1. Electro-galvanizing
Electrogalvanizing is the most common coating used for commercial fasteners. It looks good and can be available in black, military green, etc. However, its anti-corrosion performance is average, and its anti-corrosion performance is the lowest among zinc plating (coating) layers. Generally, the neutral salt spray test for electroplated zinc is within 72 hours. Special sealants are also used to make the neutral salt spray test last for more than 200 hours. However, the price is expensive, 5 to 8 times that of general galvanizing.
The electro-galvanizing process is prone to hydrogen embrittlement, so bolts above grade 10.9 are generally not galvanized. Although an oven can be used to remove hydrogen after plating, the passivation film will be destroyed above 60°C, so hydrogen removal must Carry out after electroplating and before passivation. This results in poor operability and high processing costs. In reality, general production plants will not take the initiative to remove hydrogen unless mandated by specific customers.
The torque-pretightening force consistency of electro-galvanized fasteners is poor and unstable, and they are generally not used for connections in important parts. In order to improve the torque-preload force consistency, the method of coating lubricating substances after plating can also be used to improve and improve the torque-preload force consistency.
2. Phosphating
The corrosion resistance of phosphating is worse than that of galvanizing. Oil should be applied after phosphating, and its corrosion resistance has a great relationship with the performance of the oil applied. For example, if you apply general anti-rust oil after phosphating, the neutral salt spray test will only last 10 to 20 hours. Applying high-grade anti-rust oil can last up to 72 to 96 hours. But its price is 2 to 3 times that of ordinary phosphating oil.
There are two commonly used types of firmware phosphating, zinc-based phosphating and manganese-based phosphating. Zinc-based phosphating has better lubrication performance than manganese-based phosphating, and manganese-based phosphating has better corrosion resistance and wear resistance than zinc plating. Its operating temperature can reach 225 degrees Fahrenheit to 400 degrees Fahrenheit (107~204℃).
Many industrial fasteners are treated with phosphating and oiling. Because its torque-pretightening force consistency is very good and it can ensure that the expected fastening requirements of the design are met during assembly, it is often used in industry. Especially the connection of some important parts. For example, steel structure connecting pairs, engine connecting rod bolts and nuts, cylinder heads, main bearings, flywheel bolts, wheel bolts and nuts, etc.
Phosphating high-strength bolts can also avoid hydrogen embrittlement problems, so bolts above grade 10.9 in the industrial field generally use phosphating surface treatment.
3. Oxidation (blackening)
Blackening + oiling is a very popular coating for industrial fasteners. Since blackening has almost no anti-rust ability, it will rust quickly without oil. Even in the presence of oil, the neutral salt spray test can only last for 3 to 5 hours.
Blackened fasteners also have poor torque-preload consistency. If you need to improve, you can apply grease on the inner threads during assembly and then screw them together.
4. Hot dip galvanizing
Hot-dip galvanizing is a thermal diffusion coating of zinc heated to liquid. The thickness of its coating is 15~10μm, and it is difficult to control, but it has good corrosion resistance and is mostly used in engineering. There is serious pollution during the hot-dip galvanizing process, including zinc waste and zinc vapor.
Due to the thick coating, the problem of internal and external threads in fasteners being difficult to screw is caused. There are two ways to solve this problem. One is to tap the internal thread after plating, although the problem of thread screwing is solved. But it also reduces the anti-corrosion performance. One is to make the thread larger than the standard thread by about 0.16~0.75mm (M5~M30) when tapping the nut, and then hot-dip galvanizing. Although this can also solve the screwing problem, it comes at the cost of reduced strength.
Due to the temperature of hot-dip galvanizing, it cannot be used for fasteners above grade 10.9.
5. 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 at 10%. Its bonding strength with the substrate and anti-corrosion performance are the best among zinc coatings (electro-galvanized, hot-dip galvanized, Dacromet). Its processing process is pollution-free and the most environmentally friendly.
6. 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 most suitable for high-strength fasteners with high anti-corrosion requirements.
