Why Automakers Choose Dacromet Over Zinc Plating

The short answer: an automotive fastener faces four demands at once that electroplated zinc cannot meet together: high-strength steel that must never absorb process hydrogen, years of road-salt exposure, automated torque-controlled assembly that needs a stable friction window, and substance rules that ended the original chromate chemistry. Zinc-flake coatings—Dacromet and its chrome-free successors—answer all four in one thin layer. That is why OEM specifications, not marketing, moved the industry to them.

1. Four constraints meet at one chassis bolt

A general-industrial bracket bolt may face one severe requirement. A chassis, brake or seat-belt bolt faces several simultaneously:

Constraint	Where it comes from	What it rules out
High strength, class 10.9/12.9	Compact joints and vehicle weight targets	Coating routes that introduce hydrogen
Road salt and wet–dry cycling	Winter de-icing and coastal climates	Thin decorative zinc builds
Automated torque tightening	Assembly-line cycle times and quality targets	Uncontrolled friction scatter
Restricted-substance rules	EU ELV directive and OEM material standards	Hexavalent-chromium coatings

Zinc plating remains the sensible, economical answer for enormous volumes of general hardware. The automotive question is narrower: which finish survives when all four constraints land on the same part?

2. Hydrogen: the plating process itself is the problem

Property classes 10.9 and 12.9 are routine in chassis, suspension and safety-related joints, and steel at these strength levels is sensitive to hydrogen embrittlement. Acid cleaning and electrodeposition can both charge the steel with atomic hydrogen; under sustained tensile stress the result can be a delayed brittle fracture hours after installation. ISO 4042 sets out controls for electroplated coatings, including post-plating baking—but baking is a process control, never proof that a poorly chosen route is safe.^[1]

Zinc-flake systems are non-electrolytic: the zinc and aluminium flakes are applied by dip-spin or spray and then cured. There is no deposition current and no plating bath, so the coating step itself generates no hydrogen. ISO 10683 notes that these systems are used especially for fasteners with tensile strengths of 1,000 MPa and above for exactly this reason.^[2] The caveat that survives is pretreatment: acid pickling ahead of coating reintroduces the risk, so a controlled zinc-flake process for high-strength parts starts with mechanical cleaning, not acid.

3. Salt spray at road-salt level

In a zinc-flake film, overlapping zinc and aluminium flakes form a layered, conductive barrier in which the zinc also protects sacrificially. Because protection comes from the flake structure and coating mass rather than a thin deposited layer, a film of roughly 5–12 µm reaches corrosion performance that bright zinc plating does not approach at comparable thickness. A widely used system such as GEOMET 321 is rated above 720 hours of neutral salt spray at its standard coating mass, and multi-coat systems are specified beyond 1,000 hours.^[3]

Salt-spray hours are a consistency check, not a service-life forecast. ISO 9227 defines the test method and explicitly does not assign exposure times to products or rank materials for field life. A figure such as "720 hours" is only meaningful with the method, coating mass and evaluation criterion (white rust vs red rust) stated.^[4]

4. A friction window, not just a finish

On an automated line, most of the tightening torque is consumed by friction in the threads and under the head; scatter in the coefficient of friction becomes scatter in clamp load. OEMs therefore specify a total coefficient-of-friction window—commonly within a band around 0.08–0.18, with many targets near 0.12—and verify it with torque/clamp-force testing per ISO 16047. In the German supply chain the requirement is typically expressed through VDA 235-203.^[5–7]

Zinc-flake systems are built for this. The GEOMET 321 family, combined with lubricated topcoats, publishes adjustable friction coefficients across roughly 0.06–0.20 measured per ISO 16047, so a finished lot can be tuned to the OEM window.^[3] Plated zinc can be lubricated too, but the sealer-plus-lubricant stack is harder to hold in a narrow window lot after lot—which is why a coating substitution offered on equal salt-spray hours alone should be rejected until torque/clamp behaviour is proven on production parts.

5. Thin build, real threads and engine-bay heat

Fastener threads live on tolerances. A zinc-flake build of a few micrometres per coat preserves nut fit and recess engagement without re-tapping, provided the specification covers post-coating gauging and limits pooling in recesses on dip-spin parts.^[2]

Heat is the quieter advantage. Zinc-flake systems are commonly rated by their suppliers for continuous service around 250–300 °C, while the conversion layers and sealers that give plated zinc its corrosion performance degrade well below that range. Near-engine and exhaust-adjacent fixings are therefore natural zinc-flake territory.

6. The Dacromet name trap

The original Dacromet chemistry relied on hexavalent chromium. The EU End-of-Life Vehicles Directive 2000/53/EC restricted hexavalent chromium in vehicles, and the exemption that had covered corrosion-preventive coatings expired for new vehicles on 1 July 2007. OEMs and their coaters migrated to chrome-free systems—GEOMET 321, DELTA-PROTEKT and others—so "Dacromet" on a modern automotive drawing usually means "a zinc-flake system meeting my specification," not the original product.^[8]

The practical consequence: "Dacromet or equivalent" is not a specification. ISO 10683 covers systems with or without hexavalent chromium, so citing the standard alone does not establish a chrome-free finish; ASTM F3393-24, by contrast, applies specifically to zinc-flake systems supplied without hexavalent chromium.^{[2] [9]} If the part goes into a vehicle, state the restricted-substance requirement and demand the material documentation that supports it.

7. Where it goes on a vehicle

The pattern is consistent: high-strength, exposed to the environment, frequently safety-related. Chassis and subframe bolts, suspension links, brake hardware, seat-belt anchorages, steering connections and wheel-area fixings are the classic zinc-flake positions. Interior trim screws and general hardware stay on plated zinc, where its cost and appearance win.

8. What to put on the RFQ

Coating standard and edition with the required designation (for example an ISO 10683 class), or the OEM material-specification number
Restricted-substance statement: chrome-free, with documentation covering both basecoat and topcoat
Corrosion test method, duration and evaluation criterion—for example ISO 9227 neutral salt spray, hours to red rust
Total coefficient-of-friction window and test method: ISO 16047, with the VDA 235-203 class where the customer references one
Pretreatment route for property class 10.9 and above: mechanical cleaning, no acid pickling
Post-coating thread gauging, recess-fill limits and coating mass
Approved applicator and a lot-level test certificate

Choosing a finish outside automotive? Read Dacromet and Zinc-Flake Coatings for High-Strength Fasteners for the full process comparison, or Bolt Grades 8.8, 10.9 and 12.9 for the strength side of the decision.

References

From the Catalog

Dacromet Grade 12.9 Hexagon Socket Cap Screws M16/M18/M20/M27 Alloy Steel Bolts DIN 912 Gb70.1 Compliant View Product

Dacromet Coated Button Head Socket Cap Screw Alloy Steel 35CrMo UNC ASME B18.3 View Product

High Strength 35CrMo Alloy Steel Wedge Lock Washer Dacromet DIN25201 Double Fold Self-Locking Design View Product

Why Automakers Choose Dacromet Over Zinc Plating: Hydrogen, Salt Spray and Friction