Fasteners are an indispensable part of machinery manufacturing, and their surface treatment plays an important role in improving the corrosion resistance, wear resistance, fatigue resistance and aesthetics of fasteners. Among them, common fastener surface treatment methods include phosphate treatment, galvanizing treatment, Dacromet treatment, hot dip galvanizing treatment, electrostatic spraying treatment, electrochromic plating treatment, silver plating, and nickel plating treatment.

1.Phosphating surface treatment

Phosphating is cheaper than galvanizing, and its corrosion resistance is worse than galvanizing. Oil should be applied after phosphating. The 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 72-96 hours. But its price is 2 to 3 times that of ordinary phosphating oil.

1.1 Types of phosphating surface treatment methods

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°C).

Many industrial fasteners are treated with phosphating and oiling. Because its torque-pretightening force consistency is very good, it can ensure that the expected fastening requirements of the design are met during assembly, so it is often used in the I industry. Especially the connection of some important parts. Such as 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. Therefore, bolts above grade 10.9 in the I industry generally use phosphating surface treatment.

2. Galvanized surface treatment

The most commonly used coating for commercial fasteners is electroplated zinc. It is more cost-effective and has a more beautiful appearance. It comes in a variety of colors such as black and military green. However, its anti-corrosion ability is actually average, and its anti-corrosion performance is the worst among zinc plating (coating) layers. Under normal circumstances, the neutral salt spray test of electroplated galvanizing takes 72 hours. Special sealants will also be used to make the neutral salt spray test last for more than 200 hours. However, the cost is higher, which is 5 to 8 times that of ordinary galvanizing.

Electro-galvanizing is prone to hydrogen embrittlement during the manufacturing process. However, bolts above grade 10.9 are not galvanized under normal circumstances. Even if the hydrogen can be removed in an oven after plating, the passivation film will remain at temperatures higher than 60°C. will be destroyed, so hydrogen removal must be implemented after passivation after electroplating. The operability is not good and the processing cost is high. But in our lives, under normal circumstances, production plants will not take the initiative to remove hydrogen, except for the mandatory requirements of specific customers.

Electro-galvanized fasteners have poor torque-pretightening force consistency and are not stable, so they are not normally used for connections in important parts. In order to change the torque-preload force consistency, the method of coating lubricating substances after plating can also be used to change and enhance the torque-preload force consistency.

Galvanizing is a process that forms a zinc coating on a metal surface, mainly used to improve the corrosion resistance of fasteners. The galvanized layer has good corrosion resistance and aesthetics, and can effectively protect the metal surface from corrosion. Galvanizing treatment is suitable for fasteners made of various metal materials, such as steel, aluminum, copper, etc.

3. Dacromet surface treatment

Dacromet treatment is a process of coating metal surfaces with Dacromet coating, which is mainly composed of metal oxides such as zinc, aluminum and magnesium. Dacromet coating has excellent anti-corrosion properties and wear resistance, and can also improve the fatigue resistance of fasteners. Dacromet treatment is suitable for fasteners made of various metal materials, such as steel, aluminum, copper, etc.

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

4. Hot dip galvanized surface treatment

Hot-dip galvanizing is a process in which metal materials are immersed in high-temperature molten zinc liquid, and the zinc and metal surfaces penetrate each other to form an alloy layer through diffusion. This alloy layer has excellent corrosion resistance and aesthetics, and can effectively protect the metal surface from corrosion. Hot-dip galvanizing is suitable for fasteners made of various metal materials, such as steel, aluminum, copper, etc.

5. Electrostatic spraying surface treatment

Electrostatic spraying is a process in which paint is evenly applied on the metal surface through an electrostatic spray gun to form a coating. This coating has good corrosion resistance and aesthetics, and can effectively protect metal surfaces from corrosion. Electrostatic spraying treatment is suitable for fasteners made of various metal materials, such as steel, aluminum, copper, etc.

6. Chrome surface treatment

Chromium plating is very stable when used in the atmosphere, is not easy to change color and lose luster, has higher hardness and good wear resistance. The use of chromium plating in the fastener industry is generally for decorative purposes. It is rarely used in industrial industries with high anti-corrosion requirements because good chromium-plated fasteners are as expensive as stainless steel. However, when the strength of stainless steel is not enough, chromium-plated fasteners are generally used instead.
To resist corrosion, copper and nickel plating should be used before chromium plating. Chromium plating can withstand extremely high temperatures of 1,200 degrees Fahrenheit (650 degrees Celsius). But it also has the same hydrogen embrittlement problem as electroplating zinc.

7. Silver plating, nickel plating surface treatment

Silver plating not only prevents corrosion, but also acts as a solid lubricant for fasteners. Due to cost reasons, nuts are generally silver-plated, and sometimes small bolts are also silver-plated. Silver loses its luster in the air, but can survive temperatures up to 1,600 degrees Fahrenheit. Therefore, people use its high temperature resistance and lubrication characteristics to fasteners that work at high temperatures to prevent bolts and nuts from oxidation and seizure.

Nickel-plated fasteners are mainly used in places where corrosion resistance and good electrical conductivity are required. For example, the lead-out terminals of vehicle batteries, etc.

There are seven common surface treatments for fasteners. Each surface treatment method has its specific application scenarios and performance characteristics, and the selection should be determined based on the specific use environment and requirements.