Zinc Plating for Fasteners
Zinc electroplating is the most widely used surface treatment for steel fasteners worldwide. It provides sacrificial corrosion protection, is cost-effective for high-volume production, and meets RoHS environmental compliance when using trivalent chromium passivation systems.
What Is Zinc Plating?
Zinc plating (electrogalvanizing) is an electrochemical process that deposits a thin layer of zinc onto steel or iron substrates. The zinc coating acts as a sacrificial anode — it corrodes preferentially to protect the underlying steel, even when the coating is scratched or damaged.
For fastener applications, zinc plating is typically performed in barrel plating lines for standard hardware (bolts, nuts, screws, washers) or rack plating for large or delicate components. The process involves alkaline or acid zinc baths, followed by chromate conversion coating (passivation) to further enhance corrosion resistance and provide the desired color finish.
Modern zinc plating operations use trivalent chromium (Cr3+) passivation exclusively for RoHS-compliant applications, replacing the older hexavalent chromium (Cr6+) systems that are now restricted under EU RoHS, ELV, and REACH regulations.
Passivation Types & Performance
| Passivation | Color | Salt Spray (hours) | Typical Use |
|---|---|---|---|
| Clear / Blue (Cr3+) | Bright silver-blue | 96–200 | General hardware, indoor, light outdoor |
| Yellow / Iridescent (Cr3+) | Yellow iridescent | 200–400 | Outdoor hardware, moderate corrosion environments |
| Black (Cr3+) | Black | 96–200 | Decorative, automotive interior, electronics |
| Olive Drab (Cr3+) | Army green | 200–400 | Military, defense applications |
Salt spray hours measured per ISO 9227 (neutral salt spray test). Actual performance depends on coating thickness, passivation quality, and sealant application.
Coating Thickness Standards
Zinc plating thickness for fasteners is specified by international standards. The most commonly referenced are:
| Standard | Thickness (µm) | Application |
|---|---|---|
| ISO 4042 / Fe/Zn 5 | 5 | Indoor, minimal corrosion risk |
| ISO 4042 / Fe/Zn 8 | 8 | Standard indoor/light outdoor |
| ISO 4042 / Fe/Zn 12 | 12 | Outdoor, moderate corrosion |
| ISO 4042 / Fe/Zn 25 | 25 | Severe outdoor, marine adjacent |
Thread tolerance impact: For fasteners with tight tolerance (e.g., 6g/6H), zinc thickness above 8 µm requires thread tolerance compensation. Standard practice is to plate after threading with thickness not exceeding the tolerance-allowed deviation per ISO 4042.
Hydrogen Embrittlement Considerations
Electroplating introduces hydrogen into steel substrates during the acid pickling and plating steps. For high-strength fasteners (property class 10.9 and above, or hardness ≥ 320 HV), this hydrogen must be removed through a baking process to prevent delayed brittle fracture.
Baking Requirements (per ISO 9588 / ASTM B850)
- Temperature: 190–230°C (typically 200°C)
- Duration: Minimum 4 hours at temperature (8+ hours for class 12.9)
- Timing: Must begin within 4 hours of plating completion
- Applies to: Class 10.9, 12.9 bolts; grade 10, 12 nuts; spring washers; all fasteners ≥ 320 HV
Our facility performs hydrogen embrittlement relief baking as a standard step for all high-strength fastener orders. Baking records are maintained and available upon request.
When to Choose Zinc Plating
Good Fit
- General hardware and indoor applications
- Cost-sensitive high-volume orders
- Mild outdoor environments (sheltered)
- Decorative finish requirements
- Property class 4.6–8.8 (no baking needed)
- RoHS/REACH compliance required
Consider Alternatives
- Harsh outdoor / marine → Hot-dip galvanizing
- Automotive underbody → Zinc-nickel plating
- Temperature > 120°C continuous exposure
- Salt spray > 400 hours required
- Direct contact with dissimilar metals
Zinc Plating vs Other Coatings
| Property | Zinc Plating | Zinc-Nickel | Hot-Dip Galv. |
|---|---|---|---|
| Salt spray (hours) | 96–400 | 720–1500 | 500–1000 |
| Thickness (µm) | 5–25 | 8–20 | 45–85 |
| Cost (relative) | $ | $$$ | $$ |
| Thread impact | Low | Low | High (retapping needed) |
| Max temperature | ~120°C | ~200°C | ~250°C |
| RoHS compliant | Yes (Cr3+) | Yes | Yes |
Need Zinc-Plated Fasteners?
We provide barrel and rack zinc plating services for fasteners of all sizes. From standard hardware to custom specifications, our plating lines handle orders from 1,000 to 1,000,000+ pieces with consistent quality and full traceability.