For a specific design, which surface treatment to choose is a problem faced by every designer. There are many types of surface treatments to choose from, ranging from plain oil to gold. But the designer should choose the surface treatment that can achieve the purpose economically.
Since almost all commercial fasteners are made of carbon steel, alloy steel, some types of fasteners are expected to be protected from corrosion, and even fasteners using corrosion-resistant materials still require surface treatment to prevent corrosion of dissimilar materials. In addition, the coating of the surface treatment must be firmly adhered and cannot fall off during installation and removal. For threaded fasteners, the coating must be thin enough so that the threads can still be screwed in after plating. Generally, the temperature limit of the coating is lower than that of the fastener material, so the operating temperature requirements of the fastener also need to be considered. For surface treatment, people generally pay attention to aesthetics and anti-corrosion, but the main function of fasteners is to fasten parts, and surface treatment also has a great influence on the fastening performance of fasteners. Therefore, when choosing surface treatment, The factor of fastening performance should also be considered, that is, the consistency of installation torque-pretightening force. A high-level designer should not only consider the design, but also pay attention to the manufacturability of assembly and even environmental protection requirements. The following is a brief introduction to some commonly used coatings for fasteners based on the above factors, for reference by designers and fastener practitioners.
Galvanized
Electrogalvanizing is the most commonly used coating for commercial fasteners. It is cheaper and looks better, and it can be black or army green. 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 of electric galvanizing 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.
Electrogalvanizing process is prone to hydrogen embrittlement, so bolts above grade 10.9 generally do not use galvanized treatment. Although the oven can be used to dehydrogenate after plating, the passivation film will be destroyed when the temperature is above 60°C, so dehydrogenation must be done. Carried out after plating 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 it is mandatory for a specific customer.
Electrogalvanized fasteners have poor torque-preload consistency and are unstable, so they are generally not used for important parts of the connection. In order to improve the torque-preload consistency, the method of coating lubricating substances after plating can also be used to improve and enhance the torque-preload consistency.
Phosphating
Phosphating is cheaper than galvanizing, and its corrosion resistance is worse than galvanizing. Oil should be applied after phosphating, and its corrosion resistance has a great relationship with the performance of the applied oil. For example, after phosphating and general anti-rust oil, the neutral salt spray test is only 10-20 hours. Apply high-grade anti-rust oil, it can reach 72~96 hours. But its price is 2~3 times of general phosphating oil.
There are two commonly used fastener phosphating, zinc phosphating and manganese phosphating. Zinc-based phosphating has better lubricity than manganese-based phosphating, and manganese-based phosphating has better corrosion resistance and wear resistance than galvanizing. It can be used at temperatures ranging from 225°F to 400°F (107°C to 204°C).
Many industrial fasteners are treated with phosphating and oiling. Because its torque-pretightening force is very consistent, and it can ensure the fastening requirements expected by the design during assembly, it is widely used in industry. Especially the connection of some important parts. For example, steel structure connection pairs, engine connecting rod bolts and nuts, cylinder heads, main bearings, flywheel bolts, wheel bolts and nuts, etc.
Phosphating is used for high-strength bolts, which can also avoid the problem of hydrogen embrittlement, so in the industrial field, bolts above grade 10.9 generally use phosphating surface treatment.
Oxidation (blackening)
Blackened+oiled is a popular plating for industrial fasteners because it's 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 state of oil, the neutral salt spray test can only reach 3~5 hours.
Blackened fastener torque-preload consistency is also poor. If it needs to be improved, you can apply grease to the internal thread during assembly and then screw it in.
Cadmium plating
The corrosion resistance of cadmium coating is very good, especially in the marine atmospheric environment, the corrosion resistance is better than other surface treatments. The cost of waste liquid treatment in the process of electroplating cadmium is high, and the cost is high, and its price is about 15 to 20 times that of electroplating galvanized. Therefore, it is not used in general industries and is only used in some specific environments. For example, it is used for fasteners used in oil drilling platforms and HNA aircraft.
Chrome plating
The chrome coating is very stable in the atmosphere, not easy to change color and tarnish, and has high hardness and good wear resistance. Chromium plating on fasteners is generally used as a decoration. It is rarely used in industrial fields with high anti-corrosion requirements, because good chrome-plated fasteners are as expensive as stainless steel, and only when the strength of stainless steel is not enough, chrome-plated fasteners are used instead.
To prevent corrosion, copper and nickel should be plated first before chrome plating. The chrome plating can withstand temperatures up to 1200 degrees Fahrenheit (650 degrees C). But it also has the same problem of hydrogen embrittlement as electro-galvanizing.
Silver plated, Nickel plated
The silver plating both protects against corrosion and acts as a solid lubricant for fasteners. For cost reasons, the nuts are silver-plated, but the bolts are not, and sometimes small bolts are also silver-plated. Silver tarnishes in air but works at 1600 degrees Fahrenheit. Therefore, people use its high temperature resistance and lubrication characteristics for fasteners working at high temperatures to prevent bolts and nuts from oxidative seizure.
Nickel-plated fasteners are mainly used in places that require both anti-corrosion and good electrical conductivity. Such as the lead-out terminal of the vehicle battery, etc.
Hot dip galvanized
Hot-dip galvanizing is a heat-diffusion coating in which zinc is heated into a liquid. The thickness of the coating is 15~10μm, and it is not easy to control, but it has good corrosion resistance and is mostly used in engineering. The pollution during hot-dip galvanizing is serious, including zinc waste and zinc vapor.
Due to the thick coating, the problem that the internal and external threads are difficult to screw in the fastener 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 problem of screwing, it has paid the price of reduced strength. At present, there is a kind of anti-loosening thread - American "Speelock" internal thread that can solve this problem. Because of the large tolerance between the internal thread and the external thread when they are not fastened, it can be used to accommodate thick coatings, so it does not affect the screw-in performance, and the anti-corrosion performance and strength remain the same without being affected.
Due to the temperature of hot-dip galvanizing, it cannot be used for fasteners above grade 10.9.
Zinc sherardization
Zinc sheathing is a solid metallurgical thermal diffusion coating of zinc powder. Its uniformity is good, and a uniform layer can be obtained in threads and blind holes. The coating thickness is 10~110μm, and the error can be controlled within 10%. Its bonding strength with the substrate and anti-corrosion properties are the best among zinc coatings (electrogalvanized, hot-dip galvanized, Dacromet). Its processing process is pollution-free and the most environmentally friendly.
Dacromet
There is no hydrogen embrittlement problem, and the torque-to-preload consistency is very good. If the environmental protection of hexavalent chromium is not considered, it is actually most suitable for high-strength fasteners with high anti-corrosion requirements.
Other coatings
In addition to the above coatings, there are many metallic or non-metallic coatings. These coatings are specified by the respective companies. Such as MAGNI, RUSPERT, NanoMate, aluminum volton and titanium volton from TIODIZE of the United States.
Chuangtuo Jinggong (Jiangsu) Co., LTD is profession manufacturer of various types of Hexagon Nuts, everything is customized in our factory, Should you have any questions please feel free to come back to us.
Contact: Lorena
Email: market@chtofastener.com
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