What is Dacromet?
Dakro coating process is a new surface treatment technology, also known as Dakle, Dake rust, zinc chromium film, Dakman, etc. It was invented by Americans in the early 1970s and transferred to M.C.II 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 component of the socio-economic and trade fields. In 1996, the former Ministry of Machinery Industry listed Dakro technology as a key development project for sustainable and clean production in the machinery industry, which is a type of "green electroplating". In industrialized countries, Dacromet metal surface anti-corrosion technology has been used as an alternative to traditional anti-corrosion treatment processes such as electroplating zinc, hot-dip galvanizing, cadmium plating, zinc based alloy coating, phosphating, etc., which cause serious pollution. This is a new process that fundamentally reduces environmental pollution.
1. Definition and origin
DACROMET is the transliteration of "zinc chromium coating", also known as Dacron or Dixon. 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 immersion coating or spray coating process, with corrosion resistance up to 7-10 times that of traditional electroplating.
Origin: Developed by American scientist Mike Martin in the late 1950s, it was used to solve the corrosion problem of road snow melting agents (sodium chloride) on vehicle chassis. The military anti-corrosion technology that later became the US military standard (MTL-C-87115) was improved by Japan in the 1970s and promoted globally.
Introduction from China: Introduced from Japan in 1994, initially used for national defense and automotive components, later expanded to fields such as electricity, construction, and home appliances.
2. Core components and processes
2.1. Ingredients:
Metal objects: composed of substances such as zinc and aluminum, mainly consisting of ultrafine flake like zinc and ultrafine flake like aluminum.
Solvent: It is an inert organic solvent such as ethylene glycol. Inorganic acid components: such as chromic acid, etc.
Special organic matter: It is a thickening and dispersing component of the coating liquid, mainly composed of cellulose white powder for anti-corrosion mechanism.
2.2 Process flow:
1) Key points of pretreatment process
(1) Surface cleaning
Remove oil stains: Use organic solvents (such as acetone, ethanol) or alkaline cleaning agents (such as sodium hydroxide solution) to thoroughly remove oil stains such as engine oil and cutting fluid from the surface of the workpiece, avoiding affecting the adhesion of the coating.
Rust removal treatment: Remove the oxide scale and rust on the surface of the workpiece by sandblasting (commonly using quartz sand or glass beads), shot blasting or acid washing (such as hydrochloric acid or sulfuric acid solution), ensuring that the surface roughness reaches Ra 6.3~12.5 μ m, providing a good mechanical anchoring foundation for the coating.
(2) Washing and drying
Water washing: Rinse the workpiece with deionized water to remove residual cleaning agents or rust residue, and avoid impurities affecting the coating performance.
Drying: Use hot air drying (temperature about 60-80 ℃) or natural air drying to ensure that the surface of the workpiece is free of moisture and prevent the formation of bubbles or uneven coating during coating.
2) Key points of coating process
(1) Paint blending
Composition ratio: Dacromet coating is mainly composed of zinc powder (60%~80%), aluminum powder (5%~15%), chromic acid (2%~5%), and deionized water. It needs to be strictly formulated to ensure that the zinc and aluminum particles are evenly dispersed and avoid precipitation.
Viscosity control: By adding deionized water to adjust the viscosity of the coating, it is usually controlled within 18-25 seconds (tested with 4 cups). If the viscosity is too high, it can easily cause the coating to be too thick, while if it is too low, it can cause the coating to be too thin and the anti-corrosion performance to decrease.
(2) Coating method
Dip coating: Dip the workpiece into the coating tank, remove and drain excess coating, suitable for workpieces with simple structure and batch production, with good uniformity of coating thickness.
Spraying: using air spraying or electrostatic spraying, suitable for complex structural workpieces, can accurately control the coating thickness, but attention should be paid to the spraying pressure (generally 0.3~0.5MPa) and distance (15~25cm) to avoid sagging or missed coating.
Brushing: Manual operation, suitable for local repair or small batch workpieces, ensuring uniform coating and avoiding missed brushing.
(3) Coating thickness control
Choose the coating frequency according to application requirements:
One coating one drying: thickness 2-5 μ m, suitable for light corrosive environments;
Two coats and two baking: thickness 6-10 μ m, according to conventional anti-corrosion standards;
Three coatings and three baking: thickness 8-13 μ m, suitable for heavily corrosive environments (such as marine and chemical fields).
3) Key points of curing process
(1) Curing temperature and time
Low temperature pre curing: After coating, dry at 80-120 ℃ for 10-15 minutes to remove moisture from the coating and prevent the formation of bubbles during curing.
High temperature curing: Place the workpiece in a curing furnace and keep it at 300-320 ℃ for 20-30 minutes to allow chromic acid to react chemically with zinc and aluminum, forming a dense inorganic ceramic coating. Low temperature can lead to incomplete curing and decreased corrosion resistance; Excessive temperature may cause discoloration and performance degradation of the coating.
(2) Curing atmosphere
During the curing process, it is necessary to maintain air circulation in the furnace to avoid the formation of a reducing atmosphere (such as carbon monoxide) and prevent insufficient oxidation of zinc and aluminum particles, which may affect the performance of the coating.
4) Key points of post-processing technology
(1) Cooling and Inspection
After solidification, the workpiece should be naturally cooled or forced air cooled to room temperature. The appearance of the coating should be checked (uniform, without missing or bubbles) and the thickness should be measured using an electromagnetic thickness gauge. Non conforming products need to be reprocessed and coated.
(2) Closed processing (optional)
For high corrosion resistance scenarios, a layer of organic sealing agent (such as epoxy resin, silane coupling agent) can be coated on the surface of the coating to further improve salt spray resistance (up to 1000 hours or more) and wear resistance, while enhancing insulation performance.
3. Dakro anti-corrosion mechanism
Dacromet coating, with a matte silver gray appearance, is composed of extremely fine sheet metal components such as zinc, aluminum, and chromate. After oil removal and shot blasting treatment, the workpiece is immersed in Dacromet solution. Dacromet solution is a water-based treatment solution, in which metal parts are immersed or sprayed in the water-based treatment solution, then cured in a furnace, and baked at around 300 ℃ to form a film, forming inorganic coatings of zinc, aluminum, and chromium. During solidification, the volatile components such as water and organic (cellulose) substances in the coating film evaporate while relying on the oxidizing properties of high valent chromium salts in the Dacromet mother liquor, causing the zinc and aluminum paste with large negative electrode potentials to react with the iron matrix, forming chromium compounds of Fe, Zn, and Al. Due to the fact that the film layer should be obtained directly from the substrate, the anti-corrosion layer is extremely dense (compared to the anti-corrosion layer obtained by galvanizing or zinc dipping methods). In a corrosive environment, the coating will form countless primary cells, which first corrode the Al and Zn salts with negative potential until they are consumed before they may corrode to the substrate itself. Because the corrosion resistance mechanism of Dacromet involves sacrificing the anode and cathode protection to form an integrated coating, its corrosion resistance is directly proportional to the thickness of the film.
The protective effect of Dacromet film on steel substrate can be summarized as follows:
Barrier effect: Due to the overlapping of layered zinc and aluminum, the process of corrosive media such as water and oxygen reaching the substrate is hindered, which can act as an isolation and shielding effect.
Passivation effect: During the Dacromet treatment process, chromic acid reacts chemically with zinc, aluminum powder, and the base metal to form a dense passivation film, which has excellent corrosion resistance.
Cathodic protection function: The main protective function of zinc aluminum chromium coating is the same as that of galvanized layer, which is to provide cathodic protection to the substrate.
4. Coating characteristics
Dacromet coating has high corrosion resistance and a matte silver gray appearance, similar to silver powder paint. It is mainly composed of small zinc flakes and inorganic chromium polymers bonded to zinc flakes. It has excellent characteristics compared to other surface treatment methods listed:
4.1. Excellent heat and corrosion resistance
The curing temperature of Dacromet anti-corrosion film is around 300 ℃, so even if the workpiece is exposed to high temperature for a long time, its appearance will not change color, and it has excellent heat and corrosion resistance. Traditional galvanized coatings may exhibit small cracks at temperatures above 70 ℃, discoloration at temperatures between 200-300 ℃, and significantly reduced corrosion resistance.
4.2. Excellent weather resistance and chemical stability
According to the experiment, the unpurified galvanized layer generally corrodes off 1 micron in ten hours during the salt spray test; The rainbow colored purification membrane with a thickness of 3 microns was corroded through after 200 hours of salt spray test. After 100 hours of salt spray testing, the Dacromet coating was corroded off by 1 micron. The Dacromet process improves the corrosion resistance of workpieces by seven to ten times compared to traditional surface galvanizing treatment processes. A car lamp factory in Shanghai uses Dacromet technology to coat the car lamp frame, and the salt spray resistance test has reached more than 1000 hours.
4.3. No hydrogen embrittlement
Hydrogen embrittlement is a drawback that traditional galvanizing processes cannot completely overcome. Due to the characteristics of the process, Dacromet does not undergo any acid treatment during the treatment process, and there is no hydrogen permeation problem during electroplating. In addition, the coating solidifies at high temperatures, ensuring that the Dacromet coating does not have hydrogen embrittlement from a process perspective. This makes it applicable for anti-corrosion treatment of high-strength parts with high tensile strength requirements.
4.4. Non polluting and pollution-free
As a "green electroplating" process, the Dacromet process adopts a closed-loop circulation method. In the pre-treatment, the removed oil and dust are collected and processed by specialized equipment; During coating and curing, there is no problem of acid, alkali, chromium containing and other heavy metal wastewater generated during traditional electroplating processes. What is generated is only water vapor evaporated from the coating, which has been determined to be free of harmful substances controlled by national regulations.
4.5. Soft appearance and can be further coated
The Dacromet coating is matte silver gray and can be recoated on top. This meets the coating requirements of workpieces with high appearance quality requirements, such as fasteners on bicycles, mesh baskets, etc. After coating with Dacromet coating, a layer of gloss paint is also required.
4.6. Excellent permeability
Suitable for various metals, Dacromet liquid can penetrate into subtle voids, such as blind holes with complex geometric shapes, and its permeability is very good. The spring treated with it can produce a good corrosion-resistant coating. It can not only handle iron and its alloys, but also light metals (such as aluminum) and their alloys, suitable for surface anti-corrosion treatment of components such as steel, cast iron, and aluminum alloys.
5. Application Fields and Cases
Dacromet is widely used in industries with high anti-corrosion requirements, and typical cases are as follows:
| Industry | Application Case | Effect |
| Wind Power | The salt spray test for the bolts of offshore wind turbines reached 2000 hours (national standard is 480 hours), and the replacement cycle was extended from half a year to 5 years | Annual savings of maintenance costs: 2.2 million yuan |
| Agricultural Machinery | The wear resistance of gears in Northeast agricultural machinery was increased by 3 times, and the wear amount was reduced by 80% | Annual savings of material costs: 1.5 million yuan |
| Chemical Industry | Flange bolts can withstand 98% concentrated sulfuric acid immersion for 240 hours and have no leakage for 18 consecutive months | Avoided environmental protection fines of over 5 million yuan |
| Automobile | Engine components, chassis parts (withstanding high temperature of 300°C) | Solves the problem of traditional zinc plating failure at high temperatures |
6. Comparison with traditional technology
| Properties | Dacromet | Traditional galvanization (electro-galvanization / hot-galvanization) |
| Corrosion resistance | Salt spray test for 2000+ hours, thickness of only 6-8 μm | Salt spray test for 100-200 hours, thickness of 5-15 μm |
| Environmental friendliness | No wastewater or exhaust gas, but contains hexavalent chromium (cancer-causing); chromium-free technology is being promoted | Wastewater contains heavy metals, requires strict treatment |
| Hydrocracking risk | No | There (high-strength steel is prone to) |
| coating uniformity | excellent (surface uniformity of complex parts) | poor (thin coating in blind holes and gaps) |
| Heat resistance | stable at 300℃ | peeling and shedding at 100℃ |
| Cost | higher single-piece processing cost (e.g. 3 yuan per bolt) | low cost, but frequent maintenance required |
7. Environmental disputes 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 solution will become a key 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 and ceramic coatings) to further improve corrosion resistance, wear resistance, and functionality, meeting high-end application needs.
Process optimization and cost reduction: By improving coating equipment (such as automated spraying lines) and shortening curing time, production efficiency can be increased and application costs can be reduced.
Intelligence and digitalization: Introducing intelligent detection technology (such as online monitoring of coating thickness and prediction models for corrosion resistance) to enhance process stability and product quality controllability.
Future trend: Expanding towards green processes (such as water-based coatings), anti-corrosion of new energy vehicle components, and other fields.
8. precautions
Coating repair: If the coating is partially damaged, a specialized repair agent should be used for repair to avoid exposure of the base metal.
Compatibility with other coatings: The Dacromet coating has a smooth surface. If subsequent organic coatings such as paint are required, surface roughening treatment should be carried out first to improve adhesion.
Wastewater treatment: Although the Dacromet process has been reduced in chromium, strict treatment is still required for the wastewater generated during the production process to ensure compliance with discharge standards.
9. Summary of Dacromet Coating Technology
Due to its excellent performance and good environmental characteristics, Dacromet coating technology has developed from a few production lines to dozens of production lines in just a few years since its introduction to China, and many manufacturers are constantly joining in.
Dakro technology has become an important choice for metal surface treatment due to its excellent corrosion resistance, hydrogen free brittleness, and high temperature resistance. However, its environmental issues have driven the development of chromium free technology. Users can weigh the application of traditional and new Dacromet processes based on specific needs such as cost and environmental standards.
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