Scope
ASTM F2329 / F2329M specifies the requirements for hot-dip zinc coating applied to carbon steel and alloy steel externally and internally threaded fasteners. It covers coating thickness, adhesion, continuity, appearance, and the thread accommodation required to fit coated bolts to coated nuts.
F2329 is the fastener-specific HDG spec. For structural steel members, ASTM A123 applies; for bulk hardware and small parts, ASTM A153 applies. F2329 consolidates the fastener-relevant provisions and updates them for modern practice.
How HDG works
The process is simple in concept, demanding in execution:
- Clean. Fasteners are degreased, acid-pickled to remove mill scale and rust, and fluxed.
- Dip. Clean parts are immersed in molten zinc at approximately 840°F (450°C).
- React. Iron from the steel and zinc from the bath form a series of zinc-iron intermetallic layers at the interface, topped by a layer of nearly pure zinc.
- Withdraw and drain. Parts are lifted slowly and either centrifuged (for smaller items) or allowed to drain.
- Cool. Natural air cooling or controlled quench.
The result is a coating that is metallurgically bonded — not just mechanically adhered — to the steel substrate. Zinc corrodes preferentially to the steel (galvanic or cathodic protection), meaning scratches in the coating do not lead to under-film rust the way they do on paint or plating.
Coating thickness
F2329 defines coating thickness based on fastener diameter and thread engagement requirements:
| Fastener diameter | Typical minimum coating thickness |
|---|---|
| 3/8" and smaller | 0.0017" (43 μm) — minimum average |
| Over 3/8" through 1" | 0.0020" (50 μm) — minimum average |
| Over 1" | 0.0020" (50 μm) — minimum average |
For anchor bolts and large items, thicker coatings are common and often extend to 0.003"–0.005" in practice. Measurement is per magnetic gauge (ISO 2178) or equivalent non-destructive method.
Thread accommodation — nuts are overtapped
The critical detail about HDG threaded fasteners: zinc builds up on threads. A coated bolt has more material on its threads than an uncoated bolt. To make the coated bolt and coated nut fit, nuts are overtapped to remove excess thread material that would otherwise prevent assembly.
F2329 specifies overtap amounts in conjunction with the mating nut specifications (A563, A194). Typical overtap is approximately:
- 0.016" on the pitch diameter for diameters ≤ 1"
- 0.020"–0.030" for larger diameters
The overtap is applied by tapping the nut after galvanizing (common practice), or tapping before galvanizing with oversize taps calibrated for the coating allowance. Proof-load testing is performed post-overtap to confirm the nut still develops full bolt tension before stripping.
Coating appearance
HDG is aesthetically uneven. Coated fasteners show:
- Matte silver to dull gray color
- Spangles (crystalline zinc patterns) on slower-cooled items — purely cosmetic
- Occasional drain marks, icicles, or fins where zinc cooled before flowing off
- Variation between lot-to-lot
Coating appearance is not the primary acceptance criterion. Thickness and adhesion are. F2329 explicitly disallows rejection for cosmetic reasons when mechanical requirements are met.
Hydrogen embrittlement — who's at risk
The acid pickling step that precedes galvanizing introduces hydrogen into the steel. In low-strength steels (below about 150 ksi ultimate tensile), the hydrogen diffuses out during the hot zinc dip (which essentially bakes the part at 840°F) and causes no trouble.
In high-strength steels (roughly 150+ ksi ultimate tensile), residual hydrogen can cause delayed brittle fracture — the bolt looks fine for weeks, then cracks under load. For this reason:
- F3125 A325 (120 ksi) — HDG acceptable
- F3125 A490 (150+ ksi) — HDG prohibited by the F3125 standard
- F1554 Grade 105 — HDG acceptable with hydrogen relief baking
- A354 BD (150 ksi) — HDG discouraged; mechanical galvanizing (B695) or zinc-flake preferred
- A574 socket heads — HDG not suitable (coating too thick for SHCS precision)
For A325, A307, A449, F1554 36/55, the risk is negligible and HDG is the default coating.
Inspection and acceptance
Per F2329, coated fasteners are inspected for:
- Coating thickness — magnetic gauge, minimum average across multiple readings
- Coating adhesion — the zinc should not flake off under moderate handling
- Appearance — free of bare spots, rust spots, and coating discontinuities
- Thread function — coated bolt threads must gauge within tolerance per ASME B1.1 or B1.13M
- Nut fit — overtapped nuts must run onto coated bolts by hand past the end of the threaded portion
Mill certificates show average and minimum thickness readings, nut overtap verification, and lot-level conformance.
Applications
- Anchor bolts (F1554 all grades — most common HDG fastener)
- Structural bolts (F3125 A325 only; never A490)
- Exterior infrastructure: highway, utility, transmission tower hardware
- Fence, sign, and light-standard hardware
- Bridge construction (non-weathering members)
- Exterior mechanical assembly
- Marine dock hardware (inland; for severe marine use, move to stainless or Monel)
Service life
HDG's service life depends on coating thickness and atmospheric aggressiveness. Rough expectations:
- Rural and suburban exposure: 50–75+ years
- Urban industrial: 30–50 years
- Coastal atmospheric: 20–30 years
- Marine immersion: not recommended (use stainless or copper alloy)
The zinc "sacrificial" behavior means small scratches and mill marks don't compromise protection — the surrounding zinc continues to protect the steel via galvanic action.
Related specifications
- A123 — HDG for structural steel members (not fasteners)
- A153 — HDG for iron and steel hardware (older fastener spec; F2329 is the modern replacement)
- B695 — Mechanical galvanizing (alternative for high-strength fasteners)
- A780 — Repair of damaged HDG coatings (touch-up)
- A563 — Nuts with overtap provisions for HDG
- F3125 Annex A1 — HDG provisions for structural bolts
- RCSC Specification — Structural-joint requirements including coating
Documentation
California Fastener F2329 orders ship with mill certificates for the base material plus coating certification showing average coating thickness readings, heat numbers, and nut overtap verification. Rotational capacity testing is performed on galvanized structural bolt assemblies when required by F3125.