Hot-dip galvanizing has protected structural steel for over 150 years and remains the default for onshore tower bolts. Zn-Al flake coatings — sold under brand names including Geomet and Dacromet — have become the standard for offshore projects and any application using grade 12.9 bolts. The difference comes down to coating thickness, process chemistry, and what each does to hydrogen embrittlement risk.
Hot-dip galvanising (HDG) gives 45–85 µm zinc thickness and excellent outdoor corrosion resistance, but the acid pickle process before galvanising creates hydrogen embrittlement risk for bolts above grade 8.8. For grade 10.9 bolts, HDG is standard in most European wind markets (ISO 10684) with controlled de-embrittlement baking. For grade 12.9, zinc flake coatings (Geomet or Dacromet) are preferred — no acid pickle, no embrittlement risk, 8–12 µm applied mechanically with tight thread tolerance maintained.
- Best for
- HDG: tower flange bolts, foundation anchors, and structural hardware up to M36 where thread tolerance allows. Zinc flake: grade 12.9 nacelle bolts and threaded components where tight thread tolerance is required
- Not suitable for
- HDG on grade 12.9 bolts — hydrogen embrittlement risk is too high. Zinc flake as an HDG substitute on main structural joints without engineering change approval
- Selection steps
- 1 — Confirm fastener grade → 2 — Check OEM coating specification → 3 — Select HDG for grade 10.9 structural bolts per ISO 10684 → 4 — Select zinc flake for grade 12.9 secondary bolts → 5 — Confirm thread gauge after coating
- RFQ information
- Fastener grade, dimensions, coating thickness required, salt spray hours, thread tolerance class required after coating, quantity
§ 01 How each coating process works
Hot-dip galvanizing (HDG) immerses cleaned steel in a bath of molten zinc at approximately 450 °C. The zinc reacts metallurgically with the steel surface to form a series of iron-zinc alloy layers, topped by a pure zinc outer layer. The result is a robust, thick coating (typically 45–85 µm on fasteners per ISO 1461) that bonds permanently to the substrate.
The process requires acid pickling to remove mill scale and rust before galvanizing. This pickling step is the source of the hydrogen embrittlement risk: atomic hydrogen is absorbed into the steel during acid immersion, and if not fully baked out, it can cause delayed brittle fracture — especially in steels above approximately 1000 MPa tensile strength.
Zn-Al flake coatings (ISO 10683) are applied by dip-spin or spray: the fastener is dipped in a slurry of zinc and aluminium flakes suspended in an inorganic binder, then centrifuged and cured in an oven at approximately 240–300 °C. No acid is involved. Multiple passes build the coating to typically 8–15 µm — far thinner than HDG while achieving comparable or superior corrosion protection due to the lamellar barrier structure of the flakes and the aluminium content.
The two dominant commercial brands are Geomet (NOF Metal Coatings Group) and Dacromet (MacDermid Enthone). Both meet ISO 10683; their specific formulations differ slightly in aluminium content and topcoat options.
§ 02 Corrosion protection comparison
| Property | Hot-Dip Galvanizing | Zn-Al Flake (Geomet / Dacromet) |
|---|---|---|
| Governing standard | ISO 1461, EN ISO 10684 | ISO 10683, EN ISO 10683 |
| Typical coating thickness | 45–85 µm | 8–15 µm |
| Salt-spray endurance (neutral) | 500–1 000 h (ISO 9227) | 720–1 500 h (ISO 9227) |
| Corrosion protection mechanism | Sacrificial zinc + barrier | Barrier (lamellar) + sacrificial Zn/Al |
| Suitability for C4 environment | Yes | Yes |
| Suitability for C5-M offshore | Limited — consult OEM | Yes — preferred |
| Acid pickling in process | Yes — HE risk | No |
| Coefficient of friction (K-factor) | ~0.16–0.20 (varies with lubrication) | ~0.12–0.16 (more consistent) |
The tighter friction coefficient of Zn-Al flake coatings is practically significant: more consistent K-factor means more predictable preload from a given torque, which matters for critical structural joints where the torque-preload relationship is tightly specified.
§ 03 Thread fit and nut compatibility
The thickness difference has a direct consequence on thread geometry. An HDG coating of 45–85 µm applied to a bolt thread will consume all of the clearance in a standard 6H/6g thread fit, making the nut impossible to run on without gauling.
The solution for HDG fasteners is over-tapped nuts (tolerance class TD per ISO 10684 / EN ISO 10684), where the internal thread is tapped oversize to accommodate the coating. This is standard practice for HDG tower bolts, but it means:
- HDG bolts and standard nuts are not interchangeable — TD nuts must be specified and ordered together.
- Mixing HDG bolts with non-TD nuts in the field is a common site error that results in seized or cross-threaded connections.
- Replacement bolts on a project must match the original coating to avoid thread incompatibility.
Zn-Al flake coatings at 8–15 µm are thin enough to remain within the standard 6H/6g thread tolerance. Standard nuts are used, simplifying procurement and reducing the risk of field errors.
§ 04 Bolt grade compatibility
| Bolt grade | Hot-Dip Galvanizing | Zn-Al Flake | Reason for restriction |
|---|---|---|---|
| 8.8 | Permitted | Permitted | — |
| 10.9 | Permitted (with controlled pickling and bake-out) | Permitted | HE risk at 10.9 is manageable with correct process |
| 12.9 | Prohibited | Permitted | Acid pickling causes unacceptable HE risk at 1200 MPa |
| Stainless A2 / A4 | Not applicable | Not required | Stainless uses its own passivation layer |
This is the clearest decision rule: if the bolt is grade 12.9, Zn-Al flake is the only option. If the bolt is grade 10.9 and the project is onshore at C3–C4, HDG remains a cost-effective and proven choice. See Grade 10.9 vs 12.9 bolts for the full grade selection discussion.
§ 05 Selection guide for wind turbine projects
| Application | Recommended coating | Notes |
|---|---|---|
| Onshore tower flange bolts (C3) | HDG or Zn-Al flake | HDG lower cost; Zn-Al if 12.9 specified |
| Onshore foundation anchor bolts (C3–C4) | HDG | Proven, cost-effective; TD nuts required |
| Offshore tower flange bolts (C5-M) | Zn-Al flake | Salt spray >1000 h; no HE risk; consistent K-factor |
| Offshore foundation bolts (C5-M / Im2) | Zn-Al flake + topcoat | Splash zone may require additional organic topcoat |
| Blade root / pitch bearing (grade 12.9) | Zn-Al flake only | HDG prohibited at 12.9; standard nuts |
| Cable clamps and pipe clamps (tower interior) | 316L stainless or Zn-Al flake | Interior of tower is C2–C3; stainless often specified |
For guidance on how corrosion environment categories (C1–C5/CX) are defined and how to determine which category applies to your project, see Corrosion categories C1–C5/CX and fastener selection.
Evidence and decision boundary
- Direct evidence
- ISO 10683 specifies non-electrolytically applied zinc-flake systems for fasteners. ISO 1461 covers hot-dip coatings on fabricated iron and steel articles but notes that products such as fasteners can have specific standards and different requirements.
- Engineering inference
- Compare coating systems by substrate, fastener strength, hydrogen-embrittlement route, thread allowance, friction control, damage/repair, galvanic interfaces and exposure. Trade names such as Geomet or Dacromet still require the offered system specification.
- Typical or indicative value
- Coating thickness, salt-spray hours and life claims are system-specific screening values, not interchangeable proof of field durability.
Primary sources checked
- ISO 10683:2018 - zinc-flake coating systems for fasteners
- ISO 1461:2022 - hot-dip galvanized coatings on fabricated steel articles
- ISO 14713-2:2019 - design guidance for hot-dip galvanizing
- ISO 9227:2022 - artificial-atmosphere corrosion tests
Related commercial route: Compare wind-turbine clamp systems and RFQ inputs.
For the related release decision, use Zinc-Flake Coating Changes: Must Clamp Bolt Torque Be Retested?.
For the related release decision, use Hydrogen Embrittlement in Offshore Wind Clamp Fasteners: Coating and RFQ Controls.