1. Introduction of zinc-aluminum coating
Zinc-aluminum coating, commonly known as "Dacromet", is a multi-layer ultra-fine flake zinc, aluminum and inorganic salts as the main composition, which is cured by baking at a certain temperature and time. Anti-corrosion coating technology.
At present, there are mainly two types of zinc-aluminum coatings in the world: black and silver-gray.
Figure 1 Silver-gray Dacromet
Figure 2 Black Dacromet
2. The development history of zinc-aluminum coating
In the late 1960s, the automobile industry developed rapidly. Due to the use of salt to melt snow in winter to dredge roads, a large number of automobile chassis were corroded, resulting in traffic accidents. Accidents happen frequently.
For this reason, researchers have conducted a lot of research with the goal of preventing salt water and salt spray corrosion. Among them, in 1973, Diamond Shamrock Company of the United States applied for a coating patent consisting of metal powder, chromic acid, surfactant, pH regulator, dispersant and reducing agent.
In 1976, the company transferred this technology to DACRO in France and NDS in Japan. my country also introduced this technology from Japan's NDS company in 1993, and it has been rapidly promoted because of its many advantages that cannot be compared with traditional electroplating.
In recent years, with the improvement of VOC regulations in various countries in the world and the environmental protection requirements stipulated by the automobile industry, the development and production of chromium-free zinc-aluminum coatings have gradually begun and have been generally recognized and accepted by the automobile manufacturing industry.
At present, many automobile companies in the world, such as Volkswagen in Germany, General Motors, Ford, Daimler-Chrysler in the United States, Hyundai in South Korea, Toyota, Nissan, Honda, Mitsubishi in Japan, etc., have adopted Dacromet technology and clearly specified certain Parts can only be coated with Dacromet for corrosion protection.
3. Application of zinc-aluminum coating on fasteners
When designing automotive fasteners, zinc-aluminum coating technology is often used for anti-corrosion, and some high-strength fasteners stipulate that only zinc-aluminum coatings can be used for corrosion protection.
The application of zinc-aluminum coating has many advantages on fasteners:
good corrosion resistance,
chemical stability and temperature resistance,
no hydrogen embrittlement,
No additional dehydrogenation heat treatment is required, and
it is environmentally friendly. Figure 3 Application of zinc-aluminum coated
fasteners on automobile chassis An inorganic coating wrapped in a special binder, as shown in Figure 4. Figure 4 Zinc-aluminum coating structure (1) Physical shielding: During the coating film-forming process, substances with relatively low surface tension will move towards the coating surface, and flake zinc powder and aluminum powder will float and be oriented parallel to the surface, forming layers The state of stacking layers, and there is an amorphous binder filling in between. Such a structure forms a multi-layer almost continuous barrier, which effectively prevents the corrosive medium from reaching the substrate and significantly improves the anti-corrosion ability of the coating. Figure 5 Schematic diagram of physical shielding of zinc-aluminum coating (2) Cathodic protection: Since the potential of zinc-aluminum is negative than that of iron, there is an electrode potential difference between zinc-aluminum and steel substrates, which can be used as cathode and anode to form an internal battery, and flake zinc and aluminum flakes will act as Sacrificial anodes provide cathodic protection to the steel substrate and are first corroded away, thereby protecting the substrate steel. (3) Passivation: The passivation agent in the coating can form a dense oxide film on the metal surface, passivate the metal surface, and greatly reduce the corrosion rate of zinc and the substrate. (4) Self-healing: When the coating is damaged due to external scratches, on the one hand, there may be residual passivation agent to oxidize the bare zinc-aluminum surface to form a passivation film; on the other hand, the nearby flake zinc may be related to corrosion The medium reacts to form insoluble zinc salts such as oxides, hydroxides, and carbonates, which fill fine coating losses and act as a corrosion inhibitor, thereby preventing further damage to the coating. Figure 6 Schematic diagram of zinc-aluminum coating self-healing 5. Coating process The process of zinc-aluminum coating mainly includes: pre-treatment of parts, coating and curing, see Figure 7. For auto parts, the application and working environment are different, and the anti-corrosion requirements are also different, so the coating method, baking times, etc. should also be different. There are three main coating methods of dacromet coating:













Dipping-drying
electrostatic spraying
dipping-draining
is currently the most commonly used method of dipping-drying coating.
Figure 7 Coating process
(1) Pre-treatment of parts: Generally, there are chemical cleaning and mechanical cleaning. Mechanical cleaning is to remove heat treatment scales and red rust on the surface of parts after cleaning the surface grease. The main method is shot blasting.
(2) Coating: The coating of small parts adopts the method of dip coating-spinning centrifugation. Large parts are coated by dip-drain or spray methods. Air spray or electrostatic spray can be used.
(3) Curing: Generally, pre-curing is baked at a lower temperature to make the coating form a coating, forming good adhesion and flatness. It is then baked at a higher temperature for complete reaction curing.
6. The type of Volkswagen silver zinc-aluminum coating and the requirements of standard TL245

In the above table, the surface protection type corresponding to TL245 is
zinc-aluminum coating, which is suitable for heavy corrosion protection under high corrosion load.
Due to the structure of the coating, zinc flake coatings can be used in high temperature environments such as engine parts.
Since this coating process does not cause the risk of hydrogen embrittlement, the coating is suitable for high-hardness and high-strength steel parts with a tensile strength > 1000MPa or a surface hardness > 320HV. Below, the surface protection of the most commonly used
fasteners Type Ofl-t647 describes the standard requirements:
(1) Appearance: The protective layer must be free from voids, cracks, damage or other defects that will affect the anti-corrosion performance and/or function.
(2) Coating thickness: independent measurements of coating thickness can vary from 6 μm to 20 μm, with an average minimum coating thickness of 8 μm.
Figure 8 Metallographic diagram of silver-colored zinc-aluminum coating
(3) Adhesion strength: If the size of the part permits, first scribe according to DIN EN ISO 9227, and then use adhesive tape with an adhesion strength of (10 ± 1) N per 25mm width Attached, press it tightly on the surface of the test piece by hand, and then quickly peel it off perpendicular to the surface. The coating must not have extensive peeling off.
(4) Salt spray test:
No base metal corrosion after 600 hours of test.
After 96 hours of thermal storage (baking at high temperature/circulating air) at +180°C, no base metal corrosion after 480 hours of test time.
(5) Condensation water test: Generally, it is only approved for new coating systems, and commonly used coatings are listed in the appendix of TL245. Experiments were carried out on coated test panels. After the test time of 720 hours, the base metal on the surface has no corrosion; the base metal at the scribed line is allowed to corrode, but there is no corrosion expansion.
(6) Friction coefficient test: Ofl-t647 is generally lubricated and needs to meet the friction coefficient requirements. Volkswagen's friction coefficient requirement is that the head, teeth, and total friction coefficient of the 10 parts should be in the range of 0.08-0.16.