Steel plants, coal fields and railway track are hard on fasteners. Moisture, dust, heat and chemicals all attack the bolt, and a corroded thread is hard to tighten, inspect or remove. Choosing the right finish at the ordering stage is cheaper than replacing seized bolts later.
Common finishes compared
| Finish | What it is | Where it fits |
|---|---|---|
| Black / plain | As-manufactured surface | Indoor or protected locations, or where the structure is painted after assembly |
| Oiled or phosphated and oiled | Light protection for storage and handling | Short-term protection, machinery assemblies |
| Hot-dip galvanised | Zinc coating applied by dipping in molten zinc | Outdoor structures, humid and industrial atmospheres |
| Electroplated zinc | Thin zinc layer applied electrically | Light-duty, indoor or mildly corrosive conditions |
| Painted after assembly | Bolts painted with the structure | Structures with a full paint system |
Galvanised bolts: details that matter
- Nuts are tapped oversize to fit over the zinc on the bolt thread. Galvanised bolts should be used with nuts made for galvanised bolts.
- Specify the coating by standard — in India, hot-dip galvanised coatings on threaded fasteners are covered by IS 1367 (Part 13).
- Do not mix galvanised and plain fasteners in the same joint; the dissimilar surfaces can corrode faster.
A caution for high-strength bolts
Very high-strength bolts, such as property class 10.9 and above, can be affected by hydrogen embrittlement when electroplated. If high-strength bolts need corrosion protection, agree the coating method with the manufacturer at the enquiry stage.
Specifying the finish
State the finish, and the coating standard if there is one, on every order line. S A Engineering makes hex bolts and foundation bolts with the finish specified on your order — include it in your enquiry and we will confirm.
