Selection of parts for fastener surface treatment(2)

Update:03 Apr 2020
Summary:

3. Oxidation (blackening) Oiling is a popular coating f […]

3. Oxidation (blackening)
Oiling is a popular coating for industrial fasteners because it is the cheapest and looks good until the oil is depleted. Since the blackening has almost no anti-rust ability, it will rust quickly after no oil. Even in the presence of oil, the neutral salt spray test can only reach 3 to 5 hours.
Torque of blackened fasteners—the consistency of the preload is also poor. If you need to improve, you can apply grease to the internal threads during assembly and then screw them together.
4. Cadmium electroplating
The corrosion resistance of cadmium coating is very good, especially in the marine atmospheric environment, the corrosion resistance is better than other surface treatments. The treatment of waste liquid in the process of electroplating cadmium is expensive and expensive, and its price is about 15-20 times that of electroplating zinc. Therefore, it is not used in general industries and only used in some specific environments. For example, used in fasteners for oil drilling platforms and HNA aircraft.
5. Chrome plating
The chromium coating is very stable in the atmosphere, not easy to change color and lose luster, high hardness and good wear resistance. The use of chrome plating on fasteners is generally used as a decorative effect. It is seldom 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 insufficient, chrome plated fasteners are used instead.
In order to prevent corrosion, copper and nickel should be plated before chrome plating. The chromium coating can withstand high temperatures of 1200 degrees Fahrenheit (650 ° C). But it also has the problem of hydrogen embrittlement like electro-galvanized zinc.
6. Silver and nickel plating
The silver coating can not only prevent corrosion, but also serve as a solid lubricant for fasteners. For cost reasons, nuts are silver plated, bolts are not used, and sometimes small bolts are also silver plated. Silver loses its luster in the air, but it can act at 1600 degrees Fahrenheit. Therefore, people use its high temperature resistance and lubrication characteristics for fasteners that work at high temperatures to prevent the bolts and nuts from being oxidized.
Fasteners are nickel plated and are mainly used in places where corrosion resistance and good conductivity are required. Such as the lead-out terminals of vehicle batteries.