In the fastener market, the most commonly used are Grade 4.8 ordinary hexagon head bolts. Except for their natural finish, the vast majority of these Grade 4.8 bolts undergo electrogalvanizing or hot-dip galvanizing treatment. Among bolts primarily used for electrogalvanizing, hex socket cap bolts are the main type-electrogalvanizing not only improves the product's corrosion resistance but also transforms the originally black hex socket cap bolts into a brand-new appearance, making them more aesthetically pleasing and practical. The principle of electrogalvanizing is using electrolysis to deposit zinc ions, forming a thin protective zinc layer on the bolt surface, thereby changing the bolt's color and enhancing its corrosion resistance.
Electrogalvanizing is also known as cold galvanizing. It gets this name because other common galvanizing processes (such as hot-dip galvanizing and Dacromet treatment) all require processing in high-temperature environments, while electrogalvanizing is carried out at room temperature in the electroplating bath, without relying on high temperatures to adhere the zinc layer to the bolt surface. The specific principle is as follows: the bolt acts as the cathode, and a zinc plate as the anode, both immersed in an electroplating solution containing zinc ions. When an electric current passes through, electrochemical reactions occur at the cathode and anode, and zinc ions are uniformly deposited on the bolt surface to form a bright coating-this process is called electrogalvanizing (or cold galvanizing). It should be noted that the electrogalvanized layer is relatively thin; it can only repair minor scratches or damage on the bolt surface and cannot fully cover severe wear.
Hex socket cap bolts cannot be directly placed into the electroplating bath before cold galvanizing. Their surface must be thoroughly cleaned first: for Grade 4.8 bolts, they can be directly placed into the electroplating bath after cleaning; for Grade 8.8 medium and high-strength bolts, an additional pickling process is required before galvanizing. The bolts are soaked in a pickling bath for a period of time to remove surface oxide scale and oil, followed by secondary cleaning, and finally placed into the electroplating bath for cold galvanizing. A special reminder: after pickling, Grade 8.8 bolts must undergo timely hydrogen relief treatment to avoid hydrogen embrittlement risks that could affect bolt strength.
In the industrial field, electroplating is an indispensable auxiliary processing technology. Electroplating does not only refer to cold galvanizing with a white or blue-white surface but also includes various methods such as nickel plating, gold plating, and silver plating. The processing principles and technological processes of these electroplating methods are roughly similar.
Among various surface treatment methods, one is relatively special: anodizing (the original term "anodic hydrogenation" was incorrect). Although it also falls under the category of surface treatment, anodizing is only applicable to aluminum fasteners. In daily applications, aluminum fasteners are relatively rare, so such demands are not common.
Compared with other surface treatments, electrogalvanizing has the lowest cost but offers prominent advantages: the process is simple and easy to operate, the bolt surface has high finish, it is suitable for mass production (with large output), the zinc layer is fine and uniform, it has basic corrosion resistance, and it can present different colors according to requirements. Meanwhile, the zinc layer has good adhesion and is not easy to fall off during normal use and minor impacts.
