Technically reviewed by Vincent, Founder at HEXVARO
Last reviewed
Side-by-side comparison
| Factor | Hot-dip galvanized (HDG) | Zinc flake (Dacromet / Geomet type) |
|---|---|---|
| Process | Dipped in molten zinc (about 450 °C), then spun to clear threads | Zinc and aluminium flakes in a binder, applied by dip-spin or spray and oven-cured |
| Typical thickness | Mean ≥ 50 µm on threaded parts above 6 mm | About 5–15 µm |
| Thread fit | Nuts tapped oversize, or bolt threads cut undersize | Standard ISO 965 threads; no oversizing |
| Hydrogen embrittlement | Acid pickling before galvanizing is a risk for high-hardness parts | No hydrogen generated by the coating process; cleaning is alkaline and by blasting |
| Salt spray (ISO 9227) | Not normally specified; performance follows zinc thickness | About 240 to 1,500 hours, depending on system and thickness |
| Friction control | Variable; galvanized nuts are usually waxed | Integrated lubricant or top coat can set a defined friction coefficient |
| Appearance | Dull grey, rougher surface | Smooth silver or black |
| Main standards | ISO 10684, ASTM F2329 | ISO 10683, ASTM F1136 |
| Typical uses | Structural steel, anchor bolts, towers, poles, guardrails | Automotive, machinery, wind and solar, high-strength bolts |
Why the difference in thickness matters
Zinc protects steel by corroding slowly in its place, so the amount of zinc largely sets outdoor life. A 50 µm galvanized coating holds several times more zinc than a 10 µm flake coating. In a heavily corrosive coastal or industrial site, that extra zinc usually means longer life before first maintenance.
Zinc flake makes up for its thinness with aluminium flakes and a sealing binder, which slow the zinc's own corrosion. That is why a 10 µm flake coat can pass hundreds of hours of salt spray where an 8 µm electroplated zinc coat may show red rust within 100–200 hours. Salt spray is a quality-control test, though, not a direct prediction of years outdoors.
High-strength bolts: the deciding factor
- Property class 12.9 and ASTM F3125 Grade A490 bolts are not hot-dip galvanized. Zinc flake is the standard corrosion-protection choice for them.
- Property class 10.9 can be hot-dip galvanized under ISO 10684, but the risk of hydrogen embrittlement and the thread fit need to be managed. Many engineers specify zinc flake for 10.9 to avoid the question.
- Electroplated zinc on 10.9 and 12.9 needs baking after plating and still carries some residual risk; zinc flake avoids the electrolytic step entirely.
- Class 8.8, ASTM A325 and F1554 Grade 36 / 55 are routinely hot-dip galvanized.
When to choose hot-dip galvanizing
Choose HDG for structural bolts (8.8, A325), anchor bolts, threaded rod and outdoor steelwork where a thick barrier and long service life matter more than appearance or tight thread fit. Order bolts, nuts and washers galvanized as a set so the oversize-tapped nuts match, and specify the galvanizing standard.
When to choose zinc flake
Choose zinc flake for high-strength bolts (10.9, 12.9, A490), small and fine threads, socket screws, and parts where a controlled friction coefficient or a clean appearance is needed. Zinc flake is also the usual choice when standard nuts must fit without overtapping, for example on parts assembled with automated tools.
- Cure temperatures of roughly 200–320 °C apply depending on the system; check that every part of the assembly tolerates it.
- Modern zinc flake systems are chromium(VI)-free. Specify Cr(VI)-free if your market requires RoHS or REACH compliance.
- Thin coatings scratch more easily in transit; ask for suitable packaging for bulk shipments.
Specify the coating clearly
- HDG: ISO 10684 or ASTM F2329, coating thickness, and how thread fit is achieved (oversize-tapped nuts).
- Zinc flake: ISO 10683 or ASTM F1136, system name or generic description, colour (silver or black) and top coat.
- Required salt-spray hours (neutral salt spray, ISO 9227) and when they are measured.
- Friction coefficient range for torque-controlled joints, and whether nuts and bolts are coated as a set.
- Inspection: coating thickness method and report.
Frequently asked questions
Can 10.9 bolts be hot-dip galvanized?
Yes under ISO 10684, with controls for hydrogen embrittlement and thread fit, but many engineers prefer zinc flake for 10.9. 12.9 and A490 bolts should not be hot-dip galvanized.
Which lasts longer outdoors?
Hot-dip galvanizing's thicker zinc layer generally gives longer life in harsh outdoor and coastal exposure. Zinc flake performs well in salt-spray tests for its thickness and is preferred where thread fit matters.
How thick is zinc flake coating?
Typically about 5–15 µm, compared with 50 µm or more for hot-dip galvanizing on bolts. That is why zinc-flake-coated bolts fit standard nuts.
Is Dacromet chrome-free?
Modern zinc flake systems are available chromium(VI)-free, and Cr(VI) versions are no longer used in Europe. Specify a Cr(VI)-free system if your market requires RoHS or REACH compliance.
Do zinc flake bolts need oversize nuts?
No. Zinc flake is thin enough to keep standard ISO 965 thread tolerances, so bolts and nuts assemble normally.
What is the MOQ for coated fasteners?
Reference MOQ is 500 pcs per specification. Stock sizes typically ship in 10–15 days and made-to-order sizes in 25–30 days, subject to quotation. We reply to every RFQ within 2 working hours.