Key takeaways
- Salt spray hours compare coatings in a lab; they do not translate directly into years outdoors.
- Electro-zinc suits indoor use; ceramic and zinc-flake coatings suit exterior roofing; marine sites need stainless.
- Always ask whether a quoted figure is time to white rust or time to red rust.
“1,000-hour coating” is one of the most quoted and least understood numbers in the fastener trade. Here is what it measures and how to use it when you buy.
What the test is
The neutral salt spray test (ISO 9227, ASTM B117) puts parts in a chamber filled with a fine 5 % sodium chloride mist at 35 °C. Inspectors record when white rust (zinc corrosion) and red rust (steel corrosion) appear. The test is fast and repeatable, which makes it good for comparing coatings and checking lots, but real weather includes UV, wet–dry cycles and pollutants it does not reproduce.
Common coatings
| Coating | Typical red rust | Notes |
|---|---|---|
| Blue-white zinc (Cr III) | 72–120 h | Indoor, dry use; RoHS compliant |
| Yellow zinc | 96–192 h | Better than clear zinc; common on chipboard screws |
| Black phosphate | 24–48 h | Indoor only; standard for drywall screws |
| Zinc flake (Dacromet type) | 500–1,000 h | No hydrogen embrittlement risk; thin and uniform |
| Ceramic (Ruspert type) | 500–2,000 h | Multi-layer; colour options; roofing and decking |
| Hot-dip galvanised | Depends on zinc weight | Thick; for lag screws and structural timber |
White rust vs red rust
A supplier quoting “1,000 h” might mean time to red rust, while another means time to white rust. The difference can be several hundred hours. Always ask which, and ask for the test report with photos.
Matching coating to exposure
ISO 9223 classifies atmospheres from C1 (heated interiors) to CX (extreme offshore). As a rough guide: C1–C2 interiors are fine with zinc; C3 urban and light industrial exteriors need ceramic or zinc flake; C4 coastal and industrial sites need high-grade ceramic or bi-metal; C5 and marine call for 316 stainless.
Do not forget the drive recess
Drivers scrape coating from the recess and the edge of the head. That is where rust starts on most roofs. Colour-painted heads and stainless bi-metal screws avoid the problem.
Frequently asked questions
Is AS 3566 Class 3 equal to 1,000 hours?
No. AS 3566 Class 3 uses a cyclic test and specific acceptance criteria. Ask for a Class 3 test report rather than converting from salt spray hours.
Does coating thickness matter for screws?
Yes, but too thick a coating fills the thread and raises driving torque. Screw coatings are engineered thin, typically 5–15 µm.
