⚡ Key takeaways
- Dacromet is a trademark. The technology is a non-electrolytic zinc flake coating; the generic equivalents include Geomet, Delta-Protekt and zinc-chromium coating.
- Core advantages: 720–1,000 h of salt spray, ten times electroplated zinc; no hydrogen embrittlement; 300 °C resistance; stable friction.
- Core drawbacks: about 2.5× the cost of zinc plating, electrically insulating so not for grounding, needs curing ovens, and 8–12 μm of coating affects thread fit.
- The chrome-free trend: traditional Dacromet contains hexavalent chromium, banned by RoHS. Chrome-free zinc flake is now standard, with slightly lower salt spray.
- Automotive mandates it: VW, BMW and Tesla specify zinc flake on chassis fasteners, and zinc plating has been eliminated because of hydrogen.
- Thread fit must be recalculated: 8–12 μm on the flanks reduces the effective major diameter, so use a 6g/6h clearance fit or chase the thread after coating.
- Never for conductive joints: battery grounding and EMC shielding cannot use zinc flake.
1. Why automotive dropped zinc plating
2. How the coating works
3. Zinc flake versus zinc plating
4. The chrome-free shift
5. Thread fit and coating thickness
6. Conductivity and grounding
7. Cost and lead time
8. Decision tree and FAQ
1. Why automotive dropped zinc plating
In the mid-2000s European OEMs suffered a run of delayed fractures in chassis bolts. The cause was class 10.9 bolts electroplated without complete hydrogen removal: hydrogen collected at grain boundaries and the bolts snapped after months of road testing with no warning.
The industry response was to move every high-strength chassis bolt to zinc flake and eliminate electroplated zinc:
- The coating is applied by dip and cure, with no electrolysis and therefore no hydrogen.
- Salt spray requirements rose from 240 h to 720 h, which zinc flake meets comfortably.
- Friction is stable, so preload scatter is small.
That is why the bolts under any European car are a matt silver-grey: zinc flake.
2. How the coating works
2.1 Composition
- Zinc flakes 5–10 μm across.
- Aluminium flakes.
- A chromate or silane binder.
- Deionised water and organic solvent.
2.2 Process
- Pre-treatment: degrease, then blast or pickle to remove oxide.
- Dip: immerse the parts and spin off the excess.
- Pre-dry: 80–100 °C to evaporate water.
- Cure: 300 °C for 20–30 minutes to crosslink the binder.
- Cool.
2.3 Why it protects
- Barrier: overlapping flakes stack like tiles and keep air out.
- Sacrificial: zinc corrodes before iron.
- Passivation: the binder slows zinc consumption.
3. Zinc flake versus zinc plating
| Dimension | Zinc flake | Electroplated zinc |
|---|---|---|
| Salt spray | 720–1,000 h | 24–96 h |
| Hydrogen | None | Yes, requires baking |
| Heat resistance | 300 °C | 200 °C, zinc melts |
| Friction coefficient | 0.08–0.12, stable | 0.10–0.16, wide scatter |
| Thickness | 8–12 μm | 5–8 μm |
| Conductivity | Insulating | Conductive |
| Appearance | Matt silver-grey | Blue-white, yellow, black |
| Relative cost | 2.5× | 1× |
| Lead time | Hours including cure | Tens of minutes |
4. The chrome-free shift
Traditional Dacromet contains hexavalent chromium, a carcinogen restricted by the EU RoHS directive. The mainstream is now chrome-free zinc flake:
- Silane or titanate binders replace chromate.
- Salt spray falls slightly, to 500–720 h.
- Complies with RoHS and ELV.
Common chrome-free brands are Geomet, Delta-Protekt and Zintek; domestic zinc-aluminium coatings largely follow Geomet.
5. Thread fit and coating thickness
An 8–12 μm coating changes the actual thread dimensions, and this is where engineers most often trip.
5.1 The problem
An M10 external thread has a major diameter of 9.97–10.00 mm. Add 10 μm of coating and it becomes 10.02 mm, out of tolerance, and it will not enter a standard M10 nut.
5.2 Solutions
- Specify a 6g/6h clearance fit: g for the external thread, H for the internal.
- Chase or roll the thread after coating.
- Mask the thread and coat the head only.
6. Conductivity and grounding
The coating is insulating because of the chromate or silane binder. It cannot be used for:
- Battery grounding bolts, where contact resistance matters.
- EMC shielding, which needs continuous metal contact.
- Electrical bonding points.
Those joints use conductive zinc plating or passivated stainless.
7. Cost and lead time
| Item | Zinc plating | Zinc flake |
|---|---|---|
| Unit price (M6 bolt) | 1× | 2.5× |
| Minimum batch | 1 kg | 50 kg, one oven load |
| Lead time | 1–2 days | 3–5 days |
| Suits small batches | Yes | No, needs volume to amortise |
8. Decision tree and FAQ
Step 1 — Above class 10.9?
Yes → zinc flake, because there is no hydrogen. No → zinc plating is acceptable.
Step 2 — Outdoor, coastal or chassis?
Yes → zinc flake at 720 h. No → zinc plating at 240 h is enough.
Step 3 — Conductive grounding?
Yes → not zinc flake; use zinc plating or stainless.
Step 4 — Tight thread fit?
Yes → 6g clearance fit or post-coating machining.
Step 5 — Enough volume?
Under 50 kg → zinc plating is cheaper. Above 50 kg → zinc flake.
FAQ
Q. Why is zinc flake silver-grey? It is the colour of stacked zinc and aluminium flakes, and it cannot be dyed blue or yellow like plating.
Q. Does the coating flake off? Not in normal service, but repeated assembly wears the thread locally. Recoat after heavy rework.
Q. Can it be used on aluminium? Not recommended: zinc against aluminium is a galvanic couple. Anodise aluminium instead.
Q. Is Geomet the same as Dacromet? Geomet is the chrome-free version of the same family, under a different brand.
Q. How is 1,000 h measured? Neutral salt spray to GB/T 10125, time to red rust. Real outdoor exposure also includes UV.
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