Zinc plating is the cheapest way to stop a steel housing or insert rusting: 5–8 µm of zinc, 96–240 hours of salt spray, and a price that barely moves the part cost.
| Thickness | 5–8 µm typical · 3–5 µm for tight-tolerance parts |
|---|---|
| Salt spray | 96–240 h to red rust (ASTM B117), depending on thickness and topcoat |
| Max service temperature | ≈120 °C |
| Process | Alkaline or acid trivalent zinc, with optional clear or yellow passivation |
| RoHS / REACH | Compliant in trivalent form — hexavalent chromium is prohibited |
| Effect on dimension | Adds thickness to each coated surface; 10 µm across a diameter at 5 µm |
| Hydrogen embrittlement | Possible on hardened parts — post-bake at 190–200 °C required |
| Typical use | Housings, stamped inserts, plummer blocks, exposed brackets |
Zinc is less noble than steel, so when the coating is scratched the zinc around the scratch corrodes first and protects the exposed steel. That sacrificial action is why a scratched zinc coating still protects, while a scratched paint film does not. The passivation layer on top — clear or yellow — slows the zinc's own corrosion and extends the salt-spray life.
On a bearing, zinc is almost always applied to the housing or the insert, not to the rings. A coated raceway would change the rolling contact, and the coating is far softer than hardened steel. Where rings must be protected, the usual approaches are black oxide, phosphate, or a stainless grade.
| Property | Zinc (trivalent) | Zinc-nickel | Black oxide |
|---|---|---|---|
| Thickness | 5–8 µm | 8–12 µm | 1–3 µm |
| Salt spray | 96–240 h | 500–1000 h | 24–72 h (oiled) |
| Max temp | 120 °C | 150 °C | 200 °C |
| Dimensional effect | Significant | Significant | Negligible |
| Cost index | 1.0× | 2.5–3.5× | 1.2× |
| Best for | Indoor, sheltered | Marine, outdoor | Appearance, tight tolerance |
Plummer blocks and flanged housings are the classic zinc-plated part.
Read →Accessories are usually plated steel.
Read →When the duty is outdoor or marine.
Read →Sheltered indoor steelwork at low cost.
Read →Trivalent zinc is. Hexavalent chromium passivation (the old yellow iridescent finish) is prohibited under RoHS, which is why modern yellow passivates are trivalent and slightly less protective.
You can, but it is rarely right: the coating is soft, changes the raceway geometry and can embrittle a hardened ring. Rings are normally protected by black oxide, phosphate or a stainless grade instead.
Thin coverage on edges and thread crests is normal in electroplating — current density concentrates and the deposit is uneven. Specify a minimum thickness on edges, or use a mechanical or dip-spin process for edge-critical parts.
Zinc oxide forming on the surface in damp, poorly ventilated storage. It looks alarming but the steel is still protected. Dry storage and a passivation layer prevent it.
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