Nine surface treatment options for fasteners

Editor:Jiaxing UNITEK Machinery Technology Co., Ltd. │ Release Time:2021-11-10 

1. Electro-galvanized

Electro-galvanizing is the most commonly used coating for commercial fasteners. It is cheaper: it looks good, and it can be black or army green. However, its anti-corrosion performance is general. Its anti-corrosion performance is the lowest among zinc plating (coating) layers. Generally, the neutral salt spray test of electrogalvanization is within 72 hours, and special sealants are also used, which makes the neutral salt spray test more than 200 hours, but the price is expensive, which is 5-8 times that of general galvanizing.

Hydrogen embrittlement is prone to occur during the electro-galvanizing process, so bolts above 10.9 grade are generally not treated with galvanizing. Although the oven can be used to remove hydrogen after plating. However, the passivation film will be destroyed when the passivation film is above 60°C. Therefore, dehydrogenation It must be carried out after electroplating and before purification. So poor operability: high processing cost. In reality, the general production plant will not take the initiative to remove hydrogen: unless mandatory requirements of specific customers.

Electro-galvanized fasteners have poor torque-pre-tightening consistency: they are unstable, and are generally not used for connecting important parts. In order to improve the torque-pre-tightening force consistency, the method of coating lubricating substances after plating can also be used to improve and increase the torque-pre-tightening force consistency.

2. 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. For example: After phosphating, apply general anti-rust oil, and the neutral salt spray test only lasts 10-20 hours. Apply high-grade anti-rust oil: up to 7246 hours. But its price is 2 to 3 times that of general phosphating oil.

There are two commonly used firmware phosphating: zinc phosphating and manganese phosphating. Zinc-based phosphating has better lubricating properties than manganese-based phosphating: manganese-based phosphating Corrosion resistance: galvanizing with better wear resistance. Its operating temperature can reach 225°F to 400°F (107~204°C).

Many industrial fasteners are treated with phosphating oil. Because it has a good consistency between torque and pre-tightening force, 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. Such as. Steel structure connection, engine connecting rod bolts, nuts, cylinder heads, main bearings, flywheel bolts, wheel bolts and nuts, etc.

High-strength bolts are phosphated, which can also avoid hydrogen embrittlement. Therefore, bolts above 10.9 in the industrial field generally use phosphating surface treatment.

3. Oxidation (blackening)

Blackening + oiling is a very popular coating for industrial fasteners: because it is the cheapest and looks good before the oil runs out. Due to the blackening, it has almost no anti-rust ability: so 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.

The torque-preload consistency of blackened fasteners is also poor. If you need to improve, you can apply grease to the inner thread during assembly and then screw it on.

4. Cadmium electroplating

The corrosion resistance of cadmium coating is very good: especially in the atmospheric environment of Meiyang, the corrosion resistance is better than other surface treatments. The cost of waste liquid treatment in the process of cadmium electroplating is high: the cost is high: its price is about 15-20 times that of electrogalvanized zinc. 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.

5. Chrome plating

The chromium coating is very stable in the atmosphere: it is not easy to change color and lose its luster: it has high hardness and good wear resistance. Chrome plating on fasteners is generally used for decoration. It is rarely used in industrial fields with high anti-corrosion requirements: because good chrome-plated fasteners are as expensive as stainless steel: only when the strength of stainless steel is not enough, chrome-plated fasteners are used instead.

In order to prevent corrosion, copper and nickel should be plated before chromium plating. The chromium plating can withstand high temperatures of 1200°F (650°C). But it also has the same hydrogen embrittlement problem as electro-galvanized.

6. Silver-plated, nickel-plated

Silver plating can prevent corrosion: it can also be used as a solid lubricant for fasteners. For cost reasons, nuts are silver-plated, but bolts are not used: sometimes small bolts are also silver-plated. Silver loses its luster in the air: but it can work at 1600 degrees Fahrenheit. Therefore: People use its high temperature resistance and lubrication characteristics: fasteners used at high temperatures to prevent bolts and nuts from oxidizing and seizing.

Fasteners are plated with nickel, which are mainly used in places where both anti-corrosion and electrical conductivity are required. Such as the lead-out terminal of the vehicle battery.

7. Hot dip galvanizing

Hot-dip galvanizing is a thermal diffusion coating of zinc heated to a liquid. The thickness of the coating is 15~10μm: and it is not easy to control: but the corrosion resistance is good: it is mostly used in engineering. Pollution is serious in the hot-dip galvanizing process: there are zinc waste and zinc steam.

Due to the thick plating, the problem that the internal and external threads are difficult to screw together is caused in the fastener. There are two ways to solve this problem. One is to tap the internal thread after plating, although the thread screwing problem is solved. But it also reduces the corrosion resistance. One is when the nut is tapped: make the thread larger than the standard pattern by about 0.16~0.75mm (M5~M30): then hot-dip galvanize this way, although the screwing problem can be solved, but the price of strength reduction is paid. At present, there is a kind of anti-loosening thread-the American "Spiro" internal thread can solve this problem. Because of its large tolerance when the internal thread and external thread are not tightened: it can be used to accommodate a thick coating, so it does not affect the twistability, while the anti-corrosion performance and strength remain the same: not affected.

Due to the temperature of hot-dip galvanizing: it cannot be used for fasteners above 10.9.

8. Sherardizing

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), and its processing is pollution-free and the most environmentally friendly.

9. Dacromet

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