Release time：2020-01-16 18:44 Browse：
This is a very professional and practical question. There are significant differences between environmental-friendly Dacromet (i.e., chromium-free Dacromet) and ordinary Dacromet (traditional Dacromet) for extended bolts in several aspects.
Simply put, the core difference lies in whether it contains chromium (especially hexavalent chromium), which directly leads to major differences in environmental friendliness, corrosion resistance mechanism, appearance, coating performance, and application fields.
Below is a detailed comparison from several key dimensions:
| Feature Dimension | Ordinary Dacromet (Traditional Dacromet) | Environmental-Friendly Dacromet (Chromate-Free Dacromet) |
|---|---|---|
| Environmental Friendliness | Contains hexavalent chromium (Cr6+), which is toxic and hazardous, classified as a carcinogen. Poses significant risks to the environment and human health during production and use. Wastewater and residue treatment costs are high. | Chromium-free (no chromates), non-toxic or low-toxicity. Complies with environmental regulations like EU RoHS and REACH. It is an environmentally friendly technology. |
| Corrosion Resistance Mechanism | Zinc and aluminum flakes provide sacrificial anode protection. Chromate passivation film provides dense passive protection—dual protection. The passivation film has self-healing ability. | Zinc and aluminum flakes provide sacrificial anode protection. Relies on physical barrier effect (lamellar overlapping structure) and the stability of new chromium-free passivators (e.g., silane, zirconium-based, rare earth, etc.). Weaker self-healing ability. |
| Coating Appearance | Silvery-gray, slightly matte, color is relatively uniform. | Silvery-gray, but the shade may be brighter or darker depending on the chromium-free formula. Color options might be more diverse. |
| Corrosion Resistance Performance | Excellent. Neutral salt spray test typically reaches 720 hours or more without red rust. | Good to Excellent. Depends on the formula technology. Modern high-quality chromium-free coatings can achieve 600-1000 hours or even higher, comparable to traditional products. But early or low-end products may be slightly inferior. |
| Heat Resistance | Outstanding. High curing temperature (approx. 300°C), coating can withstand high temperatures up to 300°C long-term. | Good. But usually has a lower curing temperature (approx. 200-250°C). Long-term high-temperature resistance (>300°C) might not be as good as traditional Dacromet. |
| Coating Process | Mature and stable process, but high sintering temperature, high energy consumption. | Similarly mature process, lower sintering temperature, more energy-efficient. |
| Adhesion / Torque Coefficient | Excellent, strong coating adhesion to substrate, stable torque coefficient, suitable for high-strength connections. | Also excellent, new chromium-free formulas can also provide outstanding adhesion and stable friction coefficient. |
| Cost | Raw material cost is lower, but environmental compliance and disposal costs are very high. | Raw material cost (chromium-free passivators) is higher, but comprehensive environmental cost is lower. |
| Main Application Fields | Being phased out and restricted. Still used in some special fields with extremely high requirements for heat and corrosion resistance not limited by environmental regulations. | Mainstream trend. Widely used in almost all fields requiring high-performance corrosion resistance: automobiles, wind power, high-speed rail, construction machinery, aerospace, outdoor hardware, etc. |
For extended bolts, choosing which coating also requires considering some specific factors:
Hydrogen Embrittlement Risk:
The Dacromet process (both ordinary and environmental-friendly) is inherently a hydrogen embrittlement-free process because it does not involve hydrogen evolution from cathode reactions during pickling and electroplating. This is crucial for high-strength bolts (often the application scenario for extended bolts), avoiding the risk of delayed fracture due to hydrogen embrittlement. In this aspect, both types of Dacromet are the same and superior to zinc electroplating.
Throwing Power:
Dacromet uses a dipping-centrifuging process, which provides excellent coverage for blind holes, deep recesses, and complex structures (no blind spot coating). The threaded parts of extended bolts can also receive uniform coating protection. Both Dacromet types perform consistently in this regard.
Coating Thickness and Tolerance:
The fit and tolerance of extended bolts need precise control. The Dacromet coating thickness is usually around 5-8μm, which has a minimal impact on bolt tolerance. Regardless of the Dacromet type, checking with thread go/no-go gauges after coating is necessary.
Environmental-friendly Dacromet is the absolute mainstream and future trend. As global environmental regulations (like EU's RoHS, REACH) become increasingly strict, ordinary chromium-containing Dacromet is being rapidly replaced. Unless there are extremely special heat or corrosion resistance requirements without regulatory restrictions, environmental-friendly Dacromet should be chosen.
Performance is very close. Modern, high-quality environmental-friendly Dacromet technologies (such as well-known brands like GEOMET, Delta Protekt, Zintek) offer corrosion resistance that can match or even surpass traditional Dacromet, fully meeting the long-term anti-corrosion needs of extended bolts in harsh environments.
How to Choose:
Check customer requirements: If your products are exported to regions like the EU, you must use environmental-friendly Dacromet.
Check industry standards: Industries like automotive and wind power have already fully implemented chromium-free policies.
Check performance needs: If the bolt needs to withstand continuous temperatures above 300°C, confirm the high-temperature limit of the environmental-friendly product with the coating supplier. For most application scenarios, environmental-friendly Dacromet is the optimal choice.
