A comparison of three coatings in terms of thickness, salt spray resistance and hydrogen embrittlement.
The life of a fastener is largely determined by its coating. Choosing the wrong one can lead to rust, loosening and visual deterioration within months. Let us compare three common coatings.
1. Electro-galvanizing (Zn)
5-12 µm of zinc is deposited in an electrolyte bath. It may be bright or have blue/yellow passivation. It is economical for dry indoor applications. Salt spray resistance is 48-96 hours (white rust). For classes 10.9 and above, baking is essential because of the hydrogen embrittlement risk.
2. Hot-dip galvanizing (HDG)
The bolt is dipped in a molten zinc bath at 450 °C; coating thickness is 45-80 µm. It is the standard for outdoor, steel construction and infrastructure projects. Because of the thick layer, the nut thread is tapped oversize and bolts and nuts must be certified together. Use above 10.9 is limited.
3. Geomet / zinc flake
A coating in which zinc and aluminum flakes are sprayed with an inorganic binder and baked. At 5-10 µm thickness it gives 480-1,000 hours of salt spray resistance. As it is not applied electrolytically, there is no hydrogen embrittlement risk; it is ideal for automotive and high-strength (10.9, 12.9) connections. Friction-controlled versions (K = 0.12-0.18) provide consistency in torque assembly.
Comparison
| Coating | Thickness | Salt spray | Use |
|---|---|---|---|
| Electro-galvanized | 5-12 µm | 48-96 hours | Indoor |
| Hot-dip | 45-80 µm | 720+ hours | Outdoor, structures |
| Geomet | 5-10 µm | 480-1,000 hours | Automotive, 10.9 / 12.9 |
Other options
- Black oxide: Cosmetic with low corrosion resistance; when oiled, used for indoor socket screws.
- Phosphate + oil: For friction control on assembly lines.
- Zn-Ni: Automotive engine compartment, 720+ hours; with Cr(VI)-free passivation.
Coating thickness and thread tolerance
Coating thickness affects thread tolerance. With electro-galvanizing the 6g/6H tolerance is maintained after coating; with hot-dip, the nut thread must be tapped oversize by the coating allowance (for example about +0.4 mm for M16). Do not mix bolts and nuts made with different coatings; order them as a set.
Galvanic corrosion warning
When stainless bolts and galvanized sheet are used in the same joint, zinc corrodes faster. In aluminum-to-steel joints, use insulating washers or coatings.
For a coating suited to your environment, use the sample request form or consult our engineers.
Related Posts
Choosing an Anchor: Mechanical or Chemical?
Anchor selection and installation quality based on concrete condition, edge distance and load type.
Read More