the surface treatment processes for fasteners
1. Electrogalvanizing.
Electro galvanizing is the most commonly used coating for Screw Clamp Terminal. It is relatively cheap looks good, and can be black or military green. However, its anti-corrosion performance is average, and its anti-corrosion performance is the lowest among zinc plating (coating). Generally, the neutral salt spray test of electro-galvanizing is within 72 hours. Special sealants are also used to make the neutral salt spray test reach more than 200 hours, but the price is expensive, which is 5 to 8 times that of general zinc plating.
2. Phosphating.
A basic principle is that phosphating is cheaper than zinc plating, but its corrosion resistance is worse than that of zinc plating. After phosphating, oil should be applied, and its corrosion resistance is closely related to the performance of the oil applied. For example, after phosphating, general anti-rust oil is applied, and the neutral salt spray test is only 10 to 20 hours. Applying high-grade anti-rust oil can take 72 to 96 hours. But its price is 2 to 3 times that of general phosphating oil.
3. Oxidation (blackening).
Blackening + oiling is a popular coating for industrial fasteners because it is the cheapest and looks good until the oil runs out. Since blackening has almost no rust resistance, it will rust quickly without oil. Even in the presence of oil, the neutral salt spray test can only reach 3~5 hours.
4. Cadmium plating.
Cadmium plating has good corrosion resistance, especially in marine atmospheric environments, which is better than other surface treatments. The waste liquid treatment cost in the process of cadmium plating is large and the cost is high. Its price is about 15~20 times that of zinc plating.
5. Chromium plating.
Chromium plating is very stable in the atmosphere, not easy to change color and lose gloss, and has high hardness and good wear resistance.
6. Nickel plating.
Mainly used in places that require both corrosion protection and good conductivity. Such as the lead-out terminals of vehicle batteries.
7. Hot-dip galvanizing.
Hot-dip galvanizing is a thermal diffusion coating of zinc heated to liquid. The coating thickness is 15~100μm, and it is not easy to control, but it has good corrosion resistance and is mostly used in engineering. The hot-dip galvanizing process is seriously polluted, with zinc waste and zinc vapor.
8. Zinc infiltration.
Zinc infiltration is a solid metallurgical thermal diffusion coating of zinc powder. It has good uniformity, and a uniform layer can be obtained in threads and blind holes. The coating thickness is 10~110μm, and the error can be controlled within 10%. Its bonding strength and anti-corrosion performance with the substrate are the best among zinc coatings (electroplated galvanizing, hot-dip galvanizing, Dacromet). Its processing process is pollution-free and the most environmentally friendly.
9. Dacromet.
There is no hydrogen embrittlement problem, and the torque-preload consistency performance is very good. If the environmental protection problem of hexavalent chromium is not considered, it is most suitable for high-strength fasteners with high anti-corrosion requirements.
