Application and Technical Requirements of Zinc-Aluminum Coating on Fasteners

1. Introduction to Zinc-Aluminum Coating
Zinc-aluminum coating, commonly known as "Dacromet," is a corrosion-resistant coating technology primarily composed of multi-layer ultrafine flake zinc, aluminum, and inorganic salts, which is cured through baking at a certain temperature and time.
Currently, the zinc-aluminum coatings available in the market are mainly black and silver-gray.
Figure 1 Silver-gray Dacromet
Figure 2 Black Dacromet
2. Development History of Zinc-Aluminum Coating
In the late 1960s, the automotive industry developed rapidly. Due to the use of salt for snow melting on roads in winter, a large number of automobile chassis suffered from rust, leading to frequent traffic accidents.
To address this, researchers conducted extensive studies aimed at preventing saltwater and salt mist corrosion. Among them, in 1973, the American Diamond Shamrock Company applied for a patent for a coating composed of metal powder, chromate, surfactants, pH regulators, dispersants, and reducing agents.
In 1976, the company transferred this technology to France's DACRO Company and Japan's NDS Company. In 1993, China also introduced this technology from Japan's NDS Company, which was rapidly promoted due to its many advantages that traditional electroplating could not match.
In recent years, with the increasing environmental protection requirements of VOC regulations in various countries and the automotive industry, research and production of chromium-free zinc-aluminum coatings have gradually begun, and they are widely recognized and accepted in the automotive manufacturing industry.
Currently, many automotive companies around the world, such as Germany's Volkswagen, America's General Motors, Ford, Daimler-Chrysler, South Korea's Hyundai, and Japan's Toyota, Nissan, Honda, and Mitsubishi, have adopted Dacromet technology and explicitly specified that certain parts can only use Dacromet coatings for corrosion protection.
3. Applications of Zinc-Aluminum Coating on Fasteners
In the design of automotive fasteners, zinc-aluminum coating technology is often used for corrosion protection, and certain high-strength fasteners are specified to only use zinc-aluminum coatings for corrosion protection.
The application of zinc-aluminum coatings on fasteners has many advantages, including:
Good corrosion resistance,
Chemical stability and temperature resistance,
No hydrogen embrittlement issues,
No need for additional dehydrogenation heat treatment,
Environmentally friendly, etc.
Figure 3 Application of Zinc-Aluminum Coated Fasteners on Automotive Chassis
4. Corrosion Protection Principles of Zinc-Aluminum Coating
The appearance of zinc-aluminum coating is generally matte silver-gray and black, which is an inorganic coating that wraps ultrafine zinc flakes and aluminum flakes in a special binder, as shown in Figure 4.
Figure 4 Structure of Zinc-Aluminum Coating
(1) Physical Shielding: During the film formation process of the coating, substances with relatively low surface tension move to the surface of the coating, and flake zinc and aluminum powders float and orient parallel to the surface, forming a layered state with amorphous binders filling the gaps. This structure forms a multi-layered, almost continuous barrier that effectively prevents corrosive media from reaching the substrate, significantly enhancing the corrosion resistance of the coating.
Figure 5 Schematic Diagram of Physical Shielding of Zinc-Aluminum Coating
(2) Cathodic Protection: Since the potential of zinc-aluminum is more negative than that of iron, the electrode potential difference between zinc-aluminum and the steel substrate can form an internal battery as an anode and cathode. The flake zinc and aluminum will act as sacrificial anodes to provide cathodic protection for the steel substrate and will be corroded first, thus protecting the substrate steel.
(3) Passivation: The passivators in the coating can form a dense oxide film on the metal surface, passivating the metal surface and significantly reducing the corrosion rate of zinc and the substrate.
(4) Self-Repair: When the coating is damaged due to external scratches, there may be residual passivators that oxidize the exposed zinc-aluminum surface to form a passivation film; on the other hand, nearby flake zinc may react with corrosive media to form insoluble zinc salts such as oxides, hydroxides, and carbonates, which will fill small losses in the coating and act as a corrosion inhibitor, thus preventing further damage to the coating.
Figure 6 Schematic Diagram of Self-Repair of Zinc-Aluminum Coating
5. Coating Process
The process of zinc-aluminum coating mainly includes: part pretreatment, coating, and curing, as shown in Figure 7.
For automotive parts, the corrosion protection requirements vary depending on their applications and working environments, so the coating methods, baking times, etc., should also differ.
The main coating methods for Dacromet coating are three:
Dipping - Spinning Drying
Electrostatic Spraying
Dipping - Draining
Currently, the most commonly used method is the dipping - spinning drying coating method.
Figure 7 Coating Process
(1) Part Pretreatment: Generally includes chemical cleaning and mechanical cleaning. Alkaline cleaning removes grease by immersing or spraying alkaline aqueous solutions on the surface of the parts. Mechanical cleaning is performed after cleaning the surface grease to remove heat treatment scale and red rust from the surface of the parts, mainly using shot blasting.
(2) Coating: The coating of small parts uses the dipping-rotating centrifugal method. The coating of large parts uses the dipping-draining or spraying method. Air spraying or electrostatic spraying can be used.
(3) Curing: Generally, pre-curing at a lower temperature allows the coating to form a layer, resulting in good adhesion and smoothness. Then, it is fully cured by baking at a higher temperature.
6. Types and standards of Volkswagen silver zinc-aluminum coating as per TL245 requirements

The table above shows the surface protection types corresponding to TL245.
Zinc-aluminum coating, suitable for heavy corrosion protection under high corrosion loads.
Due to the structure of the coating, the zinc sheet coating can be used in high-temperature environments such as engine components.
Since this coating process does not pose a risk of hydrogen embrittlement, the coating is suitable for high-hardness and high-strength steel parts with tensile strength >1000MPa or surface hardness >320HV.
Below, the standard requirements for the most commonly used fastener surface protection type Ofl-t647 are described:
(1) Appearance: The protective layer must not have voids, cracks, damage, or other defects that would affect corrosion resistance and/or functionality.
(2) Coating thickness: The independently measured coating thickness can range from 6μm to 20μm, with an average minimum thickness of 8μm.
Figure 8 Metallographic image of silver zinc-aluminum coating.
(3) Adhesion strength: If the part size allows, first score according to DIN EN ISO 9227, then adhere with tape having an adhesion strength of (10 ± 1) N per 25mm width, pressing it firmly against the test piece surface, followed by a quick peel-off perpendicular to the surface. The coating must not have large areas of peeling.
(4) Salt spray test:
No corrosion of the base metal after 600 hours of testing.
After 96 hours of heat storage at +180℃ (baked under high temperature/circulating air), no corrosion of the base metal after 480 hours of testing.
(5) Condensation water test: Generally only recognized for new coating systems, commonly used coatings are in TL245 appendix. The test is conducted on coated test panels. After 720 hours of testing, there is no corrosion of the surface base metal; corrosion of the base metal is allowed at scored areas, but no corrosion expansion is permitted.
(6) Friction coefficient test: Ofl-t647 generally comes with lubrication and needs to meet friction coefficient requirements. Volkswagen's friction coefficient requirement is that the head, thread, and overall friction coefficient of 10 parts must be in the range of 0.08-0.16.