Choosing which surface treatment to use is a problem faced by every designer. There are many types of surface treatments to choose from. A high-level designer should not only consider the economy and practicality of the design, but also pay attention to the craftsmanship of assembly and even environmental protection requirements. The following is a brief introduction to some commonly used coatings for fasteners based on the above principles for reference by fastener practitioners.

1.Electro-galvanized

Electrogalvanizing is the most common coating used for commercial fasteners. The price is cheaper and the appearance is better. Black and military green are common. 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 when the temperature is above 60°C, so hydrogen removal must be carried 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 a specific customer.
  
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

A basic principle is that phosphating is cheaper than galvanizing and has worse corrosion resistance than 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 general anti-rust oil is applied 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 phosphating methods for fasteners: 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 galvanizing. Its operating temperature can reach 225 degrees Fahrenheit to 400 degrees Fahrenheit (107~204 degrees Celsius). Especially the connection of some important parts. Such as engine connecting rod bolts and nuts, cylinder head, main bearing, flywheel bolts and 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)

Blackened + oiled 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 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.
  
4. Cadmium electroplating

The corrosion resistance of cadmium coating is very good, especially the corrosion resistance in marine atmospheric environment is better than other surface treatments. The waste liquid treatment during the electroplating of cadmium is expensive and costly, and its price is about 15 to 20 times that of electroplated zinc. Therefore, it is not used in general industries and is only used in some specific environments. Such as fasteners used on oil drilling platforms and HNA aircraft.
  
5. Chrome plating

Chromium plating is very stable in the atmosphere, not easy to change color and lose luster, and has high hardness and good wear resistance. The use of chromium plating on fasteners is generally for decorative purposes. It is rarely used in industrial fields with high anti-corrosion requirements, because good chromium-plated fasteners are as expensive as stainless steel. Only when the strength of stainless steel is not enough, chromium-plated fasteners are used instead.

To prevent corrosion, copper and nickel should be plated before chromium plating. Chromium plating can withstand temperatures up to 1,200 degrees Fahrenheit (650 degrees Celsius). But there is also the same problem of hydrogen embrittlement as electroplating zinc.

    
6.Nickel plating
It is mainly used in places where both anti-corrosion and good electrical conductivity are required. Such as vehicle battery lead-out terminals, etc.
  
7. Hot dip galvanizing

Hot-dip galvanizing is a thermal diffusion coating of zinc heated to liquid. The thickness of its coating is 15~100μm, 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 together is caused. Due to the temperature of hot-dip galvanizing (340~500℃), it cannot be used for fasteners above grade 10.9.

  
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 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.
  
9.Dacromet

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