Key Learnings
Dacromet (zinc‑chromium coating) is an environmentally‑friendly anti‑corrosion surface‑treatment alternative to electro‑galvanizing and hot‑dip galvanizing. It follows the sequence: degreasing‑mechanical rust‑removal‑coating‑preheating‑high‑temperature sintering‑cooling. Zero acid‑pickling eliminates hydrogen‑embrittlement risk, making it ideal for high‑strength bolts. It delivers superior salt‑spray performance, heat‑resistance and uniform thin‑layer coverage for complex‑geometry parts.
| Process Step | Key Operating Points |
|---|---|
| Degreasing | Organic‑solvent / water‑based / high‑temperature carbonization; thorough degreasing guarantees coating adhesion |
| Rust‑removal | Adopt mechanical cleaning; avoid acid‑washing to prevent hydrogen‑embrittlement |
| Coating | Dip / spray / brush; control solution gravity, pH, viscosity, temperature and centrifugal‑spin parameters |
| Pre‑heating | 120‑200 ℃, hold 10‑15 min to evaporate moisture |
| Sintering | Approx. 300 ℃, hold 20‑40 min for coating curing |
| Cooling | Fully cool before inspection and subsequent operations |
Major performance merits:
- Anti‑corrosion capacity is 7‑10 times higher than electro‑galvanizing, salt‑spray resistance reaches 500‑1200 hours.
- No hydrogen‑embrittlement risk; highly suitable for high‑strength fasteners.
- Heat‑resistant up to 300 ℃; uniform coating thickness; good friction‑wear performance, weather‑resistance and chemical stability.
Originating in the 1970s, Dacromet is widely‑deployed in automotive industry and has expanded to construction, military, shipping, railway, power, agricultural‑machinery, mining and bridge sectors.
Note: Process parameters of coating bath and thermal‑treatment directly determine final coating quality; mechanical rust‑removal is strongly preferred over acid‑pickling.
In industrialized countries, the Dacromet metal surface anti-corrosion technology has been adopted as a new anti-corrosion treatment process to replace many traditional processes such as electro-galvanizing, hot-dip galvanizing, electroplating of cadmium, zinc-based alloy plating, and phosphating, which are highly polluting. This is a new process that fundamentally reduces environmental pollution.
Dacromet Process Flow
Substrate – Degreasing – Rust Removal – Coating – Preheating – Curing – Cooling
1. Degreasing: The surface of workpieces with grease must be degreased. There are generally three methods: degreasing with organic solvents, degreasing with water-based degreasing agents, and degreasing by high-temperature carbonization. Whether the degreasing is thorough and effective will directly affect the adhesion and corrosion resistance of the coating.
2. Rust removal: Workpieces with rust or burrs are strictly prohibited from being directly coated. They must go through the rust removal and burr removal process. This process is best done mechanically to avoid acid washing, which can cause hydrogen embrittlement and also affects the corrosion resistance of the Dacromet coating.
3. Coating: Clean workpieces after degreasing and rust removal must be coated as soon as possible by dipping, spraying or brushing. During the coating process, the specific gravity, pH, viscosity, Cr6+ content, temperature and flow of the coating solution will directly affect the coating effect and performance of the workpiece.
Therefore, during the coating process, the relationship between temperature, solution indicators and the speed of the centrifuge during dipping must be properly adjusted.
4. Pre-baking: Workpieces with wet Dacromet coatings must be pre-baked at 120-200°C for 10-15 minutes (depending on the heat absorption of the workpiece) to evaporate the moisture in the coating solution and make it flow evenly.
5. Sintering: Workpieces after pre-baking must be sintered at a high temperature of about 300°C for 20-40 minutes (depending on the heat absorption of the workpiece). The sintering time can also be shortened by appropriately increasing the temperature.
6. Cooling: After sintering, the workpieces must be fully cooled through a cooling system before subsequent processing or final inspection.
Advantages of Dacromet Process
Dacromet is a new type of anti-corrosion coating. Compared with traditional electro-galvanizing, the zinc-chromium coating has extremely strong anti-corrosion performance, which is 7 to 10 times that of galvanizing.
The coating process does not produce hydrogen embrittlement and is particularly suitable for surface anti-corrosion treatment of high-strength bolts and components. It can withstand a temperature of up to 300℃ and has a salt spray resistance of 500 to 1200 hours.
In addition, it has the advantages of uniform coating, low friction and wear resistance, as well as good weather resistance and chemical stability.
Dacromet technology has gained widespread application since its introduction in the 1970s, especially in the automotive industry of developed countries such as the United States and Japan, due to its advantages of easy operation, energy conservation, low environmental pollution and the ability to avoid hydrogen embrittlement, which are unmatched by traditional electro-galvanizing and hot-dip galvanizing technologies.
It has now expanded to fields such as construction, military, shipping, railways, power, home appliances, agricultural machinery, mines and bridges.
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