Magnet Coatings: Nickel or Epoxy?
How to choose a coating for NdFeB magnets: typical options compared, the temperature and size effects, and how to specify it in an order.
1. Why NdFeB is coated
Sintered NdFeB corrodes on its own. One supplier says uncoated magnets in humid air begin to degrade in days to weeks [1], and another does not recommend uncoated magnets at all [2]. The coating also helps protect the brittle material from chipping [2]. So the real question is not whether to coat, but which coating fits your environment, temperature and assembly.
2. The main options side by side
The figures below are typical ranges quoted by suppliers. They differ between sources and between factories, so use them to compare, not as a specification.
| Coating | Typical thickness | Corrosion resistance | Main limit |
|---|---|---|---|
| Ni-Cu-Ni (nickel) | about 10–25 µm [1][2][3] | Good indoors; salt spray 24–96 h in one guide [1]; poor in salty environments [3] | Chips at sharp edges; bonds poorly [1][3] |
| Ni-Cu-Ni + epoxy | about 15–30 µm [1][3] | Good; salt spray 96–240 h in one guide [1] | Softer; the epoxy limits temperature (120–180 °C depending on the source) [1][3] |
| Epoxy only | about 10–25 µm [1][3] | Good [3]; 72–120 h in one guide [1] | Softer than plating; temperature limit around 120 °C [1][3] |
| Zinc | about 4–15 µm [1][2][3] | Weaker than nickel [2] | Short life; suits dry or captive use [1] |
3. Nickel (Ni-Cu-Ni)
This is the standard coating and one supplier says it covers most NdFeB applications [1]. It is hard, conductive and predictable in size. It suits enclosed equipment, motors, sensors and consumer assemblies [1]. Its weak points are chipping at sharp edges and poor performance under sustained wet or salty exposure [1][3]. Bright nickel also bonds poorly unless it is roughened or primed [1][3].
4. Epoxy
Epoxy, alone or over nickel, is the usual choice for outdoor, marine, washdown and condensation-cycling use [1]. It is non-conductive, which removes one source of eddy-current loss, and it bonds well [1][3]. The cost is that it is softer and can crumble under impact; damage, even if invisible, can let moisture in and harm the magnet over time [2][3]. It also limits the working temperature: one source gives the example of an N42SH rated to 150 °C that becomes a 120 °C part with epoxy [1]. Ask for the temperature limit of the actual coating, not just of the grade.
5. Other coatings in brief
Parylene gives a thin, pinhole-free layer for medical or complex shapes; PTFE gives non-stick and high-temperature use but poor bonding; gold over nickel is for body contact or stable electrical contact and is costly [1][2][3]. Phosphate is only a primer and is not standalone protection [1]. These are special cases; for most industrial magnets the choice is nickel or epoxy.
6. The coating changes the size
The coating adds to every surface, so a diameter grows by twice the thickness. One guide gives about 0.020–0.040 mm on diameter for nickel and 0.030–0.060 mm for epoxy over nickel, and notes that this can consume most of a ±0.05 mm tolerance band [1]. State on the drawing whether the dimensions apply before or after coating; the same guide recommends “after coating” [1]. Plating also thins at sharp edges, so the edges have the least protection [1].
7. What a salt spray test tells you
A neutral salt spray test (ASTM B117 or ISO 9227) exposes the parts to continuous salt fog. It is useful for comparing coatings and for catching process defects, but it is not a prediction of service life, and results vary with geometry and edge condition [1]. If you want a test, write the duration and the acceptance criterion in the order, for example the number of hours and “no red rust or substrate corrosion”. A longer test costs more and takes longer, so choose the duration to match the coating and the use.
8. What to write in your order
- Coating type and, if it matters, minimum thickness.
- Whether dimensions apply before or after coating.
- Operating temperature including the coating's limit.
- Environment: dry indoor, humid, outdoor, washdown, salt.
- Whether the magnets will be glued, and to what.
- Test and acceptance: salt spray duration and criterion, if required.
Frequently asked questions
Why are neodymium magnets coated?
Sintered NdFeB corrodes on its own. One supplier says uncoated magnets in humid air begin to degrade in days to weeks, and the coating also helps protect the brittle material from chipping. So the real question is which coating fits your environment, temperature and assembly.
Should I choose nickel or epoxy?
Nickel (Ni-Cu-Ni) is the standard coating and suits enclosed equipment, motors, sensors and consumer assemblies. Epoxy, alone or over nickel, is the usual choice for outdoor, marine, washdown and condensation use, but it is softer and limits the temperature: one source gives the example of an N42SH rated to 150 °C that becomes a 120 °C part with epoxy.
Do the dimensions apply before or after coating?
The coating adds to every surface, so a diameter grows by twice the thickness. One guide gives about 0.020–0.040 mm on diameter for nickel and 0.030–0.060 mm for epoxy over nickel, which can use most of a ±0.05 mm tolerance band. State on the drawing whether the dimensions apply before or after coating; that guide recommends “after coating”.
References
- [1] Radial Magnets, Inc. (n.d.). Magnet coatings guide. https://radialmagnet.com/magnet-coatings-guide/
- [2] supermagnete (n.d.). Which magnet coatings are there? https://www.supermagnete.it/eng/faq/Which-magnet-coatings-are-there
- [3] Goudsmit Magnetics (n.d.). Magnet coating selection guide [PDF]. https://www.goudsmitmagnetics.com/uploads/pdf/MagnetCoating_SelectionGuide_A3RGB.pdf
Important notice: This guide is general technical information based on the cited supplier publications. Thicknesses, test hours and temperature limits are typical figures that differ between sources and factories; they are not a specification, an offer or a commitment by Rami Technics. Confirm the coating for your application against the datasheet of the actual product and test a sample under real conditions.