Dacromet coating is considered one of the advanced metal surface coating techniques to prevent corrosion and damage even in harsh conditions. Dacromet is actually a water-based solution of zinc flakes, aluminium powder and an inorganic binder such as Chromic salts. Dacromet coating forms a multilayer barrier through a precise dip-spine and curing process to achieve its protective properties.
The standard salt spray test found that the Dacromet-coated sample consumed only 1 μm of coating thickness every 100 h, while the electro-galvanized sample consumed 1 μm every 10 h.
Contents
Dacromet Coating Process
Here are the basic steps of Dacromet coating process:
Cleaning and degreasing
First of all, the coating of the components to be done is decreased by using alkaline cleaners to make the adhesion effective. Mechanical cleaning for any sort of scaling and rusting is done by shot blasting.
Coating Dacromet Solution
After thorough cleaning and drying, the components are coated with dip-spin, dip-drain, or spray coating. Dipped into a solution of Dacromet with the help of a metal basket
- Dip-Spin: Small parts are dipped into the slurry and spun to remove excess.
- Dip-Drain: Larger parts whose weight is more than 0.5 kg. or longer than 200 mm are dipped and allowed to drain, suited for tubes or stampings.
- Spray Coating: Used for complex geometries or touch-up
Pre-Drying
Once the coating is applied, a flash-off step is carried out to remove water and light solvents:
Here, the temperature is maintained between 50 to 80°C for 10 to 20 minutes. This step helps prevent defects such as bubbling, streaks or peeling during curing.
Thermal Curing
The final step for Dacromet coating involves curing. After flash-off, parts are baked in a convection oven to cure the coating and bind the metal flakes. The temperature of the oven is set at 300°C – 350°C for about 20–40 minutes, depending on part mass and oven design.
At this stage, all chemical reactions between Zn Al and binder occur; as a result, zinc and aluminium oxides are formed, and the binder undergoes crosslinking. Finally, a hard, durable, and corrosion-resistant film forms.
How Dacromet Coating Works
Galvanic Protection
Dacromet coating involves galvanic action in order to protect the underlying steel from corrosion. As the slight rusting strat, zinc acts as an anode and oxidizes forming zinc oxides (ZnO) and zinc carbonates (ZnCO₃). These oxides help in sealing micro-cracks which are responsible for corrosion.
Passivation Layer
During the curing process of Dacromet coating, a dense and adherent passive film.of oxides such as ZnO, Al₂O₃, and Cr₂O₃ forms. The film is chemically stable, insoluble in water, and non-conductive, which limits the corrosion rate of zinc and steel.
Barrier Protection
Barriers protection is one of the primary defence mechanisms of Dacromet plating. In this process, the Zinc and aluminium flakes align in overlapping layers, and the binder reacts by locking them in place. As a result, a glass-like film is produced, which lowers the moisture and other chemical penetration to the steel.
Advantages of Dacromet Coating
High Corrosion Resistance
Dacromet plating develops a multilayer barrier that can withstand over 1,000 hours of salt spray testing, indicating its superior corrosion resistance even in harsh, corrosive environments. The combination of zinc and aluminium flakes provides both physical and galvanic protection that blocks salts, moisture and other chemical penetration.
Heat Resistance
Dacromet finish is highly resistant to heat, even at temperatures over 300°C (572°F). This makes it suitable for components that are exposed to high-heat environments, such as in automotive and aerospace.
No Hydrogen Embrittlement
Like other coating methods, dacromet coating does not involve hydrogen embrittlement. This makes Dacromet ideal for high-strength steel fasteners, springs, and components that would be vulnerable to embrittlement.
Self-lubricating (for Dacromet 500)
The Dacromet 500 variant offers low friction that eliminates the need for additional lubricants.
Excellent Adhesion
Dacromet coatings offer strong adhesion to the base material, making it an excellent base for painting or further coatings without the risk of flaking.
Environmentally Friendly
Dacromet Coating does not use the hazardous chemicals typically found in traditional coatings, such as lead, cadmium, or hexavalent chromium. Its water-based nature reduces its environmental impact during application and disposal because it does not involve any VOC.
Low maintenance:
Dacromet plating has self-repairing capabilities due to the presence of zinc particles. The galvanic action of zinc ensures long-term protection without the need for frequent recoating. As a result, the surface retains its lustre and structural integrity over time.
Limitations of Dacromet Coating
Contains Hexavalent Chromium (Cr⁶⁺)
Some Dacromet coatings, such as the standard types (320, 500), may contain hexavalent chromium. The presence of Cr⁶⁺ can pose environmental risks.
Surface Appearance
The natural finish of Dacromet coatings is dull and silver-grey, which may not be aesthetically pleasing. As a result, it may not be useful for all applications, especially for consumer products, where appearance is a priority.
Not as Hard as Electroplated Coatings
Although it provides good corrosion resistance, it may not offer the same level of hardness as electroplated coatings like hard chrome. This makes it less suitable for high-wear applications.
Higher Cost than Traditional Zinc Plating
Dacromet coatings tend to be more expensive per part compared to traditional zinc plating.
Types of Dacromet
Here are three main types of Dacromet coating with their composition and use:
| Type | Composition | Coating Thickness | Special Characteristics |
|---|---|---|---|
| Dacromet 320 Coating | 75% Zinc, 10% Aluminium flakes, Trivalent chromate passivation matrix | 5–7 µm (Grade A) | Basic corrosion protection, silver-grey color |
| Dacromet 500 Coating | Similar to Dacromet 320 + dry lubricant incorporated in matrix | 8–10 µm (Grade B) | Self-lubricating, reduced staining, ideal for fasteners |
| Dacromet LC | Modified version of Dacromet 320/500 with significantly reduced Cr⁶⁺ (<0.3 µg/cm²) | 5–10 µm (depending on grade) | Environmentally improved, meets stricter regulations |
What is Dacromet Coating Used For?
Automotive Sector
In the Automotive sector, Dacromet coating is used to protect brake system parts, fasteners, and chassis components from corrosion. These components remain in contact with salts and high-temperature conditions to increase their durability and reduce maintenance; they are coated with Dacromet.
Marine Sector
Dacromet-coated components like bolts, fittings, and hardware offer high resistance to salt spray and continuous humidity. It prevents rusting in offshore platforms, ship parts, and port equipment.
Powder Metallurgy
In powder metallurgy, sintered components are porous and vulnerable to moisture ingress that is why Dacromet finish is used to enhance their corrosion resistance. The coating forms a uniform, inorganic barrier. It penetrates micro-pores and surface irregularities, sealing them against rust and oxidation.
Agriculture and Heavy Machinery
In Agriculture, the Dacromet coating is applied to machinery components such as bolts, fasteners, linkages, and hydraulic parts. Due to its excellent resistance to corrosion and chemical attack, this coating protects it from mud, fertilizers, and extreme weather conditions.
Dacromet vs Zinc Plating
The following table shows the comparison between Dacromet coating and zinc plating.
| Attribute | Dacromet Coating | Zinc Electroplating |
|---|---|---|
| Process Type | Inorganic, non-electrolytic (dip-spin, spray) | Electrolytic (cyanide, alkaline non-cyanide, acidic chloride solutions) |
| Base Metal Compatibility | Suitable for a wide range including steel, cast iron, and alloys | Mainly used on steel and stainless steel |
| Corrosion Resistance | Very high (800–1000+ hrs in salt spray) | Moderate to good (96–500+ hrs depending on thickness and post-treatment) |
| Coating Thickness | 7–8 µm typically | 5–25 µm (depending on method and part spec) |
| Hydrogen Embrittlement Risk | None (non-electrolytic) | High risk, especially with high-strength steels and cyanide baths |
| Heat Resistance | High (over 300°C) | Low to moderate (typically <120°C) |
| Environmental Impact | May contain hexavalent chromium (toxic), Cr-free versions available | Cyanide baths highly toxic |
| Appearance | Matte silver-gray finish | Bright, shiny metallic |
| Coating Adhesion | Strong chemical bond with substrate | Good adhesion, depends on surface prep and bath type |
| Cost | Higher | Lower |
| Application Areas | Automotive (bolts, brake components), marine, aerospace, industrial fasteners | Automotive (interior parts, fasteners), consumer goods, undercoat for paint |
Future of Dacromet
Low Cr(VI) Content
Cr(VI) is highly toxic and carcinogenic, which has caused certain restrictions on the application of Dacromet technology.
Fine Flaky Zn powder
Metal powder is one of the most important components of Dacromet coating, and its quality plays a key role in determining the performance of the coating. When preparing Dacromet coating, flaky zinc powder with a thickness of 0.2~0.5μm and an average particle size of 15~20μm is used.
Reduced the Drying Temperature and Time
Reducing the baking temperature and time helps save energy and reduce costs.
Increased Hardness and Wear Resistance
Due to the low hardness of the Dacromet film layer, its wear resistance is poor. This makes the workpiece vulnerable to external impact and scratches, ultimately affecting the surface integrity and corrosion resistance of the product.
