Guide
Zinc flake coating vs zinc plating: which should your steel parts call for?
Zinc flake coating is a non-electrolytic corrosion coating made of zinc flakes, usually with aluminum flakes, in a binder that is inorganic in most basecoats, applied to steel parts by dip-spin or spray and then cured in an oven, usually with a sealing topcoat that can carry an integral lubricant. Zinc plating is the familiar electroplated coating, built up from a bath with electric current and finished with a passivate. Both protect steel sacrificially, but they differ on hydrogen risk, curing heat, thread fit, friction and how they are specified.
By Saola Manufacturing engineering team · Published · Updated
This guide is for design and quality engineers who see zinc flake on an automotive or heavy-equipment customer spec and need to decide whether it should replace zinc plating on their own part, and for buyers comparing quotes for both. It does not repeat the bake schedule for electroplated parts or the salt spray hours of clear and yellow zinc, which have their own guides linked below. It focuses on the decision itself: where each coating is the better engineering answer, where zinc flake causes problems people only discover at assembly, and how to write either one onto a drawing so that two suppliers quote the same thing.
On this page
What is the real difference between a zinc flake coating and zinc plating?
Zinc plating grows a dense layer of pure zinc on the part from an electrolyte, while a zinc flake coating is a paint-like dispersion of overlapping metal flakes that is applied mechanically and cured with heat. That difference in how the coating gets onto the part drives almost every other difference between them.
Electroplated zinc on iron and steel is covered by ASTM B633 in North American drawings (with ASTM F1941/F1941M for threaded fasteners) and by ISO 4042 for metric fasteners, with a thickness class and a supplementary passivate type. Because it is electrolytic, plating thickness follows current density: edges and corners collect more zinc, and deep recesses and the bottoms of internal threads collect less.
Zinc flake systems are covered for fasteners by ISO 10683:2018, which applies to coatings with or without hexavalent chromium, with or without topcoat and with or without lubricant, and by automotive company standards that name specific basecoat and topcoat pairs. The long-standing North American callout, ASTM F1136/F1136M, covers zinc/aluminum dispersion basecoats applied by dip-spin, dip-drain or spray. ASTM lists it as withdrawn in 2025 and replaced by ASTM F3393, which consolidates F1136, F2833 and F3019 into one zinc flake specification for fasteners and requires systems supplied without hexavalent chromium, so check which document your customer actually references before copying an old callout. A flake coating behaves the opposite way to plating at the geometry: the liquid drains off sharp edges and pools in recesses, drive sockets and internal threads.
Appearance is a weak guide. Both come in silver and black, and a silver flake topcoat can look much like clear trivalent zinc, so the drawing note, not the color, has to carry the choice.
How do salt spray results compare between zinc flake and zinc plating?
Zinc flake systems are normally ordered by hours of neutral salt spray to red rust, and those numbers are much larger than the hours written against electroplated zinc. Plated zinc is normally ordered by thickness class and passivate type, with its salt spray requirement stated as time to white corrosion, so the two figures measure different failures.
That difference in what each standard measures is the first trap in a side-by-side comparison. A B633 Type II yellow zinc part that meets its white-corrosion requirement and a zinc flake part that meets a red-rust requirement are not being judged on the same failure, so a quote sheet that lists both as "salt spray hours" without the end point is comparing different things. Both are run in the same kind of cabinet, typically to ASTM B117 or ISO 9227, and ISO 9227 itself states that it does not set the exposure period or the interpretation of results; those come from the product specification. Treat the hours as a lot-to-lot quality check, not as a service-life forecast.
The flake coating earns its longer cabinet result from two things: the aluminum flakes and the layered, overlapping structure slow the consumption of zinc, and the sealing topcoat adds a barrier on top. A thin flake system can therefore reach red rust later in the cabinet than a thicker plated layer, which, together with the hydrogen point below, is why many automotive fastener specs adopted it.
| Property | Electroplated zinc (ASTM B633 / ISO 4042) | Zinc flake (ISO 10683 / ASTM F1136) |
|---|---|---|
| How it is applied | Electrolytic bath, rack or barrel, then passivate | Dip-spin, dip-drain or spray, then oven cure |
| Thickness ordered | Classes Fe/Zn 5, 8, 12, 25 (µm) | Set by the salt spray class ordered; ISO 10683 gives reference thicknesses of about 4 to 10 µm for guidance |
| Salt spray end point usually specified | White corrosion (passivate type), red rust by agreement | Red rust on significant surfaces |
| Typical neutral salt spray ordered | Hours to white corrosion set by finish type; the colored Types II and VI carry the longest | 240 to 960 h to red rust in the ISO 10683 classes |
| Hydrogen from the coating step | Yes; bake needed on high-strength steel | None from coating; acid pretreatment must be avoided |
| Process heat seen by the part | Bath near room temperature; relief bake on high-strength steel per ASTM B850 | Cure up to 330 °C (F1136 scope) |
| Where coating builds up | Edges and corners | Recesses, drive sockets, internal threads |
| Electrical conductivity | Good | Limited, especially with organic topcoat |
Why do automotive and heavy-equipment specs use zinc flake to avoid hydrogen embrittlement?
Because the flake coating is applied without electric current and without hydrogen evolution at the steel surface, it removes the coating step as a source of internal hydrogen embrittlement. That matters most on high-strength bolts, springs and clips, where a plated part can crack under load days after assembly.
ISO 10683 states that its coatings are used especially for high-strength fasteners of 1,000 MPa and above to avoid the risk of internal hydrogen embrittlement, per the published scope of ISO 10683:2018. Electroplated zinc on the same steel can be made safe with a post-plating bake, but a bake reduces the risk rather than removing it, and it depends on the plater starting the oven promptly and logging it. The details of that bake are in the guide on hydrogen embrittlement baking after zinc plating.
The flake route has its own condition, and it is the one most often missed on a purchase order. The ASTM F1136/F1136M scope says the coating process does not induce internal hydrogen embrittlement provided the parts have not been cleaned or pretreated with acid or phosphate, and that alkaline cleaning or vapor degreasing with shot blasting is used to remove rust and scale. A coater who pickles parts in acid before a flake coat has put the hydrogen back in. Write the pretreatment restriction on the PO, not only the coating name.
The second condition is heat. The same F1136 scope limits the process to parts not adversely affected by baking temperatures up to 330 °C, so check the tempering temperature of the steel. Quenched and tempered property class 10.9 bolts are tempered at no less than 425 °C per ISO 898-1, above that limit, but case-hardened self-tapping screws, low-temperature-tempered springs and parts with a heat-sensitive insert, adhesive patch or polymer seal may not tolerate the cure. On those, ask the coater for the actual cure cycle before approving the change.
How do the two coatings affect thread fit and tightening torque?
Both coatings consume thread clearance, but zinc plating builds most on the crests and flank edges of an external thread while zinc flake fills the roots and the inside of internal threads. Flake coatings also change friction, so a joint designed around plated-fastener torque values must be rechecked.
Industry guidance for a 60° thread is that pitch diameter changes by about four times the coating thickness on the flanks, which is why a fine-pitch thread with a small allowance runs out of room quickly. ISO 4042 and ISO 10683 both tie the permissible coating thickness to the thread tolerance position: a standard 6g external thread has a fundamental deviation that can absorb a thin coating, and heavier coatings call for thread tolerance positions with more allowance, agreed before the parts are rolled or cut. On a Unified inch thread the same idea applies to the class 2A allowance. The guide on threads after anodizing or plating covers the arithmetic for plated and anodized parts.
Zinc flake adds a specific problem at internal threads. Because the liquid pools in the hole and in drive recesses, small tapped holes and Torx or hex sockets can come out partly filled after dip-spin. Practical options are to mask or plug internal threads, to tap them after coating, to coat nuts and tapped parts by spray rather than dip-spin, or to specify a different finish for the internal-thread part and keep flake for the bolt. Small, flat or cupped parts can also nest together in a dip-spin basket and come out with bare contact marks, so ask the coater how the part will be racked or tumbled.
On friction, many flake systems carry an integral lubricant or a lubricated topcoat with a target friction coefficient window, verified by torque-tension testing per ISO 16047. Changing from lubricated flake back to dry plated zinc, or the reverse, on the same tightening torque changes the clamp load the joint actually gets. If the assembly line tightens to torque, state the friction requirement or the test method on the drawing, and re-qualify the joint when the coating changes.
When is zinc plating still the better choice?
Zinc plating remains the better choice on low-strength steel parts that do not face long corrosive exposure, on parts that must conduct electricity, and on parts with fine internal features or tight bores that a flake coat would fill. It is also simpler to source and inspect in most supply chains.
Use zinc plating where the steel is soft enough that hydrogen is not a concern, which for most turned bushings, low-carbon stamped brackets and machined fittings means the main reason to switch does not apply. Keep it where the part is a grounding stud, an electrical contact or anything that relies on metal-to-metal conductivity through the coating, because flake topcoats are poor conductors. Keep it where an internal thread, a small cross hole or a close-fit bore cannot be masked economically. And keep it where the service environment is indoor or sheltered, because paying for a red-rust class nobody needs adds nothing.
Use zinc flake where the steel is high strength, where the part sees road salt or outdoor exposure over a long service life, where the customer standard already names a flake system, or where a lubricated, controlled-friction fastener is part of the joint design. If neither list clearly fits, compare against zinc-nickel or hot-dip options; the guide on zinc plating vs galvanizing covers the heavier end.
How should the drawing and RFQ call out a zinc flake coating or zinc plating?
Name the standard, the specific coating system or class, the salt spray end point and hours, and the conditions that protect the part: pretreatment, bake or cure limits, thread masking and friction. A note that only says "zinc flake" or "zinc plated" will be quoted differently by every coater.
Example notes, to adapt to your own standard and edition:
- Zinc flake: "Zinc flake coating per ISO 10683 (or ASTM F3393 for inch fasteners), Cr(VI)-free basecoat with sealing topcoat, neutral salt spray per ISO 9227 to red rust on significant surfaces at the hours stated in the part spec. No acid or phosphate pretreatment. Mask internal threads. Friction per ISO 16047 as specified."
- Zinc plating: "Zinc plate per ASTM B633, class and type as specified, trivalent (Cr(VI)-free) passivate. Hydrogen embrittlement relief per ASTM B850 required for parts above the hardness stated on the drawing. Threads to gauge after plating."
On the RFQ, add the base material grade and hardness, the property class for fasteners, the significant surfaces, the expected annual volume by part number, and whether you need a salt spray report with the first article or with every lot. Ask whether the coater runs the named system in its own line, because a flake coat applied by a subcontractor needs its own process record.
Common mistakes: comparing white-corrosion hours with red-rust hours; leaving acid pickling unrestricted on a flake-coated high-strength part; approving flake on a tapped part without a plan for the hole; keeping plated-zinc torque values after switching to a lubricated flake system; and copying a withdrawn ASTM F1136 callout when the customer now references F3393 or its own standard.
How are zinc flake coating and zinc plating handled on a Saola order?
Zinc flake coating is not on the Saola finishing menu. Zinc plating with RoHS-compliant blue trivalent chromate is, applied by a partner finishing plant we manage, and the quote names where each step is done.
The surface finishing menu is fixed: zinc plating, RoHS-compliant blue trivalent chromate, electroplating, gold plating, anodizing on aluminum, powder coating, pad printing and oil-dip rust protection. If a finish is not on that list, it is not offered today, and the page says plainly that we would rather route a part correctly than improvise a coating that has not been qualified. So if your drawing calls for an ISO 10683 flake system, tell us at the RFQ stage and we will say so before quoting rather than after tooling.
Where zinc plating is the right answer, it is applied to turned bushings and shafts from CNC turning, and stamped brackets from partner press shops go to the same plating route. The salt spray test standard and duration are confirmed in the written quote, and a salt-spray test result is issued at first article and on request, as listed on the quality page; we do not claim results beyond what the test report shows. Send the drawing through the quote form with the finish note written as above.
Frequently asked
Is Geomet or Dacromet the same thing as zinc flake coating?
They are trade names for proprietary zinc flake systems from coating suppliers. The original Dacromet chemistry contains hexavalent chromium, while Geomet is a chromium(VI)-free system, which matters if the part has to meet RoHS. The generic description for a drawing is a zinc flake coating to ISO 10683 or to your customer's own standard; name a trade system only when the customer standard requires that exact system.
Does zinc flake need a hydrogen embrittlement bake?
Not for the coating step itself, because no hydrogen is generated by applying it. The oven cure is a curing step, not an embrittlement-relief bake. The protection only holds if pretreatment avoids acid pickling and phosphating, so put that restriction on the order.
Can a zinc flake coated part be painted or powder coated?
Often yes, but adhesion depends on the topcoat chosen, and ISO 10683 does not set requirements for paintability. If the part will be overcoated, ask the coater for a topcoat intended for that and run an adhesion test on first articles.
Will switching from zinc plating to zinc flake change part dimensions?
Yes, mostly at threads and recesses rather than on flat faces. Recheck external thread allowance, gauge internal threads after coating, and confirm that sockets and drive recesses still accept the tool.
Sources
- ISO 10683:2018, Fasteners: Non-electrolytically applied zinc flake coating systems, ISO, 2018
- ASTM F1136/F1136M-11(2019), Zinc/Aluminum Corrosion Protective Coatings for Fasteners (listed as withdrawn 2025), ASTM International, 2019
- ASTM F3393-24, Zinc-Flake Coating Systems for Fasteners, ASTM International, 2024
- ISO 4042:2022, Fasteners: Electroplated coating systems, ISO, 2022
- ASTM B633-23, Electrodeposited Coatings of Zinc on Iron and Steel, ASTM International, 2023
- ISO 9227:2022, Corrosion tests in artificial atmospheres: Salt spray tests, ISO, 2022
- ASTM B117-26a, Operating Salt Spray (Fog) Apparatus, ASTM International, 2026