Since almost all commercial fasteners are made of carbon steel and alloy steel, general fasteners are expected to prevent corrosion, in addition, the surface treatment of the coating must be firmly attached, can not fall off during the installation and removal process, for threaded fasteners, the coating needs to be thin enough to make the thread can still be spun. Generally, the temperature limit of the coating is lower than that of the fastener material, so it is also necessary to consider the operating temperature requirements of the fastener. For surface treatment, people are generally concerned about beauty and anti-corrosion, but the main function of fasteners is to tighten parts, and the surface treatment has a great impact on the fastening performance of fasteners, so when choosing surface treatment, the factors of fastening performance should also be considered, that is, the consistency of installation torque - preload force. A high-level designer should not only consider the design, but also pay attention to the manufacturability of the assembly, and even environmental requirements. The following is a brief introduction of some commonly used coatings for fasteners according to the above factors for reference by fastener practitioners.
1: Galvanizing Galvanizing is the most commonly used coating for commercial fasteners. It is cheaper and looks better. It can be black or army green. However, its corrosion resistance is general, and its corrosion resistance is the lowest in the zinc plating (coating) layer. General galvanizing neutral salt spray test within 72 hours, there are also special sealers, making the neutral salt spray test of more than 200 hours, but the price is expensive, is 5 to 8 times that of general galvanizing. Electrogalvanizing process is easy to produce hydrogen embrittlement, so bolts above 10.9 are generally not galvanized. Although it can be dehydrogenated by oven after plating, dehydrogenation must be carried out before passivation after plating because the passivation film will be destroyed at more than 60 ° C. So poor operability, high processing costs. In reality, the general production plant will not actively dehydrogenate unless mandated by a specific customer. Electrogalvanized fasteners torque - preload consistency is poor, and unstable, generally not used for important parts of the connection. In order to improve the consistency of torque-preload, the method of coating lubricating substance after plating can also be used to improve and enhance the consistency of torque-preload.
2: Phosphating a basic principle phosphating is relatively cheap galvanized, corrosion resistance is worse than galvanized. After phosphating should be coated with oil, the level of its corrosion resistance and the performance of the oil has a great relationship. For example, after phosphating, general anti-rust oil is applied, and the neutral salt spray test is only 10 to 20 hours. Coating high-grade anti-rust oil, it can reach 72 to 96 hours. However, its price is 2 to 3 times that of general phosphating oil. Two commonly used fastener phosphating, zinc phosphating and manganese phosphating. The lubrication performance of zinc phosphating is better than that of manganese phosphating, and the corrosion resistance and wear resistance of manganese phosphating are better than that of galvanizing. It can be used at temperatures ranging from 225 to 400 degrees Fahrenheit (107 to 204 degrees Celsius). Especially the connection of some important parts. For example, connecting rod bolts, nuts, cylinder heads, main bearings, flywheel bolts, wheel bolts and nuts of the engine. The phosphating of high-strength bolts can also avoid the problem of hydrogen embrittling, so in the industrial field, bolts above 10.9 are generally treated with phosphating surface.
3: Oxidized (blackened) blackening + oiling is a popular coating for industrial fasteners because it is the cheapest and looks good before the oil runs out. Since blackening has almost no anti-rust ability, it will rust quickly after no oil. Even in the state of oil, the neutral salt spray test can only reach 3 to 5 hours.
4: Cadmium plating cadmium coating corrosion resistance is very good, especially in the Marine atmosphere environment corrosion resistance is better than other surface treatment. The waste liquid treatment cost in the processing of cadmium plating is large and the cost is high, and its price is about 15 to 20 times that of electrogalvanizing. Therefore, it is not used in the general industry, and is only used in some specific environments. For example, used in oil drilling platforms and sea aircraft fasteners.
5: electroplating chromium chromium coating in the atmosphere is very stable, not easy to discolor and lose luster, high hardness and good wear resistance. The use of chromium plating on fasteners is generally used as a decorative function. It is rarely used in industrial fields with high corrosion resistance requirements, because good chrome plated fasteners are as expensive as stainless steel, but when the strength of stainless steel is not enough, chrome plated fasteners are used instead. In order to prevent corrosion, copper and nickel should be plated before chromium plating. Chromium plating can withstand temperatures as high as 1200 degrees Fahrenheit (650 degrees Celsius). However, it also has the same hydrogen embrittlement problem as electrogalvanizing.
6: Nickel plating is mainly used for both anti-corrosion and good electrical conductivity. Such as the leading terminal of the vehicle battery.
7: Hot dip zinc Hot dip zinc is zinc heated to the liquid under the thermal diffusion coating. Its coating thickness is 15~100μm, and it is not easy to control, but it has good corrosion resistance and is mostly used in engineering. Hot dip zinc processing is seriously polluted, there are zinc waste and zinc steam and so on. Because of the thick coating, it causes the problem that the internal and external threads are difficult to screw in fasteners. Due to the temperature of hot dipping zinc processing,(340-500C) it cannot be used for fasteners above 10.9 level.
8: Zincizing zincizing zinc powder solid metallurgical thermal diffusion coating. The uniformity is good, and the uniform layer can be obtained in the thread and blind hole. The coating thickness is 10~110μm, and the error can be controlled at 10%. Its binding strength and anti-corrosion properties with the substrate are the best in zinc coatings (electrogalvanizing, hot dipping zinc, dacromet). Its processing process is pollution-free and the most environmentally friendly.
9: Dacromet does not have hydrogen embrittleness problems, and the torque-preload consistency performance is good. If you do not consider chromium and environmental protection issues, it is actually most suitable for high-strength fasteners with high corrosion protection requirements.
