Technical Library
Salt-spray hours are the number everyone quotes and the number that correlates least well with how a part behaves in service. Useful for comparing two coatings against each other, close to meaningless as an absolute prediction of field life.
| Coating | Typical thickness | Indicative salt spray to red rust | H2 embrittlement risk | Notes |
|---|---|---|---|---|
| Electroplated zinc clear or yellow passivate |
5-12 µm | 96-240 h | Yes | Cheapest and most common. Fine indoors and for general industrial use. |
| Zinc-nickel 10-15% Ni |
8-15 µm | 500-1000 h | Yes | Automotive standard where plain zinc is not enough. Good at temperature. |
| Zinc flake non-electrolytic, e.g. zinc-aluminium lamellar |
8-20 µm | 500-1000 h | No | Applied as a dispersion and cured, so no atomic hydrogen is generated. Usually carries an integrated lubricant, which changes torque. |
| Hot dip galvanizing | 45-85 µm | 1000 h+ | Low | Construction and structural. Thick, so threads must be tapped oversize to fit. |
| Passivation stainless only |
- | n/a | No | Not a coating. Removes free iron so the chromium oxide layer re-forms evenly. |
Salt-spray figures are indicative ranges to red rust under neutral salt spray testing. Actual results depend heavily on the passivate and sealer used, part geometry, and the specific process. Where a customer standard specifies hours, that standard governs, and we will supply the plant's test report.
Electroplating generates atomic hydrogen, some of which diffuses into the steel. In a high-strength fastener that hydrogen collects at grain boundaries and can cause a sudden, brittle failure hours or days after installation, at a load the part carried without complaint on assembly.
The risk becomes real above roughly property class 10.9, or a core hardness above about 320 HV. The mitigations:
If you are specifying 10.9 or 12.9 with an electroplated finish, the bake requirement belongs on the drawing. It is not something to leave to the plating shop's discretion.
Coating changes torque. A zinc flake coating with integrated lubricant can need around 30% less torque than plain zinc for the same clamp load. Do not carry a torque spec across a coating change without rechecking it. See tightening torque.
How to use these tables. The values here are standard reference figures published to help you specify and compare parts. They are guidance, not a substitute for your own engineering standard, and several of them (torque above all) depend on assumptions about friction, lubrication and joint material that will differ in your application. Where a number matters, confirm it against the governing specification and against the part drawing. We will supply the plant's actual test data with any quotation.