Magnet Coatings Compared: Ni-Cu-Ni, Zinc, Epoxy and Parylene

Sintered NdFeB is one of the strongest permanent magnet materials made and one of the least corrosion-resistant. Left bare, the neodymium-rich phase at the grain boundaries oxidizes in ordinary humidity, the surface powders, and the magnet eventually crumbles from the outside in. Every production NdFeB magnet therefore carries a coating, and the choice of coating affects corrosion life, dimensional tolerance, adhesive bonding, temperature capability, biocompatibility and cost.

This guide compares the four coatings that cover the overwhelming majority of applications — nickel-copper-nickel, zinc, epoxy and parylene — plus the specialty options, and explains how to choose and specify one.

Why NdFeB Needs a Coating at All

A sintered NdFeB magnet is roughly 85–90% Nd₂Fe₁₄B grains bound together by a neodymium-rich intergranular phase. That boundary phase is what makes the material sinter densely and gives it coercivity, but it is highly reactive. Exposed to moisture it oxidizes and expands, which loosens the surrounding grains. The result is the characteristic “pitting and powdering” of a failed magnet: a chalky surface that sheds grains, loses flux, and contaminates whatever it’s mounted in.

Rates vary with grade and environment, but an uncoated magnet in 85% relative humidity can show visible surface corrosion in days to weeks. The coating’s job is to keep water and oxygen off the boundary phase for the life of the product. Hcj and temperature class don’t change this — a high-coercivity EH grade is no more corrosion resistant than a plain N grade, and in some formulations slightly less.

Nickel-Copper-Nickel (Ni-Cu-Ni)

The industry default. Three electroplated layers: a nickel strike for adhesion, a copper layer to fill porosity and provide ductility, and a bright nickel top layer for hardness and appearance. Total thickness is typically 10–20 µm (0.4–0.8 mil), though 15–25 µm is common on larger parts.

Strengths

• Hard, bright, scratch-resistant surface that survives handling, assembly and repeated contact with steel.
• Good corrosion resistance in dry and moderately humid indoor environments; typically 24–72 hours in ASTM B117 neutral salt spray.
• Thin and dimensionally consistent, which keeps tolerance stack-ups manageable.
• Available on every size and shape, from every supplier, at the lowest cost of any plated finish.
• Thermally stable well beyond the magnet’s own temperature limit.

Weaknesses

• Poor adhesive bonding. Bright nickel is smooth and chemically passive; epoxies and acrylics bond to it weakly unless the surface is abraded or primed. This is the single most common complaint about Ni-Cu-Ni in assembled products.
• Porosity at edges and corners. Electroplating thins at sharp edges, which is where corrosion starts. Chamfers help; a radius helps more.
• Not suitable for sustained water immersion or salt exposure without an additional barrier layer. Pinholes in the plating become galvanic corrosion sites with the NdFeB substrate.
• Nickel is ferromagnetic. The outer layer shunts a small amount of flux. Irrelevant on a thick magnet; measurable on a thin one, where it can reduce surface field by a percent or two.
• Nickel sensitization. Skin-contact consumer products and some medical devices avoid exposed nickel.
• Electrically conductive, which matters for eddy-current losses in high-frequency motors and for any design that needs the magnet isolated.

Where it fits

Sensors, encoders, motors, couplings, holding assemblies, consumer electronics — anything indoors, dry to moderately humid, handled or press-fit rather than glued, and not exposed to salt or immersion. If the application has no specific reason to choose something else, Ni-Cu-Ni is the answer.

Zinc (Zn)

Electroplated zinc, 5–15 µm, with a bluish-white to dull gray appearance. Almost always finished with a chromate or trivalent passivation layer that noticeably improves corrosion performance.

Strengths

• Excellent adhesive bonding. The slightly matte, chemically active zinc surface takes epoxies well. This is the main reason to specify it.
• Sacrificial protection. Zinc is anodic to the NdFeB substrate, so a scratch in the coating corrodes the zinc rather than the magnet, at least initially. Nickel is cathodic and does the opposite — a scratch in nickel accelerates attack on the exposed magnet.
• Lowest cost of the common coatings.
• Non-magnetic, so no flux shunting.

Weaknesses

• Soft and easily scuffed; shows handling marks and white corrosion product quickly.
• Lower salt spray performance than nickel: roughly 12–24 hours bare, 24–48 hours with a good chromate.
• Thin, so it provides less barrier protection in prolonged humidity.
• Appearance is not a selling point.

Where it fits

Magnets that will be bonded into an assembly and then protected by the assembly itself — potted sensors, glued rotor segments, magnets encapsulated in plastic housings. Also a reasonable choice for very low-cost, short-exposure applications.

Epoxy

An organic coating, typically black (gray and other colors are available), applied by electrophoretic deposition or spray and then cured. Usually 10–25 µm, and most often applied over a Ni-Cu or Zn underplate rather than directly on the magnet. “Epoxy coated” on a datasheet frequently means Ni-Cu-epoxy or Zn-epoxy; confirm the stack.

Strengths

• Best humidity and salt resistance of the common coatings — 48–96 hours salt spray for epoxy alone, more than 100 hours for a Ni-Cu-epoxy stack. A continuous polymer film has no pinholes of the kind a plated metal has.
• Bonds well with adhesives. Epoxy-to-epoxy bonding is strong and well-characterized.
• Electrically insulating. Useful in motors and in any assembly where the magnet shouldn’t conduct.
• Non-magnetic, no flux shunting.
• Black finish hides the magnet and reads as “engineered” in visible applications.

Weaknesses

• Soft. Epoxy scratches, chips at edges and corners under impact, and wears through where the magnet slides against steel. A chipped epoxy coating on a bare magnet exposes substrate; on a Ni-Cu-epoxy stack the underplate buys time.
• Temperature limit. Standard epoxies soften and degrade somewhere around 150–180 °C — often below the magnet’s own limit on SH and higher grades. High-temperature epoxies exist but should be specified explicitly.
• Thicker and less dimensionally consistent than plating, especially at edges where the coating builds up.
• Can outgas during cure or at elevated temperature, which matters for vacuum and some medical environments.

Where it fits

Outdoor equipment, marine and coastal environments, pumps, HVAC, automotive underbody and anything that sees condensation, salt or repeated wet-dry cycling. Also the standard choice when the magnet will be bonded and the assembly isn’t sealed.

Parylene

A vapor-deposited polymer (poly-para-xylylene; parylene C is the common grade for magnets). The magnet is placed in a vacuum chamber and the coating forms molecule by molecule on every exposed surface simultaneously, including inside bores and under edges. Typical thickness is 5–25 µm, and it’s often applied over Ni-Cu-Ni rather than on the bare magnet.

Strengths

• Truly conformal and pinhole-free at thicknesses where plating isn’t. Uniform coverage on sharp edges, in holes and inside complex geometries.
• Biocompatible. Parylene C meets USP Class VI and ISO 10993 requirements and has a long history inside implantable and body-contact devices. It’s the default coating for medical magnets.
• Chemically inert and an excellent moisture barrier — better water vapor transmission resistance per micron than epoxy.
• Electrically insulating, non-magnetic, optically clear, and compatible with sterilization (EtO and gamma; autoclave with limits).
• Very thin options (5 µm) for tight-tolerance parts.

Weaknesses

• Soft and low abrasion resistance. Parylene scratches easily and must not be relied on where the magnet slides or impacts. Handling after coating needs care.
• Cost. Batch vacuum deposition is slow and the raw dimer is expensive; parylene is several times the cost of Ni-Cu-Ni.
• Temperature. Parylene C is rated to roughly 125 °C continuous in air; parylene N lower; the HT variant handles 350 °C but is less common and more costly.
• Adhesive bonding is only fair; the surface is low-energy and often needs plasma treatment for a strong bond.
• Clear coating means a bare NdFeB substrate looks bare — which can be mistaken for an uncoated magnet.

Where it fits

Medical devices (implants, catheters, surgical instruments, wearables with sustained skin contact), laboratory and analytical equipment, food-contact and clean environments, and any part where conformal coverage of a complex shape matters more than scratch resistance.

Side-by-Side

	Ni-Cu-Ni	Zinc	Epoxy	Parylene
Typical thickness	10–20 µm	5–15 µm	10–25 µm	5–25 µm
Salt spray (ASTM B117)*	24–72 h	12–48 h	48–96 h (100+ over Ni-Cu)	100+ h over Ni-Cu-Ni
Hardness / scratch resistance	High	Low	Low–medium	Low
Adhesive bonding	Poor without prep	Excellent	Good	Fair, better with plasma
Max continuous temp	> magnet limit	> magnet limit	~150–180 °C	~125 °C (C); 350 °C (HT)
Electrical	Conductive	Conductive	Insulating	Insulating
Magnetic effect	Slight shunting (Ni)	None	None	None
Biocompatibility	Nickel concern	No	No	USP VI / ISO 10993
Appearance	Bright silver	Blue-white / gray	Black (others available)	Clear
Relative cost	1×	0.8×	1.2–1.5×	3–6×

*Salt spray figures are typical ranges from supplier data and vary widely with plating quality, thickness, edge geometry and test lab. Use them for ranking, not for design; specify your own acceptance test.

Specialty and Stacked Coatings

• Ni-Cu-Ni + Gold flash. A thin (0.1–0.5 µm) gold layer over nickel for medical and electronic-contact applications where exposed nickel is unacceptable. Not a corrosion coating in itself — it’s cosmetic and nickel-blocking.
• Ni-Cu-Ni + Epoxy. Hard underplate for handling and edges, polymer top coat for moisture. The best general-purpose stack for harsh environments.
• Ni-Cu-Ni + Parylene. Medical-grade barrier with a hard base. The standard for implantable and body-contact magnets.
• Ni-Cu-Ni + Tin (Sn). For solderable magnets.
• PTFE / Teflon. Low-friction, chemically inert, and non-stick; used in some fluid-handling and release applications. Soft.
• Everlube and other dry-film lubricants. Over nickel, for magnets that slide against steel.
• Chrome, titanium nitride. Hard, decorative or wear-specific; rare and expensive.
• Phosphate passivation. A thin conversion layer that provides a few weeks of storage protection, used on magnets that will be coated or potted by the customer. Not a service coating.
• Uncoated. Only for magnets going straight into a hermetic encapsulation or an overmold within days of receipt, and even then at some risk. Order with passivation and plan the handling.

How to Choose

1. Environment. Dry indoor → Ni-Cu-Ni. Humid, condensing, outdoor, salt → epoxy, ideally over Ni-Cu. Immersion → epoxy or parylene over nickel, plus a sealed assembly.
2. Body contact or implant. Parylene over Ni-Cu-Ni, or gold flash where only nickel exposure is the concern. Confirm the specific regulatory requirement (ISO 10993 part and contact category).
3. Assembly method. Press-fit or mechanically retained → Ni-Cu-Ni. Adhesive bonded → zinc or epoxy, or Ni-Cu-Ni with a specified surface prep. Potted or overmolded → zinc or passivated.
4. Temperature. Above ~150 °C, organic coatings are suspect. Check the coating’s rating against the magnet’s class; a UH magnet with a standard epoxy coat has an epoxy-limited assembly. See NdFeB Temperature Classes.
5. Handling and wear. If the magnet is handled in production, slides against steel, or is assembled by machine, specify a hard coating (nickel) or a hard underplate.
6. Tolerance. Coating thickness consumes dimensional tolerance. On a part with ±0.05 mm tolerances, 20 µm of epoxy per side is a meaningful fraction. Thinner plating or parylene may be necessary.
7. Electrical. Need insulation → epoxy or parylene. Need conductivity → nickel.

What to Specify on the Drawing

• The full stack, not just the top layer: “Ni-Cu-Ni, 15–20 µm total” or “Ni-Cu-Ni + Parylene C, 10 µm.” “Epoxy coated” alone leaves the underplate to the supplier.
• Thickness range and where it’s measured. Specify whether dimensions are before or after coating — “dimensions apply after coating” is the usual intent and should be written down.
• Edge treatment. A minimum chamfer or radius improves coating coverage on every finish and reduces chipping on organic coatings.
• Acceptance test. Salt spray hours per ASTM B117, or a pressure cooker test (PCT: 121 °C, 100% RH, 2 atm, typically 96 h) for demanding applications, or 85 °C / 85% RH for a stated duration. Specify the pass criterion — “no visible corrosion, no blistering, no powdering” — and the sample size. PCT is the test that separates well-adhered nickel plating from poorly adhered; it is far more severe than salt spray and is worth requiring on anything safety- or reliability-critical.
• Adhesion test if relevant — tape test per ASTM D3359 for organic coatings, or a thermal shock cycle count.
• Appearance and cosmetic requirements if the magnet is visible, including allowable contact marks from plating racks.
• Nickel-free, RoHS, REACH or other material restrictions where they apply.

Our guide to reading a magnet datasheet covers the rest of the specification; the Compliance Document Center has our RoHS, REACH and related statements.

Frequently Asked Questions

What is the best coating for neodymium magnets?

There isn’t one. Ni-Cu-Ni is the best default for dry, handled, press-fit parts. Epoxy over nickel is best for humid and outdoor use. Parylene is best for medical and conformal coverage. Zinc is best for adhesive bonding at low cost. The best coating is the one that matches the environment, the assembly method and the temperature.

Does the coating weaken the magnet?

Not in any meaningful way, with one small exception: the nickel outer layer of Ni-Cu-Ni is ferromagnetic and shunts a small fraction of flux at the surface. On thin magnets this can be a 1–2% reduction in surface field. Zinc, epoxy and parylene have no magnetic effect. All coatings add thickness that increases the effective air gap, which matters more than any material effect.

Can I glue a nickel-plated magnet?

Yes, but expect weaker bonds than with zinc or epoxy unless the surface is abraded, plasma-treated or primed. Many assembly failures trace to an unprepared nickel surface with an off-the-shelf epoxy. If bonding is the retention method, specify zinc, epoxy, or nickel with a defined surface prep.

Are coated magnets waterproof?

Coated magnets are water-resistant for a duration that depends on the coating; none are waterproof indefinitely. For sustained immersion, use epoxy or parylene over nickel and seal the assembly as well. Treat the coating as the second line of defense, not the first.

How long does a magnet coating last?

Indoors in a dry assembly, Ni-Cu-Ni routinely lasts the life of the product. In condensing or salt environments, an unsealed nickel-plated magnet can fail in months; epoxy or parylene stacks extend that substantially. Accelerated tests (salt spray, PCT, 85/85) estimate relative life but don’t translate directly into years.

Is the coating applied before or after magnetization?

Before. Plating lines and coating chambers can’t process magnetized parts — they’d stick to fixtures and each other. Magnets are coated unmagnetized, then magnetized as a final step, which is also why last-minute coating changes delay delivery.

Related Coating & Specification Resources

• How to Read a Magnet Datasheet
• NdFeB Temperature Classes Explained
• Compliance Document Center (RoHS, REACH, Prop 65)
• Magnet Terminology Glossary
• Magnets 101
• Submit a Bulk Magnet RFQ

Radial Magnets stocks over 10 million NdFeB magnets in the U.S. in Ni-Cu-Ni, zinc, epoxy and parylene finishes, and supplies custom coatings and stacks for medical, automotive and defense programs. Request a quote or contact us with your environment and assembly requirements.