Commonly used Surface Treatment of Fasteners? Detailed introduction of 6 methods!

Treatment of Fasteners

– fastenera.com

Surface treatment of fasteners refers to the process of applying a coating or finish to the surface of fasteners, such as screws, bolts, nuts, and washers. The purpose of surface treatment is to improve the performance, durability, and appearance of fasteners. There are various surface treatment methods used in the fastener industry, each offering unique benefits.

The main purpose of fastener surface treatment is aesthetics and anti-corrosion. Since the main function of fasteners is to fasten parts, surface treatment also has a great influence on its fastening performance. When choosing a surface treatment process, factors such as the torque of the fastener and the consistency of the preload should be considered.

For high-quality fasteners, not only the structural design and manufacturing process of the product must be considered, but also the manufacturability of assembly, and even the requirements for environmental protection and economy. Next, this article will briefly introduce some commonly used surface treatment methods for fasteners for readers' reference.

Surface Treatment of Fasteners: Electroplating

Electroplating refers to the coating of a metal coating on a fastener, thereby changing the surface properties of the fastener. The electroplating treatment process makes the fasteners not easy to oxidize and corrode. The metal used for the coating is generally a corrosion-resistant metal. Electroplating can enhance the corrosion resistance of fasteners, increase hardness, prevent wear, and improve electrical conductivity and heat resistance. At the same time, the electroplating process can make the surface of the fastener more smooth and beautiful. Electroplating is a process that uses electrolysis to attach a layer of metal film to the surface of metal or other material parts. Next, the article will introduce several commonly used coating treatments for fasteners.

Galvanized

Zinc, the symbol of the element Zn. Zinc is a light gray transition metal, which is relatively brittle at room temperature. A thin and dense basic zinc carbonate film will form on the surface of zinc at room temperature, which can prevent further oxidation. Electro-galvanizing is the most common coating for fasteners. Compared with other metal coatings, zinc is relatively cheap and easy to electroplate. Its appearance is more beautiful, and it can be available in white zinc (ZINC CLEAR, ZI), blue zinc (ZINC BLUE, ZU), color zinc (ZINC YELLOW, ZC), black zinc (ZINC BLACK, ZB) and other colors. However, its anti-corrosion properties are mediocre. The anti-corrosion performance of electro-galvanized is the lowest among zinc-coated coatings. Generally, the neutral salt spray test of galvanized electroplating is within 72 hours. Of course, there are also special sealing agents that make the neutral salt spray test more than 200 hours, but its price is expensive, which is 5 to 8 times that of ordinary galvanized.

Electrogalvanizing is prone to hydrogen embrittlement during processing, so bolts above grade 10.9 generally do not use galvanizing treatment. Although the hydrogen can be removed by an oven after galvanizing, because the passivation film will be destroyed when the temperature is above 60°C, the dehydrogenation must be carried out before passivation after electroplating. Such a process has poor operability and will have higher processing costs. Under normal circumstances, ordinary manufacturers will not take the initiative to dehydrogenate unless it is mandatory for a specific customer.

Electro-galvanized fastener torque - the consistency of the pre-tightening force is poor and unstable, and it is generally not used for the connection of important parts. In order to improve the consistency of torque-preload force, the method of coating lubricant after galvanizing can also be used to improve and increase the consistency of torque-preload force.

Cadmium Plating

Cadmium is a metal element with the element symbol Cd. Cadmium is silvery white with light blue luster, soft and wear-resistant, tough and malleable, flammable and irritating. Electroplated cadmium is a strong and versatile coating on metals. When it is plated on steel, cast iron, malleable iron, copper and powdered metals, electroplated cadmium acts as a "sacrificial coating" that corrodes before the substrate. To enhance the corrosion protection of cadmium plating, chromate conversion coatings can be applied to the plated metal to produce the familiar golden color. At the same time, it is also available in other colors, such as olive.

The cadmium electroplating process is a method of depositing a layer of cadmium on the surface of steel parts by means of electrolysis. Cadmium plating can be used to prevent corrosion from seawater or salt spray, thereby improving assembly tightness and friction, and increasing aesthetics.

The cadmium coating has good corrosion resistance. Especially in marine atmospheric environment, the corrosion resistance of cadmium coating will be better than other surface treatment methods. However, the waste liquid produced in the process of electroplating cadmium has a relatively high treatment cost. Its waste liquid treatment price is about 15 to 20 times that of electro-galvanizing, and the cost is relatively high. Therefore, cadmium plating is rarely used in general industries and is only used in some specific environments. For example, it is often used in fasteners used in oil drilling platforms, aviation, navigation, electronics and other fields.

Chrome Plating

Chromium, the symbol for the element Cr. Chromium is a silver-white metal with high hardness, relatively brittle, and corrosion resistance. Chromium is an inactive metal that is stable to oxygen and moisture at room temperature. The thickness of the chrome plating is generally 0.25 μm to 2 μm. Because the chrome coating is very stable in the atmosphere, it is not easy to change color and tarnish, has high hardness, good wear resistance, and good reflective performance. The chrome plating of fasteners is generally used as a decorative effect. Chromium plating is rarely used in industrial areas with high corrosion protection requirements. Chromium-plated fasteners are as expensive as stainless steel fasteners, so chrome-plated fasteners are used instead only when the strength of stainless steel fasteners does not meet the requirements.

To prevent corrosion, copper and nickel should be plated first before chrome plating. The chrome plating can withstand temperatures of 1200 degrees Fahrenheit (650 degrees C). But it also has the same problem of hydrogen embrittlement as electro-galvanizing.

Nickel Plating

Nickel, the symbol for the element is Ni. Nickel is a silver-white metal, which is easily oxidized by air in the air, and a somewhat black oxide film is formed on the surface. Nickel is hard, malleable, magnetic, corrosion-resistant, and highly polishable.

Nickel-plated fasteners are generally used in occasions with high corrosion resistance and good electrical conductivity. Electroplated nickel layer has high stability in air. Metal nickel has a strong passivation ability, and a very thin passivation film can be quickly formed on the surface of the fastener. This passivation film is resistant to corrosion by air, alkalis and certain acids. Electroplated nickel has excellent polishing properties. After polishing, the luster of electroplated nickel can be maintained for a long time. In addition, the hardness of nickel plating is higher, which can improve the wear resistance of fasteners.

Surface Treatment of Fasteners: Phosphating

Phosphorus, the symbol for the element is P. Phosphating treatment is one of the earliest treatment methods used in modern metal surface treatment. As early as 1869 in England, someone discovered that phosphating film can be used on the metal surface, which can effectively protect the metal from corrosion for a long time. Phosphating has been used in industrial products since the beginning of the 20th century.

Phosphating treatment is cheaper than galvanizing. However, the corrosion resistance of phosphating is worse than that of galvanizing. Oil should be applied after phosphating, and its corrosion resistance has a great relationship with the performance of the applied oil. For example, after phosphating and general anti-rust oil, the neutral salt spray test is only 10-20 hours. Apply high-grade anti-rust oil, it can reach 72~96 hours. But its price is 2~3 times of general phosphating oil.

There are two commonly used phosphating methods for fasteners, namely zinc phosphating and manganese phosphating. The lubricity of zinc phosphating is better than that of manganese phosphating. Manganese phosphating has better corrosion resistance and wear resistance than galvanized. Phosphating can be used at temperatures ranging from 225°F to 400°F (107°C to 204°C). Phosphate-treated fasteners are widely used in the connection of some important parts, such as engine connecting rod bolts, nuts, cylinder heads, main bearings, flywheel bolts, wheel bolts and nuts, etc.

Phosphating process is adopted for high-strength bolts, which can avoid the problem of hydrogen embrittlement. Therefore, high-strength bolts above grade 10.9 in the industrial field generally use phosphating surface treatment.

Surface Treatment of Fasteners: Oxygenated Black

The oxidation blackening treatment process of fasteners is one of the commonly used surface treatment methods. Oxidation blackening treatment is widely used in bolts, screws, studs or nuts, washers and other fasteners. Fasteners treated with oxidation and blackening have good anti-rust effect, low cost and high production efficiency. At the same time, the oxidation blackening process can reduce the quenching stress, so that the fastener is not easy to break when it is stressed.

The oxidation and blackening treatment of fasteners is to heat and oxidize the fasteners in the sodium hydroxide and sodium nitrite oxidants in the solution tank. The oxidation and blackening process mainly includes degreasing with boiling alkali solution, water cleaning, derusting with hydrochloric acid solution, water cleaning, boiling water heating, oxidation, water cleaning, saponification, water cleaning, self-drying, soaking in oil, oil control, inspection, etc. After blackened fasteners, a layer of magnetic ferric oxide (Fe3O4) film will be formed on the surface. The thickness of the film is generally 0.6μm~0.8μm, and it is black or blue-black.

Surface Treatment of Fasteners: Hot-dip Galvanizing

Hot-dip galvanizing is one of the commonly used surface treatment methods for fasteners. It is a surface treatment method in which the fastener is immersed in molten zinc to form a dense zinc layer on the surface of the fastener. Hot-dip galvanizing originated at the end of the 19th century, when it was mainly used to protect iron products from corrosion. With the development of technology, hot-dip galvanizing has gradually become an important metal surface treatment process, which is widely used in various fields.

The hot-dip galvanized treatment has good corrosion resistance. In marine environments, industrial atmospheres, soil and corrosive media, the zinc layer can effectively protect fasteners from corrosion. The zinc layer of hot-dip galvanizing has a high hardness and can resist the wear of the surface of the fastener. Therefore, the hot-dip galvanized fasteners have a good service life in the fields of mining and chemical industry. The hot-dip galvanized zinc layer can still maintain high strength and hardness at high temperature, which is beneficial to improve the service life of the product in high temperature environment. The hot-dip galvanized zinc layer can significantly increase the tensile strength of the fastener, which is conducive to improving the stability of the fastener under external force. The hot-dip galvanized galvanized layer is silvery white, has good gloss, and has high decorative value. At the same time, the hot-dip galvanizing process is relatively simple, the production cycle is short, and the cost is low.

Due to the thick coating of hot-dip galvanizing, it will be difficult to twist the thread after hot-dip galvanizing. After hot-dip galvanizing, the remaining zinc sticks to the thread and is not easy to remove, and the thickness of the zinc layer is uneven, which affects the fit of the threaded parts. At the same time, the high operating temperature of hot-dip galvanizing will reduce the mechanical strength of high-strength fasteners. The strength of some threads of 8.8 bolts after hot-dip galvanizing is lower than the standard requirements. Therefore, hot-dip galvanizing cannot be used for fasteners above grade 10.9.

Although there are many defects in the hot-dip galvanizing treatment process of fasteners, the coating of hot-dip galvanizing is thick, the bonding strength is good, and the long-term corrosion effect is good. Therefore, hot-dip galvanized fasteners are widely used in power facilities, transportation, communication equipment, petrochemical and other fields.

Surface Treatment of Fasteners: Zinc Sherardizing Treatment Process

Zinc shedding is a surface protection process that obtains a zinc-iron alloy layer on the surface of fasteners by thermal diffusion. The surface layer obtained by zinc sherardization is composed of zinc-iron alloy layer. The uniformity of the galvanized coating is good, and a uniform layer can be obtained in threads and blind holes. The coating thickness is 10~110μm, and the error can be controlled within 10%. The bonding strength and anti-corrosion performance of galvanized and substrate are the best among zinc coatings (electrogalvanized, hot-dip galvanized, Dacromet).

Depending on the equipment and process used, sherardization can be divided into solid sherardization (powder sherardization), vacuum sherardization, plating diffusion sherardization, and gas sherardization.

Powder zinc sherardizing is to bury the fastener with clean surface in a sealed container filled with powder zinc sherardizing agent, heat it to 300-400 degrees, keep it warm for a period of time to obtain a certain thickness of seepage layer, and then cool it to room temperature with the furnace. The equipment required for powder sherardizing is simple and easy to operate. The most prominent advantage of powder zinc sherardizing is uniform coating, no hydrogen embrittlement, and almost no deformation. Therefore, powder sherardizing is suitable for fastener products with complex shapes.

Zinc-treated fasteners are widely used in communication equipment, electric power fittings, railway construction, subway engineering, tunnel pipe fittings, building steel structures, highway guardrails, etc.

Surface Treatment of Fasteners: Dacromet Treatment Process

Dacromet (zinc-chromium coating) is a new type of anti-corrosion coating with zinc powder, aluminum powder, chromic acid and deionized water as the main components. The anti-corrosion performance of Dacromet is dozens of times that of traditional galvanizing. After passing the salt spray test, it was found that the corrosion resistance of ordinary Dacromet can reach 20 times that of galvanized. Long-acting dacromet will have longer corrosion resistance. Some components in the nuclear industry use the Dacromet process, and the service life can reach 30 years. The traditional electroplating process has cracks or fractures after being subjected to tension and stress. Dacromet can avoid this from happening. In high temperature environment, dacromet has good stability. Its high temperature resistance is better than traditional galvanizing process. The fastener products after using Dacromet process have very good impact resistance. Due to the good permeability of Dacromet, it can combine the surface coating with the fastener well. Therefore, the dacromet process is very suitable for fasteners that need to be stressed. Dacromet does not have the problem of hydrogen embrittlement, and the torque-preload consistency is very good. Dacromet is very suitable for high-strength fasteners with high anti-corrosion requirements.

The surface color of Dacromet process is only silver and white, which is relatively simple. If other colors are required, it must be post-processed again to obtain different colors. At the same time, the conductivity of Dacromet coating is relatively poor. In the production process, the time is long and the energy consumption is high. The above reasons also limit its scope of use.

Conclusion

Fasteners come in a variety of surface treatments. When choosing the surface treatment method of fasteners, we need to fully consider the characteristics of the fasteners themselves. At the same time, factors such as assembly process, processing cost, and use environment should also be considered. For example, for fasteners with high thread accuracy requirements or small dimensional tolerances, a surface treatment process with good performance, thin plating/coating film, and good uniformity should be selected as much as possible. For fasteners of grade 10.9 and above and high-hardness standard parts, we should try our best to choose a surface treatment method without the risk of hydrogen embrittlement during processing. For fasteners that require weldability, we should try our best to use "natural color" anti-rust treatment. If surface treatment must be used, plating/coating with better conductivity should be selected, but "closed" treatment should not be used. Only by choosing the surface treatment method that conforms to the characteristics of the fastener and the use environment, the fastener can play a better role.