Fastener electroplating considerations

Fasteners (screws, bolts, nuts, washers) require specialized electroplating to balance corrosion resistance, thread fit, and durability. Here are critical factors:

 

Author: Anna

 

Fastener Electroplating: Key Considerations

 

Fasteners (screws, bolts, nuts, washers) require specialized electroplating to balance corrosion resistance, thread fit, and durability. Here are critical factors:

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1. Plating Selection Based on Application

Plating Type	Thickness	Pros	Cons	Common Uses
Zinc	5–25 μm	Low cost, sacrificial protection	White rust risk	Automotive, construction
Zinc-Nickel	5–15 μm	High corrosion resistance (1,000+ hrs salt spray)	Higher cost	Aerospace, marine
Nickel	10–50 μm	Wear resistance, decorative	Poor galvanic protection	Electronics, food industry
Chromate Conversion	0.5–2 μm (post-Zn)	Enhances corrosion resistance	Hexavalent Cr (RoHS concerns)	Military, outdoor hardware
Tin	3–10 μm	Good conductivity, non-toxic	Soft coating	Electrical connectors

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2. Critical Process Considerations

① Pre-Treatment

• Degreasing: Ultrasonic cleaning for blind holes.

• Acid Pickling: Remove oxides (HCl/H₂SO₄), but avoid hydrogen embrittlement (bake at 200°C for 4 hrs if needed).

• Barrel Tumbling: Deburring for consistent plating.

② Plating Method

• Barrel Plating: Cost-effective for bulk small fasteners (<100 mm).

• Rack Plating: For precision threads or large bolts (avoids contact marks).

③ Post-Treatment

• Passivation: Chromate (yellow/black) or trivalent Cr alternatives for Zn coatings.

• Sealing: Silicate/PTFE seals to reduce porosity.

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3. Dimensional & Functional Checks

• Thread Fit: Plate thickness must not exceed 5–8% of thread pitch (e.g., M6 bolt: max ~8 μm coating).

• Torque-Tension Testing: Ensure plated threads don’t gall under load.

• Hydrogen Embrittlement Risk: High-strength fasteners (≥10.9 grade) require baking within 1 hr post-plating.

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4. Industry Standards

• ASTM B633 (Zn/Ni), ISO 4042 (Fastener plating)

• RoHS/REACH Compliance: Avoid hexavalent chromium (use trivalent Cr or organic passivates).

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5. Common Defects & Solutions

Defect	Cause	Fix
Uneven Coating	Poor current distribution	Optimize barrel rotation speed (2–10 RPM)
White Rust	Inadequate passivation	Improve drying, add silicate sealant
Flaking	Poor adhesion	Enhance cleaning/activation steps

 

Pro Tip: For critical applications (e.g., automotive), specify micro-cracked chromium over nickel to prevent corrosion creep.