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A bolt’s corrosion resistance directly determines its service life in the field. The mainstream surface treatments on the market — hot-dip galvanizing, electro-galvanizing, and Dacromet — differ significantly in process, corrosion performance, cost, and application. This article compares them side by side and gives recommendations for high-humidity, high-salinity environments in the Middle East and Central Asia.
1. Process Comparison
| Process | Principle | Coating Thickness | Appearance |
|---|---|---|---|
| Hot-Dip Galvanizing | Parts dipped in ~450°C molten zinc, forming a zinc-iron alloy layer | 45-85μm | Rougher surface, gray color |
| Electro-Galvanizing | Electrolytic deposition, pure zinc layer adheres to the surface | 5-25μm | Smooth surface, silver/rainbow finish |
| Dacromet | Zinc-chromium coating cured by high-temp baking, no hydrogen embrittlement risk | 6-12μm (multi-layer) | Silver-gray, matte finish |
2. Corrosion Resistance Comparison (Neutral Salt Spray Test NSS)
| Process | Salt Spray Duration (until red rust) | Corrosion Resistance |
|---|---|---|
| Electro-galvanizing (no passivation) | ~24-72 hours | Lower |
| Electro-galv + blue/white passivation | ~96 hours | Fair |
| Electro-galv + color passivation | ~150-200 hours | Moderate |
| Hot-dip galvanizing | ~500-1000 hours (depending on thickness) | Good |
| Dacromet | ~720-1000+ hours | Excellent |
Note: Actual salt spray test duration is affected by coating thickness, substrate condition, and the test standard used (ISO 9227 / ASTM B117). The figures above are industry experience ranges — always refer to the supplier’s actual test report.
3. Hydrogen Embrittlement Risk Comparison
This is a critical concern when selecting surface treatment for high-strength bolts (grade 10.9 and above):
- Electro-galvanizing: The electrolytic process causes the part to absorb hydrogen atoms, which penetrate the metal lattice, creating a significant hydrogen embrittlement risk. Grade 10.9+ bolts using electro-galvanizing require strict post-treatment de-hydrogenation (e.g. baking at 190-220°C);
- Hot-dip galvanizing: The high-temperature dipping process has relatively low hydrogen embrittlement risk, but the thicker coating may affect thread precision — allowance must be made in advance;
- Dacromet: No electrolytic process, so essentially no hydrogen embrittlement risk — the preferred surface treatment for high-strength bolts (especially grade 12.9).
4. Cost and Application
| Process | Relative Cost | Typical Application |
|---|---|---|
| Electro-galvanizing | Lowest | General indoor industrial use, furniture hardware, light-load connections |
| Hot-dip galvanizing | Medium | Structural steel, outdoor facilities, anchor bolts, railings |
| Dacromet | Higher | High-strength bolts, automotive industry, high-corrosion environments, precision threaded parts |
5. Recommendations for High-Humidity, High-Salinity Environments (Middle East & Central Asia)
For coastal Saudi Arabia, Dubai, and other high-humidity, high-salinity, high-temperature environments, as well as saline-alkaline soil areas in parts of Central Asia, we recommend:
- Standard structural bolts (grade 8.8 and below): Hot-dip galvanizing is preferred, with thickness ≥65μm recommended — a good balance of cost and corrosion performance;
- High-strength bolts (grade 10.9 and above): Dacromet is preferred, avoiding hydrogen embrittlement risk while achieving excellent corrosion resistance;
- Extreme corrosion environments (offshore platforms, chemical plants): Consider stainless steel 316L, or a sealing topcoat applied over Dacromet;
- For precision connections requiring high thread accuracy, avoid thick hot-dip galvanizing coatings, as they may affect assembly.
6. Purchasing Recommendations
- When requesting quotes, specify: the surface treatment name, coating thickness requirement, salt spray duration requirement, and testing standard (ISO 9227/ASTM B117);
- For high-strength bolts (grade 10.9+), proactively ask the supplier whether de-hydrogenation treatment is performed, and request a hydrogen embrittlement test report;
- For large-volume orders, request coating thickness sampling reports and salt spray test sample photos from the supplier;
- When mating bolts and nuts with different surface treatments, ensure adequate thread clearance to avoid assembly difficulties.
Need Fasteners With a Specific Corrosion Grade?
Rongrun Fasteners offers hot-dip galvanizing, electro-galvanizing, Dacromet and other surface treatments, with recommendations tailored to your operating environment
Get a Quote Now →Related Products
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