Surface treatment of fasteners

The main function of a fastener is to fasten parts, and surface treatment has a great influence on its fastener performance, so factors such as its surface treatment process, the torque of the fastener and the consistency of pre-tightness should be considered when selecting a fastened device.
The core function of fasteners surface treatment can be divided into corrosion and rust prevention, functional strengthening, process adaptation and beautification. The specific functions and corresponding processes are as follows:
1. Electrolytic zinc plating is the most commonly used coating for fasteners. The cost is lowest, the appearance is diverse (white zinc, color zinc, blue zinc, etc.) but the corrosion protection is general, and its corrosion protection performance is the lowest among zinc plating (coated) layers.
Electrolytic zinc plating process is easy to produce hydrogen embrittlement, so the bolts of grade 10.9 and above are generally not galvanized. Electrolytic zinc plating fastener torque and preload consistency is poor, and unstable, generally not used in important parts of the connection.
2. Heated zinc heats zinc to a thermally diffused coating under a liquid. Its coating thickness is not easy to control, but good corrosion resistance, salt spray performance is significantly better than electrolytic zinc plating, long life, widely used in engineering.
However, the pollution during the heat immersion of zinc processing is serious, and there are zinc waste and zinc steam. Due to the thick plating, the problem of internal and external screws being difficult to rotate in the fastener caused. Due to the temperature of the processing, it cannot be used for fasteners above Grade 10.9.
3. Phosphate to prevent rust and lubricate the substrate to improve the precision of the screw fitting. Relatively zinc coating is cheap and has poor corrosion resistance than zinc polishing. After phosphorization, oil should be coated, and the high or low corrosion resistance performance has a great relationship with the performance of the coated oil. High-strength bolts are phosphorized to avoid hydrogen brittle problems, so in the industrial field, bolts above grade 10.9 are generally phosphorized surface treatment.
4. Blacking (oxidation) is a common plating for industrial fasteners. Low cost rust protection and aesthetics, and industrial use. However, the black coating is easy to rust without oil, and the neutral salt atmosphere test can only reach 3 to 5 hours under the condition of oil.
5. Corrosion resistance of cadmium plating is good, especially in marine atmospheres. The cost of processing waste liquid is high, the cost is about 15 ~ 20 times of electrolytic zinc plating. Usually only used in some specific environments, such as oil rigs.
6. The chrome plating has high surface hardness, good wear resistance, resistance to high temperatures of 650 ° C, and is both decorative, improving wear resistance and gloss. The price is high, and it is usually used when stainless steel is not strong enough. It can withstand high temperatures, but has hydrogen brittleness problems.
7. The method of coating metals or certain non-metals with a layer of nickel by electrolysis or chemical methods is called nickel plating. Nickel plating is electronickel plating and chemical nickel plating. Considering both corrosion protection and electrical conductivity, it is suitable for electronic equipment fasteners.
8. Zinc emulsion is a solid metallurgical heat dispersion coating of zinc powder, and its uniformity is good, and a uniform layer can be obtained in screws and blind holes. The thickness of the coating is 10 ~ 110um, and the error can be controlled within 10%. It is the best in the zinc coating (electrolytic zinc plating hot dip zinc, dacromet). Its processing process is non-polluting and environmentally friendly.
9. Dacro is a corrosion coating with zinc powder, aluminum powder, Chromic Acid and deionized water as the main components. There is no hydrogen brittle problem, high heat resistance and corrosion resistance, and good torque-pretightness consistent performance. Used for high strength fasteners with high corrosion requirements.

Summary: Surface treatment extends the lifespan by preventing corrosion (accounts for 90% of applications), enhances functionality to meet specific conditions (e.g., wear resistance, electrical conductivity), while also ensuring compatibility with processes (e.g., resistance to hydrogen embrittlement, thin coatings) and aesthetic considerations.