Dacromet Coating Guide for High-Strength Fasteners

The short answer: zinc-flake coating is usually the safer starting point for high-strength steel fasteners when corrosion resistance and hydrogen-embrittlement control are both important. Electroplated zinc is thinner and often more economical, with a brighter cosmetic finish. The correct choice still depends on the specified coating system, pretreatment, topcoat, friction window and test requirement—not on the words “Dacromet” or “zinc plated” alone.

Dacromet is a brand; zinc flake is the coating family

Dacromet is a trade name associated with a particular zinc-flake technology. Zinc-flake coating is the generic category: a dispersion of zinc flakes, sometimes with aluminium, is applied to steel and cured to form a conductive, layered barrier. GEOMET and DELTA-PROTEKT are other commercial systems, but they are not automatically interchangeable. Each has its own basecoat, topcoat, lubricant and approved performance classes.

This distinction matters on a drawing. “Dacromet or equivalent” leaves unanswered questions about chromium chemistry, coefficient of friction, corrosion hours, sealer, colour and approved applicator. Specify the standard and required performance, then name an approved system if the programme requires one.

How zinc flake differs from electroplated zinc

Feature	Zinc-flake coating	Electroplated zinc
Application	Non-electrolytic dip-spin, spray or rack application followed by curing	Electrochemical deposition from a plating bath
Typical build	Multiple basecoat/topcoat layers; thickness depends on the specified class and geometry	Generally thinner and more uniform on simple exposed surfaces
Hydrogen concern	No hydrogen generated by electrodeposition; pretreatment still needs control	Cleaning and electroplating can introduce hydrogen; risk controls are required
Corrosion performance	High performance is achievable with the correct system and coating mass	Depends on zinc or zinc-alloy deposit, conversion layer and sealer
Torque behaviour	Topcoat or integral lubricant can be engineered to a friction window	Sealant and lubricant strongly affect friction; process control is still essential
Appearance	Usually matte silver-grey or black	Often bright clear, blue, yellow, black or other converted finishes
Common use	Automotive chassis, wind, heavy equipment and high-strength fasteners	General industrial hardware and parts needing a thin decorative-protective finish

Why high-strength bolts need a hydrogen review

Hydrogen introduced during acid cleaning or electroplating can diffuse into susceptible high-strength steel. Under sustained tensile stress, delayed brittle failure may occur hours after installation. Risk generally becomes more serious as strength and hardness rise, which is why the coating route for 10.9 and 12.9 fasteners deserves explicit review.

ISO 10683 notes that non-electrolytic zinc-flake systems are especially used for fasteners with tensile strength of 1,000 MPa or above to avoid internal hydrogen embrittlement. That does not mean every zinc-flake process is automatically hydrogen-free. Acid pickling, unsuitable phosphate treatment or prior manufacturing steps can still introduce hydrogen. Mechanical cleaning or another approved low-risk pretreatment has to be part of the controlled process.^[1]

For electroplating, ISO 4042 specifies requirements and recommendations intended to minimise embrittlement risk. Post-plating baking can reduce risk when properly timed and controlled, but it cannot be used as proof that a poorly chosen process is safe.^[2]

Chromium-free claims need precise wording

Modern systems are often promoted as chromium(VI)-free. ASTM F3393-24 specifically covers zinc-flake coating systems supplied without hexavalent chromium. ISO 10683, however, is broader and applies to systems with or without hexavalent chromium. Therefore “ISO 10683 coating” does not by itself establish a Cr(VI)-free finish.^{[1] [3]}

If regulatory compliance matters, state the restricted substance requirement and require supporting documentation for the selected basecoat and topcoat. A brand family may contain more than one chemistry.

Salt-spray hours are a quality check, not a service-life forecast

Neutral salt spray is useful for checking coating consistency and detecting defects. ISO 9227 explicitly does not define exposure time for a particular product and does not recommend using the test to rank materials or predict long-term corrosion life. A result such as “720 hours” only has meaning when the test method, coating system, substrate, evaluation point and acceptance criteria are all stated.^[4]

Real fasteners see cyclic wetting, drying, temperature, chemicals, installation damage and galvanic contact. If field life matters, pair accelerated tests with the customer specification, relevant cyclic-corrosion testing and evidence from the actual service environment.

Friction control is as important as corrosion

Most tightening torque is consumed by friction. Changing the topcoat or lubricant can change clamp load more than changing the bolt property class. Zinc-flake systems are commonly supplied with an integral or subsequent lubricant so the finished lot meets a defined coefficient-of-friction range.

Do not accept a coating substitution solely because the salt-spray result is equal or higher. Ask whether the replacement meets the same torque/clamp behaviour, prevailing-torque requirements, colour, electrical conductivity and compatibility with adhesives. ISO 16047 is the common reference for controlled torque/clamp-force testing.^[5]

Thickness and thread fit cannot be separated

Coating builds on thread flanks, recesses and bearing faces. Too much build can reduce nut fit, fill small internal drives or change prevailing torque; too little can miss the corrosion class. Dip-spin parts may also show pooling in recesses or parts sticking together if loading and centrifuging are poorly controlled.

The coating specification should therefore state the thread tolerance before coating, the gauging condition after coating and any masking requirement. A generic thickness copied from a brochure is not enough, especially on small threads.

When each finish is the better starting point

Requirement	Start with	Confirm before release
High-strength bolt with substantial corrosion exposure	Specified zinc-flake system	Pretreatment, IHE control, friction and coating class
Thin, bright finish on general industrial hardware	Electroplated zinc or zinc alloy	Material strength, hydrogen controls and corrosion class
Controlled assembly coefficient of friction	Either, with qualified lubricant/topcoat	Lot torque/clamp test using production parts
Electrical grounding through the joint	Application-specific conductive finish	Contact resistance after assembly and corrosion exposure
Weldability or paint adhesion	Application-specific system	Neither property is guaranteed merely by citing ISO 10683

What to put on the RFQ

Coating standard and edition, plus the required coating designation
Basecoat, topcoat, colour and restricted-substance requirement
Corrosion test method, hours and evaluation criteria
Coefficient-of-friction or torque/clamp-force window
Pretreatment and hydrogen-embrittlement controls for high-strength parts
Post-coating thread gauging, recess fill and coating-thickness requirements
Approved applicator and lot-level certificate or test report

Not sure whether the bolt itself should be 8.8, 10.9 or 12.9? Read our metric bolt property-class comparison. It explains the strength code, matching nuts, preload and why the highest class is not always the safest selection.

References

From the Catalog

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

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

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

Dacromet and Zinc-Flake Coatings for High-Strength Fasteners