1. Why Surface Treatment Matters for Fasteners
Bare carbon steel fasteners rust quickly in atmospheric, humid or corrosive environments. Proper surface treatment protects the base material and extends service life. However, different coating processes deliver very different anti-corrosion performance. Wrong selection leads to early rust, thread interference or unnecessary cost increase.
2. Four Common Surface Treatments Compared
2.1 Electro Zinc Plating (EG)
Electrochemical process deposits a thin zinc layer on fastener surface. It is the most common and economical treatment for indoor and general-purpose fasteners like DIN933 hex bolts. Salt spray performance typically ranges from 48 to 100 hours with chromate passivation. Not recommended for long-term outdoor exposure.
2.2 Dacromet Coating
Water-based zinc-flake coating applied by spraying and baking. It provides excellent corrosion resistance without hydrogen embrittlement risk. Salt spray can reach 500–1000 hours. Suitable for high-strength bolts and automotive fasteners. No thread interference issue due to thin coating thickness.
2.3 Hot Dip Galvanizing (HDG)
Fasteners are immersed in molten zinc at around 450°C, forming a thick zinc alloy layer. Salt spray exceeds 1000 hours. Widely used for outdoor structural steel, power transmission and infrastructure. Note: thick coating may cause thread interference and requires oversize tapped nuts for matching.
2.4 Mechanical Galvanizing
Zinc powder is cold-bonded to fastener surface by impact media. It avoids hydrogen embrittlement and has lower temperature than hot dip galvanizing. Salt spray performance is between EG and HDG. Suitable for fasteners where hydrogen embrittlement is a concern.
3. Core Comparison Table
Item | Electro Zinc Plating | Dacromet | Hot Dip Galvanizing | Mechanical Galvanizing |
Typical Salt Spray | 48–100 hrs | 500–1000 hrs | 1000+ hrs | 200–500 hrs |
Coating Thickness | 5–12 μm | 8–15 μm | 45–80 μm | 10–30 μm |
Thread Interference | Low | Very low | High (needs oversize nut) | Medium |
Hydrogen Embrittlement Risk | Yes | No | No | No |
Cost Level | Low | Medium-high | Medium | Medium |
Best Scenario | Indoor, general machinery | Automotive, high-strength bolts | Outdoor structure, infrastructure | Hardware, roofing |
4. Selection Guide by Application
Indoor dry environment: Electro zinc plating is sufficient and cost-effective.
Outdoor rural / urban atmosphere: Dacromet or mechanical galvanizing recommended.
Coastal / marine environment: Hot dip galvanizing or Dacromet, avoid standard EG.
High-strength bolts (10.9 and above): Avoid electroplating due to hydrogen embrittlement, choose Dacromet.
Thread interference sensitive: Dacromet preferred, HDG needs oversize tapped nuts.
5. Common Mistakes
Mistake: Longer salt spray hours always mean better outdoor performance. Correction: Salt spray test is an accelerated lab test, not equal to real outdoor service life. Real environment includes acid rain, humidity and pollution.
Mistake: Any zinc coating works for coastal projects. Correction: Standard EG rusts quickly near the sea due to chloride. Use HDG or Dacromet.
Mistake: Dacromet and HDG are interchangeable. Correction: Dacromet is thin and non-thread-interfering, HDG is thick and needs nut re-tapping.
Mistake: Higher strength bolts can be electroplated like regular bolts. Correction: High-strength bolts have hydrogen embrittlement risk after acid pickling and electroplating.
6. FAQ
Q1: Which surface treatment has the longest salt spray hours? A1: Hot dip galvanizing generally reaches the highest salt spray duration, but Dacromet also delivers excellent performance with thinner coating.
Q2: Can I use electro zinc plated DIN933 bolts for outdoor fencing? A2: Short-term OK for mild climate. For long-term outdoor, especially coastal area, HDG or Dacromet is recommended.
Q3: Does Dacromet cover thread well and cause no interference? A3: Yes. Dacromet coating is thin and uniform, thread gauge still passes after coating.
