Key Takeaways
- ◆Epoxy coating thickness is 15-25 microns per side; zinc is 5-10 microns. The thinner zinc profile matters when magnet-to-slot clearance is tight, but epoxy's thickness is still well under the adhesive bond line in most motor assemblies.
- ◆Salt spray life (ASTM B117): epoxy-coated NdFeB typically survives 48-72 hours; zinc survives 24-48 hours. Neither matches NiCuNi plating (72-200+ hours), but epoxy is the better choice for environments with humidity cycling.
- ◆Epoxy provides full electrical isolation between the magnet and surrounding steel, which helps suppress eddy-current paths through the magnet-rotor interface. Zinc is conductive and does not provide this benefit.
- ◆Zinc coating max operating temperature is approximately 120 C before the zinc layer begins to oxidize and lose adhesion. Epoxy coatings (depending on resin) are rated to 150-200 C, making epoxy the better match for SH and UH grade applications.
- ◆Zinc is approximately 20-30% cheaper than epoxy per coated piece. At volumes above 50,000 pieces per year, the difference is $0.02-0.08 per magnet depending on size.
Overview
Epoxy and zinc are the two most common alternatives to NiCuNi plating for NdFeB magnet corrosion protection. Both are specified when NiCuNi is either too expensive, too thick, or inappropriate for the assembly process (e.g., adhesive bonding where epoxy provides a better glue surface). The choice between them depends on the operating environment, the thermal envelope, whether electrical isolation matters, and cost sensitivity. This comparison uses real production data to help procurement engineers make the right call.
Side-by-Side Comparison
| Criterion | Epoxy Coating | Zinc Coating |
|---|---|---|
| Coating thickness per side | 15-25 microns | 5-10 microns |
| Salt spray life (ASTM B117) | ✓48-72 hours | 24-48 hours |
| Humidity resistance (85 C / 85% RH) | ✓Good (500+ hours typical) | Moderate (200-300 hours typical) |
| Max operating temperature | ✓150-200 C (resin dependent) | ~120 C |
| Electrical isolation | ✓Yes (insulating layer) | No (zinc is conductive) |
| Adhesive bond strength | ✓Excellent (epoxy-to-epoxy bond) | Good (requires primer or roughening) |
| Cost per coated piece | Baseline | ✓20-30% less than epoxy |
| Surface appearance | Matte black or grey | Bright metallic silver |
Green tick indicates the better option for the criterion. Winner assignment reflects typical engineering practice; your application may weight criteria differently.
When Epoxy Coating Is the Right Choice
- •Motor operates above 120 C or uses SH/UH/EH grade magnets where the zinc thermal limit would be the weak link in the coating system
- •Application requires electrical isolation between the magnet and rotor steel to suppress parasitic eddy-current paths through the contact interface
- •Magnets will be adhesive-bonded into rotor slots, where the epoxy surface provides a natural glue surface without additional primer
- •Operating environment involves humidity cycling, condensation, or intermittent water exposure (outdoor motors, marine, HVAC)
When Zinc Coating Is the Right Choice
- •Magnet-to-slot clearance is extremely tight and every micron of coating thickness matters (zinc at 5-10 microns vs epoxy at 15-25 microns)
- •Application operates below 100 C in a dry, indoor environment where corrosion risk is minimal (consumer electronics, indoor automation)
- •Budget is the primary constraint and the 20-30% coating cost savings matters at high volume
- •Magnets are mechanically retained (press-fit, clips, retaining sleeve) rather than adhesive-bonded, removing the adhesion advantage of epoxy
Decision Framework
Start with temperature. If your magnet operates above 120 C, zinc is disqualified and epoxy (or NiCuNi) is the minimum. Next, check electrical isolation: if your motor design benefits from breaking eddy-current paths at the magnet-rotor interface, epoxy wins by default. Third, check the assembly method: adhesive bonding favors epoxy because the surface bonds directly without primer. If none of these factors are decisive (low temperature, mechanical retention, dry environment), zinc is the cheaper option and works fine. One common middle-ground approach is to use epoxy on the two pole faces (the magnetically active surfaces that sit against steel) and leave the non-critical edges uncoated or zinc-coated to save cost. Ask your supplier whether split-coating is available at your volume.
Related NdFeB Grades
N42SH
150°CWorkhorse SH-grade NdFeB for 150°C traction motors, robotics actuators, and high-duty servo drives.
N42H
120°CWorkhorse high-temperature NdFeB grade for automotive motors, pumps, and industrial drives up to 120°C.
N48H
120°CPremium high-temperature NdFeB grade for the most power-dense motors running up to 120°C.
N45SH
150°CHigh-flux SH-grade NdFeB for compact, high-torque motors operating continuously up to 150°C.
Related Applications
EV Motors
High-performance NdFeB magnets for electric vehicle traction motors, auxiliary drives, and e-axle systems — with the temperature stability and flux density required for continuous high-torque service.
Robotics
Radial multi-pole rings, joint-motor magnets, and high-torque servo-motor assemblies for humanoid robots, collaborative robots, and industrial robotic systems.
Industrial Automation
NdFeB magnets for stepper motors, servo drives, linear actuators, magnetic couplings, and factory automation equipment across European and North American manufacturing.
Frequently Asked Questions
Is epoxy coating the same as powder coating?
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Electrophoretic epoxy (e-coat) and electrostatic powder epoxy are both used on NdFeB magnets, but they differ in application method and thickness. E-coat produces a uniform 15-20 micron layer through an electrodeposition bath and is the standard process for motor magnets. Powder epoxy is sprayed and cured, producing 20-40 micron layers with slightly less uniformity. When a magnet datasheet says 'epoxy coating,' it typically means e-coat unless otherwise specified. Always confirm the process with your supplier.
Can I use zinc-coated magnets in an outdoor application?
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Zinc-coated magnets can work outdoors in dry climates or sealed enclosures, but they are not recommended for direct outdoor exposure in humid or coastal environments. The 24-48 hour salt spray life means the zinc layer will begin to fail within months in a marine or tropical environment. For outdoor applications, epoxy or NiCuNi provides significantly better protection. If zinc is chosen for cost reasons, ensure the magnet is sealed inside a housing with IP65 or better ingress protection.
Does coating affect magnetic performance?
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Neither epoxy nor zinc coatings are magnetic, so they do not alter the magnet's remanence, coercivity, or energy product. The only impact is dimensional: coating adds thickness to every surface, which slightly increases the effective air gap if the magnet sits in a slot or against a pole piece. At 15-25 microns per side (epoxy) or 5-10 microns per side (zinc), the flux loss from the increased air gap is typically less than 0.5% and is negligible in most motor designs.
Which coating is better for medical device magnets?
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Medical devices that require biocompatibility testing (ISO 10993) typically use Parylene conformal coating rather than epoxy or zinc, because Parylene is chemically inert and pinhole-free. For non-implant medical devices (MRI components, external diagnostic equipment), epoxy is generally preferred over zinc for its superior corrosion resistance and electrical isolation. Always check whether your regulatory pathway requires specific coating biocompatibility data.
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