When 240 hours of salt spray resistance is demanded for a bolt, ordinary zinc plating won’t suffice. This level of corrosion protection requires specialized surface finishes engineered for severe environments—marine, automotive underbody, industrial, or coastal applications.
Here are the four most reliable surface treatments that consistently achieve 240+ hours of salt spray resistance, with detailed performance data and application guidance.
Understanding the 240-Hour Requirement
Before diving into specific finishes, it’s important to understand what “240-hour salt spray” means. This refers to testing per ASTM B117 or ISO 9227, where specimens are exposed to a continuous salt fog (5% NaCl) at 35°C . Achieving 240 hours to red rust (base metal corrosion) or white rust (zinc corrosion) depends on both the coating type and thickness.
The finishes below have been validated through laboratory testing and real-world applications to meet or exceed this threshold.
1. Zinc-Nickel Alloy Plating (Zn-Ni)
Zinc-nickel is widely regarded as the premier electroplated finish for high corrosion resistance, particularly in automotive applications.
How It Works
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Nickel content is precisely controlled between 12-15% for optimal corrosion performance
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Applied via alkaline or acid electroplating processes
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Usually combined with trivalent passivation (clear, yellow, or black) and optional topcoat/sealer
Salt Spray Performance
| Condition | Hours to White Rust | Hours to Red Rust |
|---|---|---|
| Zinc-nickel + passivation | 120+ hours | 1,000+ hours |
| With premium sealers | 720+ hours | >1,000 hours |
Best Applications
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Automotive chassis and engine fasteners
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High-strength bolts (10.9/12.9 grade) requiring hydrogen embrittlement safety
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Applications with temperature requirements >120°C
2. Dacromet® Coating (Zinc-Aluminum Flake, Chrome-Containing)
Dacromet is the original non-electrolytic zinc flake coating, developed more than 35 years ago and still widely specified for demanding applications .
How It Works
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Water-based dispersion of zinc and aluminum flakes in a chromium-containing binder
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Applied by dip-spin or spray, then baked at approximately 300°C
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Forms a multi-layer barrier with sacrificial protection
Salt Spray Performance (per ISO 9227)
| Grade | Coating Weight | Hours to White Rust | Hours to Red Rust |
|---|---|---|---|
| Dacromet 320/500 Grade A | 24 g/m² | 240 hours | 600 hours |
| Dacromet 320/500 Grade B | 36 g/m² | 240 hours | 1,000 hours |
| High thickness | 80 g/m² | 15 Kesternich cycles |
Best Applications
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Applications where Cr6+ is still permitted (some industrial, military, or legacy specifications)
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High-temperature environments up to 300°C
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Assemblies requiring precise torque-tension control
3. Delta Coatings (Zinc-Aluminum Flake, Chrome-Free)
Delta coatings are the chrome-free evolution of zinc flake technology, developed to meet stringent environmental regulations while maintaining corrosion performance.
How It Works
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Similar mechanism to Dacromet—zinc and aluminum flakes in a binder
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Completely chromium-free, using proprietary inorganic or organic binders
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Applied via dip-spin or spray, followed by baking
Salt Spray Performance
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Standard thickness (5-12μm): 240+ hours to red rust
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Increased thickness (9-25μm): 500-1,000+ hours available
Best Applications
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Automotive fasteners requiring Cr6+ compliance (virtually all modern vehicles)
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Export products to EU markets
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Wind turbine and renewable energy equipment (C5 corrosion environments)
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High-strength structural bolts
4. Geomet® Coating (Zinc-Aluminum Flake, Chrome-Free)
Geomet is a leading brand of chrome-free zinc flake coating, developed as a direct replacement for Dacromet in environmentally sensitive applications.
How It Works
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Similar zinc-aluminum flake technology in a water-soluble, chrome-free binder
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Closed-loop processing minimizes environmental impact
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Multiple layers can be applied for enhanced protection
Salt Spray Performance
| Coating Variant | Salt Spray Resistance |
|---|---|
| Geomet standard | 240 hours to red rust |
| Geomet 321/500 | 720-1,000 hours |
| Geomet (with PLUS topcoat) | Up to 1,500 hours |
Best Applications
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European automotive OEMs (VW, Ford, GM approvals)
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High-strength fasteners requiring maximum safety against hydrogen embrittlement
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Brake system components (Geomet 360 for 400°C resistance)
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Offshore and marine applications (1,440+ hours available for C5-H environments)
Comparison Summary
| Finish | 240h Achieved? | Max Hours | Chrome-Free? | H Embrittlement Risk | Max Temp | Cost Level |
|---|---|---|---|---|---|---|
| Zinc-Nickel + passivation | ✅ Yes | 1,000+ | Yes (with trivalent) | Low (baking required) | 180°C | High |
| Dacromet | ✅ Yes | 1,000 | No (contains Cr6+) | None | 300°C | Moderate |
| Delta | ✅ Yes | 1,000+ | Yes | None | 300°C | Moderate-High |
| Geomet | ✅ Yes | 1,500+ | Yes | None | 400°C | Moderate-High |
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