Electro-galvanizing is the most commonly used coating for commercial fasteners. It is cheap, the appearance is also good-looking, you can have black, military green. However, its anti-corrosion properties in general, its corrosion resistance is the zinc coating (coating) the lowest layer. Neutral salt spray test in general within 72 hours, but also the use of special sealing agent, making neutral salt spray test for 200 hours or more, but the price is expensive, is generally 5 to 8 times the zinc.
Electroplating process is easy to produce hydrogen embrittlement, so beyond grade 10.9 the bolts are generally not treated with galvanized.Although the plating can be used after the oven to hydrogen, but the passive film at 60 ℃ or more will be destroyed, thus the dehydrogenation must be processed before passivation. Poor operability, high processing costs. In reality, the general manufacturer will not take the initiative to dehydrogenation, unless a specific customer's mandatory requirements.
Electro-galvanized fasteners torque - preload force consistency is poor, and unstable, generally not used for important parts of the connection. In order to improve the torque - preload uniformity, can also be coated with lubricating material after plating method to improve and improve the torque - preload force consistency.
Phosphating is cheaper than galvanized, and corrosion resistance is worse. Phosphating should be coated with oil, its corrosion resistance and the performance have a great relationship with oil. For example, after phosphating anti-rust oil coated in general, neutral salt spray test is only 10 to 20 hours. Coated high-grade anti-rust oil, it can reach 72 to 96 hours. But its price is generally 2 to 3 times of the phosphate coating oil.
Phosphating fasteners commonly used in two, zinc-based phosphate and manganese-based phosphating. Zinc phosphating lubrication performance is better than manganese phosphate, manganese phosphate is better in corrosion resistance. It can be used at temperatures between 225 ° F and 400 ° F (107 ° C to 204 ° C).
Industrial fasteners are treated with phosphating oils. Because of its torque-tightening force consistency is very good, the assembly can guarantee the design to achieve the expected tightening requirements, so it's widely used in the industry. Especially some important parts of the connection. Such as, steel connection, the engine connecting rod bolts, nuts, cylinder head, the main bearing, flywheel bolts, wheel bolts and nuts.
High-strength bolts use phosphating to avoid the hydrogen embrittlement problem, so in the industrial field above grade 10.9 generally use of phosphate coating surface treatment.
3) Oxidation (black oxide)
Black + oil 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 rust resistance, it will rust quickly after oil-free. Is in the oil state, the neutral salt spray test can only reach 3 to 5 hours.
The blackened fastener torque - the preload force consistency is also poor. For improved, can be in the assembly at the internal thread on the oil and then screwed.
4) Electroplating cadmium
Corrosion resistance of cadmium coating is very good, especially in the marine atmosphere it's much better than other surface treatment. Electroplating cadmium takes high cost in the processing of waste liquid, its price is about 15 to 20 times of the electro-galvanized. So in the general industry it's seldom used, only for some specific environment, like oil drilling platforms and fasteners for HNA aircraft.
5) Electroplating chromium
Chromium coating in the atmosphere is very stable, hard to lose luster, high hardness and good corrosion resistance. Chromium plating on fasteners is generally used as a decorative effect. In the industrial areas of high corrosion resistance it's rarely used, because a good chrome plating fasteners nearly has the same cost of stainless steel, only when stainless steel strength is not enough, chrome fasteners take the replacement.
In order to prevent corrosion, chrome plating should be preceded by copper and nickel. Chromium coatings can withstand temperatures as high as 1,200 ° F (650 ° C). However it also exist hydrogen embrittlement as a problem .
6) Silver, nickel
Silver plating can be both corrosion-resistant and solid lubricant in fasteners. For reasons of cost, nuts are silver plated, bolts are not used, and sometimes small bolts are also silver plated. Silver loses its luster in the air, but it works at 1600 degrees Fahrenheit. Therefore, people use its high temperature and lubrication characteristics, for working at high temperature fasteners, to prevent oxidation of bolts, nuts killed.
Fastener nickel plated, mainly used for both anti-corrosion, but also a good electrical conductivity of the place. Such as vehicle battery terminals and so on.
7) Hot-dip Galvanized
Hot-dip galvanized is heated to a liquid under heat diffusion coating. The coating thickness is 15 ~ 100μm, and it's difficult to control, but engage in corrosive resistance, and more for the project. Hot dip zinc processing in the process of serious pollution, zinc waste and zinc vapor.
As the thickness of the coating in the fasteners, it caused internal and external screw in fitting problem. There are two ways to work around this problem. One is plating first and then attack the internal thread, although the screw can solve the problem of screw, but it also reduced the corrosion resistance. The other one is when tapping the nut, the thread is about 0.16 ~ 0.75mm greater than standard (M5 ~ M30), and then hot dip galvanized, although it can solve the problem of rotation, but pay the cost of reducing strength. At present, there is a locking thread - the United States, an internal thread can solve this problem. Because of its internal and external threads are not tightened when the gap is large, can be used to accommodate the thickness of the coating, it does not affect the screw coupling, while preserving the performance and strength remains intact, unaffected.
Due to the temperature of hot-dip galvanizing processing, it can not be used for fasteners beyond grade 10.9.
Sherardizing is a zinc powder solid-state metallurgical thermal diffusion coating. Its uniformity is good, thread, blind hole can get a uniform layer. Plating thickness is generally between 10 ~ 110μm, and the error can be controlled less than 10%. Its bonding strength to the substrate and corrosion resistance in zinc coating (electro-galvanized, hot-dip zinc, dacromet) is the best. The process of pollution-free, the most environmentally friendly.
There is no hydrogen embrittlement problem, and the torque - preload force consistency is very good. If you do not consider the environmental problems of hexavalent chromium, it is actually the most suitable for high corrosion requirements of high-strength fasteners.