How long the zinc lasts here
Galvanizing does not stop corroding — it corrodes instead of the steel, and it lasts until it is used up. How long that takes depends almost entirely on where the bolt is. The same coating that gives twenty-five years inland can be gone in six on a jetty. Press play to watch the zinc go, then change the environment and see the timeline move.
Life = zinc thickness ÷ zinc loss rate. The rate is the first-year figure from ISO 9223, which is the conservative end — on most sites the rate falls after the first year as the corrosion products build up. Treat these as a planning range, not a guarantee.
The corrosivity categories, and which one you are in
ISO 12944-2 sorts environments by how fast they eat a standard specimen in its first year. The zinc column is the one that matters for a galvanized bolt.
Thickness loss in the first year of exposure, per ISO 9223 as reproduced in ISO 12944-2.
Reading the Gulf honestly
Most of the populated UAE coast is C5: high humidity, airborne chloride, and a daily condensation cycle that keeps surfaces wet for hours after dawn even when it has not rained in months. Jetties, splash zones, desalination plants and anything offshore sit in CX. Inland industrial areas away from the coast are usually C3 to C4 — genuinely less aggressive, and worth checking rather than assuming, because over-specifying a coating costs money too.
Two local effects the category tables do not capture. Night-time condensation means time-of-wetness here is higher than the rainfall figures suggest. And sand abrasion physically removes coating on exposed edges and threads, which no corrosion rate accounts for.
What MF supplies, and the thickness you actually get
| Finish | Local min. | Mean min. | Where it applies |
|---|---|---|---|
| Zinc plated, ISO 4042 | — | 5–8 µm | Indoor, dry, or a temporary finish. Not a coastal coating. |
| Spun, thread ≤ 6 mm | 20 µm | 25 µm | Small fasteners, centrifuged after dipping. |
| Spun, thread > 6 mm | 40 µm | 50 µm | M8 and up: nuts, bolts, screws. Nut must be tapped oversize. |
| General article, 1.5–3 mm | 45 µm | 55 µm | Thin sections, light brackets. |
| General article, 3–6 mm | 55 µm | 70 µm | Medium plate and light fabrications. |
| General article, > 6 mm | 70 µm | 85 µm | Anchor bolts, U-bolts, plate washers. The usual MF case. |
| Grit-blasted first | 140 µm nominal | Specified in addition to ISO 1461 where the environment demands it. | |
| Stainless A4-80, ISO 3506-1 | no coating | Where no thickness of zinc will do. Several times the cost. | |
Two things follow from that table. A spun M20 hex bolt carries about 50 µm while an M20 anchor bolt from the same plant carries about 85 µm — 70% more zinc and about 70% more years, from the same process, purely because one is spun and one is not. And every figure above is a minimum: real coatings on thick or reactive steel routinely come out at 100 µm or more, so a life calculated from the minimum is the conservative answer, not the expected one.
Can you get 30 years out of it?
This is the question we are actually asked, so here is the arithmetic rather than a brochure answer. To last 30 years the coating has to be at least 30 × the annual zinc loss. Read the left column if you want to be confident, the right if you are willing to bet on the slower end of the range.
So: on the UAE coast, 85 µm does not give 30 years. In C5 it gives roughly 10 to 20. Thirty years in C5 needs somewhere between 126 and 252 µm of zinc, which no standard galvanizing run will give you. Inland at C4 the same 85 µm gives roughly 20 to 40 years, so 30 is a reasonable expectation there. The honest options for a 30-year coastal life are three:
A nominal 140 µm, specified in addition to ISO 1461. In C5 that is roughly 17–33 years — 30 only at the optimistic end, and it changes thread fit, so threads are cut after coating or masked.
The two coatings work together: time to first maintenance is 1.5 to 2.3 times the sum of what each would give alone. In a high-salt, high-humidity environment the factor to use is the bottom of that range, 1.5. This is the usual answer for a 30-year coastal requirement.
No coating to consume. Costs several times more, has lower strength than 8.8 unless you pay for a high-strength grade, and brings its own crevice and galling problems — but it does not have a service life measured in zinc.
Duplex multiplier per the American Galvanizers Association and Van Eijnsbergen: Mduplex = 1.5 to 2.3 × (Mgalvanizing + Mpaint), the lower factor for aggressive environments. A galvanized coating is also an excellent primer — it prevents the under-film corrosion that undercuts paint on bare steel.
Questions we are asked
Does a thicker coating really last proportionally longer?
Broadly yes, and that is the useful thing about zinc: it is consumed at a rate set by the environment, so twice the thickness is close to twice the life in the same place. It is not exactly linear — the rate typically falls after the first year — but the proportionality is reliable enough to plan maintenance on, which is why ISO 12944 works the way it does.
What happens when the zinc runs out?
Red rust appears, and it appears suddenly rather than gradually, because the zinc protects the steel electrochemically right up until there is not enough of it left. That is the point for the first maintenance coat — and repainting galvanized steel before the zinc is exhausted is far cheaper than after, because the surface is still sound.
Is stainless always the safer choice on the coast?
Not automatically. A4 / 316 resists chloride far better than A2, but it is not immune: crevice corrosion in stagnant, oxygen-starved joints and chloride stress corrosion cracking at elevated temperature are both real. Stainless also galls on assembly and has lower strength than 8.8 unless you pay for a high-strength grade. It is the right answer often, not always.
Can I galvanize a high-strength bolt?
Class 8.8 is routinely hot-dip galvanized. Above that the hydrogen embrittlement risk rises sharply — 10.9 requires the coater to manage it deliberately, and ASTM A490 prohibits it outright. If the drawing calls for a high-strength galvanized bolt, that is a conversation to have before the order, not after.
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Coating & corrosion selector · Galvanizing & thread fit · How anchor bolts fail · Back to the Bolt Lab