Dacromet Bolts vs Hot-Dip Galvanized vs Zinc-Plated: Which Coating Should You Specify?

Dacromet bolts are carbon or alloy steel fasteners protected by a zinc flake coating, a non-electrolytic zinc-aluminium film that delivers higher corrosion resistance than conventional zinc plating without the hydrogen embrittlement risk that threatens hot-dip galvanized high-strength fasteners. For engineers specifying strength class 10.9 or 12.9 hardware in corrosive or safety-critical service, the coating choice is often more consequential than the steel grade. This review compares Dacromet (zinc flake), hot-dip galvanized, and zinc-plated bolts across salt spray performance, thread fit, and procurement criteria.

Dacromet (zinc flake) coating applies a 5–15 μm zinc-aluminium flake film that provides roughly 240–1,500 hours of salt spray protection and generates no hydrogen during processing, making it the safest finish for high-strength bolts. Hot-dip galvanized bolts build a much thicker 80–200 μm zinc layer but carry a hydrogen embrittlement risk on grade 10.9/12.9 fasteners and usually require thread re-tapping. Zinc-plated bolts are economical to apply but typically survive only 96–200 hours in salt spray and are unsuitable for high-strength parts.

Quick Takeaways

• Dacromet is a trademarked zinc flake coating; the generic term is "non-electrolytically applied zinc flake coating" (ISO 10683).
• Zinc flake coatings avoid hydrogen embrittlement because processing is non-electrolytic and uses no acid pickling.
• Typical coating thickness is 8–12 μm, preserving ISO 965 thread tolerances without re-tapping.
• Salt spray resistance commonly ranges from 240 to 1,500 hours, far beyond zinc plating's 96–200 hours.
• Chrome-free (Cr(VI)-free) variants are now the default, after Cr(VI) coatings were withdrawn from the European market in 2017.
• Hot-dip galvanizing remains the better choice mainly for heavy, low-strength structural hardware in aggressive outdoor exposure.

What Is a Dacromet Bolt? (Zinc Flake Coating, Explained)

A Dacromet bolt is a steel fastener whose surface is protected by a zinc flake coating rather than by electroplating or hot-dip galvanizing. According to the reference definition of zinc flake coating, these coatings consist of a mixture of zinc and aluminium flakes bonded together by an inorganic matrix and applied non-electrolytically. The result is a thin, uniform, silver-grey film that protects the underlying steel through cathodic (sacrificial) action: the less-noble zinc corrodes preferentially, shielding the steel substrate beneath it.

Zinc flake systems come in three chemistry groups: chromium(VI)-containing, solvent-based chromium-free, and water-based chromium-free. Cr(VI) coatings offer strong protection at minimal thickness but are carcinogenic and were withdrawn from the European market in June 2017, so modern specifications are chromium-free by default. The water-based, chrome-free systems now used across the automotive industry deliver the same sacrificial protection through zinc and aluminium flakes without the environmental burden.

Zinc Flake Coating: Composition and How It Works

The coating starts as a liquid dispersion of microscopic zinc and aluminium flakes suspended in an inorganic binder. When the fastener is dipped or sprayed and then cured in an oven, the binder hardens and the flakes pack into an overlapping, lamellar barrier. This structure delivers corrosion protection through two mechanisms at once: a physical barrier that slows moisture and oxygen ingress, and sacrificial (cathodic) protection in which the zinc and aluminium flakes corrode in place of the steel. Because the flakes overlap like scales, a film of only 8–12 μm can outperform a much thicker electroplated zinc layer, and unlike paint, the coating resists sub-surface corrosion creep.

Dacromet vs Generic Zinc Flake: The Trademark Question

"Dacromet" is a registered brand name, not a technical specification. The technology it represents is called, generically, a non-electrolytically applied zinc flake coating, standardized as ISO 10683. Other brand names in the same family include Geomet, a chrome-free zinc flake system. For procurement, this distinction matters: specifying "Dacromet" by brand can lock a project into a single source, whereas specifying "zinc flake coating to ISO 10683, Cr(VI)-free" keeps sourcing open while guaranteeing an equivalent, standards-defined finish. AOZHAN applies zinc flake coating to fastener blanks in-house and validates every batch against the ISO 10683 performance classes rather than against a single brand formula.

Dacromet vs Hot-Dip Galvanized vs Zinc-Plated: Head-to-Head Comparison

The three finishes are often treated as interchangeable, but they differ fundamentally in process, thickness, and failure mode. Whether the fasteners in question are hexagon head bolts, flange bolts, or studs, the coating logic is identical. The table below compares the three finishes across the dimensions a buyer actually cares about.

For further detail on how these surface treatments compare in day-to-day selection, see AOZHAN's guide to bolt surface treatment comparison.

Coating Thickness and Thread Fit

Coating thickness is where the three finishes diverge most dramatically. Zinc flake coatings sit at 5–15 μm, thin enough that a standard metric thread manufactured to ISO 965 tolerances still mates correctly after coating. Hot-dip galvanizing, by contrast, deposits 80–200 μm, which can completely consume the allowance on a standard nut-bolt assembly; galvanized high-strength bolts are typically tapped oversize or re-tapped after coating to restore fit. Electroplated zinc is thin and relatively uniform, but its protection is shallow. When thread fit and torque-tension behavior matter — as they do on any controlled-torque assembly — the thin, uniform zinc flake layer is the more predictable choice.

Corrosion Resistance and Salt Spray Hours

Salt spray testing, run in accordance with ISO 9227 (equivalent to ASTM B117), is the industry's standard proxy for coating durability. The salt spray test exposes samples to a continuous neutral fog and records hours to first red rust. Typical results are revealing:

The exact zinc flake figure depends on the coating class specified — thinner base coats fall near the low end, while top-coated, multi-layer systems reach four figures. This is the single most persuasive reason to choose zinc flake on a fastener that must survive years outdoors at a small cross-section.

Why Dacromet Bolts Avoid Hydrogen Embrittlement (and Galvanized Bolts Don't)

Hydrogen embrittlement is the hidden failure mode that separates zinc flake from both galvanizing processes. High-strength steel — bolts of strength class 10.9 and 12.9, or parts above 1,000 N/mm² tensile strength or 320 HV — is susceptible to hydrogen embrittlement: atomic hydrogen diffuses into the steel lattice and causes delayed, brittle fracture, sometimes days after installation.

The hydrogen comes from the coating process itself. Acid pickling and electrolytic plating both generate atomic hydrogen at the steel surface, which then migrates into the metal. Zinc flake coating eliminates the problem by design. Pre-treatment uses alkaline cleaning and mechanical shot blasting instead of acid, and the coating is applied non-electrolytically, so no atomic hydrogen is produced at any stage. This is why the automotive and wind industries specify zinc flake finishes for critical high-strength fasteners: the coating can be applied to a 12.9 bolt without degrading its load-carrying capacity.

Hydrogen-induced failures are especially dangerous because they are delayed. A bolt can be torqued to specification, pass visual inspection, and then fracture hours or days later under a load well below its rated capacity. For this reason, quality programs that use galvanized high-strength fasteners must add post-plating bake-out steps to drive off absorbed hydrogen, an extra operation that zinc flake coating makes unnecessary. Specifying zinc flake on class 10.9 and 12.9 hardware removes the embrittlement variable from the joint design entirely, which is why automotive OEMs and wind turbine builders treat it as the default finish for safety-critical bolted joints.

How Dacromet Bolts Are Coated: Dip-Spin vs Spray (Factory Perspective)

The performance of a zinc flake coating depends as much on process control as on the chemistry. At AOZHAN, coating begins with alkaline degreasing to remove forming lubricants, followed by fine shot blasting to create a clean, lightly textured surface for adhesion. No acid is used at any point, which preserves the hydrogen-embrittlement-free property of the finished bolt.

Bulk fasteners are then coated by dip-spin: a basket of bolts is dipped into the prepared zinc flake dispersion and spun to sling off excess material, leaving a thin, uniform film. Larger or awkward parts are racked and spray-coated instead. Coated parts are cured in a controlled oven at curing temperatures around 200 °C, 240 °C, or 320 °C depending on the coating class. AOZHAN technicians then check coating thickness with a gauge and run periodic salt spray sampling against the specified ISO 10683 class. This step-by-step control — cleaning, blasting, coating, curing, verification — is documented in AOZHAN's production and inspection workflow, which is why the finished coating is consistent from first bolt to last.

Curing temperature is not a trivial detail. Each coating class has a defined cure window, and drifting outside it changes both adhesion and corrosion performance. AOZHAN runs coated batches through a controlled oven and verifies the result in two ways: a thickness check on a sampling plan tied to batch size, and periodic neutral salt spray coupons that confirm the coating still meets its class hours-to-red-rust. This dual verification, thickness plus performance, is what separates a repeatable zinc flake finish from a cosmetic one that looks right but fails early in service.

Standards to Specify: ISO 10683 vs ASTM F1136

Putting the right standard on the drawing is what makes a zinc flake requirement enforceable. Two standards govern the coating, and a third governs the test.

ISO 10683:2018 is the primary reference for threaded fasteners and defines the coating classes by corrosion-resistance performance rather than brand. ASTM F1136/F1136M plays the equivalent role in the North American market. When an application must be Cr(VI)-free — which is effectively mandatory in Europe since 2017 and increasingly the default worldwide — the specification should state "zinc flake coating, Cr(VI)-free" explicitly, because both standards permit both chrome-containing and chrome-free chemistries.

Where Dacromet Bolts Are Used: Automotive, Wind Power, Construction

Zinc flake coatings began in the automotive industry, where the combination of high strength, thin film, and hydrogen-embrittlement freedom solved a problem that neither electroplating nor galvanizing could. Today the finish spans wind power, truck and trailer manufacturing, electrical equipment, and construction.

Wind turbines are a representative case: tower and nacelle bolted joints use high-strength fasteners that must survive decades of coastal humidity and salt-laden air without fracture. A zinc flake finish provides the corrosion resistance the environment demands while preserving the mechanical integrity of a grade 10.9 or 12.9 bolt. AOZHAN has supplied zinc-flake-coated high-strength fasteners for industrial and outdoor structural applications where salt spray resistance and reliable torque-tension behavior were both specified on the drawing. You can review representative installations in AOZHAN's customer cases.

The same logic extends across industries. In automotive assembly, zinc flake fasteners carry brake, chassis, and powertrain hardware where a fractured bolt is unacceptable and corrosion from road salt is constant. In electrical and plant construction, the finish protects bolted connections that must remain serviceable outdoors for decades. In construction, zinc flake is chosen where thin, uniform protection and precise torque behavior matter more than the heavy sacrificial layer of hot-dip galvanizing. Across all of these, the common requirement is identical: corrosion resistance without sacrificing the mechanical integrity of a high-strength fastener.

How to Choose the Right Dacromet Bolt for Your Project

Selection should follow the environment first, the mechanical requirement second, and the coating specification third. Working in that order prevents the common mistake of choosing a finish before the load case is known.

A 4-Step Selection Path

1. Define the environment. Map exposure to moisture, salt, chemicals, and temperature cycling. Indoor, dry assemblies may not need zinc flake at all; coastal, de-icing-salt, or humid-service assemblies usually do.
2. Fix the strength class. If the joint requires 10.9 or 12.9 fasteners, zinc flake is effectively the default safe finish, since electroplating and galvanizing both introduce hydrogen embrittlement risk.
3. State the coating class. Specify "zinc flake coating to ISO 10683, Cr(VI)-free" and name the required salt spray hours (for example, 480 hours to red rust) rather than naming a brand.
4. Confirm thread and torque requirements. Verify the coating thickness stays within the ISO 965 allowance for your thread size and that the specified coating supports the required coefficient of friction.

Pre-Order Inspection Checklist

• Coating type and class (ISO 10683 or ASTM F1136/F1136M) stated on the purchase order
• Cr(VI)-free requirement written explicitly where compliance matters
• Required salt spray hours to red rust named as a pass/fail criterion
• Coating thickness range agreed (typically 8–12 μm for threaded fasteners)
• Batch traceability and a coating test report requested with the shipment
• Thread fit verified against ISO 965 after coating

For a quotation or to confirm a coating class against your project spec, contact AOZHAN's engineering team.

Dacromet Bolts FAQ

Is Dacromet the same as galvanizing?

No. Galvanizing refers to zinc applied by hot-dipping or electroplating. Dacromet is a non-electrolytic zinc flake coating with different thickness, process, and hydrogen-embrittlement behavior.

Can Dacromet coating be used on grade 10.9 and 12.9 bolts?

Yes — this is its principal advantage. Because the process generates no hydrogen and uses no acid pickling, it can be applied to high-strength fasteners without embrittlement risk.

How many hours of salt spray does Dacromet coating provide?

Typically 240–1,500 hours depending on the coating class and thickness, versus roughly 96–200 hours for electroplated zinc.

What is the difference between Dacromet and Geomet?

Both are zinc flake coating systems. Dacromet is the long-established brand; Geomet is a chrome-free zinc flake brand in the same technology family. Specifying "zinc flake coating to ISO 10683" avoids brand lock-in.

Is Dacromet coating environmentally compliant?

Modern zinc flake systems are chrome-free (Cr(VI)-free). Cr(VI)-containing coatings were withdrawn from the European market in 2017, and chrome-free variants are now the standard specification.

What coating thickness does a Dacromet bolt have?

Threaded fasteners typically carry 8–12 μm, within a 5–15 μm range, thin enough to preserve standard ISO 965 thread tolerances without re-tapping.