Finish & Coating

STARWDH delivers a variety of different Finish & Coating to meet customer requirements for fasteners in different industrial fields. The common methods of fastener surface treatment mainly include the following:

1. Hot dip galvanizing of fasteners

Fastener hot-dip galvanizing is to immerse fasteners made of carbon steel into a molten zinc plating tank with a heat of about 510℃, so that the iron-zinc alloy on the surface of the fasteners will be converted into passivated zinc, thereby Obtain the surface treatment effect.

2. Mechanical plating of fasteners

The mechanical plating of fasteners refers to the use of specific physical and chemical means to impact the surface of the fastener with the powder of the plated metal, so that the plated metal forms a coating on the surface of the fastener by cold welding to reach the surface Treatment effect. The mechanical plating of fasteners is mainly applicable to spare parts such as screws, nuts and washers.

3. Electroplating of fasteners

Fastener electroplating refers to immersing the part of the fastener that needs to be electroplated into a specific aqueous solution. The aqueous solution will contain some deposited metal compounds, so that after the current passes through the aqueous solution, the metal substances in the solution will precipitate and adhere to On the immersed part of the fastener. Electroplating of fasteners generally includes zinc, copper, nickel, chromium, copper-nickel alloys, etc.

Note for selection of surface treatment of fasteners:

1. Electro-galvanized

Electro-galvanizing is the most commonly used surface treatment method for fasteners. Not only is the cost low, the appearance of fasteners that are often galvanized is also more beautiful, and can be electroplated into black or army green. However, electro-galvanizing has one disadvantage, that is, its anti-corrosion performance is general, and its anti-corrosion performance is the lowest among the electroplating (coating) layers. Generally, fasteners after electro-galvanizing can pass the neutral salt spray test within 72 hours , There are also special sealants used to make the salt spray test reach more than 200 hours after electrogalvanizing, but its price is expensive, 5-8 times that of general galvanizing.
Fasteners are prone to hydrogen embrittlement during the surface treatment process of electro-galvanizing. We do not recommend the use of galvanized surface method for bolts with general strength above 10.9. Although the electro-galvanizing can be used to remove hydrogen in an oven, but the passivation film is It will be damaged when the temperature is above 60℃, so dehydrogenation must be carried out after electroplating before passivation, but this operation will increase the processing cost, and the operability is poor, so generally if the customer does not require dehydrogenation treatment, the manufacturer will not take the initiative Dehydrogenation, this also requires special attention.
In addition to the above two shortcomings of electro-galvanized fasteners, its torque-pre-tightening force consistency is poor and unstable. Generally, electro-galvanized products will not be used in the connection of important parts of the product, but if electro-galvanized Screws are used in important product connections, so they can also be coated with lubricating substances after electroplating to improve and increase the torque-preload consistency of fasteners.

2. Phosphating of fastener surface

Phosphating on the surface of fasteners is cheaper than galvanizing, but the corrosion resistance is worse than galvanizing. Fasteners should be coated with oil after phosphating, because the corrosion resistance and the performance of the oil applied have a lot relationship. For example, if you apply general-quality anti-rust oil after phosphating, the neutral salt spray test can only take 10-20 hours. If you apply some high-quality anti-rust oil, the fastener can reach 72-96 hours. But the price is 2-3 times that of phosphating general quality anti-rust oil.
There are two commonly used fasteners phosphating, zinc phosphating and manganese yarn phosphating, zinc phosphating has better lubricity than manganese phosphating, and manganese phosphating has better corrosion resistance and wear resistance than plating. Zinc is better, and its resistance to high temperatures can reach 225°F to 400°F (107-204°C)
Most of the fasteners used in industry are phosphated and oiled because of its consistent performance of torque and preload. In the assembly of components, it is ensured that the tightening requirements expected during the design can be met, so it is used more in industrial construction. Especially the connection of some important parts: some high-strength bolts are phosphated, which can also avoid hydrogen embrittlement. Therefore, in the industrial field, bolts with strength above 10.9 are generally treated with phosphating surface treatment.

3. Oxidation (blackening)

Fastener blackening + oiling is a very common coating for industrial fasteners. Because it is cheap, it is cheaper than all surface treatments. And it looks good before the oil runs out, but the blackened surface treatment basically has no anti-rust ability, so it will quickly rust without oil. Even in the state of oil, the salt spray test can only Up to 3-5 hours.
After the fastener is blackened, its torque and pre-tightening force consistency is also very poor. To improve its torque and pre-tightening force consistency, it is only possible to apply grease on the internal thread during assembly and then screw it together.

4. Cadmium electroplating

The corrosion resistance of fasteners electroplated cadmium is very good, especially in the marine atmosphere. However, the cost of waste liquid treatment during the processing of cadmium electroplating is relatively high and the cost is high, and its price is about 15*20 times that of electrogalvanized zinc. So it is not used in general industries, but only used in some specific environments.

5. Chrome plating

The chromium plating of fasteners is stable in the atmosphere, not easy to change color and lose luster, and has high hardness and good wear resistance. Chrome plating on fasteners is generally used as a decorative effect, and is rarely used in industrial fields with high corrosion resistance. Because high-quality chrome-plated fasteners are as expensive as stainless steel, chrome-plated fasteners are only used when the strength of stainless steel is insufficient. In order to improve the corrosion resistance of electroplated chromium, copper and nickel should be plated before chromium plating. The chromium plating layer can withstand a high temperature of 1200 degrees Fahrenheit (650°C), but it also has the same hydrogen embrittlement problem as electroplating zinc.

6. Silver and nickel plating on the surface of fasteners

Fasteners using silver plating can not only prevent corrosion, but also serve as a solid lubricant for fasteners. For cost reasons, silver-plated nuts are generally used, but bolts are not used, and sometimes small bolts are also silver-plated. Silver loses its luster in the air, but can work at 1600 degrees Fahrenheit. Therefore, people take advantage of its high temperature resistance and lubrication characteristics for fasteners that work at high temperatures to prevent bolts and nuts from oxidizing and seizing. Fasteners are plated with nickel, which are mainly used in places where anti-corrosion and good conductivity are required. Such as the lead-out terminal of the vehicle battery.

7. Fastener galvanizing (hot-dip galvanizing and zinc infiltration)

The surface treatment method of hot-dip galvanizing of screw fasteners is: the zinc is heated to the thermal diffusion coating under the liquid. The thickness of the coating is 15~10μm, and it is not easy to control, but it is corrosive and is mostly used in engineering. During the hot-dip galvanizing process, pollution is serious, including zinc waste and zinc vapor. Due to the thick galvanized coating of fasteners, the problem that the internal and external threads are difficult to screw together is often caused in fasteners. There are two ways to solve this problem. One is to tap the internal thread after plating, although the thread screwing problem is solved. But it also reduces the corrosion resistance. One is to make the thread larger than the standard pattern by about 0.16~0.75mm (M5~M30) when tapping the nut, and then hot-dip galvanize. Although it can also solve the screwing problem, it affects the strength of the screw. Due to the temperature of hot-dip galvanizing, it cannot be used for fasteners with strength above 10.9.

8. Surface zinc infiltration of fasteners

Generally in the range of 10-190μm, the thickness of the sherardizing layer can be controlled according to user requirements, and the error is ≤±10%. For workpieces with threads and matching requirements, the interchangeability can be guaranteed, while the thickness of the hot-dip galvanizing process is difficult to control, and it is difficult to ensure interchangeability, and the workpiece needs to reserve a large matching gap, which will damage the bonding strength of the matching parts. . Vacuum powder sherardizing overcomes the defects such as zinc nodules and burrs of hot-dip galvanizing, and makes the appearance of the sherardized parts smooth and smooth. Even without any treatment after sherardizing, it is also very beautiful. The thickness of the sherardizing layer is uniform, and the thickness of the vacuum powder sherardizing even in the thread, blind hole, and corner parts is consistent with the thickness of other parts. During use, local premature corrosion will not occur, and the tip discharge hazard of high-voltage power transmission can be prevented.

This is a problem that cannot be solved by hot-dip galvanizing and other processes, especially the threads and blind holes are more prominent. The sherardizing layer is a zinc-iron alloy layer with a very dense structure and a firm combination with the steel matrix. The electrode potential is lower than that of iron and higher than that of zinc. The surface hardness is high and the wear resistance is good. Its hardness is more than twice higher than that of the steel body and more than four times higher than the hot-dip galvanizing layer. During transportation, assembly and disassembly, the galvanizing layer is not easy to wear, scratch or fall off. The sherardizing layer has good coating performance. The sherardizing layer can be directly painted, plasticized or coated with polymer materials without any treatment. The coating binding force is 3-4 levels higher than that of hot-dip galvanizing, and the binding force reaches the national standard. The sherardizing process does not change the mechanical properties of the steel matrix material. The formation temperature of the sherardizing layer is lower than the phase transition temperature of steel, does not damage the mechanical properties of the steel matrix, has no hydrogen embrittlement, and can handle calcined parts, assemblies, special-shaped parts, high-strength steel parts, cast iron and steel castings (not Fracture), spring steel (without loss of elasticity) and section steel (without deformation).

9. Surface treatment Dacromet

The surface treatment of fasteners made of Dacromet has no hydrogen embrittlement problem, and the torque-pre-tightening force is consistent. But its pollution is serious. If chromium and environmental issues are not considered, it is actually most suitable for high-strength fasteners with high anti-corrosion requirements.