Neodymium Magnet Coatings Guide

Sintered NdFeB is a composite of Nd₂Fe₁₄B grains held together by a neodymium-rich boundary phase — and that boundary phase is highly reactive. Exposed to humidity, it oxidizes preferentially, and because it forms the network between grains, corrosion doesn't just discolor the surface: it travels along grain boundaries, liberating whole grains as powder. An uncoated magnet in a humid environment visibly degrades in weeks — surface bloom first, then flaking, then dimensional swelling that can crack surrounding assemblies.

Corrosion also destroys magnetic performance twice over: oxidized material is non-magnetic (permanent flux loss), and the swelling opens air gaps in the magnetic circuit. This is why coating failure appears in the demagnetization guide as a root cause — and why the coating deserves the same specification rigor as the grade.

*Coating temperature limits — the magnet grade sets its own limit (see the grades chart); the working limit is the lower of the two. Corrosion ratings are relative; environment-specific performance is specified in salt-spray or humidity-test hours (section 09).

The industry default is a three-layer electroplated system: nickel, then copper, then nickel again, typically 15–25 µm total. Each layer earns its place:

• Inner nickel bonds to the NdFeB substrate and provides the first barrier.
• Copper middle layer is ductile and fine-grained — it seals the pores that inevitably occur in any single plated layer, interrupting the pinhole paths that let moisture reach the substrate. This is the layer that separates good plating from cheap plating.
• Outer nickel supplies hardness, abrasion resistance, and the bright cosmetic finish.

Properties worth knowing: nickel is slightly ferromagnetic (negligible effect at these thicknesses), hard enough to survive handling and repeated contact cycles, and stable well beyond any NdFeB grade's temperature limit. Its weaknesses: it is a barrier coating — scratches through to the substrate become corrosion initiation sites — and salt-spray endurance is modest compared with epoxy systems. Also note nickel is a common contact-allergen and a California Prop 65-listed substance, relevant for skin-contact consumer products (see the compliance guide).

• Direct epoxy (electrophoretic or spray, usually black): a polymer barrier with excellent moisture and chemical resistance and the best adhesive-bonding surface of any coating. Softer than nickel — it scratches and abrades more easily, so it suits parts that are assembled once and left alone.
• NiCuNi + epoxy: the belt-and-suspenders system — metallic layers underneath for adhesion and mechanical robustness, epoxy on top for environmental sealing. The standard call for automotive underhood, marine-adjacent, and outdoor products; typical salt-spray endurance is several times bare NiCuNi.
• Color and marking: epoxy takes colors (useful for polarity or grade identification at assembly) and laser-marks cleanly.

• Parylene C — vapor-deposited polymer that grows conformally on every surface, edge, and hole with zero pinholes at just 5–20 µm. The choice for medical devices (established biocompatibility record), aggressive chemical exposure, and tolerance-critical parts where 20 µm of plating won't fit. Costs come from the batch vapor process (minimum-lot charges dominate small orders) and its low-energy surface makes adhesive bonding difficult without plasma treatment.
• Gold over NiCu — biocompatible, chemically inert, cosmetic, and solderable/conductive. Standard for implant-adjacent and dental applications and premium consumer products. The gold is thin (1–3 µm); the NiCu underneath does the corrosion work.
• PTFE — thick, slick, chemically inert; used where magnets slide against surfaces, in washdown food-adjacent equipment, and where non-stick release matters. Adds real thickness — account for it dimensionally.
• Rubber jacketing — an assembly-level over-mold rather than a plating; seals the whole magnet system and transforms shear performance. Covered fully in the assemblies guide.

Zinc plating (8–15 µm, blue-gray) costs less than NiCuNi and — unlike nickel's barrier-only protection — corrodes sacrificially, protecting small scratches galvanically. Its limits: softer and shorter-lived than NiCuNi, duller cosmetics, and lower practical temperature. Right for dry indoor, cost-sensitive, high-volume applications; wrong for humidity or handling-intensive duty.

When the environment is severe enough that coating selection turns into coating engineering — repeated salt-spray failures, sterilization cycling, chemical cocktails — step back and re-run the material choice: bare SmCo or ferrite is sometimes the cheaper system answer than the fourth coating iteration on NdFeB.

The coating is one of the two surfaces in every bonded magnet joint, and the joint is only as strong as the coating's own adhesion to the NdFeB beneath. The short version of the bonding guide's coating logic: epoxy coatings bond best (like bonds to like), bright NiCuNi needs light abrasion to bond reliably, zinc's own softness becomes the limit, and parylene requires surface treatment. If a joint is structural, specify a coating-adhesion requirement (cross-hatch class or thermal-shock cycles) to the magnet supplier — a perfect glue bond to plating that peels is still a failed joint.

• Name the system, not just "coated": "NiCuNi, 15–25 µm total" or "NiCuNi + black epoxy, 25–40 µm total." For zinc, add "trivalent passivation."
• Dimensional basis: state whether drawing dimensions apply before or after coating — after-coating is the safe default for fit-critical parts (a recurring theme from the RFQ guide).
• Performance requirement: for corrosive duty, specify the test — salt spray hours per ASTM B117 with acceptance criteria (no red rust/blistering), or 85 °C/85 %RH duration — rather than trusting the coating name alone.
• Adhesion requirement where the part is bonded or thermally cycled.
• Verification: per-lot plating certificates stating coating type and measured thickness (XRF resolves the individual Ni/Cu/Ni layers); periodic audit testing for salt spray and adhesion. The measurement side lives in How Magnets Are Tested.