The choice of surface treatment is a problem faced by every designer. There are many types of surface treatment available, but only one principle is "economical and practical".
As almost all commercial fasteners are made of carbon steel and alloy steel, general fasteners are expected to prevent corrosion. In addition, the coating of the surface treatment must be firmly attached and cannot fall off during installation and removal. For threaded fasteners, the coating must be thin enough so that the threads can still be rotated after plating. Generally, the temperature limit of plating is lower than that of fastener materials, so the operating temperature requirements of fasteners should 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 surface treatment also has a great impact on the fastening performance of fasteners. Therefore, when selecting surface treatment, the factors of fastening performance should also be considered, that is, the consistency of installation torque and preload.
A high-level designer should not only consider the design but also pay attention to the assembly process and even environmental requirements. According to the above factors, the plating commonly used for some fasteners is briefly introduced below for the reference of fastener practitioners.
Galvanizing is the most commonly used coating for truck fasteners. It is relatively cheap and looks good. It can be black or military green. However, its anti-corrosion performance is general, and its anti-corrosion performance is the lowest among zinc plating (coating) layers. In general, a neutral salt spray test of galvanization is conducted within 72 hours, and the special sealing agent is also used to make a neutral salt spray test for more than 200 hours, but the price is expensive, which is 5~8 times that of general galvanization.
Hydrogen embrittlement is easy to occur in the process of galvanization, so truck wheel bolts above Grade 10.9 are generally not galvanized Although the oven can be used to remove hydrogen after electroplating, the passivation film will be damaged when the temperature is above 60 ℃, the hydrogen removal must be carried out after electroplating and before passivation. So the operability is poor and the processing cost is high. In reality, general manufacturers will not actively remove hydrogen unless required by specific customers.
Phosphating
One basic principle is that phosphating is cheaper than galvanizing, and its corrosion resistance is worse than galvanizing. After phosphating, it shall be coated with oil, and its corrosion resistance is greatly related to the performance of the oil coated. For example, after phosphating, general antirust oil is applied, and the neutral salt spray test is only 10-20 hours. It can take 72~96 hours to apply high-grade antirust oil. But its price is 2~3 times that of general phosphating coating oil.
There are two kinds of phosphating commonly used for truck hub 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 operating temperature can reach 225 ° F to 400 ° F (107~204 ° C). Especially the connection of some important components. For example, connecting rod bolts and nuts of the engine, cylinder head, main bearing, flywheel bolts, wheel bolts, and nuts, etc.
Phosphating is adopted for high-strength bolts, which can also avoid hydrogen embrittlement. Therefore, phosphating surface treatment is generally adopted for bolts above 10.9 in the industrial field.
Oxidation (blackening)
Blackening+oiling is a popular coating for industrial fasteners because it is the cheapest and looks good before the oil is used up. Since blackening has almost no anti-rust ability, it will rust soon after there is no oil. Even in the presence of oil, the intermediate salt spray test can only reach 3-5 hours.
Cadmium electroplating
The corrosion resistance of cadmium coating is very good, especially in the marine atmospheric environment. The cost of waste liquid treatment in the process of cadmium electroplating is high, and its price is about 15-20 times that of electroplating. Therefore, it is not used in general industries, but only for some specific environments. For example, it is used for fasteners for oil drilling platforms and HNA aircraft.
Chromium plating
The chromium coating is stable in the atmosphere, does not easy to change color and lose luster, and has high hardness and good wear resistance. Chromium plating on fasteners is generally used as decoration. It is rarely used in industries with high corrosion resistance 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 can be used instead.
To prevent corrosion, copper and nickel should be plated before chrome plating. The chromium coating can withstand the high temperature of 1200 degrees Fahrenheit (650 ℃). However, hydrogen embrittlement is the same as that of electro-galvanizing.
Platingplate with nickel
It is mainly used in places with good corrosion resistance and conductivity. Such as the lead-out terminal of the vehicle battery.
Hot-dip galvanizing
Hot dip galvanizing is a thermal diffusion coating of zinc heated to liquid. The coating thickness is 15~100 μ m. It is not easy to control, but has good corrosion resistance and is mostly used in engineering. The hot dip galvanizing process is seriously polluted, including zinc waste and zinc vapor.
Due to the thick coating, it is difficult to screw the internal and external threads in fasteners. Due to the temperature of hot dip galvanizing, (340-500C) cannot be used for bolts and nuts above 10.9.
Zincifying
Zinc diffusion coating is zinc powder solid metallurgy thermal diffusion coating. Its uniformity is good, and even layers can be obtained in threads and blind holes. Plating thickness is 10~110 μ m. The error can be controlled within 10%. Its bonding strength with the substrate and anti-corrosion performance is the best among zinc coatings (galvanization, hot-dip galvanizing, Dacromet). Its processing process is pollution-free and the most environmentally friendly.
Dacromet
There is no hydrogen embrittlement problem, and the torque preload consistency is very good. If the problems of chromium and environmental protection are not considered, it is most suitable for high-strength truck fasteners with high anti-corrosion requirements.
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