Specify the test, not the name
Write the corrosion requirement as hours to red rust in a neutral salt spray test, and the coating system that achieves it. Zinc with a trivalent passivation might give 96 hours; zinc-nickel can exceed 720. Both are 'zinc plated'.
Plating thickness versus coating weight
For small parts, thickness in microns is easier to verify by XRF than coating weight. Specify both the under-layer (nickel, copper) and the top layer where the application needs it — silver on copper contacts is a system, not a single coat.
Hydrogen embrittlement
Acid cleaning and plating can introduce hydrogen into high-strength steel. Above roughly property class 10.9, parts should be baked after plating. This is not optional on safety-critical fasteners, and it is one of the first things an automotive auditor checks.
What to expect from a supplier
A plating report per batch, with thickness measurement on the finished part and salt spray hours to the drawing limit. A supplier who quotes salt spray from a brochure rather than from a test report is telling you something.
Common systems
| System | Typical salt spray | Where it belongs |
|---|---|---|
| Zinc + clear passivation | 48–96 h | Indoor, dry environments |
| Zinc + thick film passivation | 120–240 h | Appliances, occasional moisture |
| Zinc-nickel | 720 h+ | Outdoor, automotive, marine |
| Nickel + silver | n/a (contact function) | Current-carrying contacts |
If the drawing says 'zinc plated' and nothing else, the supplier chooses the corrosion performance for you. Write the hours.