For fasteners surface treatment, people generally pay attention to beauty and corrosion, but the main function of fasteners is to tighten parts, and surface treatment has a great impact on fasteners fastening performance, so when choosing the surface treatment, should also consider the fastening performance factor, that is, the installation torque – pretightening force consistency.
The surface treated coating must be firmly attached and cannot fall off during installation and removal. For threaded fasteners, the coating must be thin enough to allow the thread to spin after plating.Generally, the temperature limit of the coating is lower than that of the fastener material, so it is necessary to consider the working temperature requirement of the screws fastener.
An excellent screw man should not only consider the design, but also pay attention to the assembly process, and even environmental requirements.There are many types of surface treatments to choose from, but only one principle is “economical and practical”.The following is a brief introduction of some commonly used fastener coatings according to the above factors for the reference of fastener practitioners.
Electrogalvanizing is the most commonly used coating for commercial fasteners.It is cheaper and looks better. It can be in black or military green.However, its anticorrosion performance is general, its anticorrosion performance is the lowest in zinc plating (coating) layer.General electric galvanizing neutral salt mist test is in 72 hours, also have use special sealant, make neutral salt mist test amounts to 200 hours above, but the price is expensive, it is 5~8 times of general galvanizing.
Electrogalvanizing process is easy to produce hydrogen embrittlement, so the bolt above grade 10.9 generally does not use galvanized treatment, although after plating can be used to dehydrogenate oven, but due to the passivation film in the above 60℃ will be damaged, so dehydrogenation must be carried out before passivation after electroplating.So operability is poor, processing cost is high.In reality, the general production plant will not take the initiative to remove hydrogen, unless the specific customer’s mandatory requirements.
Electrogalvanized fasteners have poor torque-pre-tightening consistency and are unstable, and are generally not used for the connection of important parts. In order to improve the torque-pre-tightening force consistency, the method of coating lubricating substances after plating can also be used to improve and increase the torque-pre-tightening force consistency.
Phosphating is cheaper than galvanizing and has worse corrosion resistance 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 10-20 hours after phosphating with general antirust oil. Apply high-grade anti-rust oil for 72~96 hours. But its price is 2~3 times that of general phosphating oil.
Two common types of fastener phosphating are zinc and manganese series phosphating.Zinc phosphating lubrication performance than manganese phosphating, manganese phosphating corrosion resistance, wear resistance is better than galvanized.It can be used at temperatures from 225 to 400 degrees Fahrenheit (107 to 204 degrees Celsius).Especially the connection of some important parts.For example, engine connecting rod bolts, nuts, cylinder heads, main bearings, flywheel bolts, wheel bolts and nuts, etc.
Phosphating is used for high-strength bolts, which can also avoid hydrogen embrittement. Therefore, phosphating surface treatment is generally used for bolts above grade 10.9 in the industrial field.
Blackening + oiling is a popular coating for industrial fasteners because it is the cheapest and looks good until the oil runs out.Since blackening has almost no rust resistance, it rusts quickly when it is oil-free.Even in the oil state, the neutral salt spray test can only reach 3~5 hours.
Cadmium coating corrosion resistance is very good, especially in the Marine atmosphere environment corrosion resistance is better than other surface treatment.In the process of electroplating cadmium, the waste liquid treatment cost is high, the cost is high, its price is about 15~20 times that of electroplating zinc.So it is not used in the general industry, only for some specific environment.For example, fasteners for oil RIGS and Hainan aircraft.
The chromium plating layer is stable in the atmosphere, is not easy to change color and lose luster, has high hardness and good wear resistance. Chrome plating is generally used for decoration on fasteners. It is rarely used in industrial fields with high anti-corrosion requirements, because good chrome-plated fasteners are as expensive as stainless steel. 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 before chromium plating. The chromium coating can withstand high temperatures of 1200 degrees Fahrenheit (650°C). But it also has the same hydrogen embrittlement problem as electro-galvanized.
It is mainly used in places with good anticorrosion and electrical conductivity. Such as the vehicle battery terminals, etc.
Hot dip zinc is a thermal diffusion coating of zinc heated to liquid.Its coating thickness is 15~100 m, and not easy to control, but good corrosion resistance, used in engineering.Hot dip zinc processing in the serious pollution, such as zinc waste and zinc steam.
Due to the thickness of the coating, the internal and external threads are difficult to screw in the fastener.It is not suitable for fasteners of class 10.9 or above due to the temperature of hot dip zinc processing.
Zincizing is a zinc powder solid metallurgical thermal diffusion coating.Its uniformity is good, and the uniform layer can be obtained in thread and blind hole.The coating thickness is 10~110 m, and the error can be controlled in 10%.Its bonding strength and anti-corrosion properties with the matrix are the best among zinc coatings (electric galvanizing, hot dipping zinc, dacromet).Its processing process pollution-free, the most environmental protection.
There is no hydrogen embrittlement problem, and the torque-preload consistency is very good. If price and environmental issues are not considered, it is actually most suitable for high-strength metal fasteners with high anti-corrosion requirements.
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