The choice of surface treatment is a problem that every designer faces. There are many types of surface treatment options available, and a high-level designer should not only consider the economy and practicality of the design, but also pay attention to the assembly process and even environmental requirements. Below is a brief introduction to some commonly used coatings for fasteners based on the above principles, for reference by fastener practitioners.
1. Electrogalvanizing
Zinc is the most commonly used coating for commercial fasteners. The price is relatively cheap, and the appearance is good. Common colors include black and military green. However, its anti-corrosion performance is average, and its anti-corrosion performance is the lowest among zinc plating (coating) layers. Generally, the neutral salt spray test of galvanized steel is carried out within 72 hours, and special sealing agents are also used to ensure that the neutral salt spray test lasts for more than 200 hours. However, the price is expensive, which is 5-8 times that of ordinary galvanized steel.
The process of electrogalvanizing is prone to hydrogen embrittlement, so bolts above grade 10.9 are generally not treated with galvanizing. Although hydrogen can be removed using an oven after plating, the passivation film will be damaged at temperatures above 60 ℃, so hydrogen removal must be carried out after electroplating and before passivation. This has poor operability and high processing costs. In reality, general production plants do not actively remove hydrogen unless mandated by specific customers.
The consistency between torque and pre tightening force of galvanized fasteners is poor and unstable, and they are generally not used for connecting important parts. In order to improve the consistency of torque preload, the method of coating lubricating substances after plating can also be used to improve and enhance the consistency of torque preload.
2. Phosphating
A basic principle is that phosphating is relatively cheaper than galvanizing, but its corrosion resistance is worse than galvanizing. After phosphating, oil should be applied, and its corrosion resistance is closely related to the performance of the oil applied. For example, after phosphating, applying a general anti rust oil and conducting a neutral salt spray test for only 10-20 hours. Applying high-grade anti rust oil can take up to 72-96 hours. But its price is 2-3 times that of general phosphating oil.
There are two commonly used types of phosphating for fasteners, zinc based phosphating and manganese based phosphating. Zinc based phosphating has better lubrication performance than manganese based phosphating, and manganese based phosphating has better corrosion resistance and wear resistance than zinc plating. It can be used at temperatures ranging from 225 to 400 degrees Fahrenheit (107-204 ℃). Especially for the connection of some important components. Such as connecting rod bolts and nuts of the engine, cylinder head, main bearing, flywheel bolts, wheel bolts and nuts, etc.
High strength bolts use phosphating, which can also avoid hydrogen embrittlement issues. Therefore, bolts above grade 10.9 in the industrial field generally use phosphating surface treatment.
3. Oxidation (blackening)
Blackening+oiling is a popular coating for industrial fasteners because it is the cheapest and looks good before fuel consumption. Due to its blackening, it has almost no rust prevention ability, so it will rust quickly without oil. Even in the presence of oil, the salt spray test can only last for 3-5 hours.
4. Electroplating partition
Cadmium plating has excellent corrosion resistance, especially in marine atmospheric environments, compared to other surface treatments. The waste liquid treatment cost in the process of electroplating cadmium is high, and its price is about 15-20 times that of electroplating zinc. So it is not used in general industries, only for specific environments. Fasteners used for oil drilling platforms and HNA aircraft.
5. Chromium plating
The chromium coating is very stable in the atmosphere, not easy to change color and lose luster, and has high hardness and good wear resistance. The use of chromium plating on fasteners is generally used for decorative purposes. It is rarely used in industrial fields with high corrosion resistance requirements, as good chrome plated fasteners are equally expensive as stainless steel. Only when stainless steel strength is insufficient, chrome plated fasteners are used instead.
To prevent corrosion, copper and nickel should be plated first before chrome plating. The chromium coating can withstand high temperatures of 1200 degrees Fahrenheit (650 ℃). But there is also a problem of hydrogen embrittlement, similar to electrogalvanizing.
6. Nickel plating
Mainly used in areas that require both anti-corrosion and good conductivity. For example, the outgoing terminals of vehicle batteries.
7. Hot-dip galvanizing
Hot dip galvanizing is a thermal diffusion coating of zinc heated to a liquid. The coating thickness is between 15 and 100 μ m. And it is not easy to control, but has good corrosion resistance and is often used in engineering. During the hot dip galvanizing process, there is severe pollution, including zinc waste and zinc vapor.
Due to the thick coating, it has caused difficulties in screwing in internal and external threads in fasteners. Due to the temperature of hot-dip galvanizing processing, it cannot be used for fasteners above grade 10.9 (340~500 ℃).
8. Zinc infiltration
Zinc infiltration is a solid metallurgical thermal diffusion coating of zinc powder. Its uniformity is good, and a uniform layer can be obtained in both threads and blind holes. Plating thickness is 10-110 μ m. And the error can be controlled at 10%. Its bonding strength and anti-corrosion performance with the substrate are the best in zinc coatings (such as electrogalvanizing, hot-dip galvanizing, and Dacromet). Its processing process is pollution-free and the most environmentally friendly.
9. Dacromet
There is no hydrogen embrittlement issue, and the torque preload consistency performance is very good. Without considering chromium and environmental issues, Dacromet is actually the most suitable for high-strength fasteners with high anti-corrosion requirements.
Post time: May-19-2023
