Scope
Three ASTM specifications govern zinc-flake coating systems for fasteners:
- ASTM F3019 — Nonelectrolytically Applied Zinc-Flake Composite Corrosion-Protective Coatings for Fasteners
- ASTM F1136 / F1136M — Chromium/Zinc Corrosion Protective Coatings for Fasteners
- ASTM F2833 — Corrosion Protective Coatings for Fasteners (inorganic zinc-flake systems)
Each spec covers slightly different coating chemistries and performance characteristics, but the dominant commercial systems (Geomet 500/720, Magni 565/577, Dacromet, Zintek 300, PPG HI-KOTE) generally conform to one or more.
Why zinc-flake exists
Zinc-flake systems solve a very specific problem: F3125 A490 structural bolts need corrosion protection, but HDG and electroplated zinc both carry unacceptable hydrogen embrittlement risk at the 150-ksi tensile strength of A490. Mechanical galvanizing is one alternative; zinc-flake coatings are the other. Both avoid the acid-pickling and thermal-dipping steps that introduce or retain hydrogen.
Beyond structural bolts, zinc-flake has become a mainstream coating for:
- Automotive chassis fasteners (widely specified by GM, Ford, VW, Toyota)
- Wind turbine bolting (top of tower, blade-root, nacelle)
- Heavy industrial equipment
- High-strength aerospace ground support
How zinc-flake coatings are applied
The process is paint-like rather than immersion or electrolytic:
- Clean. Mechanical cleaning (shot blast) plus degreasing — no acid pickling, avoiding hydrogen introduction.
- Apply base coat. The base coat is a liquid containing zinc flakes (and often aluminum flakes) suspended in an inorganic binder. Applied by dip-spin, spray, or dip-drain.
- Cure base coat. Oven cured at 550–600°F to drive off solvents and bond the flakes to the substrate.
- Apply topcoat (optional, usually). A compatible polymer topcoat adds color (gray, silver, black), additional corrosion resistance, and friction modification for controlled K-factor.
- Cure topcoat. Second bake at ~400°F.
The finished coating consists of overlapping metallic flakes bonded through a cured binder matrix — mechanically and chemically anchored, functioning as both a barrier coating and a galvanic sacrificial layer.
The three ASTM specs compared
ASTM F1136 — Chromium/Zinc Corrosion Protective Coatings
The original zinc-flake spec. Covers coatings containing hexavalent chromium (Cr-VI) as a passivation layer. Grade 3 is the specific variant approved for F3125 A490 structural bolts.
Performance benchmarks:
- Salt spray per ASTM B117: typically 500–720 hours to 5% red rust
- Hydrogen embrittlement: passes F519 / F1940 requirements
- Trivalent-chromium versions are increasingly common for RoHS/REACH compliance
ASTM F2833 — Inorganic Zinc-Flake Coatings
Specifies inorganic zinc-flake systems without topcoat (or with specific topcoat requirements). Grade 1 is approved for F3125 A490 bolts.
Distinguishing feature: inorganic binder chemistry provides higher temperature resistance than polymer-bound systems — useful in under-hood automotive and some engine-mount applications.
ASTM F3019 — Nonelectrolytically Applied Zinc-Flake Composite Coatings
The newer, broader spec covering modern zinc-flake systems including those with chromium-free and trivalent-chromium chemistries. Designed to accommodate evolving commercial products (Geomet, Magni, Zintek) under a standardized framework.
Performance benchmarks:
- Salt spray: 480–1000+ hours to 5% red rust depending on system and thickness
- Hydrogen embrittlement: pass
Corrosion performance
Zinc-flake systems outperform conventional zinc coatings at equivalent or lighter coating thicknesses:
| Coating | Typical thickness | Salt-spray hours to 5% red rust |
|---|---|---|
| Fe/Zn 8 (F1941) + Type II chromate | 8 μm | 72–96 hours |
| HDG per F2329 | 43+ μm | 240–500 hours |
| Mechanical zinc Class 50 | 50 μm | 240–500 hours |
| Zinc-flake (base coat only) | 5–10 μm | 240–500 hours |
| Zinc-flake + topcoat | 8–25 μm | 500–1000+ hours |
The critical advantage: a 10 μm zinc-flake coating performs like a 50 μm HDG coating, without the hydrogen issues and without the thread-fit challenges of thick coatings.
Friction and K-factor control
Zinc-flake topcoats can be formulated with lubricants built into the polymer binder, producing fasteners with specified and repeatable K-factors. This is particularly valuable in:
- Automotive assembly — robots must achieve consistent tension across millions of fasteners
- Structural bolting — turn-of-nut and torque methods rely on predictable torque-tension behavior
- Wind turbine installation — large fasteners with tight installation tolerances
K-factors for zinc-flake coated assemblies typically range from 0.09 to 0.14, comparable to fluoropolymer-coated fasteners.
Hydrogen embrittlement — why zinc-flake is approved for A490
The process avoids hydrogen because:
- No acid pickling (mechanical cleaning only)
- Cure temperature ~600°F — enough to drive off any trace moisture but below tempering temperatures that would alter steel properties
- No electrolytic deposition
- Water-based or solvent-based chemistries without aggressive acids
F3125 explicitly permits F1136 Grade 3 and F2833 Grade 1 coatings on Grade A490 bolts. F3019 systems are increasingly accepted as they demonstrate F1940 hydrogen embrittlement compliance.
Common commercial brand names
Spec writers regularly specify zinc-flake coatings by brand name:
- Geomet 500 / Geomet 720 (NOF Metal Coatings) — F3019 and F1136 compliant systems
- Magni 565 / Magni 577 (Magni Group) — F1136 and F3019 compliant
- Dacromet (NOF) — the original chromium/zinc flake system
- Zintek 300 (ZINTEK) — base + topcoat system
- HI-KOTE 1 (PPG) — fluoropolymer-topcoated zinc flake
A drawing that says "Geomet 720 or equivalent" is invoking the performance class of that specific product; equivalents from other manufacturers are generally acceptable if they meet the same ASTM spec and performance benchmarks.
Limitations
- Thickness. Zinc-flake coatings are thin; they're a corrosion layer, not a structural dimension-building coating. Use HDG for heavy sections where coating mass is part of the design.
- Abrasion. The coating is harder than paint but softer than zinc metal. Service where parts are repeatedly abraded (machinery contact points, heavy-traffic areas) can wear through the coating locally.
- Temperature. Most polymer-topcoated systems rate to roughly 500°F; above that, the topcoat degrades. Inorganic binders (F2833) handle higher temps.
- Touch-up. Field touch-up is possible but matches the original less perfectly than with other systems. Dedicated touch-up products exist (e.g., Geomet touch-up).
Applications
- F3125 A490 and A325 structural bolting where corrosion resistance is critical
- Automotive chassis, engine compartment, and underbody hardware
- Wind turbine tower, blade-root, and nacelle bolting
- Heavy equipment (construction, agriculture) fastener hardware
- A574 socket-head cap screws requiring corrosion protection beyond black oxide
Related specifications
- F3125 — Structural bolts (references F1136 Grade 3 and F2833 Grade 1 as approved coatings for A490)
- F1940 — Fastener coating HE testing method
- F519 — HE testing method
- F2329 — HDG alternative (for A325 only; prohibited on A490)
- B695 — Mechanical galvanizing (alternative for A490)
- F1941 — Electroplating (not approved for A490)
Documentation
California Fastener zinc-flake-coated orders ship with mill certificates for the base material plus coating certification showing system used (brand and grade), coating thickness, salt-spray test results, and hydrogen-embrittlement compliance. For F3125 A490 orders, the specific coating approval (F1136 Grade 3, F2833 Grade 1, or F3019-compliant equivalent) is documented.