1. Electro-galvanized
Electrogalvanizing is the most commonly used coating for commercial fasteners. It's cheaper: it's also good looking and can come in black or army green. However, its anti-corrosion performance is average. Its anti-corrosion performance is the lowest among the zinc plating (coating) layers. Generally, the neutral salt spray test of electro-galvanizing is within 72 hours, and special sealants are also used to make the neutral salt spray test more than 200 hours, but the price is expensive, which is 5-8 times that of general galvanizing.
The electro-galvanizing process is prone to hydrogen embrittlement, so the bolts above grade 10.9 are generally not treated with galvanizing. Although the oven can be used to remove hydrogen after plating. However, the passivation film will be destroyed when the temperature is above 60 °C. Therefore, hydrogen removal It must be done before post-plating purification. So poor operability: high processing cost. In reality, general production plants do not actively dehydrogenate: unless mandated by specific customers.
The torque-preload consistency of electro-galvanized fasteners is poor: and unstable, and is generally not used for the connection of important parts. In order to improve the torque-preload consistency, the method of coating lubricating substances after plating can also be used to improve and enhance the torque-preload consistency.
2. Phosphating
Phosphating is cheaper than galvanizing, and its corrosion resistance is worse than galvanizing. After phosphating, oil should be applied, and the level of its corrosion resistance has a great relationship with the performance of the oil applied. For example: after phosphating, apply general anti-rust oil, and the neutral salt spray test is only 10~20 hours. Apply high-grade anti-rust oil: up to 7246 hours. But its price is 2 to 3 times that of general phosphating oil.
There are two commonly used firmware phosphating: zinc-based phosphating and manganese-based phosphating. The lubricating performance of zinc-based phosphating is better than that of manganese-based phosphating: manganese-based phosphating Corrosion resistance: galvanized with better wear resistance. Its operating temperature can reach 225°F to 400°F (107~204°C).
Many industrial fasteners are treated with phosphating and oiling. Because its torque-preload consistency is very good, it can ensure that the tightening requirements expected by the design can be met during assembly. Therefore, it is widely used in the industry. Especially the connection of some important parts. Such as. Steel structure connection, engine connecting rod bolts, nuts, cylinder heads, main bearings, flywheel bolts, wheel bolts and nuts, etc.
Phosphating is used for high-strength bolts to avoid the problem of hydrogen embrittlement, so bolts above grade 10.9 in the industrial field generally use phosphating surface treatment.
3. Oxidation (blackening)
Blackened + oiled is a popular finish for industrial fasteners: because it is the cheapest and looks good until the oil runs out. Due to the blackening, it has almost no anti-rust ability: so it will rust very quickly without oil. Even in the state of oil: its neutral salt spray test can only reach 3 to 5 hours.
Blackened fasteners also have poor torque-preload consistency. If you need to improve, you can apply grease to the inner thread during assembly and then screw it together.
4. Electroplating cadmium
Cadmium coating has good corrosion resistance: especially in the atmospheric environment, the corrosion resistance is better than other surface treatments. The waste liquid treatment cost in the process of electroplating cadmium is high: the cost is high: its price is about 15-20 times that of electroplating. Therefore, it is not used in general industries, and is only used in some specific environments. For example, it is used in fasteners for oil drilling platforms and HNA aircraft.
5. Chrome electroplating
Chromium plating is very stable in the atmosphere: not easy to change color and tarnish: high hardness and good wear resistance. Chrome plating on fasteners is generally used for decorative purposes. 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 first before chrome plating. Chrome plating can withstand high temperatures of 1200 degrees Fahrenheit (650 degrees Celsius). But it also has the same problem of hydrogen embrittlement as electrogalvanizing.
6. Silver-plated, nickel-plated
The silver coating can not only prevent corrosion: it can also act as a solid lubricant for fasteners. For cost reasons, the nuts are silver-plated and the bolts are not: sometimes small bolts are also silver-plated. Silver tarnishes in air: but works at 1600 degrees Fahrenheit. Therefore: people use its high temperature resistance and lubrication characteristics: fasteners used to work at high temperatures to prevent the oxidation of bolts and nuts from seizing.
Fasteners are nickel-plated, mainly used in places where both anti-corrosion and good electrical conductivity are required. Such as vehicle battery lead-out terminals, etc.
7. Hot dip galvanizing
Hot dip galvanizing is a thermal diffusion coating of zinc heated to a liquid. Its coating thickness is 15~10μm: and it is not easy to control: but it has good corrosion resistance: it is mostly used in engineering. Serious pollution during hot-dip galvanizing: zinc waste and zinc vapor, etc.
Due to the thickness of the coating, the problem of difficulty in screwing the internal and external threads in the fastener is caused. There are two ways to solve this problem. One is to tap the internal thread after plating, although the problem of threading is solved. But it also reduces the corrosion resistance. One is when the nut is tapped: make the thread larger than the standard thread about 0.16~0.75mm (M5~M30): and then hot-dip galvanized, although this can also solve the problem of screwing, but it pays the price of reduced strength. At present: there is a kind of anti-loosening thread, the American "spirit" internal thread can solve this problem. Because the internal thread and external thread are not fastened, the tolerance is large: it can be used to accommodate thick coatings, so it does not affect the screwability, and the anti-corrosion performance and strength are also unchanged: unaffected.
Due to the temperature of the hot dip galvanizing process: it cannot be used for fasteners above 10.9.
8. Sherardization
Sherardizing is a solid metallurgical thermal diffusion coating of zinc powder. Its uniformity is good, and a uniform layer can be obtained in the thread and blind hole. The thickness of the coating is 10-110μm: and the error can be controlled within 10%. Its bonding strength with the substrate and anti-corrosion performance are the best among zinc coatings (electrogalvanized, hot-dip galvanized, dacromet), and its processing is pollution-free and the most environmentally friendly.
9. Dacromet
There is no hydrogen embrittlement problem, and torque-preload consistency is good. If the environmental protection issues of hexavalent chromium are not considered: it is actually most suitable for high-strength fasteners with high anti-corrosion requirements.
