For a specific design, the choice is any surface treatment, is each designers are faced with the problem. Can choose the surface treatment of many types, from the ordinary oil to gold. But the designer should choose the surface treatment can achieve the purpose of the economy.
Since almost all commercial fasteners are made from carbon steel, alloy steel, some kinds of fasteners, hope to be able to prevent corrosion, even if the use of anti-corrosion materials fasteners, but still need to surface treatment to prevent the corrosion of different materials. In addition, the coating surface treatment must be attached firmly, cannot fall off in the process of installation and remove, of threaded fasteners, coating still need thin enough, make the threaded can still after plating. General plating temperature limit is lower than fastener materials, therefore also take into account the working temperature of fasteners are required. For surface treatment, people tend to focus on is beautiful and corrosion, but the main functions of the fastener is fastening parts, and the surface treatment of fastener tighten performance also has a big impact, so choosing surface treatment, also should consider the factor of fastening performance, namely install torque - the consistency of the prestressing force. A high level of design, not only should consider to design, also because of the attention to the technology of assembly, and environmental requirements. According to the above factors under the brief introduction to some commonly used coating, fasteners, to provide the reference for the designers and fasteners practitioners.
Electric galvanizing is the most commonly used coating business fasteners. It is cheaper, appearance is good-looking, also can have black and army green. However, its anticorrosion performance in general, the anticorrosion performance is among the lowest in the zinc plating (tu) layer. General electric galvanized salt spray test within 72 hours, also have the use of special sealing agent, the neutral salt spray test of 200 hours, but the price is expensive, 5 ~ 8 times that of general galvanized.
Electric galvanized process is easy to produce hydrogen embrittlement, so 10.9 magnitude of bolt is generally not used galvanized processing. Although after plating can use oven to hydrogen, but because of the passivation membrane will be destroyed when 60 ℃ above, therefore to the hydrogen must be conducted before electroplating after passivation. So poor maneuverability, high processing cost. In reality, the general factory does not take the initiative to hydrogen, unless specific customer required.
Electric galvanized fasteners - consistency pretightening torque is poorer, and unstable, and is generally not used for the important parts of the connection. Consistency in order to improve the torque - pre-tightening force, also can be used after plating coating lubrication material method to improve and enhance - consistency pretightening torque.
The corrosion resistance of phosphating galvanized relatively cheap, worse than galvanized. Should be greased after phosphating, the corrosion resistance of high and low and the performance of the oil has a lot to do. After the phosphating coating, for example, are usually rust-proof oil, neutral salt spray test is only 10 ~ 20 hours. With high-grade anti-rust oil, it can reach 72 ~ 96 hours. But its price is generally 2 ~ 3 times of phosphating coating oil.
Fasteners phosphating commonly used two kinds, zinc phosphide and manganese phosphate. Zinc series phosphating lubrication performance is better than manganese series phosphating, manganese phosphate corrosion resistance, wear resistance of galvanized is better. Its temperature can reach 225 degrees to 225 degrees f (107 ~ 204 ℃).
Industrial fasteners, a lot of phosphating coating oil processing. Because it - very good consistency pretightening torque and achieve the desired design fastening assembly time can guarantee requirements, so widely used in industry. In particular, some important parts of the connection. Vice, such as, steel structure connection of engine connecting rod bolt, nut, bolt, main bearing, flywheel bolt, wheel bolt and nut, etc.
High strength bolts used phosphating, still can avoid hydrogen embrittlement problem, so the bolt in industrial field level is above 10.9 generally adopt the phosphating surface treatment.
Black + oil is a very popular coating of industrial fasteners, because it is the cheapest, and before the oil runs out it looks good. Because black almost no rust resistance, after oil free so it will soon be rusty. Is in the condition of oil, the neutral salt spray test also can only achieve 3 ~ 5 hours.
Dark fasteners - consistency pretightening torque is also very poor. If you need to improve, during assembly, place daub grease on thread after spin.
Cadmium coating corrosion resistance is very good, especially in Marine atmospheric environment better corrosion resistance than other surface treatment. Cadmium plating liquid waste processing costs of the process, cost is high, the price is about 15 ~ 20 times that of galvanized. So do not use in general industry, only for some specific environment. For example, used in oil drilling platform and hainan airlines aircraft with fasteners.
Chromium plating in the atmosphere is very stable, not easy to change color and lose luster, high hardness, good wear resistance. On the fasteners with chromium plating is as a general adornment effect. Is rarely used in the field of anticorrosion to demand higher industry, because good chrome plating fasteners and stainless steel is also expensive, just use stainless steel strength is not enough, only replaced with chrome fasteners.
In order to prevent corrosion, should first before chromium plating copper and nickel plating. Chromium plating can withstand 1200 degrees high temperature (650 ℃). But also hydrogen embrittlement problems as well as electric galvanized.
Silver plating, nickel plating,
Silver plating can be corrosion resistant, and can be used as a solid lubricant for the fasteners. Due to cost reasons, use silver plated nut, bolts, sometimes small bolt also silver plating. Silver lose luster in the air, but can work under 1600 degrees. So, people use the characteristics of its resistance to high temperature and lubrication, fasteners for working under high temperature, in order to prevent the oxidation of bolt, nut and killed.
Fasteners, nickel plating, mainly used for both corrosion protection, and to have good electrical conductivity. Such as vehicle battery leading-out terminal.
Hot dip zinc
Hot dip zinc in zinc thermal diffusion coating under heated to a liquid. The plating thickness in 15 ~ 10 microns, and not easy to control, but make good corrosive, more used in engineering. Severe pollution in the manufacturing process of hot dip zinc, zinc waste and zinc steam, etc.
Due to the coating thickness, in fasteners, raise the question of the internal and external thread is difficult to spin. There are two ways to solve this problem. Again after plating is a kind of internal thread, solved the problem of the threaded together though. But also reduces the anticorrosion performance. When one is nut tapping, the thread is greater than the standard lines about 0.16 ~ 0.75 mm (M5 ~ (), then such hot dipping zinc although also can solve the screw problem, but at the expense of lower strength. At present, there is a locking screw, the United States "will be on" internal thread can solve this problem. Because of its thread and thread not binding capacity gap is big, can be used to hold the thick coating, so does not affect the spin and sex, anticorrosion performance and strength also remain the same at the same time, are not affected.
Due to the temperature of the hot dip zinc processing, it can not be used in 10.9 magnitude of fasteners.
Zinc impregnation for zinc powder solid metallurgy thermal diffusion coating. Its uniformity is good, screw thread, blind hole can obtain homogeneous layer. Plating thickness for 10 to 110 microns, and the error can be controlled in 10%. It combined with the matrix strength and anti-corrosion performance in zinc coating (electricity galvanized, hot dip zinc, dacromet) is one of the best. No pollution in the machining process, the most environmentally friendly.
There is no problem of hydrogen embrittlement, prestressing force and torque, and consistent performance is very good. If you don't consider the environmental problem of hexavalent chromium, it is actually the most suitable for high corrosion of high strength fasteners.
The plating, there are many metal or nonmetal coating. These coating detail by each company. Such as the strength (MAGNI), lars pat (RUSPERT), resistant to meetall (NanoMate), aluminum FuLong, titanium FuLong TIODIZE companies in the United States.