contact, for example in jewellery or clothing, it is better to choose a magnet with an epoxy or plastic coating, or a magnet that is fully sewn in. See sew-in magnets with PVC.
Can I repair a damaged coating?
You can cover small damage with a layer of clear lacquer, nail polish or epoxy glue. If the material is crumbling, it is better to replace the magnet.
Are magnets with a coloured coating weaker?
No. Gold, rose and gunmetal are thin top layers on NiCuNi; the magnet underneath is the same.
Read also
Need help choosing? Use the Magnet Advisor or contact us.
