1. Electroplating (Zinc Plating)
A layer of zinc is deposited on the nut surface through electrolysis, utilizing zinc's sacrificial anode effect to protect the base metal.
Low cost, bright appearance, suitable for general indoor environments.
Salt spray resistance is typically 72 hours, but can reach over 200 hours after sealing treatment.
Common colors include white zinc, blue zinc, and colored zinc, suitable for scenarios where aesthetics are important.
2. Dacromet (Dacron)
An electroless zinc-chromium coating technology, composed of zinc powder, aluminum powder, and chromic acid, sintered to form a dense, corrosion-resistant layer.
Salt spray resistance can reach 500–1000 hours or more, with corrosion resistance far exceeding that of electroplating, suitable for high-humidity, high-salt environments such as automotive chassis and power facilities.
No risk of hydrogen embrittlement, particularly suitable for high-strength nuts (e.g., grade 10.9 and above).
3. Blackening (Oxidation)
A layer of magnetite (Fe3O4) is formed on the surface through a chemical reaction, resulting in a black or bluish-black color.
This is the lowest cost treatment, but its rust prevention is weak, with a salt spray test duration of only 3-5 hours. It is prone to rusting in humid environments.
It is often used for internal structural components or short-term rust prevention, and usually requires the application of oil to enhance its protective effect.
4. Hot-Dip Galvanizing
The nut is immersed in molten zinc at approximately 510°C, forming a thick zinc-iron alloy layer.
The coating is thick, with excellent corrosion resistance, suitable for harsh environments such as bridges, towers, and outdoor steel structures.
The disadvantage is that it may affect the thread fit accuracy, making it unsuitable for precision assembly.
5. Phosphating
A phosphate crystal film is formed on the surface, commonly used for cold heading and assembly lubrication of high-strength nuts.
It provides good lubrication, helping to control tightening torque and reduce friction damage.
When used alone, its corrosion prevention ability is limited; it needs to be used in conjunction with grease to achieve the best results.
6. Nickel Plating
Provides excellent corrosion resistance and metallic luster, combining decorative and functional properties.
Suitable for electronic equipment, precision instruments, and other applications requiring high conductivity and aesthetics.
The higher hardness of the nickel layer improves wear resistance, but the cost is higher than zinc plating.
7. Teflon (PTFE) Coating
Coated with polytetrafluoroethylene material, it has an extremely low coefficient of friction and excellent chemical stability.
Prevents stainless steel nuts from seizing in high-temperature or highly corrosive environments, facilitating disassembly.
A variety of colors (e.g., blue, green, black) are available, also used to indicate different torque levels or applications.
8. Other Special Treatments
DLC (Diamond-like Carbon) Coating: Used for ultra-high precision assembly, providing a stable coefficient of friction and preventing sticking.
Mechanical Plating: Suitable for small fasteners (M1–M3), preventing hydrogen embrittlement and thermal deformation.
PVD Vacuum Ion Plating: Environmentally friendly and allows for colored surfaces, used for high-end decorative or marking needs.
