The fastener surface treatment basically has nine kinds of
1, electroplating Zinc
Zinc electroplating is the most commonly used coating for commercial fasteners. It is cheaper, look good, can have black, military green. However, its anti-corrosion performance is general, its anti-corrosion performance is the lowest in the zinc coating layer. General electroplating Zinc neutral salt spray test within 72 hours, there are also special sealants, making the neutral salt spray test up to 200 hours, but expensive, is the general galvanized $number times.
Electroplating Zinc processing process is easy to produce hydrogen embrittlement, so more than 10.9 bolts generally do not use galvanized treatment. Although after the plating can be used in the oven to hydrogen, but because the passivation film in 60 ℃ above will be destroyed, so the hydrogen must be in the plating after passivation. So the operability is poor, the processing cost is high. In reality, the general production plant will not take the initiative to hydrogen, unless the specific customer's mandatory requirements.
The fastener torque-preload force of electroplating zinc is poor in consistency and unstable, and is generally not used in the connection of important parts. In order to improve the consistency of torque-preload, the consistency of torque-preload can be improved and improved by the method of coated lubricating material.
Phosphating is cheaper than zinc plating and corrosion resistance is worse than zinc plating. After phosphating, oil should be applied, and its corrosion resistance is closely related to the performance of oil coating. For example, after phosphating coating general anti-rust oil, neutral salt spray test is only 10-20 hours. Coated with high-grade anti-rust oil, it can reach 72-96 hours. But the price is the general phosphating coating oil $number times.
There are two common types of the phosphating of fasteners, zinc phosphating and manganese system phosphating. Zn-Series Phosphating lubrication performance is better than manganese system phosphating, manganese system phosphating corrosion resistance, wear resistance galvanized better. Its use temperature can be Dahua 225 degrees to 400 degrees (107~204℃).
Many industrial fasteners are treated with phosphating. Because of its torque-preload consistency is good, the assembly can be guaranteed to meet the design expected tightening requirements, so in industry use more. Especially the connection of some important parts. For example, steel structure connection, engine connecting rod bolts, nuts, cylinder head, main bearing, flywheel bolt, wheel bolt nut and so on.
High-strength bolts used phosphating, but also to avoid hydrogen embrittlement, so in the industrial field of more than 10.9 bolts generally used phosphating surface treatment.
3, oxidation (blackening)
Blackened + oiled is a popular coating for industrial fasteners because it is the cheapest and looks good before the oil runs out. Because there is almost no rust-proof ability, it will soon rust without oil. In the presence of oil, the salt spray test can only reach 3-5 hours.
Black Fasteners Torque-preload consistency is also poor. If you need to improve, you can apply grease to the thread when assembling and then spin.
4. Cadmium electroplating
The corrosion resistance of cadmium coating is very good, especially in the marine atmosphere environment, the corrosion resistance is better than other surface treatments. Electroplating cadmium processing process of waste liquid treatment costs, high cost, the price of zinc electroplating is about $number times. Therefore, in the general industry is not used, only for a specific environment. For example, for the oil drilling platform and the HNA aircraft used fasteners.
5, Electroplating Chromium
Chromium coating in the atmosphere is very stable, not easy to change color and loss of luster, high hardness and wear resistance. Chrome plating on fasteners is generally used as a decorative function. It is rarely used in industrial areas where corrosion resistance is high, because good chrome plated fasteners are as expensive as stainless steel, but only when stainless steel is not strong enough to be replaced with chrome fasteners.
In order to prevent corrosion, chrome plating should be first plated with copper and nickel. Chromium coating can withstand the high temperature of 1200 degrees Fahrenheit (650 ℃). But it also has the same hydrogen embrittlement problem as electroplating zinc.
6, silver-plated, nickel-plated
Silver coating can be anti-corrosion, and can be used as a solid lubricant fasteners. For cost reasons, the NUT is silver plated, bolts are not used, and sometimes small bolts are silver plated. Silver loses luster in the air, but can function under 1600 degrees Fahrenheit. Therefore, people use its high temperature and lubrication characteristics, to work at high temperature fasteners to prevent the bolt, nut oxidation bite dead.
Fasteners Nickel-plated, mainly for both anti-corrosion and good conductivity of the place. such as vehicle battery extraction terminals.
7, Hot Dip zinc
Hot-dip zinc is heated to the liquid heat diffusion coating. Its coating thickness in 15~10μm, and not easy to control, but good corrosion, more used in engineering. Hot-dip zinc processing in the process of serious pollution, zinc scrap and zinc vapor.
Due to the thickness of the coating, it is difficult to rotate the internal and external thread in the fastener. There are two ways to resolve this problem. One is to attack the internal thread after plating, although the problem of thread rotation is solved. But also reduces the antiseptic performance. One is when the nut tapping, so that the thread is larger than the standard pattern of about 0.16~0.75mm (M5~M30), and then hot dip zinc, although it can also solve the problem of spinning, but pay the cost of strength reduction. At present, there is a kind of loose thread-the United States "in the" internal thread can solve this problem. Because the internal thread and the outer thread are not fastened when the gap is large, can be used to accommodate the thick coating, so it does not affect the rotation, while the anti-corrosion performance and strength also remain intact, unaffected.
Because of the temperature of hot dip zinc processing, it can not be used for fasteners above 10.9 level.
8, zinc infiltration
Zinc infiltration is a zinc powder solid metallurgical thermal diffusion coating. Its uniformity is good, the thread, the blind hole can obtain the uniform layer. The coating thickness is 10~110μm and the error can be controlled at 10%. Its adhesion to the matrix and anti-corrosion properties in the zinc coating (galvanized, hot dip zinc, dacromet) is the best. Its processing process non-polluting, the most environmentally friendly.
There is no hydrogen embrittlement problem, and the torque-preload force is good for consistent performance. If you do not consider the environmental problems of hexavalent chromium, it is actually the most suitable for high strength fasteners.