Compared with other metals used for electroplating, zinc is the cheapest, and the corrosion resistance of zinc is far better than that of steel in most cases. Therefore, most of the general screws or screws that are susceptible to atmospheric corrosion are galvanized, but the screws do not have the same shape or smooth surface as plates or rods. The front end of the thread mountain is second, and the thread valley is the thinnest. In order to cooperate with the mass production of screws, reduce costs and increase production, the method of galvanizing into barrels is usually carried out by electroplating. This method is to make current flow from the electrodes of the barrel rack to the wires in the barrel, and the wires are in contact with the screws to conduct electricity, and at the same time The barrel is rotated and the screw is stirred so that the screw is in even contact with the electrode. However, at this time, the current is intermittent and it is difficult to form a uniform current density, so the thickness of the galvanized is also easy to vary.
Electroplating galvanizing can use two kinds of acid electroplating solution and alkaline electroplating solution. Acid electroplating solution is mainly composed of zinc sulfate, which is cheap and efficient, electroplating speed is fast, easy to manage, and it is also less prone to hydrogen embrittlement than alkaline electroplating solution. However, the crystals are slightly coarser, the gloss is poor, and the throwing ability is poor. Alkaline electroplating solution is mainly composed of zinc oxide. Although the price is high, it has good leveling ability. As long as it is properly managed, a glossy coating can be obtained. If further chromate treatment is performed, a brighter coating can be obtained. And excellent corrosion resistance.
Hot-dip galvanizing is also used for surface corrosion protection of screws, bolts and nuts. Hot-dip galvanized screws have better corrosion resistance than electro-galvanized screws, but the cost is higher, and high-strength structural bolts are not suitable. Hot-dip galvanizing, because the surface of the coating is easy to form hard and brittle, it may break at high temperature or high load. In industrial areas containing sulfides, its corrosion resistance is poor, and in marine and tropical climates, a large number of self-chromatic corrosive products will be produced, which must be treated with chromate to prevent it.
