On high-strength wind turbine bolts you will rarely see hot-dip galvanizing — you will see a thin, grey, matte coating called Geomet or Dacromet. These are zinc-flake systems, and they exist specifically to protect bolts that galvanizing would damage. Understanding why is essential to specifying coated fasteners correctly.
Geomet and Dacromet are zinc-flake composite coatings that provide 720–1000+ hours salt spray resistance at 8–12 µm thickness — significantly better than electrolytic zinc plate at similar thickness. Key advantage over hot-dip galvanising: no acid pickle pre-treatment means no hydrogen embrittlement risk for high-strength bolts (grade 10.9, 12.9). They maintain thread tolerance (important for precision-torqued structural joints) and are the standard for wind turbine nacelle secondary bolts and clamp hardware in C4–C5 environments.
- Best for
- Grade 10.9 and 12.9 nacelle secondary bolts, DIN 3015 clamp hardware, and any high-strength fastener in C4–C5 environment where HDG thread tolerance disruption is a concern
- Not suitable for
- Replacing ISO 10684 HDG specification on main structural bolts without engineering change approval — HDG provides much greater coating thickness than zinc flake
- Selection steps
- 1 — Confirm fastener grade → 2 — Confirm salt spray hours required → 3 — Select Geomet or Dacromet for 720+ h at thin coat → 4 — Specify basecoat plus topcoat system → 5 — Confirm thread tolerance after coating
- RFQ information
- Fastener grade and dimensions, salt spray requirement, thread tolerance class required, quantity, topcoat colour if specified
§ 01 What a zinc-flake coating is
A zinc-flake coating is a thin film of overlapping aluminium and zinc flakes held in an inorganic binder, applied by dip-spin or spray and then cured in an oven at around 300 °C. The result is a 5–10 µm layer that protects in two ways: a barrier effect from the flake structure, and a sacrificial (galvanic) effect from the zinc, which corrodes preferentially to protect the steel beneath.
Crucially, the process is entirely non-electrolytic and acid-free. There is no pickling bath and no electroplating current — and therefore no mechanism to drive hydrogen into the steel. That single fact is why zinc-flake dominates high-strength fastener coating.
§ 02 Geomet vs Dacromet — what's the difference?
Both are brand families of zinc-flake coating from the same lineage (NOF Metal Coatings). The practical distinction is chemistry:
- Dacromet — the original system, which contained hexavalent chromium (Cr6+) in its formulation. Cr6+ is restricted under RoHS/REACH and the EU End-of-Life Vehicle directive, so it is now largely phased out for new work.
- Geomet — the chromium-free (Cr6-free) successor, developed to meet environmental regulations while delivering equal or better corrosion performance. It is the current standard specification for new wind projects.
In day-to-day specification, "Geomet" is what you should normally call out; "Dacromet" persists mostly as legacy terminology or in older drawings. Both are typically combined with a topcoat (e.g. Geomet + Geokote / PLUS) to add lubricity and a controlled, repeatable coefficient of friction for tightening.
§ 03 Why wind turbines use it
Two reasons dominate. First, hydrogen embrittlement: high-strength bolts (class 10.9 and especially 12.9) are vulnerable to delayed brittle fracture if hydrogen is introduced during coating. Hot-dip galvanizing of 12.9 is prohibited for this reason. Zinc-flake's acid-free process sidesteps the risk entirely. The grade-coating interaction is detailed in Grade 10.9 vs 12.9 bolts.
Second, offshore corrosion: marine atmospheres demand high salt-spray endurance in a thin, dimensionally precise coating that will not clog threads. Zinc-flake meets high corrosion categories while keeping the coating thin enough for fine threads — see offshore vs onshore fastener materials.
§ 04 Performance vs hot-dip galvanizing
| Attribute | Zinc flake (Geomet) | Hot-dip galvanizing |
|---|---|---|
| Process | Acid-free, <320 °C | Acid pickle + molten zinc |
| Hydrogen embrittlement risk | None | High on ≥10.9 |
| Coating thickness | 5–10 µm | 45–85 µm |
| Suits fine / small threads | Yes | Bulky; needs thread allowance |
| Salt-spray to red rust | 720–1000+ h | Varies with thickness |
| Use on class 12.9 | Permitted | Prohibited |
For the full HDG-vs-flake decision, including cost and where galvanizing is still the right choice, see hot-dip galvanizing vs Zn-Al flake.
§ 05 When to specify Geomet
Specify a zinc-flake coating when any of the following applies:
- The bolt is class 12.9, where galvanizing is prohibited.
- The fastener is offshore or coastal and needs high salt-spray endurance in a thin coating.
- You need a controlled friction coefficient for accurate torque-to-preload.
- Threads are fine or tolerances tight, where a thick HDG layer would interfere.
When specifying, call out the system and class (e.g. "Geomet 500 + topcoat, Cr6-free"), the required salt-spray hours, and the friction-coefficient class. Pair this with the correct base material and an EN 10204 3.1 certificate, and the coating becomes a controlled, documented part of the joint rather than a generic finish. Galvanic pairing in mixed-metal assemblies is covered in how to prevent galvanic corrosion.
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?.