Introduction: Why Coating Matters in Fastener Performance
When importing bolts, nuts, washers, and custom fasteners, one key factor often overlooked is surface treatment. A fastener’s coating doesn’t just affect its appearance—it directly impacts its corrosion resistance, durability, compliance, and suitability for your application.
Choosing the wrong coating can lead to premature rust, structural failure, or even rejection at customs. Whether you’re supplying to Europe, North America, or Australia, understanding fastener coatings is essential to meet quality standards and extend product lifespan.
In this article, we’ll break down the most commonly used coatings for industrial fasteners, their advantages, limitations, and typical applications.
1. Zinc Plating: Economical and Versatile
What It Is:
Zinc plating is an electroplating process where a thin layer of zinc is deposited on the surface of a steel fastener. This acts as a sacrificial barrier to corrosion.
Pros:
- Cost-effective
- Bright and attractive finish (silver, yellow, black)
- Smooth surface with tight tolerances
- Widely available and fast turnaround time
Cons:
- Thin layer (usually 5–10 μm) provides only mild corrosion resistance
- Not suitable for marine or acidic environments
- Prone to white rust over time
Applications:
- Indoor construction
- Light-duty machinery
- Electrical components
Compliance:
Zinc plating can be made ROHS and REACH compliant by using trivalent chromium passivation instead of hexavalent chromium.
2. Hot-Dip Galvanizing (HDG): Heavy-Duty Protection
What It Is:
HDG involves immersing steel fasteners in molten zinc, forming a thick, robust coating that bonds metallurgically with the steel.
Pros:
- Excellent corrosion resistance (up to 50–75 μm)
- Ideal for outdoor and marine environments
- Long service life (15–50 years depending on exposure)
Cons:
- Bulkier coating—may affect thread fit (typically requires oversize tapping)
- Matte, uneven surface finish
- Longer production and cooling times
Applications:
- Bridges and highways
- Outdoor structures
- Agricultural and utility equipment
Compliance:
Compliant with ISO 1461, ASTM A153, and AS/NZS 4680 for structural fasteners.
3. Dacromet® / Geomet®: Advanced Non-Electrolytic Coating
What It Is:
Dacromet and Geomet are proprietary zinc-aluminum flake coatings applied by dip-spinning. These are non-electrolytic and don’t cause hydrogen embrittlement, making them ideal for high-strength fasteners.
Pros:
- Excellent corrosion resistance without thick buildup
- Suitable for Grade 10.9 / 12.9 bolts
- Heat and chemical resistant
- Compatible with automated production
Cons:
- More expensive than traditional coatings
- Typically gray or dull silver (not for decorative use)
- Requires licensed applicators
Applications:
- Automotive and aerospace
- Wind energy
- High-strength bolts used in construction
Compliance:
Meets SAE J2334, ISO 10683, and often required by Tier 1 OEMs in automotive industries.
4. Mechanical Plating: A Safer Alternative to Electroplating
What It Is:
A cold process where zinc particles are mechanically bonded to fasteners using tumbling action with glass beads.
Pros:
- No hydrogen embrittlement risk
- Uniform thickness on complex shapes
- Environmentally friendly process
Cons:
- Limited coating thickness
- Not as corrosion-resistant as HDG or Dacromet
- Less common globally
Applications:
- High-strength steel fasteners
- Military components
- Fasteners requiring tight tolerance
5. Black Oxide: For Appearance, Not Protection
What It Is:
A chemical treatment that creates a dark black finish on steel by converting the surface into magnetite (Fe3O4).
Pros:
- Aesthetic matte-black appearance
- Minimal dimensional change
- Reduces light reflection
Cons:
- Low corrosion resistance (requires oil coating)
- Mostly cosmetic unless post-treated
Applications:
- Furniture and tools
- Decorative hardware
- Indoor applications
6. Nickel and Chrome Plating: Decorative with Limited Protection
What It Is:
Fasteners can also be coated in nickel or chrome for a shiny, premium finish.
Pros:
- High luster appearance
- Smooth, decorative finish
- Resistant to oxidation
Cons:
- Not suitable for structural or load-bearing fasteners
- Expensive and time-consuming
- Minimal corrosion protection unless used with sub-layers
Applications:
- Automotive interiors
- Decorative architecture
- Designer furniture
7. Comparison Table: Which Coating Should You Choose?
| Coating Type | Corrosion Resistance | Cost | Hydrogen Embrittlement Risk | Ideal Use Environment |
|---|---|---|---|---|
| Zinc Plating | ★★☆☆☆ | Low | Yes | Indoors, dry environments |
| Hot-Dip Galvanized | ★★★★☆ | Medium | Yes (if improperly treated) | Outdoors, bridges, utilities |
| Dacromet / Geomet | ★★★★★ | High | No | High-strength, automotive |
| Mechanical Plating | ★★★☆☆ | Medium | No | Aerospace, high-strength bolts |
| Black Oxide | ★☆☆☆☆ | Low | No | Aesthetic indoor applications |
| Chrome/Nickel | ★★☆☆☆ | High | Possible | Interior decorative applications |
8. How Coatings Affect Thread Fit and Tolerances
Be mindful that coating thickness adds to the thread profile, potentially causing issues during assembly. Here’s how to manage that:
- Specify class of fit (e.g., 6g, 6H) and tolerance allowances in technical drawings
- For HDG, use oversized nuts to accommodate coating build-up
- Confirm coating thickness with your supplier before production
9. Compliance and Regulatory Considerations
For EU Markets:
- Must be ROHS and REACH compliant
- No hexavalent chromium
- Coatings must meet EN ISO 10684, ISO 4042
For US and Canada:
- ASTM standards (e.g., A153, B633, F1941) apply
- Automotive projects may require OEM-specific specs
For Australia and New Zealand:
- Refer to AS/NZS 1214, AS/NZS 4680 for galvanizing
- Coating specs often required for public infrastructure
Always request documentation such as:
- Salt spray test reports
- Coating thickness certification
- Compliance declarations
10. Ask These Questions Before You Order
- What coating thickness will be applied?
- Is the coating compliant with my target market’s regulations?
- Will the coating impact thread tolerance or torque performance?
- Is there any hydrogen embrittlement risk?
- What corrosion test reports can the supplier provide?
Conclusion: The Right Coating = Long-Term Performance
Choosing the correct fastener coating is not just a technical detail—it’s a strategic procurement decision. The wrong coating can result in corrosion, product failure, or rejected shipments. The right one can mean decades of reliable performance.
By understanding the pros and cons of each surface treatment, you can work with your supplier to specify exactly what your project requires—no more, no less.
Need help selecting the right coating for your fasteners?
Contact Jiaxing Linkworld Fasteners. We offer a wide range of certified coatings including zinc plating, HDG, Dacromet, and more—compliant with EU, US, and Australian standards. Submit a Quick Inquiry now and receive expert support within 24 hours.