Dacromet vs Zn-Ni vs Electro-Galvanized: Which Coating to Choose?

"What coating should this bolt have?" — The answer affects corrosion resistance, preload consistency, hydrogen embrittlement risk, and cost. Three common fastener coatings each have their place.

Electro-Galvanized (Blue-White / Clear Zinc)

The most common and cheapest. Electro-deposited zinc layer (5–12 μm) with chromate passivation.

• Salt spray: 48–96 hours
• Cost: lowest
• Critical issue: hydrogen embrittlement risk. Hydrogen ions penetrate the steel during plating. Class 10.9+ bolts not baked after plating can fracture delayed during storage.
• Best for: Class 8.8 and below, indoor use, cost-sensitive parts

Dacromet (Zn-Flake Coating)

Water-based zinc-chromate coating, dip-spin applied, cured at 300°C. Layer consists of stacked zinc and aluminum flakes.

• Salt spray: 500–1000 hours (no scribe)
• Cost: medium (2–3× electro-zinc)
• Hydrogen-free: No pickling or electroplating in the process. This is why Dacromet dominates automotive.
• Stable friction coefficient: K = 0.12–0.14, batch-to-batch consistent
• Best for: Class 10.9+, automotive underbody, outdoor

Zn-Ni Alloy

Electroplated zinc-nickel alloy (12–15% nickel), the best corrosion protection among electro-deposited coatings.

• Salt spray: 720–1000+ hours (no scribe)
• Cost: higher (3–4× electro-zinc)
• Hydrogen risk: lower than pure zinc but still requires baking (it's still electroplating)
• Thin coating (5–8 μm) preserves fatigue life
• Best for: EV, coastal equipment, highest corrosion requirements

Decision Guide

1. Class 8.8 and below, indoor, cost-sensitive → Electro-zinc

2. Class 10.9+, outdoor, automotive → Dacromet (hydrogen-free)

3. Maximum corrosion, EV / marine → Zn-Ni

4. Need precise friction control → Dacromet or Zn-Ni (stable K)

Toprecision high-strength bolts come standard with Zn-Ni flake coating (hydrogen-free), 720+ hours salt spray.