Nine choices for fastener surface treatment

1. Electro-galvanized

Electro-galvanized is the most commonly used coating for commercial fasteners. It is cheaper and looks good. It can come in black or army green. However, its anti-corrosion performance is general, and 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 to make the neutral salt spray test more than 200 hours, but the price is expensive, which is 5 to 8 times that of general galvanizing.

The electro-galvanizing process is prone to hydrogen embrittlement, so bolts above 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 ℃, so Dehydrogenation must be performed after electroplating and before passivation. Such operability is poor and the processing cost is high. 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 force consistency and are unstable, and are generally not used for connection of important parts. In order to improve the consistency of torque pre-tightening force, the method of coating lubricating substance after plating can also be used to improve and increase the consistency of torque pre-tightening force.

2. Phosphating (Guide: Four major inspections of stainless steel screws after treatment)

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, the neutral salt spray test is only 10 to 20 hours after applying general anti-rust oil after phosphating. Apply high-grade anti-rust oil for 72 to 96 hours. But its price is 2 to 3 times that of general phosphating oil.

There are two commonly used fasteners phosphating, zinc phosphating and manganese phosphating. Zinc-based phosphating has better lubrication performance than manganese-based phosphating, and manganese-based phosphating has better corrosion resistance and 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 of torque 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. 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.

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

The consistency of the torque preload 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 marine atmosphere, the corrosion resistance is better than other surface treatments. The waste liquid treatment in the process of cadmium electroplating is expensive and costly, and 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 plating layer is very stable in the atmosphere, is not easy to change color and lose luster, has high hardness and good wear resistance. Chrome plating on fasteners is generally used for decorative purposes. It is rarely used in industrial fields with high anti-corrosion requirements, because good chrome-plated fasteners are as expensive as stainless steel. Chrome-plated fasteners are only used when the strength of stainless steel is insufficient.

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

6. Silver plating, nickel plating

The silver coating can not only prevent corrosion, but also can be used as a solid lubricant for fasteners. For cost reasons, nuts are silver-plated, bolts are not used, and sometimes small bolts are also silver-plated. Silver loses its luster in the air, but can work at 1600 degrees Fahrenheit. Therefore, people take advantage of its high temperature resistance and lubrication characteristics for fasteners that work at high temperatures to prevent bolts and nuts from oxidizing and seizing.

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

7, hot dip galvanizing

Hot-dip zinc is a thermal diffusion coating of zinc heated to liquid. The thickness of the coating is 15~10μm, and it is not easy to control, but it is corrosive and is mostly used in engineering. During the hot-dip galvanizing process, pollution is serious, such as zinc waste and zinc vapor.

Due to the thick plating layer, the internal and external threads are difficult to screw together 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 to make the thread larger than the standard pattern by about 0.16~0.75mm (M5~M30) when the nut is tapped, and then hot-dip galvanize. Although it can also solve the screwing problem, it pays the price of strength reduction. At present, there is a kind of anti-loosening thread, American Spiron internal thread, which can solve this problem. Because the internal thread and external thread have a large tolerance when they are not tightened, it can be used to accommodate thick coatings, so it does not affect the twistability, while the anti-corrosion performance and strength remain the same and will not be 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). The processing process is pollution-free and the most environmentally friendly.

9, Dacromet

There is no hydrogen embrittlement problem, and the torque preload is consistent and the performance 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.

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