Electroplating is to plate a layer of metal coating on the fastener to change its surface properties and make it not easy to be oxidized and corroded. The coating metal generally uses corrosion-resistant metal.
Electroplating can not only enhance the corrosion resistance of fasteners, but also increase the hardness to prevent wear, improve electrical conductivity, heat resistance, and make the surface smoother and more beautiful.
Electroplating is a process that uses electrolysis to attach a metal film to the surface of a metal or other material. The following briefly introduces some commonly used coatings for fasteners.
Electro-galvanized is the most commonly used coating for fasteners. It looks good and is relatively cheap. There are white zinc, blue zinc, color zinc, black zinc and other colors. Compared with other metal coatings, zinc is a relatively cheap and easy-to-electroplating metal. However, its anti-corrosion performance is general. The neutral salt spray test of electrogalvanized zinc is within 72 hours. Of course, special sealants are also used to make the The salt spray test is more than 200 hours, but the price is 5-8 times that of general galvanizing.
The following picture shows the screws with electroplated blue and white zinc:
The following picture shows the screws of electroplated color zinc:
2. Nickel plating
Electroplated nickel fasteners are generally used in places where both high corrosion resistance and good electrical conductivity are required. The stability of the electroplated nickel layer in the air is very high. Due to the strong passivation ability of metallic nickel, a very thin passivation film can be quickly formed on the surface, which can resist the corrosion of the atmosphere, alkali and certain acids. Electroplated nickel has excellent polishing performance, and its gloss can be maintained for a long time after polishing, and the high hardness of the nickel plating can improve the wear resistance of fasteners.
The following picture shows a nickel-plated hexagon socket bolt:
Black oxide + oil coating is a popular coating for industrial fasteners because it is the cheapest and looks good before the oil runs out. The neutral salt spray test can only reach 3 to 5 hours in the presence of oil. Blackened fastener torque-preload consistency is also poor. If you need to improve, you can apply grease to the inner thread during assembly and then screw it on.
4. Chrome plating
Chrome plating on fasteners is generally used for decoration. The chromium coating is very stable in the atmosphere, is not easy to change color and lose luster, has high hardness and good wear resistance. Good electroplated chromium fasteners are as expensive as stainless steel. Chrome-plated fasteners are only used when the strength of stainless steel is not enough. Therefore, they are rarely used in industrial fields that require high corrosion resistance. To prevent corrosion, copper and nickel should be plated before chromium plating. The chromium coating can withstand the high temperature of 650℃, but it has the same hydrogen embrittlement problem as electro-galvanized.