Knowledge

The Dacromet coating process is a brand-new surface treatment technology, also known as Dacrol, Dacron, zinc chromate film, and Dacromet, etc. It was invented by Americans in the early 1970s and was transferred to MCI Company in the United States, DACRAL Company in France, and NDS Company in Japan around 1976. With the development of the global economy, environmental protection has increasingly become an important part of social economy and trade. In 1996, the former Ministry of Machinery Industry of China listed the Dacromet process as a key development project for clean production in the sustainable development of the machinery industry, which is a kind of "green electroplating". In industrialized countries, the Dacromet metal surface anti-corrosion technology has been adopted as an anti-corrosion treatment process to replace traditional processes such as electro-galvanizing, hot-dip galvanizing, electro-cadmium plating, zinc-based alloy plating, and phosphating, which cause serious pollution. This is a new process that fundamentally reduces environmental pollution.

I. Definition and Origin

DACROMET is the transliteration of "zinc chromate coating". It is a new type of anti-corrosion coating mainly composed of zinc powder, aluminum powder, chromic acid and deionized water. The core technology is to form an inorganic anti-corrosion layer on the metal surface through dip coating or spray coating processes, with corrosion resistance up to 7-10 times that of traditional electro-galvanizing.

 

Origin: Developed by American scientist Mike Martin in the late 1950s to address the corrosion of vehicle chassis caused by road de-icing salt (sodium chloride). It later became a military anti-corrosion technology under the US military standard (MTL-C-87115) and was improved and promoted globally by Japan in the 1970s.

 

Introduction to China: Introduced from Japan in 1994, initially used in defense and automotive parts, and later expanded to the power, construction, and home appliance sectors.

 

II. Core Components and Processes

1. Components:

- Metallic substances: Composed of substances such as zinc and aluminum, mainly ultrafine flake zinc and ultrafine flake aluminum.

- Solvent: Inert organic solvents, such as ethylene glycol, etc.

- Inorganic acid components: Such as chromic acid, etc.

- Special organic substances: Viscosity-increasing and dispersing components of the coating liquid, mainly white powder of cellulose type. Anti-corrosion mechanism.

 

2. Process Flow:

1) Key Points of Pretreatment Process

a. Surface Cleaning

Removal of oil stains: Use organic solvents (such as acetone, ethanol) or alkaline cleaners (such as sodium hydroxide solution) to thoroughly remove oil, cutting fluid, and other contaminants from the surface of the workpiece to prevent affecting the adhesion of the coating.

Rust removal: Remove the oxide scale and rust on the surface of the workpiece through sandblasting (commonly using quartz sand or glass beads), shot blasting, or acid washing (such as hydrochloric acid, sulfuric acid solution) to ensure the surface roughness reaches Ra 6.3~12.5μm, providing a good mechanical anchoring base for the coating.

b. Water washing and drying

Water washing: Rinse the workpiece with deionized water to remove residual cleaning agents or rust debris, avoiding impurities that may affect coating performance. Drying: Use hot air drying (at approximately 60-80°C) or air dry naturally to ensure the workpiece surface is free of moisture, preventing air bubbles or uneven coating during application.

 

2) Key points of coating process

a. Paint mixing

Component ratio: Dacromet paint mainly consists of zinc powder (60% - 80%), aluminum powder (5% - 15%), chromic acid (2% - 5%), and deionized water. It must be mixed strictly according to the formula to ensure uniform dispersion of zinc and aluminum particles and prevent sedimentation.

Viscosity control: The viscosity of the paint is adjusted by adding deionized water, usually controlled at 18 - 25 seconds (Ford Cup test). Excessively high viscosity may lead to overly thick coating, while too low viscosity may result in overly thin coating and reduced anti-corrosion performance.

b. Coating methods

Dip coating: The workpiece is immersed in the coating tank, then taken out and excess coating is drained off. It is suitable for simple-structured workpieces in mass production, and the coating thickness is relatively uniform.

Spray coating: Air spray coating or electrostatic spray coating is used. It is suitable for workpieces with complex structures, and the coating thickness can be precisely controlled. However, attention should be paid to the spray pressure (generally 0.3 to 0.5 MPa) and distance (15 to 25 cm) to avoid sagging or missed coating.

Brush coating: Manual operation, suitable for local repair or small-batch workpieces. It is necessary to ensure uniform coating and avoid missed brushing.

c. Coating Thickness Control

Select the number of coating applications based on the application requirements:

One coat and one bake: thickness 2 to 5 μm, suitable for light corrosion environments;

Two coats and two bakes: thickness 6 to 10 μm, the conventional anti-corrosion standard;

Three coats and three bakes: thickness 8 to 13 μm, suitable for heavy corrosion environments (such as marine and chemical fields).

 

3) Key Points of Curing Process

a. Curing Temperature and Time

Low-temperature Pre-curing: After coating, the workpiece should be dried at 80~120℃ for 10~15 minutes to remove moisture from the coating and prevent bubble formation during curing.

High-temperature Curing: Place the workpiece in a curing oven and maintain it at 300~320℃ for 20~30 minutes to allow chromium acid to react chemically with zinc and aluminum, forming a dense inorganic ceramic-like coating. A temperature that is too low will result in incomplete curing and reduced corrosion resistance; a temperature that is too high may cause the coating to discolor and deteriorate in performance.

b. Solidification Atmosphere

During the solidification process, it is necessary to maintain air circulation inside the furnace to avoid the formation of a reducing atmosphere (such as carbon monoxide), which could prevent the zinc and aluminum particles from being fully oxidized and thus affect the performance of the coating.

 

4) Key Points of Post-treatment Process

a. Cooling and Inspection

After curing, the workpiece should be cooled naturally or by forced air cooling to room temperature. Inspect the coating appearance (it should be uniform, without missed coating or bubbles) and thickness (measured by an electromagnetic thickness gauge). Unqualified products need to be re-pretreated and recoated.

b. Encapsulation Treatment (Optional)

For scenarios with high corrosion resistance requirements, an organic encapsulant (such as epoxy resin or silane coupling agent) can be applied on the coating surface to further enhance salt spray resistance (up to over 1000 hours) and wear resistance, while also improving insulation performance.

 

 

III. Dacromet Anti-corrosion Mechanism

 

The Dacromet coating, with a matte silver-gray appearance, is composed of extremely fine flaky metals such as zinc and aluminum, along with chromates and other components. After the workpiece undergoes degreasing and shot blasting treatment, it is immersed in the Dacromet solution. The Dacromet solution is a water-based treatment liquid. After the metal parts are immersed or sprayed with the water-based treatment liquid, they are then cured in an oven and baked at around 300°C to form a zinc, aluminum, and chromium inorganic coating. During the curing process, the volatile components such as water and organic substances (cellulose) in the coating film evaporate. At the same time, relying on the oxidizing property of the high-valent chromium salts in the Dacromet mother liquid, the zinc and aluminum flakes with a more negative electrode potential react with the iron substrate to form chromium salt compounds of Fe, Zn, and Al. Since the coating film is directly formed by reacting with the substrate, this anti-corrosion layer is extremely dense (uncomparable to the anti-corrosion layers obtained by galvanizing or zinc immersion methods). In a corrosive environment, the coating forms countless galvanic cells, where the more negative Al and Zn salts are corroded first until they are consumed, and then the substrate itself may be corroded. Because the Dacromet anti-corrosion mechanism integrates sacrificial anode and cathodic protection into a single coating, its anti-corrosion performance is directly proportional to the thickness of the film.

 

The protective effect of the Dacromet coating on the steel substrate can be summarized as follows:

1) Barrier effect: Due to the overlapping of flaky zinc and aluminum layers, the progress of corrosive media such as water and oxygen reaching the substrate is hindered, which can play an isolating and shielding role.

2) Passivation effect: During the Dacromet treatment process, chromic acid reacts chemically with zinc, aluminum powder and the substrate metal to form a dense passivation film, which has excellent corrosion resistance.

3) Cathodic protection effect: The main protective effect of the zinc-aluminum-chromium coating is the same as that of the galvanized layer, which is to provide cathodic protection to the substrate.

 

 

IV. Characteristics of the Coating

The Dacromet coating features high corrosion resistance and a matte silver-gray appearance, somewhat resembling silver powder paint. It is mainly composed of tiny zinc flakes and an inorganic chromium polymer that binds the zinc flakes. It has outstanding characteristics that are unmatched by other surface treatment methods:

 

1. Excellent heat corrosion resistance: The curing temperature of the Dacromet anti-corrosion film is around 300°C. Therefore, even if the workpiece is exposed to high temperatures for a long time, its appearance will not change color, and it has excellent heat corrosion resistance. Traditional galvanized layers will develop tiny cracks at temperatures above 70°C and change color at 200-300°C, with a significant decrease in corrosion resistance.

 

2. Superior weather resistance and chemical stability: Tests have shown that un-purified galvanized layers typically corrode by 1 micron in a salt spray test within ten hours; a 3-micron thick rainbow-colored purified film is penetrated after 200 hours of salt spray testing. After 100 hours of salt spray testing, the Dacromet coating is only corroded by 1 micron. Compared to traditional surface galvanizing treatment, the Dacromet process enhances the corrosion resistance of the workpiece by seven to ten times. The lamp brackets we coated for our clients using the Dacromet process have passed salt spray tests for over 1,000 hours.

 

3. No hydrogen embrittlement: Hydrogen embrittlement is a drawback that traditional galvanizing processes cannot completely overcome. Due to the process characteristics, Dacromet does not undergo any acid treatment during processing and does not have the hydrogen penetration issue that occurs during electroplating. Additionally, the coating is cured at high temperatures, ensuring that the Dacromet coating does not suffer from hydrogen embrittlement. This makes it suitable for anti-corrosion treatment of high-strength parts with high tensile strength requirements.

 

4. No pollution or environmental hazards: As a "green electroplating" process, Dacromet uses a closed-loop circulation system. During the pretreatment stage, oil and dust removed are collected and treated by specialized equipment. During the coating and curing stages, there is no issue of acidic, alkaline, or heavy metal-containing wastewater as in traditional electroplating processes. The only byproduct is water vapor evaporated from the coating, which has been tested and found to contain no harmful substances regulated by the state.

 

5. Soft appearance and further coating capability: The Dacromet coating has a matte silver-gray appearance and can be further coated. This meets the coating requirements of workpieces with high appearance quality standards, such as fasteners and baskets on bicycles. After being coated with Dacromet, they still require an additional topcoat of clear paint.

 

6. Excellent penetration and applicability to various metals: The Dacromet solution can penetrate into tiny crevices, such as complex-shaped blind holes. It has excellent penetration. Springs treated with it can form a good anti-corrosion coating. It can not only treat iron and its alloys but also aluminum and its alloys, making it suitable for surface anti-corrosion treatment of steel, cast iron, aluminum alloy, and other components.

 

 

V. Application Fields and Cases

Dacromet is widely used in industries with high requirements for anti-corrosion. Typical cases are as follows:

Trade	Application case	Effect
Wind electricity	The salt spray test of offshore fan bolts reaches 2000 hours (national standard 480 hours), and the replacement cycle is extended from half a year to 5 years	The annual maintenance cost is 2.2 million yuan
Agricultural machinery	The wear resistance of agricultural machinery gears in northeast China is increased by 3 times and the wear amount is reduced by 80%	The annual material cost is 1.5 million yuan
Chemical industry	The threaded bolt can be soaked in 98% concentrated sulfuric acid for 240 hours, and there is no leakage for 18 consecutive months	Avoid environmental protection fines of more than 5 million yuan
Automobile	Engine parts, chassis parts (high temperature resistant 300℃)	Solve the problem of traditional high temperature failure of galvanizing

 

VI. Comparison with Traditional Technologies

Compared with electro-galvanizing and hot-dip galvanizing, the advantages and limitations of Dacromet are as follows:

Characteristic	Dacron	Traditional galvanizing (electro-galvanizing / hot-dip galvanizing)
Corrosion resistance	Salt spray test for more than 2000 hours, thickness only 6-8μm	Salt spray test 100-200 hours, thickness 5-15μm
Environmental friendliness	No waste water or gas, but it contains hexavalent chromium (carcinogenic); chromium-free technology is being promoted	The wastewater contains heavy metals and needs to be treated strictly
Hydrogen embrittlement risk	not have	Yes (high strength steel is prone to occur)
Coating uniformity	Excellent (uniform surface of complex parts)	Poor (thin coating in blind holes and gaps)
Heat resistance	Stable at 300℃	100℃ peeling off
Prime cost	The processing fee of a single piece is high (e.g., bolt 3 yuan/ piece)	Low cost, but frequent maintenance

 

VII. Environmental Controversies and Development Trends

Hexavalent Chromium Risk: Traditional Dacromet contains hexavalent chromium (a carcinogen), and long-term exposure may cause respiratory diseases, skin cancer, etc. Protective measures include closed production, personal protective equipment, and environmental monitoring.

Chromium-Free Technology: By 2025, chromium-free Dacromet coating liquid will become a focus in the industry, using trivalent chromium or non-chromium passivators, but cost and technological maturity remain challenges.

Composite Coating Technology: Combining Dacromet coating with other coatings (such as organic coatings, ceramic coatings) to further enhance corrosion resistance, wear resistance, and functionality, meeting the demands of high-end applications.

Process Optimization and Cost Reduction: By improving coating equipment (such as automated spraying lines) and shortening curing time, production efficiency is increased and application costs are reduced.

Intelligentization and Digitalization: Introducing intelligent detection technologies (such as online monitoring of coating thickness, corrosion resistance performance prediction models) to enhance process stability and product quality controllability.

Future Trends: Expansion into green processes (such as water-based coatings), anti-corrosion of new energy vehicle components, and other fields.

 

VIII. Precautions

Coating Repair: If the coating is locally damaged, a dedicated repair agent should be used for restoration to prevent exposure of the base metal.

Compatibility with Other Coatings: The surface of Dacromet coating is smooth. If subsequent organic coatings such as paint need to be applied, surface roughening treatment should be carried out first to improve adhesion.

Wastewater Treatment: Although the Dacromet process has been low-chromiumized, wastewater generated during production still needs to be strictly treated to ensure compliance with discharge standards.

 

IX. Summary of Dacromet Coating Technology

 

Due to its excellent performance and favorable environmental protection features, Dacromet coating technology has developed from just a few production lines when it was introduced to China to dozens of lines in just a few years. Moreover, many manufacturers are constantly joining in.

 

Dacromet technology, with its outstanding anti-corrosion, non-hydrogen embrittlement and high-temperature resistance properties, has become an important choice for metal surface treatment. However, its environmental issues have driven the development of chromium-free technologies. Users can weigh the application of traditional and new Dacromet processes based on specific needs such as cost and environmental standards.

 

Vigor has more than 20 years of experience in producing and supplying castings, forged parts, and CNC machined parts. We have a strong capability supply chain of various surface coating partners here. If anything we can help, please contact us soon. info@castings-forging.com