1. Electroplating (Zinc Plating)
A layer of zinc is plated onto the nut surface, using sacrificial anode protection to prevent corrosion of the base metal.
Low cost, bright appearance, suitable for general environments.
Salt spray resistance time is typically around 72 hours, but can reach over 200 hours after special sealing treatment.
Common colors: white zinc, blue zinc, colored zinc.
2. Dacromet (DACROMET)
An electroless coating based on zinc powder, aluminum powder, and chromic acid, which is sintered to form a dense anti-corrosion layer.
Extremely strong corrosion resistance, salt spray test results can reach over 500–1000 hours, suitable for harsh environments such as high humidity and high salinity.
No risk of hydrogen embrittlement, suitable for high-strength nuts.
Silver-gray appearance, more environmentally friendly than traditional electroplating.
3. Blackening (Oxidation Treatment)
A dense layer of magnetite (Fe3O4) film is formed on the surface through a chemical reaction, appearing black or bluish-black.
Lowest cost, offers some rust prevention, but its protective performance is weak, with a salt spray test duration of only 3-5 hours.
Mostly used for short-term rust prevention or internal structural components, often requiring the application of oil.
4. Nickel Plating
Provides good corrosion resistance and metallic luster, combining decorative and functional properties.
Suitable for precision instruments, electronic equipment, and other applications requiring high aesthetics and stability.
The high hardness of the nickel layer enhances wear resistance.
5. Phosphating Treatment
Forms a phosphate crystal film on the surface, often used as a base treatment before subsequent coating.
Good lubrication, suitable for cold heading and assembly processes of high-strength nuts.
Limited corrosion resistance when used alone; requires the use of grease to enhance the effect.
6. Teflon (PTFE) Coating
Coated with polytetrafluoroethylene material, possessing an extremely low coefficient of friction and excellent chemical corrosion resistance.
Prevents "seizing," particularly suitable for tightening stainless steel nuts in high-temperature, highly corrosive environments.
Various colors (such as black, blue, and green) are also used to indicate different torque ratings.
7. Hot-dip galvanizing
The nut is immersed in molten zinc to form a thick zinc-iron alloy layer.
Its corrosion resistance far exceeds that of electroplating, making it suitable for harsh environments such as outdoor use, bridges, and power towers.
The disadvantage is a slight impact on dimensional accuracy, making it unsuitable for precision assembly.
8. Other special treatments
DLC (diamond-like carbon) coating: Used for ultra-high precision assembly, providing stable friction coefficients and preventing sticking.
PFA coating: A perfluoroalkoxy resin coating that covers only the head, improving chemical resistance; used in semiconductor and chemical equipment.
