Almost all commercial fasteners are made of carbon steel and alloy steel, and generally meet the requirements of corrosion resistance. Therefore, the surface-treated coating must be firmly attached and cannot fall off during installation and disassembly. On the other hand, for threaded fasteners, the coating should be thin enough so that the threads can still be screwed after plating. Generally, the temperature limit of coating is lower than that of fastener material, so it is necessary to consider the working temperature requirements of fastener.
The main purpose of surface treatment is beauty and anticorrosion. Because the main function of fasteners is to fasten parts, and the surface treatment also has a great influence on their fastening performance. Therefore, when selecting the surface treatment process, we should consider the factors such as the consistency of the torque and pre-tightening force of fasteners.
A high-level designer should not only consider the structural design and manufacturing process, but also pay attention to the assembly process, even the requirements of environmental protection and economy. According to the above factors, the following briefly introduces some commonly used coatings for fasteners.
Galvanizing is the most commonly used coating for commercial fasteners. It is cheaper and looks better, and it can be black or military green. However, its corrosion resistance is average, and its corrosion resistance is the lowest among zinc plating (coating) layers. Generally, the neutral salt spray test of zinc plating is within 72 hours, and some special sealing agents are used, which makes the neutral salt spray test reach more than 200 hours, but the price is expensive, which is 5~8 times that of general zinc plating.
Hydrogen embrittlement is easy to occur in the process of electro-zinc plating, so the bolts above grade 10.9 are generally not treated with zinc plating. Although hydrogen can be removed in an oven after plating, the passivation film will be destroyed when it is above 60℃, so hydrogen removal must be carried out before passivation after plating. This has poor operability and high processing cost. In reality, the general manufacturer will not take the initiative to remove hydrogen, unless the specific customer’s mandatory requirements.
Galvanized fastener torque-preload consistency is poor and unstable, so it is generally not used for the connection of important parts. In order to improve the consistency of torque-preload, the method of coating lubricating substance after plating can also be used to improve and improve the consistency of torque-preload.
A basic principle: Phosphating is cheaper than zinc plating, and its corrosion resistance is worse than zinc plating. Oil should be applied after phosphating, and its corrosion resistance is closely related to the performance of the oil applied. For example, after phosphating, the neutral salt spray test is only 10~20 hours after applying general antirust oil. It takes 72~96 hours to apply high-grade antirust oil. But its price is 2~3 times that of ordinary phosphating oil.
There are two kinds of phosphating commonly used for fasteners, zinc phosphating and manganese phosphating. Zinc phosphating has better lubrication performance than manganese phosphating, and manganese phosphating has better corrosion resistance and wear resistance than zinc plating. Its service temperature can reach 225 to 400 degrees Fahrenheit (107~204℃). Especially the connection of some important parts. For example, connecting rod bolts and nuts of engines, cylinder heads, main bearings, flywheel bolts, wheel bolts and nuts, etc.
Phosphating is adopted for high-strength bolts, which can also avoid hydrogen embrittlement. Therefore, in the industrial field, bolts above grade 10.9 are generally treated with phosphating surface.
Blackening+oiling is a popular coating for industrial fasteners, because it is the cheapest and looks good before the oil consumption runs out. Because blackening has almost no rust-proof ability, it will rust quickly without oil. Even in the presence of oil, the neutral salt spray test can only reach 3~5 hours.
The corrosion resistance of cadmium coating is very good, especially in marine atmospheric environment, which is better than other surface treatments. The treatment cost of waste liquid in the process of electroplating cadmium is high, and its price is about 15~20 times that of electroplating zinc. Therefore, it is not used in general industries and is only used in some specific environments. For example, fasteners for oil drilling platforms and HNA aircraft.
Chromium coating is very stable in the atmosphere, not easy to change color and lose luster, high hardness and good wear resistance. Chromium plating on fasteners is generally used for decoration. It is rarely used in industrial fields with high corrosion resistance requirements, because good chromium-plated fasteners are as expensive as stainless steel. Only when stainless steel is not strong enough, chromium-plated fasteners are used instead.
In order to prevent corrosion, copper and nickel should be plated before chromium plating. Chromium coating can withstand high temperature of 1200 degrees Fahrenheit (650℃). However, there is also the problem of hydrogen embrittlement like zinc electroplating.
The method of plating a layer of nickel on metal or some nonmetals by electrolysis or chemical method is called nickel plating. Nickel plating is divided into electrolytic nickel plating and electroless nickel plating.
It is mainly used in places where corrosion protection and good electrical conductivity are required. Such as the lead-out terminal of the vehicle battery.
Hot dip zinc
Hot dip zinc is a thermal diffusion coating when zinc is heated to liquid. Its coating thickness is 15~100μm, which is not easy to control, but it has good corrosion resistance and is often used in engineering. There are serious pollution in the process of hot dip zinc processing, including zinc waste and zinc vapor.
Because of the thick coating, it is difficult to screw the internal and external threads in the fastener. Due to the temperature of hot-dip galvanizing, (340-500C) it cannot be used for fasteners above grade 10.9.
Galvanizing is a solid metallurgical thermal diffusion coating of zinc powder. Its uniformity is good, and even layers can be obtained in threads and blind holes. The thickness of the coating is 10~110μm, and the error can be controlled at 10%. Its bonding strength with the substrate and corrosion resistance are the best among zinc coatings (electro-galvanizing, hot-dip galvanizing and dacromet). Its processing process is pollution-free and the most environmentally friendly.
DACROMET is a transliteration and abbreviation of Dacromet, which is called Dacromet, Dackrust and Ducron for short. Named zinc-chromium coating in China, it is a new type of anticorrosive coating with zinc powder, aluminum powder, chromic acid and deionized water as the main components.
There is no hydrogen embrittlement problem, and the torque-preload consistency is very good. If we don’t consider the problems of chromium and environmental protection, it is actually the most suitable for high-strength fasteners with high anti-corrosion requirements.