A coating changes four things at once: corrosion resistance, friction, running-in behaviour and the dimensions of the part. This library gives each finish the same table — thickness, protection, temperature, and what it does to the fit.
| Thinnest finishes | Passivation and black oxide — 0 to 3 µm, no dimensional effect |
|---|---|
| Thickest finishes | Zinc-nickel at 8–12 µm, thin dense chrome at 2–10 µm |
| Best salt-spray performance | Zinc-nickel, 500–1000 h at 5–12 µm |
| Lowest friction | PTFE / MoS2 dry film, μ ≈ 0.05–0.15 |
| Hardest | DLC at 2000–4000 HV; TDC at 800–1000 HV |
| Dimension rule | A coating adds its thickness to every coated surface — twice the thickness across a diameter |
| Hydrogen embrittlement risk | Electroplated finishes on hardened rings — require post-bake |
| RoHS | Trivalent zinc and zinc-nickel are RoHS compliant; hexavalent chromium is not |
A 10 µm zinc-nickel layer on a 6205 bore reduces the bore by 20 µm across the diameter. The bearing still fits its nominal shaft, but the interference changes, and in a C2 or preloaded assembly that is enough to alter the running clearance. This is why bearing rings are normally plated before final grinding, or masked on the raceways, or plated only on non-functional surfaces.
The practical rule: a coating is applied to the surface that needs protecting and removed or masked from the surface that defines the fit. If a supplier quotes “zinc plated” without saying which surfaces, the answer is incomplete.
| Coating thickness | Across a diameter | Consumes | Practical consequence |
|---|---|---|---|
| 2 µm | 4 µm | ≈40% of a P6 bore band | Usually absorbed by the tolerance |
| 5 µm | 10 µm | A full standard bore band | Change the fit or mask the bore |
| 10 µm | 20 µm | Two standard bore bands | Must be ground after plating or masked |
| 25 µm | 50 µm | All normal bearing tolerances | Only for non-functional surfaces |
Cheapest corrosion protection for housings and exposed steel.
Read →Where salt spray is measured in hundreds of hours.
Read →Appearance, mild protection, no dimensional change.
Read →Oil retention and running-in on raceways.
Read →Low friction and dry running.
Read →Wear resistance and very low friction in clean duty.
Read →Raceway wear and corrosion without changing size.
Read →Stainless that must not rust at a scratch.
Read →| Finish | Thickness | Salt spray | Max temp | Friction effect | Adds dimension |
|---|---|---|---|---|---|
| Zinc (trivalent) | 5–8 µm | 96–240 h | 120 °C | None | Yes |
| Zinc-nickel | 8–12 µm | 500–1000 h | 150 °C | None | Yes |
| Black oxide | 1–3 µm | 24–72 h oiled | 200 °C | Slight | Negligible |
| Mn phosphate | 5–15 µm | 24–96 h oiled | 200 °C | Lowers, aids running-in | Yes |
| PTFE / MoS2 | 5–15 µm | Depends on binder | 200–260 °C | Strongly lowers | Yes |
| DLC | 1–3 µm | Good (barrier) | 300 °C | Strongly lowers | Minimal |
| Thin dense chrome | 2–10 µm | Moderate | 300 °C | Slight | Yes |
| Passivation | 0 µm | Restores passive layer | n/a | None | No |
Yes. Plating adds its thickness to every coated surface, so a 5 µm layer reduces a bore by 10 µm across the diameter. Standard practice is to plate before final grinding, or to mask functional surfaces such as the bore and the raceway.
Zinc-nickel at 8–12 µm, typically 500–1000 hours to red rust, against 96–240 hours for a plain trivalent zinc at 5–8 µm. For exposed marine hardware it is the standard choice.
The coating sets a new ceiling. Zinc and zinc-nickel are limited to roughly 120–150 °C; phosphate and black oxide to about 200 °C; PTFE dry films to 200–260 °C depending on the binder; DLC and TDC to about 300 °C.
It is a mild one. Black oxide is 1–3 µm of converted surface, usually oiled, and it protects against handling and condensation rather than against salt or water. Its main value is appearance and minimal dimensional change.
Passivation adds nothing. It removes free iron from a stainless surface with an acid bath and lets the natural chromium oxide layer re-form. It is the correct treatment for machined or welded 316 and 440C parts that show rust spots.
Yes, on the parts where the finish is applied before grinding or on non-functional surfaces. Send the drawing with the finish specification — thickness, standard and which surfaces are coated — and we will quote it with the dimensional implications stated.
Send the drawing and the environment — salt spray hours, temperature, friction requirement — and we will name the finish, the thickness and the surfaces to mask.