Radial Magnets, Inc.
Radial Magnets, Inc.we know magnets
Radial Magnets, Inc.
Radial Magnets, Inc.we know magnets
Radial Magnets · Technical Resource
A coating specification is the difference between a magnet that lasts twenty years and one that swells apart in a season. It is also the specification most often left blank on an RFQ, or filled in with the word "nickel" and nothing else. This guide covers what each coating actually does, what a salt spray number is worth, and how thickness quietly wrecks press fits.
Sintered neodymium is not a homogeneous material. It is a dense field of Nd2Fe14B grains held together by a neodymium-rich phase that sits along the grain boundaries. That intergranular phase is what makes the magnet sinter properly, and it is also chemically the most reactive thing in the part. Given moisture and oxygen it corrodes preferentially, and because it is the material holding the grains together, corrosion does not stay on the surface. It runs along the boundaries into the body of the magnet.
The visible result is characteristic and unmistakable: the magnet swells, the surface lifts in flakes, and the part eventually crumbles into dark powder. Because the mechanism is intergranular rather than surface oxidation, a magnet can be structurally compromised well before the damage is obvious, and a magnet that has begun to corrode cannot be recovered by stripping and re-plating.
SmCo, Alnico and ferrite are all specified without coatings routinely, and engineers coming from those materials often treat plating on NdFeB as cosmetic or optional. It is neither. An uncoated sintered NdFeB part in ordinary humid air begins visible degradation in days to weeks. There is no application in which bare sintered NdFeB is the correct answer.
Figures below are typical production ranges. Treat them as a starting point for a conversation with your supplier rather than as guaranteed values — salt spray in particular varies substantially with part geometry and edge condition.
| Coating | Thickness | Salt spray | Temp limit | Strengths | Limitations |
|---|---|---|---|---|---|
| Ni-Cu-Ni | 10–20 µm | 24–96 h | 200 °C | Industry default. Hard, bright, conductive, dimensionally predictable | Chips at sharp edges; unsuitable for sustained wet or salt exposure |
| Zinc | 5–15 µm | 24–72 h | 150 °C | Lowest-cost metallic option; sacrificial, so minor damage self-protects | Dulls and blooms white; poor in acidic conditions; short service life |
| Ni-Cu-Ni + epoxy | 15–30 µm | 96–240 h | 120 °C | The practical outdoor and washdown answer; polymer barrier over a metallic base | Thicker, so tolerances must account for it; epoxy is the temperature limit |
| Epoxy only | 15–25 µm | 72–120 h | 120 °C | Non-conductive, smooth, good bonding surface for adhesives | Softer than plating; some formulations chalk under UV |
| Gold over Ni | 0.5–5 µm Au | >200 h | 200 °C | Biocompatible, corrosion-proof, stable contact resistance | Expensive; still needs the nickel underlayer to do the structural work |
| Parylene C | 5–25 µm | 200–500+ h | 125 °C | Vapour-deposited, conformal and pinhole-free including into corners | Higher cost; not UV stable; fixturing affects coverage |
| Phosphate | 2–5 µm | 12–24 h | 200 °C | Cheap; excellent primer for paint or encapsulation | Not standalone protection. Only correct if something else follows it |
| PTFE | 10–30 µm | 120+ h | 260 °C | Non-stick, chemically inert, highest temperature of the polymers | Low adhesion without careful surface prep; specialist process |
Coating comparisons are usually settled by citing salt spray hours per ASTM B117, and the number gets treated as a service-life prediction. It is not one. B117 is a continuous neutral salt fog at 35 °C — a constant, unvarying condition that no real environment reproduces. Its value is comparative and diagnostic: it sorts coating systems against each other and it catches process defects such as thin deposits, poor adhesion or inadequate edge coverage.
What it does not do is translate into calendar time in your application. Real environments cycle — wet to dry, hot to cold — and cyclic exposure is substantially more aggressive on polymer-coated parts than steady fog, while being gentler on sacrificial metallic coatings. Two coatings with the same B117 rating can behave completely differently in service.
Electroplated deposits thin at sharp external edges and build up at corners, because current density follows geometry. A part quoted at 15 µm nominal may carry 5 µm or less on a sharp arris, and that is where corrosion starts. If the part has square external edges and lives in a wet environment, specify an edge break — a 0.2–0.3 mm chamfer or radius costs almost nothing at grinding and does more for coating life than moving up a coating grade.
Verification methods, sampling and what to write into an acceptance specification are covered in incoming inspection and acceptance and tolerances and acceptance criteria.
The most common coating-related production failure has nothing to do with corrosion. It is a magnet that will not go into its pocket, or that goes in and cracks. Coating adds material to every surface, and on a diameter it adds twice the coating thickness. A 20 µm coating adds 40 µm to an outside diameter — comparable to the entire dimensional tolerance on a precision part.
| Coating | Added per surface | Added on a diameter | Fit consequence |
|---|---|---|---|
| Zinc | 5–15 µm | 0.010–0.030 mm | Usually absorbable in standard tolerance |
| Ni-Cu-Ni | 10–20 µm | 0.020–0.040 mm | Consumes most of a ±0.05 mm band |
| Epoxy over Ni | 15–30 µm | 0.030–0.060 mm | Must be dimensioned after coating |
| Parylene C | 5–25 µm | 0.010–0.050 mm | Highly uniform, so predictable if specified |
State on the drawing that dimensions and tolerances apply after coating. It sounds obvious and it is routinely omitted. Without it the supplier is entitled to grind the substrate to print and plate on top, and every part arrives oversize by the coating stack. Where a press fit or a close-clearance pocket is involved, call out the maximum outside diameter after coating explicitly as a separate controlled dimension.
Work from the environment inward. The sequence that avoids most mistakes is: establish the worst-case exposure, then the temperature ceiling of the whole assembly, then the fit and bonding constraints, and only then choose a coating that satisfies all three.
| Environment | First choice | Alternative | Watch for |
|---|---|---|---|
| Sealed enclosure, dry, <80 °C | Ni-Cu-Ni | Zinc if cost-critical | Nothing unusual |
| General industrial, occasional condensation | Ni-Cu-Ni | Epoxy over Ni | Edge coverage on square parts |
| Outdoor, unsheltered | Epoxy over Ni | Parylene C | UV on the epoxy; 120 °C ceiling |
| Marine or salt spray | Epoxy over Ni | Full encapsulation | Any coating breach ends the part |
| Food, pharma, washdown | Epoxy over Ni | PTFE | Cleaning chemistry compatibility |
| Body contact or implantable | Parylene C | Gold over Ni | Biocompatibility documentation |
| Above 150 °C | Ni-Cu-Ni | PTFE to 260 °C | Polymers are excluded; check the grade too |
| Aggressive chemical | PTFE | Parylene C | Specific reagent testing, not general claims |
If the environment is genuinely hostile and the temperature is high, the honest conclusion is sometimes that NdFeB is the wrong material. Samarium cobalt is intrinsically corrosion-resistant and needs no coating in most industrial and aerospace service, and ferrite does not corrode at all. Both give up energy density, so the magnet gets larger — but a larger magnet that survives beats a smaller one wrapped in a coating that has one job and no redundancy. See samarium cobalt magnets and ceramic and ferrite magnets.
The full specification sequence is set out in how to prepare a magnet RFQ.
Tell us the operating environment, the temperature range and how the magnet is retained, and we will recommend a coating system and tolerance scheme that works together rather than against each other.