Zinc Plated vs Hot-Dip vs Dacromet: Salt Spray, Hydrogen Embrittlement and Thread Fit
The three most-used fastener finishes in export orders compared: electro-zinc (blue-white 48-72 h / yellow 96-240 h), hot-dip galvanizing (1000 h+, 50-100 um) and Dacromet (500-1000 h+, 4-10 um, hydrogen-free) across salt-spray life, temperature limits, hydrogen-embrittlement risk, relative cost and thread fitability.
Overview
The vaguest inquiry line is finish: zinc plated - yet within zinc plating salt-spray life spans 48 hours to over 1000 hours at 3x price difference. Coating selection starts with the environment floor, then the strength grade rules out processes (be careful electroplating 10.9+), and finally thread fit and total cost close the loop.
Parameter Comparison Table
| Property | Electro-Zinc (blue-white/yellow) | Hot-Dip Galvanized | Dacromet |
|---|---|---|---|
| Neutral salt spray (to red rust) | 48-72 h / 96-240 h | 1000 h+ | 500-1000 h+ |
| Temperature limit | ≤80°C | ≤250°C (短期更高) | ≤300°C |
| Hydrogen embrittlement risk | Medium - 10.9+ needs de-embrittlement bake | Low | None (non-electrolytic) |
| Coating thickness | — | 50-100 μm | 4-10 μm |
| Thread fit | Standard threads | Thread allowance / re-tap · nuts +0.2-0.4 mm | Standard threads direct |
| Relative cost | 低 | 中 | 中-高 |
| Typical use | Indoor machinery / auto parts | Steelwork / railing / towers | Chassis / high-strength bolts / marine |
Selection Conclusion
One-line rule: blue-white zinc indoors, yellow zinc or hot-dip outdoors, Dacromet for high-strength bolts and corrosive environments. Avoid electrodeposition on 10.9+ - pickling infuses hydrogen and a missing bake plants delayed fracture; Dacromet runs non-electrolytic and hydrogen-free, the default finish for high-strength bolts, and at only 4-10 um it mates with standard nuts, sparing you the 0.2-0.4 mm oversize nuts hot-dip demands. Judge cost on the whole ledger: Dacromet costs more per piece than blue-white zinc, but a container rejected for rust loses far more - for demanding Western markets we recommend Dacromet on 8.8 and above by default. State standard and hours in the contract (e.g. Dacromet, 720 h to red rust per ISO 9227).
FAQ
They must be. Coating performance depends on thickness and process, so the bare words zinc plated bind nothing. Specify, e.g., Dacromet, 720 h to red rust per ISO 9227 NSS, and inspect to the same standard.
The HDG layer is 50-100 um thick and piles onto the thread. External threads take allowance or are re-tapped after coating, so mating nuts must be oversize to run on - that is the HDG thread-fit cost. Dacromet at 4-10 um avoids this entirely.
Not recommended. Outdoor rain exposure starts at yellow zinc (96-240 h), but coastal, de-icing-salt or chemical service still exceeds it; steel structures normally go hot-dip (1000 h+), or Dacromet when easy thread fit matters.
Hydrogen enters during pickling and plating. Dacromet is a non-electrolytic zinc-flake process with no hydrogen uptake, so its own process needs no bake. But if you switch 10.9+ hardware to zinc plating, confirm the 190-220°C de-embrittlement hold.
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