When it comes to surface treatment, people generally focus on aesthetics and corrosion resistance, but the main function of fasteners is to fasten components, and surface treatment also has a great influence on the fastening performance of fasteners.
Author: Marisa
Choosing which surface treatment to use is a problem that every designer faces. There are many types of surface treatments to choose from, but the principle is only "economical and practical".
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 coating 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 screwed after plating. The temperature limit of the general coating is lower than that of the fastener material, so the working temperature requirements of the fastener must also be considered.
For surface treatment, people generally pay attention to aesthetics and corrosion resistance, 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 choosing surface treatment, the fastening performance factor should also be considered, that is, the consistency of installation torque-preload.
A high-level designer should not only consider the design, but also pay attention to the processability of assembly and even environmental protection requirements. The following is a brief introduction to some commonly used coatings for fasteners based on the above factors for reference by fastener practitioners.
1: Electrogalvanizing
Electroplating zinc is the most commonly used coating for commercial fasteners. It is relatively cheap and looks good, and can be black or military green.
2: Phosphating
Phosphating is cheaper than zinc plating, but its corrosion resistance is worse than that of zinc plating. After phosphating, oil should be applied, and its corrosion resistance is closely related to the performance of the oil applied. There are two common types of fastener phosphating, zinc phosphating and manganese phosphating. Zinc phosphating has better lubrication than manganese phosphating, and manganese phosphating has better corrosion resistance and wear resistance than zinc plating.
3: Oxidation (blackening)
Blackening + oiling is a very popular coating for industrial fasteners because it is the cheapest and looks good before the oil runs out. Since blackening has almost no rust resistance, it will rust quickly after oiling. Even in the presence of oil, the neutral salt spray test can only reach 3 to 5 hours.
4: Cadmium electroplating
Cadmium plating has good corrosion resistance, especially in marine atmospheric environments, where its corrosion resistance is better than other surface treatments. The cost of waste liquid treatment in the process of electroplating cadmium is high, and its price is about 15 to 20 times that of electroplating zinc. Therefore, it is not used in general industries and is only used in some specific environments.
5: Electroplating chromium
The chromium coating is very stable in the atmosphere, not easy to change color and lose gloss, high hardness and good wear resistance. The use of chromium coating on fasteners is generally used for decoration. It is rarely used in industrial fields with high corrosion resistance requirements, because good chromium electroplated fasteners are as expensive as stainless steel. Only when the strength of stainless steel is not enough, chrome-plated fasteners are used instead.
6: Nickel plating
It is mainly used in places that require both corrosion resistance and good conductivity. Such as the lead-out terminals of vehicle batteries.
7: Hot-dip galvanizing
Hot-dip galvanizing is a thermal diffusion coating of zinc heated to liquid. The coating thickness is 15 to 100 μm, and it is not easy to control, but it has good corrosion resistance and is mostly used in engineering. The hot-dip galvanizing process is seriously polluted, with zinc waste and zinc vapor.