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Which hexagonal bolt has a longer service life: galvanized or blackened?
In numerous fields such as industrial manufacturing, construction, and mechanical assembly, hexagonal bolts, as indispensable fasteners, their performance directly affects the stability and safety of the entire equipment or structure. The surface treatment method is one of the key factors determining the service life of hexagonal bolts. Among various surface treatment processes, galvanizing and blackening are the most common. So, which of these two treatment methods is better and which one can make the hexagonal bolt have a longer service life?
I. Entering the World of Galvanizing and Blackening: The Processes and Principles are quite Different
To evaluate the impact of the two treatment methods on the service life of hexagonal bolts, it is first necessary to have a thorough understanding of their processes and principles.
Galvanizing treatment is mainly divided into electro-galvanizing and hot-dip galvanizing. Electro-galvanizing is a process that uses an electrochemical method to coat a layer of zinc on the surface of carbon steel bolts, forming a sacrificial anode for protection. It has a relatively low cost and is suitable for dry indoor environments or short-term rust prevention requirements. Hot-dip galvanizing involves immersing bolts in molten zinc to form a thicker zinc-iron alloy layer. The coating is thicker, has stronger adhesion, and has a much longer corrosion resistance and service life than electro-galvanizing. It is often used in outdoor steel structures, power transmission towers, highway guardrails and other places that are exposed to the atmospheric environment for a long time. During the hot-dip galvanizing process, the zinc coating and the steel are metallurgically bonded, becoming part of the steel surface. This tight bonding method makes the zinc coating less likely to fall off, providing long-lasting and reliable protection for bolts.
Blackening treatment, also known as oxidation blackening, is a process that uses chemical or heat treatment methods to form a black oxide film on the surface of bolts. Common blackening processes include high-temperature blackening, low-temperature blackening and chemical blackening. Among them, low-temperature blackening has been basically phased out, and chemical blackening is also rarely used due to environmental protection issues. High-temperature blackening is more widely applied. Before blackening, heat treatment must be carried out first, and the surface oil stains and oxide layers should be thoroughly cleaned. The blackening process is usually carried out in multiple steps to ensure quality. This black oxide film not only enhances the appearance consistency of the bolts but also improves their anti-rust ability to a certain extent. Meanwhile, the blackened bolts have excellent wear resistance and can also play a lubricating role during installation and use.
Ii. Lifespan Showdown: Analysis from Multiple Dimensions Such as Anti-corrosion and Wear Resistance
The service life of hexagonal bolts mainly depends on their anti-corrosion performance, wear resistance and stability in different environments. Next, we will compare galvanized and blackened hexagonal bolts from these key dimensions.
(1) Anti-corrosion performance: Galvanizing is superior
Anti-corrosion performance is the core factor affecting the service life of hexagonal bolts. In this regard, galvanizing treatment has demonstrated significant advantages.
The anti-corrosion ability of hot-dip galvanized bolts is particularly outstanding. The combined structure of the pure zinc layer and the iron-zinc alloy layer formed by hot-dip galvanizing can effectively prevent direct contact and corrosion of the steel substrate by air and moisture. In suburban environments, the standard hot-dip galvanized anti-rust thickness can last for about 50 years without the need for repair. In urban areas or coastal regions, a standard hot-dip galvanized anti-rust coating can last for about 20 years without the need for repair. Even when used in the harsh conditions of coastal areas, hot-dip galvanized screws can remain rust-free for fifteen years. This is attributed to the formation of a protective film of ZnO, Zn(OH)₂ and basic zinc carbonate on the zinc layer under atmospheric corrosion conditions. This film layer can effectively slow down the corrosion of zinc and form a new film layer after being damaged, thereby continuously protecting the base metal iron.
Although the anti-corrosion performance of electro-galvanizing is not as good as that of hot-dip galvanizing, it can still meet the general rust prevention requirements. However, the anti-corrosion performance of electro-galvanizing is the lowest among zinc plating (coating) layers. Generally, the neutral salt spray test of electro-galvanizing is within 72 hours. Even if special sealants are used to make the neutral salt spray test last for more than 200 hours, the price will increase significantly, being 5 to 8 times that of ordinary galvanizing.
In contrast, the anti-corrosion performance of blackening treatment is slightly inferior. Blackening has almost no anti-rust ability. It will rust very quickly in an oil-free state. Even in an oil-filled state, its neutral salt spray test can only last for 3 to 5 hours. The oxide film formed by blackening is relatively thin and has limited protective capacity. It can only delay the rusting of bolts to a certain extent and cannot provide long-term and effective anti-corrosion protection for bolts like a galvanized layer.
(2) Wear resistance: Blackening has a slight edge
In some scenarios where frequent disassembly is required or there is friction, the wear resistance of bolts is of vital importance. Blackening treatment has demonstrated unique advantages in this regard.
After blackening, a hard black oxide film will form on the surface of the bolt. This film has excellent wear resistance and can effectively reduce the wear of the bolt during installation, disassembly and use. Meanwhile, this black film can also play a lubricating role during installation and use, making the rotation of the bolts smoother and further reducing the degree of wear. In some situations where the wear resistance of bolts is highly demanded and frequent disassembly is not necessary, blackened bolts can function better and extend their service life.
The zinc coating on the surface of galvanized bolts is relatively soft, and their wear resistance is not as good as that of blackened bolts. During long-term friction or frequent disassembly, the zinc coating is prone to wear. Once the zinc coating is damaged, the bolt base will be directly exposed to a corrosive environment, thereby affecting its service life. However, the zinc-iron alloy layer formed by hot-dip galvanizing has a relatively high hardness and better wear resistance than electro-galvanizing. But overall, it still cannot compare with blackened bolts.
(3) Environmental adaptability: Galvanizing has a wider range of applications
The performance requirements for bolts vary greatly depending on different usage environments. From the perspective of environmental adaptability, bolts treated with galvanization have a wider range of applications.
Hot-dip galvanized bolts, with their outstanding anti-corrosion performance, are widely used in various fields such as steel structures and steel frames in the bridge construction, vehicle and other industries, highway guardrails, textile machinery, petroleum machinery, food and agriculture, as well as outdoor power equipment and other occasions with high anti-rust requirements. Whether in a damp outdoor environment, an industrial site with strong corrosiveness, or an area with a large temperature difference, hot-dip galvanized bolts can maintain excellent performance and provide a stable connection for equipment and structures.
Although electro-galvanized bolts have inferior anti-corrosion performance compared to hot-dip galvanized ones, they can still be used in dry indoor environments or for short-term rust prevention needs, such as in some furniture manufacturing and interior decoration fields.
Blackened bolts, due to their limited anti-corrosion performance, are more suitable for use in situations where appearance requirements are not high but cost control is important, and the environment is relatively dry with less corrosion. For instance, the internal connections of some mechanical equipment, ordinary building structures, etc. In damp and highly corrosive environments, blackened bolts are prone to rust, which can affect their service life and the stability of the connection.
Iii. Comprehensive Consideration: Choosing what suits you best is the best
From the above comparison, it can be seen that both galvanizing and blackening treatments have their own advantages and disadvantages. It cannot be simply stated which treatment method has a longer service life for hexagonal bolts. Instead, a comprehensive consideration should be made based on specific usage scenarios, requirements, and cost factors.
If the bolts need to be used for a long time in harsh environments, such as outdoor steel structures and facilities in coastal areas, then galvanizing treatment, especially hot-dip galvanizing treatment, is undoubtedly a better choice. The outstanding anti-corrosion performance of hot-dip galvanized bolts can ensure that they are not prone to rust during long-term use, thereby extending their service life and guaranteeing the safety and stability of equipment and structures. Although the cost of hot-dip galvanizing is relatively high, in terms of long-term usage and maintenance costs, its cost performance is higher.
If there are special requirements for the appearance and texture of the bolts, or if the usage environment is relatively dry, the corrosion is relatively light, and there are certain requirements for the wear resistance of the bolts, then blackened bolts are more suitable. Blackening treatment not only makes the bolts present a uniform matte or bright black color, giving them a warm and soft visual impression that conforms to modern aesthetics, but also provides good wear resistance, while the cost is relatively low.
In addition, the strength grade of the bolts also needs to be taken into consideration. For high-strength bolts (such as grade 10.9 and above), hydrogen embrittlement is prone to occur during the electro-galvanizing process. Generally, galvanizing treatment is not adopted, while blackening treatment or Dacromet treatment is a better choice. Dacromet treatment adopts zinc-chromium coating technology. The coating contains no heavy metals and has excellent anti-corrosion performance, which is 7 to 10 times that of electro-galvanizing. Moreover, the treatment process does not require electroplating, completely avoiding the problem of hydrogen embrittlement. It is particularly suitable for surface anti-corrosion of high-strength hexagon socket bolts.
Iii. Comprehensive Consideration: Choosing what suits you best is the best
From the above comparison, it can be seen that both galvanizing and blackening treatments have their own advantages and disadvantages. It cannot be simply stated which treatment method has a longer service life for hexagonal bolts. Instead, a comprehensive consideration should be made based on specific usage scenarios, requirements, and cost factors.
If the bolts need to be used for a long time in harsh environments, such as outdoor steel structures and facilities in coastal areas, then galvanizing treatment, especially hot-dip galvanizing treatment, is undoubtedly a better choice. The outstanding anti-corrosion performance of hot-dip galvanized bolts can ensure that they are not prone to rust during long-term use, thereby extending their service life and guaranteeing the safety and stability of equipment and structures. Although the cost of hot-dip galvanizing is relatively high, in terms of long-term usage and maintenance costs, its cost performance is higher.
If there are special requirements for the appearance and texture of the bolts, or if the usage environment is relatively dry, the corrosion is relatively light, and there are certain requirements for the wear resistance of the bolts, then blackened bolts are more suitable. Blackening treatment not only makes the bolts present a uniform matte or bright black color, giving them a warm and soft visual impression that conforms to modern aesthetics, but also provides good wear resistance, while the cost is relatively low.
In addition, the strength grade of the bolts also needs to be taken into consideration. For high-strength bolts (such as grade 10.9 and above), hydrogen embrittlement is prone to occur during the electro-galvanizing process. Generally, galvanizing treatment is not adopted, while blackening treatment or Dacromet treatment is a better choice. Dacromet treatment adopts zinc-chromium coating technology. The coating contains no heavy metals and has excellent anti-corrosion performance, which is 7 to 10 times that of electro-galvanizing. Moreover, the treatment process does not require electroplating, completely avoiding the problem of hydrogen embrittlement. It is particularly suitable for surface anti-corrosion of high-strength hexagon socket bolts.