Almost every neodymium magnet leaves the factory with a protective coating. Bare NdFeB corrodes fast — rust can start within days in humid air. The coating, usually nickel, zinc or epoxy, is what keeps the magnet alive. But a coating only protects if it is thick enough: a thin spot is where rust begins. That is why coating thickness is written on the drawing and checked on every batch.
How thickness is measured
Three methods cover almost every case:
- XRF (X-ray fluorescence) — the workhorse for metal coatings like the classic nickel-copper-nickel stack. The instrument fires X-rays at the magnet; each layer reflects a characteristic signal, and the strength of that signal reveals the thickness. Fast, non-destructive, and it measures each layer of a multi-layer coating separately.
- Metallographic cross-section — the referee method. The magnet is cut, polished and examined under a microscope, and the coating is measured directly. It destroys the part, but it shows the true picture, layer by layer, defects included.
- Portable gauges — small handheld tools for quick checks. Magnetic-induction gauges suit non-magnetic coatings on a magnetic base (like zinc on NdFeB); eddy-current gauges suit insulating films. Both struggle with nickel, which is itself magnetic, so XRF or a cross-section stays the right tool for the most common coatings.
What to check as a buyer
Plating is thinner at edges and corners than on flat faces, so take readings at several points on several parts per batch — one reading is not a measurement. And thickness is not the whole story: a thick coating over poor surface preparation will blister and peel. Run thickness checks together with corrosion tests.
Read on: surface treatments compared → · ICP chemical analysis →