If you have ever tried to glue something to a freshly waxed car, you know the problem: the glue beads up and slides off instead of sticking. Most plated NdFeB magnets face exactly this — because the nickel–copper–nickel (NiCuNi) plating that makes them shiny and rust-proof also makes them very slippery for adhesives. And most plated magnets are glued into products.
What NiCuNi is
It is a three-layer metal sandwich: nickel on the magnet, a copper layer in the middle, and nickel on top. Together they give magnets the bright, hard, corrosion-resistant surface you see on most consumer magnets.
Why adhesion matters
Glue only holds if it can wet the surface — spread out and make full contact. A mirror-smooth plating makes glue bead up like water on a waxed bonnet, leaving tiny air pockets where the bond starts to fail. The shop-floor word for this is surface tension, measured in dyn/cm: the higher the number, the easier glue spreads. A fresh NiCuNi surface should reach at least 42 dyn/cm and stay above 36 dyn/cm even after months of storage.
The quick check
You do not need a laboratory: a dyne test pen, filled with ink of a known surface tension, either beads up or spreads on the surface. If it spreads, the surface is ready for bonding. It turns an invisible property into a ten-second incoming inspection.
What actually moves the number
- Drying — leftover residues contaminate the surface.
- Roughening — micro-roughness gives glue more area to grip.
- Brightener — the shinier the plating, the slipperier it gets.
- Storage — surface energy decays over time, faster when warm.
A magnet that looks perfect can still fail a glued assembly if its surface has drifted. Checking surface tension costs pennies — a failed bond in a shipped product costs far more.
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