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NdFeB Magnet Coating Guide: NiCuNi, Epoxy, Zinc and Parylene
How four common coating families change corrosion protection, handling, dimensions and the information a custom-magnet supplier needs.
Reference note: Market prices and capacity figures are volatile reference points, not quotations or guarantees of current availability.
| Coating | Useful starting point | RFQ question |
|---|---|---|
| NiCuNi | Common plated finish with abrasion resistance | Are final dimensions specified after plating? |
| Epoxy | Organic barrier used where environment drives the choice | How are edges, chips and adhesion inspected? |
| Zinc | Non-nickel metallic option; supplier data may show thinner builds | What corrosion test and service environment apply? |
| Parylene | Conformal polymer option for complex surfaces | What masked areas and finished dimensions are required? |
Selection depends on base material, preparation, geometry and environment. Names alone do not establish performance.
Why NdFeB is usually coated
Sintered NdFeB is susceptible to oxidation and corrosion, particularly when moisture reaches the base material. Arnold Magnetic Technologies recommends protective coating in humid applications and lists epoxy, nickel plating, combinations of coatings and Parylene among successful options. Coating is therefore part of the component specification, not a cosmetic choice added after the magnetic grade.
The coating cannot be evaluated independently from the magnet beneath it. Base-material corrosion resistance, surface preparation, sharp edges, chips and process control all influence the result. A supplier’s coating name describes a process family; it does not prove performance for the buyer’s humidity, chemicals, temperature, abrasion or service-life requirement.
Start with the environment
Describe whether the magnet sees indoor dry air, condensation, sustained humidity, salt, cleaning agents, coolant, adhesives, sterilization, abrasion or thermal cycling. Identify direct exposure and exposure after assembly. A magnet enclosed in a sealed housing faces a different requirement from a loose magnet handled repeatedly or mounted near an opening.
Translate the environment into an agreed test or document where one exists. Do not request “maximum corrosion resistance” without acceptance criteria. Ask the potential manufacturer which preparation, coating system, thickness range and inspection it proposes, then preserve that proposal in the quote. Different systems should not be ranked on unit price before their scope is aligned.
NiCuNi: the common plated starting point
Nickel-copper-nickel is widely offered as a standard NdFeB finish. Magnet Applications describes nickel plating as a common three-layer nickel, copper and nickel system and notes good abrasion resistance for indoor use. Its standard-grade data sheet identifies NiCuNi as the standard coating and publishes example overall thickness and environmental-test results for its own process.
Those supplier-specific numbers should not become universal requirements. The RFQ should state appearance needs, handling or abrasion, electrical contact if relevant, adhesion expectations and the finished dimensional basis. Plating adds material to each coated surface, so tight gaps, press fits and bonded interfaces need dimensions defined after coating or a clearly stated allowance.
Epoxy: an organic barrier with edge questions
Epoxy and e-coat systems are common protective options for NdFeB. They can be useful when an organic barrier aligns with the environment or when a combined system is proposed. Arnold lists dry-sprayed epoxy and e-coat among common protective coatings, while Magnet Applications lists black and grey epoxy and combinations over metallic layers.
The RFQ should ask how coating reaches corners, holes and sharp edges; how chips or holidays are detected; and whether the epoxy is applied directly or over another layer. Edge preparation matters because machined magnets can chip and coating around sharp edges is difficult. Define colour only after the protective and dimensional requirements are clear.
Zinc: a metallic alternative that needs a defined test
Zinc is available as a non-nickel metallic option. Arnold describes zinc phosphates and chromates as conversion coatings that may provide temporary protection or serve as layers in a larger system. Magnet Applications also publishes zinc plating among available finishes and shows a supplier-specific example with a thinner overall build than its NiCuNi example.
That does not establish zinc as interchangeable with NiCuNi or epoxy. Ask whether the offer is plated zinc, a conversion treatment or a combined finish. State the actual environment, corrosion test, appearance and dimensional requirements. A thinner nominal system can help a tolerance stack, but only if its protection and process control match the application.
Parylene: conformal coverage for geometry-sensitive parts
Parylene is an available polymer coating for neodymium magnets. Arnold identifies Parylene capability, and Magnet Applications lists Parylene C and combined systems. Because Parylene is applied as a conformal process, it can be considered when coverage of complex surfaces, electrical isolation or a thin polymer barrier is relevant to the application.
The request still needs precision. Identify masked faces, bonding surfaces, contact points, holes and dimensions that apply after coating. Ask how parts are fixtured, how coverage and adhesion are inspected and what test supports the proposed use. “Parylene” alone does not specify grade, thickness, pretreatment, masking or acceptance criteria.
Coating changes the tolerance conversation
Every coating occupies space and can behave differently at edges, corners and recesses. If a drawing gives a finished diameter, thickness or air gap, state whether that dimension includes the coating. If dimensions apply before coating, identify the allowed finished build. Otherwise one source may machine to nominal before plating while another targets nominal after plating.
The issue extends to assemblies. Coating thickness can affect adhesive bond-line, mechanical retention, press fit and magnetic air gap. Arnold’s technical note explains that coating adds a gap in the magnetic circuit. Quote the magnet and mating interface together when that gap matters, and preserve the offered coating build in the assembly response.
Inspection and documentation
Specify what the supplier must report with prototypes and production lots. Options may include visual criteria, thickness measurement, adhesion or an environmental test tied to an agreed method. The buyer should define the required evidence; the potential manufacturer should state what its process can provide. A generic certificate should not replace the identified acceptance requirement.
Request the coating process location and change-control expectation when origin or consistency matters. Ask whether prototypes use the proposed production coating line and base material. Record any alternative system as a deviation. This makes a later change from NiCuNi to epoxy, zinc or Parylene visible before it affects dimensions, assembly or environmental performance.
A coating-ready RFQ
Include base magnet grade, drawing revision, final dimensions and tolerances, surface finish, operating environment, temperature, cleaning or chemical exposure, assembly method, masked areas, appearance and required tests. State prototype, initial-order and annual quantities. If the coating is not approved, describe the environment and request a documented proposal rather than selecting from colour or familiarity alone.
A comparable response repeats the coating system, layer sequence where relevant, offered thickness range, dimensional basis, inspection, test assumptions and every deviation. Coating selection then becomes part of the same controlled sourcing record as grade, origin and delivery. That record is more useful than declaring one finish universally superior across unrelated applications.
Questions buyers ask
Is NiCuNi always the best NdFeB coating?+
No. It is a common starting point, but environment, abrasion, geometry, adhesion and dimensional requirements determine suitability.
Should drawing dimensions apply before or after coating?+
The RFQ must state the basis. Otherwise suppliers can quote different finished components while appearing to meet the same drawing.
Does a coating name guarantee corrosion life?+
No. Base material, surface preparation, edge coverage, process control and the agreed test method all influence performance.
Technical references
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