Three different jobs
A finish is usually chosen for one of three reasons: to resist corrosion, to look right, or to change how the part behaves in assembly. Those are different requirements and they are met by different processes. Confusing them is the most common finishing mistake.
| Finish | Primary purpose | Mechanism | Effect on dimensions |
|---|---|---|---|
| Zinc plating | Corrosion protection | Sacrificial — the zinc corrodes instead of the steel | Adds thickness to the thread |
| Zinc-nickel | Corrosion protection | Sacrificial, higher protection per unit thickness | Adds thickness to the thread |
| Nickel plating | Corrosion and appearance | Barrier plus decorative | Adds thickness to the thread |
| Passivation | Restore stainless oxide layer | Chemical, no deposit | No change |
| PVD | Wear, friction, colour | Hard ceramic film deposited in vacuum | Thin enough not to affect the fit |
Sacrificial versus barrier
Zinc and zinc-nickel are sacrificial: they corrode preferentially, which protects the steel even where the coating is scratched. That is why they are the default for steel fasteners — a barrier coating fails at the first scratch, while a sacrificial one keeps working.
PVD is a hard ceramic film, not a sacrificial coating. It resists wear, gives a stable colour and reduces friction, but it is not designed to be consumed in place of the substrate. If the requirement is measured in salt spray hours, PVD alone is not the answer.
The dimensional problem nobody expects
A plated coating adds thickness to the thread flank, and a thread that gauges correctly before plating can fail its gauge after. On a micro screw this matters more than on a large bolt because the coating is a larger fraction of the thread height.
This is why threads are re-gauged after coating, and why plating thickness is a specification rather than a detail left to the finishing shop. It is also one of the reasons PVD is chosen on micro threads: a vacuum-deposited film is thin enough that the thread still assembles normally.
Hydrogen, and when it matters
Electroplating introduces hydrogen into steel. In a hardened part that hydrogen can cause a delayed brittle fracture, sometimes days after the part is assembled. The mitigation is baking after plating, which is why that step is not optional on hardened plated fasteners.
PVD has no aqueous chemistry and therefore no hydrogen. On a high-strength hardened screw that is a genuine technical advantage, not a marketing one.