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.
§ 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.