Hot-dip galvanised bolts for outdoor use
Hot-dip galvanising is the reference coating for outdoor fasteners. The part is dipped in molten zinc at about 450 °C and comes out with a layer of 40 to 85 microns, five to ten times more zinc than electroplated zinc.
We supply it in DIN 931 and DIN 933 hex bolts, nuts and washers, from M16 to M72. As a real example, we have supplied more than 10,500 hot-dip galvanised pieces for an outdoor installation, with ISO 4017 M20 bolts and 300HV washers.
| Coating | Thickness | Use |
|---|---|---|
| Electroplated zinc | 5 to 12 microns | Indoor |
| Hot-dip galvanising | 40 to 85 microns | Outdoor and aggressive environments |
| Zinc flake (Geomet) | 8 to 12 microns | High strength with no hydrogen risk |
The nut has to be oversize
With 50 microns or more of zinc per flank, a standard nut does not fit a galvanised bolt. That is why the nut is tapped oversize after galvanising, to ISO 10684. The cross combination fails: a galvanised bolt with a standard nut does not assemble, and an ungalvanised bolt with an oversize nut assembles loose. We supply the matched set.
Up to which property class can be galvanised
ISO 10684 limits hot-dip galvanising to class 10.9 with a controlled process. 12.9 is not galvanised because of the risk of hydrogen embrittlement; there the option is zinc flake, which does not introduce hydrogen in the process.
Galvanised or zinc plated
Zinc plating is thinner and cheaper, but outdoors it lasts a short time, sometimes months in a marine or aggressive industrial environment. If the part is going to be outside, hot-dip galvanising is the default choice.
Lead times
| Case | Lead time |
|---|---|
| Standard size in stock | Immediate dispatch |
| Specific coating (galvanising, Geomet, etc.) | About 7 days |
| Made to drawing or special part | About 30 days |
No minimum order, 3.1 inspection certificate on request and delivery across Spain.
Frequently asked questions
What thickness does a hot-dip galvanised bolt have?
Between 40 and 85 microns of zinc, compared with the 5 to 12 microns of electroplated zinc.
Can a 12.9 bolt be galvanised?
No. ISO 10684 limits hot-dip galvanising to class 10.9 with a controlled process. For 12.9, zinc flake is used.
Why does the nut have to be oversize?
Because the zinc layer thickens the bolt thread. The nut is tapped oversize after galvanising so that it assembles with the right clearance.
How long does a galvanised order take?
If the size is not in stock galvanised, the specific coating takes about 7 days.