The choice of surface treatment is a problem that every designer faces. There are many types of surface treatments that can be selected, but there is only one "economic and practical" principle.
Since almost all commercial fasteners are made of carbon steel or alloy steel, general fasteners are expected to prevent corrosion, and in addition, the surface-treated coating must adhere firmly and cannot be detached during installation and removal. For threaded fasteners, the plating must be thin enough so that the plated threads can still be screwed together. In general, the temperature limit of plating is lower than the fastener material, so the working temperature requirement of the fastener must also 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 the surface treatment also has a great influence on the fastening performance of fasteners. Therefore, when selecting a surface treatment, It should also consider the fastening performance factors, namely the installation torque - the consistency of the preload.
A high-level designer should not only consider the design, but also pay attention to the assembly process and even environmental protection requirements.
According to the above factors, the following brief introduction of some commonly used coatings for fasteners, reference for fasteners practitioners.
Electrogalvanizing is the most commonly used coating for commercial fasteners. It is cheaper and looks better. It can be black and green. However, its anti-corrosion performance is general, and its anti-corrosion performance is the lowest in the zinc plating (coating) layer. In the normal electro-galvanized neutral salt spray test, within 72 hours, special sealing agents are also used, making the neutral salt spray test more than 200 hours, but the price is expensive, which is 5 to 8 times that of general galvanizing.
Galvanizing process is easy to produce hydrogen embrittlement, so bolts above grade 10.9 are generally not treated with galvanizing. Although it is possible to use the oven to dehydrogenate after plating, since hydrogen will be destroyed when the passivation film is above 60°C, dehydrogenation must be performed. It is carried out before passivation after electroplating. This has poor operability and high processing costs. In reality, the general production plant will not take the initiative to dehydrogen unless the specific customer's mandatory requirements.
Galvanized fasteners have poor torque-pretensioning and are not stable, and are generally not used for the connection of important parts. In order to improve the consistency of the torque-preload force, it is also possible to improve the torque-preloading consistency by applying post-plating lubricants.
A basic principle is that phosphating is cheaper than galvanizing, and its corrosion resistance is inferior to that of galvanizing. After phosphating, it should be oiled, and its corrosion resistance has a great relationship with the performance of the oil. For example, after the phosphating, the general anti-rust oil is applied, and the neutral salt spray test is only 10 to 20 hours. High grade anti-rust oil can reach 72~96 hours. But its price is 2 to 3 times that of ordinary phosphating oil.
Phosphorus fasteners are commonly used two kinds of zinc phosphating and manganese phosphating. Zinc phosphate lubricating performance is better than manganese phosphating, manganese phosphating corrosion resistance, wear resistance is better than galvanized. Its use temperature can reach 225 degrees Fahrenheit to 400 degrees (107~204 C).
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.
High-strength bolts use phosphating and can also avoid the problem of hydrogen embrittlement, so bolts above grade 10.9 in the industrial sector generally use phosphating surface treatment.
Blackening + oiling is a popular coating for industrial fasteners because it is the cheapest and looks good before the oil runs out. Due to the almost no anti-rust ability of blackening, it quickly rusts when there is no oil. In the oily state, the neutral salt spray test can only reach 3 to 5 hours.
The cadmium coating has good corrosion resistance, and its corrosion resistance is particularly better in marine atmospheres than other surface treatments. The effluent during the electroplating of cadmium is expensive and costly. The price is about 15 to 20 times that of galvanization. Therefore, they are not used in the general industry and are only used in some specific environments. For example, fasteners used in oil drilling platforms and HNA aircraft.
Chrome plating is very stable in the atmosphere, not easy to change color and lose luster, high hardness and good wear resistance. Chrome plating on fasteners is generally used for decorative purposes. It is rarely used in industries with high requirements for corrosion resistance because good chromium plated fasteners are as expensive as stainless steel, but instead of using chrome-plated fasteners when stainless steel is not strong enough.
To prevent corrosion, first apply copper and nickel before plating. The chrome coating can withstand temperatures of 1200 degrees Fahrenheit (650°C). However, hydrogen embrittlement also exists as in electrogalvanizing.
It is mainly used in places where both antisepsis and good conductivity are required. Such as the vehicle battery terminal and so on.
Hot dip galvanized zinc is heated to a liquid thermal diffusion coating. Its coating thickness is 15~100μm, and it is not easy to control, but it has good corrosion resistance and is used in engineering. In hot dip galvanizing process, the pollution is serious, there are zinc scrap and zinc vapor.
Due to the thick plating layer, the problem that internal and external threads are difficult to screw together is caused in the fastener.
Due to the temperature of the hot dip galvanizing process, (340-500C) it cannot be used for fasteners above 10.9.
Zinc plated zinc powder solid metallurgy thermal diffusion coating. The uniformity is good, uniform layers can be obtained in the threads and blind holes. The coating thickness is 10~110μm, and the error can be controlled at 10%. Its bonding strength to the substrate and corrosion protection are best in zinc coatings (galvanizing, hot dip galvanizing, Dacromet). The process is pollution-free and environmentally friendly.
There is no hydrogen embrittlement problem and the torque-preload force is consistent. If you do not consider the environmental issues of hexavalent chromium, it is actually best suited for high-strength fasteners that require high corrosion resistance.
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