Coating selection
Which fastener finish for which African site.
Corrosion is local, not national: a specification that lasts in Nairobi fails in Mombasa. Hot-dip galvanising to ISO 1461 deposits 45–85 microns of zinc and is the practical default for exposed structural fasteners in Africa; plated zinc suits internal work; stainless 316 is for the coast. The tables below set out the trade-offs.
Finishes compared
Coating comparison.
| Finish | Thickness | Service life | Cost | Use for | Avoid |
|---|---|---|---|---|---|
| Self colour (black) | None — oiled only | Weeks outdoors | Lowest | Internal, dry, painted-over or immediately enclosed work | Any external or humid exposure |
| Electro zinc plated | 5–12 microns | 1–3 years external inland | Low | Internal fixings, light exterior, cost-sensitive volume lines | Coastal sites and anything expected to last a decade outdoors |
| Hot-dip galvanised (ISO 1461) | 45–85 microns | 15–40 years depending on atmosphere | Medium | Structural steelwork, exposed connections, buried and cast-in items | Fine threads and tight tolerance parts without oversize tapping |
| Dacromet / zinc flake | 8–12 microns | Comparable to HDG in salt spray | Medium-high | High-strength bolts where thread fit and no embrittlement matter | Applications needing mechanical abrasion resistance |
| Class 3 / Class 4 coating | Proprietary multi-layer | Class 3 inland, Class 4 coastal roofing | Medium | Roofing and cladding screws where the head is exposed | Structural bolting — these classes are roofing-specific |
| Stainless 304 (A2) | Inherent | Decades inland | High | Exposed architectural work, food and hygiene areas inland | Direct marine exposure and chloride-rich process streams |
| Stainless 316 (A4) | Inherent | Decades coastal | Highest | Coastal, marine, chemical and desalination environments | Highly loaded structural joints without checking strength grade |
By exposure zone
Specify from the site, not the price list.
- Inland dry, internal
- Zinc plated or self colour. Lowest landed cost, adequate service life.
- Inland exposed
- Hot-dip galvanised structure, Class 3 roofing, bonded EPDM washers for UV.
- Within 5 km of coast
- Hot-dip galvanised minimum, Class 4 roofing, stainless where hard to replace.
- Shoreline and marine
- Stainless 316 (A4) throughout, budgeted at estimating stage.
- Mining and process
- Hot-dip galvanised or Dacromet; stainless 316 around acid and leach plant.
- Dissimilar metals
- Insulating washers and matched finishes to stop galvanic corrosion.
Two repeat mistakes
Both cost more than the coating.
Mismatching the washer or nut finish to the bolt. The cheapest component becomes sacrificial, rusts first and stains the connection — the client sees it long before the engineer does. We supply matched finishes across the assembly as standard.
Ordering hot-dip galvanised threaded rod with standard nuts. Galvanising thickens the thread, so nuts must be tapped oversize after coating. Buy rod and nuts from different sources and you discover this on site with a crew standing idle.