Choice of fastener surface treatment:
Considering that almost all commercial fasteners are made of carbon steel and alloy steel, some kinds of fasteners want to prevent corrosion. Even if fasteners made of corrosion-resistant products are utilized, surface treatment is still required to prevent rust of different materials. In addition, the plating layer of the surface area treatment should be securely connected and can not fall off throughout installation and removal. For threaded fasteners, the plating layer should be thin enough so that the threads can still be screwed after plating. Typically, the temperature level limitation of the coating is lower than that of the fastener product, so the operating temperature level requirements of the fastener must also be thought about. For surface treatment, individuals generally pay attention to visual appeals and anti-corrosion, but the primary function of fasteners is to secure parts, and surface area treatment likewise has a terrific impact on the attaching efficiency of fasteners. For that reason, when picking surface treatment, The aspect of tightening up performance needs to also be considered, that is, the consistency of setup torque-pre-tightening force. The designer ought to not only think about the design however likewise focus on the manufacturability of the assembly and even the environmental management requirements. The following briefly presents some frequently used finishings for fasteners based on the above elements.
Electro-galvanized
Electro-galvanizing is the most typically used coating for commercial fasteners. It is cheaper and looks better. However, its anti-corrosion efficiency in general, and its anti-corrosion efficiency are the most affordable among zinc plating (finish) layers. Generally, the neutral salt spray test of electro galvanizing is within 72 hours, and some special sealants are used to make the neutral salt spray test more than 200 hours, however, the rate is costly, which is 5 to 8 times that of basic galvanizing. Electro-galvanizing is prone to hydrogen embrittlement, so bolts above 10.9 are normally not galvanized. Although an oven can be used to get rid of hydrogen after plating, the passivation movie will be ruined when the temperature is above 60 ℃, so hydrogen elimination is needed to be Performed after electroplating before passivation. Such operability is poor and the processing cost is high. Generally, production plants will not take the initiative to get rid of hydrogen, unless mandatory requirements of specific clients. Electro-galvanized fasteners have bad torque and pre-tightening force consistency and are unstable, and are normally not utilized for the connection of important parts. In order to enhance the torque-pre-tightening force consistency, the method of coating lubricating substances after plating can also be used to enhance and increase the torque-pre-tightening force consistency.
Phosphating
Phosphating is more affordable than galvanizing, and its rust resistance is worse than galvanizing. Oil should be used after phosphating. The level of rust resistance has a terrific relationship with the performance of the applied oil. For instance, applying general anti-rust oil after phosphating, the neutral salt spray test is just 10-20 hours. Apply high-grade anti-rust oil, it can reach 72 ~ 96 hours. But its cost is 2 to 3 times that of general phosphating oil. There are 2 frequently utilized fastener phosphating, zinc phosphating, and manganese phosphating. Zinc-based phosphating has much better lubrication performance than manganese-based phosphating, and manganese-based phosphating has much better rust resistance and wear resistance. Its operating temperature can reach 225 ° F to 400 ° F. Numerous industrial fasteners are treated with phosphating oil. Since it has an excellent torque-pre-tightening force consistency, it can guarantee to meet the tightening up requirements expected by the style throughout the assembly, so it is used more in the industry, particularly the connection of some important parts. High-strength bolts are phosphate, which can also avoid hydrogen embrittlement.
Oxidation (blackening).
Blackening + oiling is a popular coating for industrial fasteners because it is the cheapest and looks good prior to the oil goes out. Considering that it has practically no anti-rust capability due to blackening, it will rust not long after it is oil-free. Even in the existence of oil, the neutral salt spray test can only reach 3 to 5 hours. Blackened fastener torque-pre-tightening force consistency is also very bad. If you need to enhance, you can apply grease to the inner thread during assembly and then screw it on.
Electroplated cadmium (the cost has to do with 15-20 times that of electro-galvanized).
The rust resistance of cadmium coating is excellent, especially in the marine environment, the corrosion resistance is better than other surface area treatments. The expense of waste liquid treatment during the processing of cadmium electroplating is high and the cost is high, and its cost is about 15-20 times that of electro galvanizing zinc. For that reason, it is not utilized in general markets but only utilized in some specific environments.
Chrome plating.
The chromium plating layer is very stable in the environment, is hard to alter color and lose luster, has high hardness and good wear resistance. Chrome plating on fasteners is typically used for decorative functions. It is seldom utilized in industrial fields with high anti-corrosion requirements since great chrome-plated fasteners are as pricey as stainless-steel. Only when the strength of stainless-steel is not enough, chrome-plated fasteners are utilized instead.
In order to avoid deterioration, copper and nickel ought to be plated prior to chromium plating. The chromium finish can stand up to high temperatures of 1200 degrees Fahrenheit. But it likewise has the same hydrogen embrittlement problem as electro-galvanized.
Silver-plated, nickel-plated.
The silver finishing can not just prevent corrosion however likewise function as a solid lube for fasteners. Silver loses its appeal in the air, however, can work at 1600 degrees Fahrenheit. Therefore, people make the most of its high-temperature resistance and lubrication attributes for fasteners that operate at high temperatures to prevent bolts and nuts from oxidizing and seizing. Fasteners are plated with nickel, which is generally utilized in places where anti-corrosion and great conductivity are required.
Hot-dip galvanized.
Hot-dip galvanizing is a thermal diffusion finishing of zinc heated to a liquid. The density of the coating is 15 ~ 10μm, and it is difficult to control, but it is destructive and is mostly utilized in engineering. Throughout the hot-dip galvanizing process, pollution is severe, such as zinc waste and zinc vapor. Due to the thick plating, the problem that the internal and external threads are tough to screw together is caused by the fastener. There are 2 methods to solve this problem. One is to tap the internal thread after plating, although the thread screwing issue is solved. However, it also reduces corrosion resistance. One is to make the thread larger than the standard pattern by about 0.16 ~ 0.75 mm (M5 ~ M30) when tapping the nut, and after that hot-dip galvanized. This way, although the screwing problem can be fixed, the cost of strength decrease is paid. Due to the temperature level of hot-dip galvanizing, it can not be used for fasteners above 10.9.
