What each fastener finish actually survives: electro-zinc for dry interiors, hot-dip galvanising outdoors, Sendzimir strip coatings on brackets, acid-resistant A4 stainless for marine and washdown — plus galvanic pairing rules and the stocked examples of each class.
| Finish/material | Typical zinc/protection | Belongs in | Stocked examples |
|---|---|---|---|
| Electro-zinc (FZB/FZV) | ~5–12 µm zinc | Dry interiors | M6 expansion bolt, zinc rod |
| Sendzimir (strip-galv.) | ~20 µm, coated before forming | Indoor brackets/rails, mild environments | pendant bracket 200 mm |
| Hot-dip galvanised (HDG) | ~45–85 µm | Outdoors, splash zones | M8 × 75 HDG bolt, M8 × 90 HDG |
| A4 / acid-resistant stainless | Alloy, not coating | Marine air, washdown, chemical | A4 rod M8, (grooved steel for contrast), acid-resistant lever ZC2JY313 |
| Polymer (nylon) | Immune, weak | Isolation, light chemical duty | nylon rod |
Life scales roughly with zinc thickness divided by the environment's corrosion rate (the ISO 9223 C-classes): electro-zinc that lasts decades in a C1 office measures its life in months in C5 coastal spray. HDG's thick layer plus sacrificial protection of cut edges is why outdoor structural fixings specify it; A4 sidesteps the whole consumption model — and adds chloride pitting resistance that plain A2 stainless lacks, which is why washdown and marine specs say A4/"syrefast".
Decide by the worst micro-environment on the path, not the room label: the 'indoor' plant room with a washdown hose is C4; the roof under a cooling-tower plume is coastal-equivalent; inside insulation on a cold line is permanently condensing (which is why insulated clamps with dry inserts matter — clamp guide). Where inspection access is poor, step one class up — the cost delta between zinc and HDG, or HDG and A4, is trivial against re-doing a suspended run. And record the class on drawings: 'M10 rod' is an incomplete specification in any environment above C2.