Do You Understand What DACROMET Is?

“Dacromet” is a transliterated term, and its English name is “DACROMET”. This surface treatment technology has a wide range of applications in the industrial field due to its unique coating properties.

History

Dacromet was first developed in the late 1950s. At that time, in the cold winters of North America and Northern Europe, salt was commonly used on roads to lower the freezing point and relieve traffic, but the chloride ions in sodium chloride eroded the steel substrate, causing serious damage to vehicles. American scientist Mike Martin developed a highly dispersed water-soluble coating with zinc flakes as the main component, along with aluminum flakes, chromic acid, and deionized water as solvents. After a fully closed-loop coating and baking process, a thin coating was formed, which successfully resisted the erosion of chloride ions, marking a new level in anti-corrosion technology and revolutionizing the short lifespan drawback of traditional anti-corrosion processes.

In the 1970s, Japan's NDS company introduced Dacromet technology from the US MCI company and bought out the usage rights in the Asia-Pacific region and took a controlling stake in the US MCI company. After being improved by Japan, Dacromet technology developed rapidly in Japan, with more than 100 coating factories and more than 70 pharmaceutical units emerging. In 1994, China officially introduced Dacromet technology from Japan. Initially, it was only used in the defense industry and domestic auto parts, and later it was applied to various industries such as power, construction, ocean engineering, household appliances, hardware and standard parts, railways, bridges, tunnels, highway guardrails, petrochemical, bioengineering, medical devices, powder metallurgy, etc.

Definition and Principle

1.Definition

The Dacromet coating, also known as the zinc-chromium coating, is a new type of metal surface treatment technology. It is an inorganic water-soluble coating made with zinc powder, aluminum powder, and chromic anhydride as the base materials.

2.Principle

After the coating is applied to the surface of a metal workpiece and then baked at a high temperature, the water in the coating evaporates, and a chemical reaction occurs between chromic acid and zinc and aluminum, forming an amorphous inorganic conversion film, which is the Dacromet coating. This coating can adhere tightly to the metal surface and provide good protection for the metal.

3.Performance Characteristics

High Corrosion Resistance: Zinc and aluminum in the Dacromet coating act as sacrificial anodes, and chromates form a passive film. It has better corrosion resistance in salt spray tests than traditional galvanizing, e.g., over 1000 hours vs. 200 - 300 hours.

No Hydrogen Embrittlement Risk: Unlike traditional electroplating, Dacromet treatment has no hydrogen

Good Heat Resistance: The coating maintains stable performance at 250 - 300°C, e.g., used in automotive exhaust parts to resist corrosion and oxidation.

Good Permeability: It can cover all parts of complex-shaped workpieces, including gaps, holes, and threads, ensuring effective protection.

Process Flow

Pretreatment

Solvent Degreasing: Use organic solvents such as trichloroethylene, ethanol, and acetone to remove oil stains on the surface of metal workpieces to avoid the influence of oil stains on the adhesion of subsequent coatings. Alkaline chemical degreasing agents can also be used for degreasing. Soak the workpiece in the degreasing agent solution at a certain temperature and then rinse it with clean water.

Shot Blasting: Through a shot blasting machine, abrasives such as steel shot and wire cut shot are projected at high speed onto the surface of the workpiece to remove impurities such as oxide scale, rust, and welding slag on the surface of the workpiece, forming a certain roughness on the surface of the workpiece and increasing the bonding force between the coating and the substrate.

Coating Treatment

Spraying or Dipping:

Praying: For workpieces with complex shapes, large sizes, or not suitable for dipping, use a spray gun to evenly spray Dacromet coating on the surface of the workpiece, paying attention to controlling the thickness and uniformity of spraying.

Dipping: Immerse the cleaned workpiece in Dacromet coating solution to make the surface of the workpiece fully adsorb the coating solution. The soaking time depends on the material, shape of the workpiece, and the formulation of the coating solution, generally ranging from a few seconds to a few minutes.

Liquid Removal or Leveling:

Liquid Removal: For workpieces treated by dipping, after taking them out, put them in a spin-drying device and remove the excess coating solution on the surface of the workpiece through centrifugal force to make the coating thickness uniform and avoid liquid accumulation and sagging.

Leveling: The sprayed workpiece can be leveled under certain temperature and humidity conditions to make the coating solution naturally level on the surface of the workpiece and further improve the uniformity of the coating.
Curing Treatment

Pre-baking: Put the workpiece coated with Dacromet coating into an oven or drying tunnel and pre-bake it at a lower temperature (such as 100 - 150°C) to make the water and organic solvents in the coating solution initially volatilize. The pre-baking time is generally 10 - 20 minutes.

High-temperature Sintering: The pre-baked workpiece continues to be sintered at a higher temperature (about 300°C) to make the metal particles such as zinc and aluminum in the coating solution react with chromic acid, etc., forming a dense zinc-chromium coating. The sintering time is usually 30 - 60 minutes. The specific time and temperature depend on the formulation of the coating solution and the requirements of the workpiece.
Post-treatment

Cooling: The temperature of the workpiece after high-temperature sintering is high and needs to be quickly cooled to room temperature through natural cooling or air cooling, water cooling, etc., to avoid the decline of coating performance or deformation of the workpiece due to long-term staying at a high temperature.

Inspection: Conduct an appearance inspection on the treated workpiece to check whether the coating is uniform and complete, and whether there are any defects such as missed coating, blistering, and cracking; at the same time, conduct relevant performance tests such as salt spray test and adhesion test to ensure that the quality of the coating meets the requirements.

Packaging and Storage: Package the inspected qualified workpiece to avoid damage and pollution of the coating during storage and transport, and store it in a dry and ventilated environment.

Characteristics

Advantages of Dacromet Coating

Excellent Corrosion Resistance: Although the thickness of the Dacromet film is only 4 - 8μm, its rust prevention effect is more than 7 - 10 times that of traditional electro-galvanizing, hot-dip galvanizing, or coating methods. No red rust has appeared after more than 1200h of salt spray test.

Good Heat Resistance: It can resist high-temperature corrosion, and the heat-resistant temperature can reach more than 300°C. The appearance hardly changes during long-term use at 250°C, and the corrosion resistance is very strong. However, the traditional galvanizing process peels and is scrapped when the temperature reaches 100°C.

No Hydrogen Embrittlement: The treatment process determines that there is no hydrogen embrittlement phenomenon, which is very suitable for coating stressed parts and avoids the danger of hydrogen embrittlement of high-strength steel during pickling and electroplating processes.

Good Adhesion and Repainting Performance: It has good adhesion to the metal substrate and strong adhesion to other additional coatings. The treated parts are easy to spray and color, and the adhesion to the organic coating even exceeds that of the phosphating film.

Good Permeability: It can enter deep holes, narrow slits of workpieces, and the inner walls of pipe fittings to form a good protective film, solving the problem of poor uniform coating and depth coating abilities in the electroplating process.

No Pollution and Public Hazard: During the entire production and processing and workpiece coating process, no wastewater or exhaust gas that pollutes the environment is produced. There is no need for three-waste treatment, reducing the treatment cost.

Good Weather Resistance: It can withstand the erosion of sulfur dioxide, acid rain, smoke, and dust. Its sulfur dioxide resistance test can reach 3 cycles, and it also has a certain resistance to chemical drugs, gasoline, and engine oil corrosion.

Disadvantages of Dacromet Coating

Some Components are Harmful: In the early stage, some Dacromet contained chromium ions harmful to the environment and human body, especially hexavalent chromium ions which are carcinogenic. However, under environmental protection policies, current Dacromet no longer contains hexavalent chromium ions.

High Energy Consumption: The sintering temperature is relatively high (about 300°C) and the time is long. The curing equipment consumes a large amount of energy and has a high cost in this process, and the consumed cost accounts for 1/4 of the total cost.

Poor Surface Hardness and Wear Resistance: The surface hardness is not high and the wear resistance is not good. It is not suitable for contact and connection with parts made of copper, magnesium, nickel, and stainless steel, as it will cause contact corrosion and affect the surface quality and anti-corrosion performance of the product.

Single Color: The surface color is usually only silver-white and silver-gray, which is relatively single and not suitable for applications requiring personalized colors. Although different colors can be obtained through post-treatment or composite coatings, the process complexity is increased.

Poor Conductivity: The coating is a poor conductor and the conductivity is not very good. Therefore, it is not suitable for parts requiring conductive connection, such as grounding bolts of electrical appliances.