A coating protects a neodymium magnet only while the barrier remains intact. A part can look flawless on its flat face yet start corroding at a sharp edge, a mounting hole, or a spot rubbed during assembly. That is why a coating name on a drawing is a starting point, not a complete specification.
For most custom NdFeB orders, the useful question is: what will the finished magnet encounter, and how will you accept or reject the coating? The comparison below helps you frame that discussion with your supplier.
Four common coating choices
| Finish | What to expect | What to check before ordering |
|---|---|---|
| Ni-Cu-Ni | A nickel–copper–nickel plated stack with a bright metallic appearance. Often considered for protected assemblies. | Specify total coating allowance, edge coverage, handling wear, and the actual exposure environment. |
| Zinc | A plated metal finish; its appearance and behavior depend on the particular passivation and process. | Ask which finish is proposed, whether passivation is included, and how it will be qualified on your part. |
| Epoxy | A polymer barrier that may be specified alone or over another coating. Formulation and application method matter. | Check coverage at corners, chips after handling, temperature and chemical exposure, and the effect on dimensions and bonding. |
| Gold | Usually a thin outer finish over an underlying plated system, chosen for a specific surface requirement or appearance. | Confirm the full layer stack, wear and porosity limits. Do not assume a gold appearance makes the finished part safe for body contact. |
There is no universal ranking of these finishes by “salt spray hours.” Coating thickness, preparation, geometry, defects and test criteria all affect the result. A comparison across suppliers is meaningful only when the test conditions and acceptance rules match.
Start with the exposure, not the color
Describe where the magnet sits in the final assembly. Is it enclosed in a dry housing, exposed to condensation, cleaned with chemicals, or handled repeatedly? Does it touch another metal or a moving part? Is there adhesive, potting compound or an overmold? These details can change the coating choice more than the magnet’s grade does.
Wet or chemically aggressive service needs particular care. Asking simply for “epoxy for outdoor use” is too broad: different epoxy systems, thicknesses and application methods can behave differently. If the magnet will be immersed, exposed to salt, or used in a safety-critical product, define the actual exposure and qualify the finished part under conditions relevant to that use. An enclosure or encapsulation may be part of the solution.
Geometry deserves its own review. Thin rings, small holes, sharp edges and countersinks can be harder to coat consistently than a broad flat face. Request a drawing review before treating a published coating description as a promise for your shape.
Make the coating measurable on the drawing
A purchasing specification should answer these questions:
- Finished dimensions: Are tolerances measured before or after coating? Allow for the proposed coating stack on mating surfaces, bores and slots.
- Coverage: Which surfaces and edges must be covered? Are rack or contact marks acceptable, and where?
- Appearance: Define acceptable color variation, scratches, exposed substrate and visible defects if appearance matters.
- Adhesion and handling: State the assembly process, any adhesive or overmold, and the checks needed after handling.
- Corrosion validation: Name the test method, duration, sample condition, sample count and pass/fail criteria. Ask for results on a representative part and coating process.
- Traceability: If your quality system requires it, specify the inspection record and lot information with the quotation.
A coating thickness figure copied from a general web chart is not a substitute for a supplier’s process specification or an approved sample. Agree on the value and measurement method for your actual geometry.
What a salt spray result can and cannot tell you
ASTM B117 describes how to operate a salt spray (fog) apparatus. ASTM’s description of the method says it does not prescribe a particular specimen, exposure period or interpretation of results. It also cautions that salt spray performance alone does not reliably predict outdoor service. A claim such as “Ni-Cu-Ni passes 24 hours” therefore needs a defined specimen, test duration, failure criterion and report; it is not a property guaranteed by the coating name.
For a design with recurring moisture, use salt spray as one defined qualification check where appropriate, alongside inspection of the actual coated part and any other application-specific testing. Record whether “failure” means first visible rust, blistering, loss of adhesion, or another agreed condition.
Gold and medical use: check the complete device
Gold-colored plating does not certify a magnet for skin contact, implantation or any other medical use. A thin outer layer can have pores or wear through to underlying materials. The complete construction, exposure duration and manufacturing process matter.
The FDA’s biocompatibility guidance assesses the final finished medical device, including processing and coatings, rather than treating a raw material or coating label as proof of safety. If your application involves body contact, define the contact type and duration, then have the finished device assessed under the requirements that apply to it. Do not substitute a generic magnet coating specification for that assessment.
What to send with a custom magnet RFQ
Send the drawing, intended magnet grade or magnetic requirement, estimated quantity, operating environment, assembly method and proposed coating if you have one. Flag critical faces, holes or edges, the finished-part tolerance, and any test method with its acceptance criteria. If your coating choice is still open, say so; comparing two qualified samples is more useful than choosing from a generic ranking.
For help reviewing coating options on a specific geometry, see our custom neodymium magnets page or send a drawing and requirements for quotation. We can discuss the specification and what needs validation before an option is approved for your application.
