Specifying the right fastener coating is as important as specifying the right grade. A grade 8.8 bolt in the wrong coating will corrode prematurely; a high-strength bolt in the wrong coating process can fail catastrophically through hydrogen embrittlement. This guide compares the three most widely used coating systems — hot-dip galvanizing, zinc electroplating, and Geomet — across the criteria that matter for procurement decisions.
Why the Coating Choice Matters
Bare carbon steel fasteners rust rapidly in outdoor, humid, or saline environments. Corrosion does not just degrade the fastener surface — it can cause bolt seizure (preventing disassembly), galvanic corrosion between dissimilar metals, and loss of clamp load as rust products expand or corrode away the thread contact area. Choosing the right coating for the environment and the bolt grade prevents all of these outcomes.
Three variables drive coating selection:
- Environment severity — indoor/dry, outdoor, coastal, industrial, fully marine
- Bolt grade — HDG and acid-based processes create hydrogen embrittlement risk on grades 10.9 and 12.9
- Thread fit requirement — thick coatings change thread dimensions and require oversize nuts
Hot-Dip Galvanizing (HDG)
Hot-dip galvanizing immerses the fastener in molten zinc at approximately 450°C after acid pickling and fluxing. The zinc metallurgically bonds to the steel surface, forming a layered coating (eta, zeta, delta, gamma layers) that provides both barrier protection and cathodic (sacrificial) protection — the zinc corrodes preferentially, protecting the steel even at scratches or cut edges.
Key characteristics:
- Coating thickness: 45–85 µm per ISO 1461 (fasteners ≥ 6mm)
- Salt-spray resistance: 500–1,000+ hours (ISO 9227)
- Outdoor service life: 20–40 years in moderate environments; 10–25 years coastal
- Thread fit: Requires nuts tapped one ISO tolerance class oversize (6AZ/6H tolerance) to accommodate zinc thickness on threads
- Appearance: Rough, matte grey; may have drip marks or uneven surface
- Grade limit: Safe on grade 8.8; not recommended on 10.9; prohibited on 12.9
- Standard: ISO 1461, ASTM A153
HDG is the dominant outdoor structural coating for grade 8.8 hex bolts, anchor bolts, and foundation bolts. TERNS EXIM supplies hot-dip galvanized fasteners per ISO 1461 with coating thickness test reports.
Zinc Electroplating
Zinc electroplating (electrodeposited zinc, ISO 4042) deposits zinc from an electrolyte solution onto the fastener using electric current. It produces a uniform, smooth, bright finish at much thinner coating depths than HDG. The acid pickling stage of the process introduces hydrogen into high-strength steel — the critical limitation of this process.
Key characteristics:
- Coating thickness: 5–25 µm (typical commercial: 8–12 µm)
- Salt-spray resistance: 96–200 hours at 8–12 µm; up to 500 hours with chromate passivation
- Appearance: Bright, uniform silver or yellow (chromate passivated) finish
- Thread fit: Minimal dimensional impact at standard 8–12 µm; no special nut sizing needed
- Grade limit: Safe on grades up to 8.8; on grade 10.9 requires hydrogen embrittlement relief bake (190°C, ≥4 hours within 4 hours of plating, per ASTM B849); not recommended on 12.9
- Standard: ISO 4042, ASTM B633
Zinc electroplating is the default finish for interior mechanical fasteners and light-duty outdoor applications. Its low corrosion resistance makes it unsuitable for structural outdoor or coastal work without additional topcoats (e.g., clear or yellow chromate passivation).
Geomet and Zinc Flake Coatings
Geomet, Delta Tone, Dacromet, and similar products are zinc flake coatings — water-based suspensions of zinc and aluminium flakes applied by dipping or spraying and oven-cured at 200–300°C. The process contains no acid pickling step and no electrolytic hydrogen generation, making it the safest choice for high-strength fasteners.
Key characteristics:
- Coating thickness: 8–12 µm (typically two coat-and-cure cycles of 4–6 µm each)
- Salt-spray resistance: 720–1,000+ hours — far exceeding zinc electroplating at the same thickness
- Appearance: Smooth, dark grey metallic; consistent and dimensionally precise
- Thread fit: Minimal impact; no oversize nut required
- Grade compatibility: Safe on all grades including 10.9 and 12.9 — no hydrogen embrittlement risk
- Temperature resistance: Up to 300°C service temperature (HDG de-zincs above ~200°C)
- REACH compliance: Chromium-free formulations comply with EU chemical regulations
- Standard: ISO 10683 (non-electrolytically applied zinc flake coatings)
Coating Comparison Table
| Property | Hot-Dip Galvanized | Zinc Electroplated | Geomet / Zinc Flake |
|---|---|---|---|
| Coating thickness | 45–85 µm | 5–25 µm | 8–12 µm |
| Salt-spray hours (ISO 9227) | 500–1,000+ | 96–200 (plain) / up to 500 (passivated) | 720–1,000+ |
| Cathodic protection | Yes (strong) | Yes (limited at thin deposits) | Yes |
| Appearance | Rough, matte grey | Smooth, bright silver/yellow | Smooth, dark grey |
| Thread tolerance impact | High — oversize nut required | Low | Very low |
| H₂ embrittlement risk | Yes (pickling stage) | Yes (acid + electrolytic H) | None |
| Max service temperature | ~200°C | ~120°C | ~300°C |
| Safe on grade 8.8 | Yes | Yes | Yes |
| Safe on grade 10.9 | Not recommended | With bake relief | Yes |
| Safe on grade 12.9 | Prohibited | Not recommended | Yes |
| Relative cost | Medium | Low | Medium–High |
| Best suited for | Outdoor structural (≤8.8), long life | Interior, light-duty, low-cost | High-strength, precision, automotive |
Grade Compatibility Summary
The interaction between coating process and bolt grade is governed by hydrogen embrittlement. High-strength steels (above ~1000 MPa tensile strength) are sensitive to atomic hydrogen absorbed during acid pickling or electrolytic processes. The hydrogen diffuses into the steel lattice and can cause delayed brittle fracture under sustained load — often hours or days after installation, with no visible warning.
- Grade 8.8: All three coatings permitted. HDG preferred for outdoor structural work.
- Grade 10.9: Avoid HDG. Electroplating permissible only with bake relief (190°C, ≥4 hours). Geomet preferred.
- Grade 12.9: HDG prohibited. Electroplating not recommended even with bake relief. Geomet or phosphate-and-oil only.
Which Coating to Specify
- Outdoor structural, construction, civil engineering (grade 8.8): Hot-dip galvanized per ISO 1461. Best long-term value. Ensure nuts are oversize grade.
- General indoor machinery, enclosures, furniture: Zinc electroplated, 8–12 µm, yellow chromate passivation. Cost-effective, good appearance.
- High-strength fasteners (10.9/12.9), automotive, precision assemblies: Geomet or Delta Tone. No HE risk, tight tolerances maintained.
- Coastal outdoor or marine-adjacent (above waterline), grade 8.8: HDG — the zinc sacrificial action handles intermittent salt spray well.
- Fully marine/submerged: A4/316 stainless steel (not a coating — a material choice). See our marine fasteners guide.
TERNS EXIM supplies plain, zinc electroplated, and hot-dip galvanized fasteners as standard finishes. Geomet and other zinc flake coatings are available on request for high-strength orders. All coated fasteners are supplied with coating test reports confirming thickness and salt-spray compliance.