The selection of fastener surface treatment method is the last process of fastener processing technology, and it is also an important aspect of comprehensive consideration in the selection process.
　

1. Galvanized

Zinc plating is the most commonly used coating for commercial fasteners. It is cheaper and looks good. It can be black or army green. However, its anti-corrosion performance is general, and its anti-corrosion performance is the lowest among zinc plating (coating) layers. The galvanized layer has good corrosion resistance in the atmosphere, but it is easy to dissolve in acid and alkali, and has poor protection performance in seawater and seawater vapor saturated atmosphere.

Features: The galvanized layer has medium hardness, and it is not easy to fall off when screwed and bent, but the elastic wear resistance and pressure resistance are slightly poor, and the working temperature of the galvanized threaded parts is lower than 300℃

2. Cadmium plating

The corrosion resistance of cadmium coating is very good, especially in the marine atmosphere, the corrosion resistance is better than other surface treatments. The cost of waste liquid treatment in the process of cadmium plating is high, and the cost is high, and its price is about 15-20 times that of galvanizing. Therefore, it is not used in general industries, but only used in some specific environments. For example, fasteners for oil drilling platforms and HNA aircraft.

3. Silver plating

Silver plating can not only prevent corrosion, but also can be used as a solid lubricant for fasteners, and it also has good welding performance, but it is expensive.

Features: Mainly used for threaded parts that require good electrical conductivity.

4. Hair Lan

Hair blue oxide film is very thin, has good chemical stability in dry air, and has poor anti-rust performance in turbid air.

5. Phosphating

Phosphating is cheaper than galvanizing, and its corrosion resistance is worse than galvanizing. Oil should be applied after phosphating. The level of corrosion resistance has a great relationship with the performance of the applied oil.

Many industrial fasteners are treated with phosphating oil. Because its torque-pre-tightening force consistency is very good, it can guarantee to meet the tightening requirements expected by the design during assembly, so it is used more in the industry. Especially the connection of some important parts. For example, the steel structure connection pair, the connecting rod bolts and nuts of the engine, the cylinder head, the main bearing, the flywheel bolts, the wheel bolts and nuts, etc.

6. Dip galvanizing

Sherardizing is zinc powder solid metallurgical thermal diffusion coating. Its uniformity is good, and a uniform layer can be obtained in threads and blind holes. The thickness of the coating is 10~110μm, and the error can be controlled within 10%. Its bonding strength and anti-corrosion performance with the substrate is the best among zinc coatings (electro-galvanized, hot-dip galvanized, dacromet). The processing process is pollution-free and the most environmentally friendly.

7. Dacromet

There is no hydrogen embrittlement problem, and the torque-preload consistency is very good. If the environmental issues of hexavalent chromium are not considered, it is actually the most suitable for high-strength fasteners with high anti-corrosion requirements.

8. Oxidation (blackening)

Blackening + oiling is a popular coating for industrial fasteners because it is the cheapest and looks good before the oil runs out. Since it has almost no anti-rust ability due to blackening, it will rust soon after it is oil-free. Even in the presence of oil, the neutral salt spray test can only reach 3 to 5 hours.

Blackened fastener torque-pre-tightening force consistency is also very poor. If you need to improve, you can apply grease on the inner thread during assembly and then screw it on.