Every year, high-strength bolts fail in service not because the steel was wrong, but because the coating process quietly made them brittle. Dacromet — the best-known name in zinc-flake coating — exists largely to solve that problem. This guide explains what the coating actually is, what the salt-spray numbers mean, where it beats zinc plating and hot-dip galvanizing, and what to put in your specification when you order.
What Dacromet actually is
Dacromet is a brand of zinc-flake coating owned by NOF Metal Coatings; Geomet (also NOF) and Delta (Dörken) are sibling systems your supplier may offer as equivalents. Despite the brand names, the technology is one family: a water-based dispersion of microscopic zinc and aluminium flakes, applied to bulk fasteners by dip-spin (dip, then centrifuge off the excess) or by spraying for large parts, then cured in an oven at roughly 180–300 °C.
Each coat is only about 4 μm thick, so a typical fastener system is two coats plus an optional topcoat — 8–12 μm in total, about a fifth of the thickness of hot-dip galvanizing. That thinness matters: threads stay within tolerance, and go/no-go gauges still pass on standard M6 and 1/4-inch threads.
How a 10-micron film stops rust
Three mechanisms work together:
- Barrier. The flakes overlap like roof shingles, forcing corrosive electrolyte through a long, tortuous path before it reaches steel.
- Sacrificial (cathodic) action. Zinc is more active than iron, so at a scratch or cut edge the zinc corrodes preferentially, protecting the substrate — the same principle as galvanizing, delivered in a far thinner layer.
- Passivation. The binder chemistry passivates the flake surfaces, slowing the zinc's own consumption and extending service life per micron.
Modern formulations (Dacromet LC, Geomet) are free of hexavalent chromium, which keeps them compliant with RoHS, ELV and REACH — one reason the automotive industry adopted them so broadly.
The real reason engineers specify it: no hydrogen embrittlement
Electroplating has a hidden cost for hard steel. Acid pickling and the plating bath itself generate atomic hydrogen, which diffuses into the steel and, in parts harder than about 390 HV (property class 10.9 and up, plus spring washers and any case-hardened part), can cause delayed brittle fracture days or weeks after installation — under a load the bolt was rated to carry.
De-embrittlement baking within a few hours of plating mitigates the risk, but never eliminates it, and it depends on the plater actually doing it. Zinc-flake coating sidesteps the problem entirely: no acid, no electrolysis, no hydrogen. For Grade 10.9 and 12.9 socket screws, and for spring and wedge-lock washers that live at extreme hardness, zinc-flake is not the premium option — it is the correct one.
Rule of thumb for buyers: if the part is property class 10.9 or 12.9, or a spring component, and the quote says "zinc plated" — stop and ask for the de-embrittlement baking record, or change the finish to zinc-flake. The few cents saved on electroplating are not worth a delayed fracture in a structural joint.
The numbers that matter
| Property | Zinc-flake (Dacromet/Geomet) | Electro zinc | Hot-dip galvanizing |
|---|---|---|---|
| Salt spray to red rust (ASTM B117 / ISO 9227) | 720–1,200+ h (basecoat + topcoat) | 96–240 h with passivate | 500–1,000 h (coating-life driven) |
| Total thickness | 8–12 μm | 5–15 μm | 40–85 μm |
| Hydrogen embrittlement risk | None (non-electrolytic) | Present above ~390 HV; baking required | Low (thermal process) |
| Friction control | Topcoat sets μ ≈ 0.09–0.14, tight window | Variable; wax or seal adds scatter | Poor; high and uneven |
| Thread fit on fine/small threads | Good | Good | Often needs overtapped nuts |
| Continuous temperature | ≈ 250–300 °C | ≈ 120 °C (passivate degrades) | ≈ 200 °C |
| Appearance | Silver-grey matte | Bright / yellow / black options | Dull grey, can be rough |
Salt-spray hours are a comparative lab metric, not a field-life guarantee — but the ranking between coatings translates reliably to real exposure.
The friction window is a bigger deal than it looks
Torque-controlled tightening converts wrench torque into clamp load through thread friction. If the friction coefficient scatters from 0.08 to 0.20 across a batch, identical torque produces wildly different preload — some bolts under-clamped, some yielded. Zinc-flake systems accept topcoats (sealers and lubricants such as Delta Seal or Delta Lube) that pin the friction coefficient into a narrow window, typically μ 0.09–0.14. This is why automotive OEM torque specs so often name a zinc-flake finish outright: the coating is part of the joint design, not just corrosion protection.
Where Dacromet earns its price
- Automotive chassis and safety parts — high-strength, torque-critical, road-salt exposed.
- Wind and solar structures — towers, trackers and PV mounting live outdoors for 20+ years with minimal maintenance.
- Rail and infrastructure hardware — vibration, weather, and no easy access for replacement.
- High-tensile and spring parts anywhere — Grade 10.9/12.9 screws, spring washers, wedge-lock washers, where electroplating is off the table.
Its honest limitations
- Cost. Clearly above electro zinc, roughly comparable to mechanical galvanizing, below premium fluoropolymer systems.
- One look. Silver-grey matte is the only color; decorative bright finishes need other processes.
- Field damage. A coating scratched through to steel cannot be touched up invisibly; severe handling damage means replacement, not repair.
- Electrical grounding. The resin-bound film is less conductive than bare electroplated zinc — check before using coated fasteners as grounding paths.
What to write in your specification
- The system and standard: "zinc-flake coating per ISO 10683 (or ASTM F1136), Dacromet/Geomet or approved equivalent."
- Salt-spray requirement: e.g. "≥ 720 h to red rust per ISO 9227."
- Total coating thickness and the thread gauge requirement.
- Friction coefficient window if the joint is torque-controlled.
- Test evidence per batch: coating thickness report, salt-spray sample results, and — for any plated alternative — de-embrittlement records.
Our Dacromet-coated fasteners cover bolts, socket screws, nuts and washers in carbon and alloy steel. If your drawing calls a different coating on a 10.9+ part, send it over — we will flag the embrittlement risk before production, not after.