Surface treatment of fasteners （1）
Surface treatment of fasteners: which one should be selected: zinc plating, phosphating, blackening, chrome plating?
The main purpose of surface treatment is aesthetics and anti-corrosion. Since the main function of the fastener is to fasten the parts, and the surface treatment also has a great influence on its fastening performance. Therefore, when choosing the surface treatment process, factors such as the torque of the fastener and the consistency of the preload should be considered.
A high-level designer must not only consider the structural design and manufacturing process, but also pay attention to the technicality of assembly, even environmental and economic requirements. The following is a brief introduction to some commonly used fastener coatings based on the above factors for the reference of fastener practitioners.
Galvanized zinc is the most commonly used coating for commercial fasteners. It is cheaper and looks better. It can be black or army green. However, its corrosion resistance is average, and its corrosion resistance is the lowest among zinc plating (coating) layers. Generally, the electroplating zinc neutral salt spray test is within 72 hours, and a special sealant is also used to make the neutral salt spray test more than 200 hours, but the price is expensive, which is 5 to 8 times that of general zinc plating.
Electroplating zinc is prone to hydrogen embrittlement, so bolts above grade 10.9 are generally not galvanized. Although the oven can be used for dehydrogenation after plating, the passivation film will be damaged when it is above 60 ℃, so dehydrogenation must After electroplating and before passivation. Such operability is poor, and the processing cost is high. In reality, general production plants will not take the initiative to dehydrogenate, unless mandated by specific customers.
Galvanized fastener torque-the consistency of the preload is poor and unstable, and it is generally not used for the connection of important parts. In order to improve the consistency of torque-pretension force, the method of coating lubricating materials after plating can also be used to improve and improve the consistency of torque-pretension force.
A basic principle is that phosphating is cheaper than galvanizing, and its corrosion resistance is worse than galvanizing. Oil should be applied after phosphating, and its corrosion resistance has a great relationship with the performance of the oil applied. For example, if you apply ordinary anti-rust oil after phosphating, the neutral salt spray test is only 10 to 20 hours. With high-grade anti-rust oil, it can reach 72 ~ 96 hours. But its price is 2 ~ 3 times that of ordinary phosphating oil.
There are two common types of fastener phosphating, zinc phosphating and manganese phosphating. The zinc-based phosphating has better lubricating properties than the manganese-based phosphating. The manganese-based phosphating has better corrosion resistance and wear resistance than zinc plating. Its use temperature can reach 225 degrees Fahrenheit to 400 degrees (107 ~ 204 ℃). Especially the connection of some important parts. For example, engine connecting rod bolts, nuts, cylinder heads, main bearings, flywheel bolts, wheel bolts and nuts, etc.
Phosphating is used for high-strength bolts, which can also avoid hydrogen embrittlement. Therefore, bolts with grade 10.9 or higher in the industrial field generally use phosphating surface treatment.
Blackening + oiling is a popular coating for industrial fasteners because it is the cheapest and looks good until the oil is depleted. Because 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.