Galvanized coils: Versatile in metal protection and application

In the vast material system of modern industry, galvanized coils occupy a pivotal position due to their unique properties and wide applications. From the grand architecture of buildings to the delicate exteriors of household appliances, from the sturdy car bodies to the reliable protection of various containers, galvanized coils are everywhere, silently contributing to the development of various fields. It is not only an effective means of protecting metal materials but also a key factor in driving the progress of numerous industries. Now, let's delve into the world of galvanized coils.

I. Definition and Basic Concepts of Galvanized Coils

From a visual perspective, galvanized coils are thin steel plates that are immersed in a molten zinc bath to form a thin layer of zinc on their surface, presented in a rolled form for subsequent transportation and processing. This seemingly simple process endows the steel plates with new properties. The zinc layer, like a loyal guardian, adheres closely to the surface of the steel plates, providing reliable protection. It effectively isolates the steel plates from contact with corrosive media such as oxygen and moisture in the environment, significantly slowing down the rate of rusting and corrosion of the steel plates and greatly extending their service life. This product obtained through the galvanizing process has unique advantages and plays an indispensable role in industrial production and daily life.

II. Production Process of Galvanized Coils

Raw material preparation:

The production of galvanized coils begins with the careful selection of high-quality raw materials. Usually, cold-rolled or hot-rolled plates are the main sources. Before entering the galvanizing process, these plates need to undergo a series of strict pre-treatment procedures. Cleaning is the first step, using professional cleaning equipment and cleaning agents to remove dust, oil stains, and other impurities from the surface of the plates, making the surface clean. Subsequently, surface treatment is carried out, which aims to optimize the microstructure of the plate surface and enhance the adhesion of the subsequent zinc layer to the plate. Degreasing is also an indispensable process, ensuring that the surface of the plates has no residual oil, creating favorable conditions for the galvanizing treatment.

Pre-treatment:

After cleaning and degreasing, the plates are sent to specific solutions for further treatment. Commonly used are ammonium chloride or zinc chloride aqueous solutions, or a mixture of ammonium chloride and zinc chloride aqueous solutions. In these solutions, the surface of the plates undergoes a series of chemical reactions, forming a conversion film that helps enhance the adhesion of the zinc layer to the plate. This conversion film is like a strong adhesive between the zinc layer and the plate, ensuring that the zinc layer remains firmly attached to the plate surface throughout the subsequent processing and use.

Galvanizing treatment:

Galvanizing processes mainly include hot galvanizing and electro-galvanizing. Hot galvanizing involves immersing the pre-treated plates in a hot molten zinc bath, allowing zinc atoms to rapidly diffuse and react with the iron atoms on the plate surface to form a solid zinc-iron alloy layer, with a thicker layer of pure zinc on top. This coating structure makes hot galvanized coils have excellent corrosion resistance and strong adhesion. Electro-galvanizing uses electrochemical principles, immersing the plates in an electrolyte containing zinc ions, and applying an electric current to deposit zinc ions on the plate surface, forming a uniform and fine zinc layer. Electro-galvanized coils have a shiny appearance and a uniform coating.

Cooling and solidification:

After galvanizing, the plates need to be cooled promptly to ensure the stability of the zinc layer structure. Cooling methods include natural cooling and water cooling. Natural cooling is slower but allows the zinc layer to gradually form a dense structure during the slow cooling process; water cooling is faster, quickly reducing the plate temperature and improving production efficiency. After cooling, a certain period and conditions of curing are required to make the bond between the zinc layer and the plate more solid, thereby obtaining a performance-stable galvanized coil.

Surface treatment:

To further enhance the performance and appearance quality of galvanized coils, the surface treatment process is crucial. Common surface treatment methods include passivation, oil coating, paint sealing, and phosphating, etc. Passivation treatment forms an extremely thin oxide film on the surface of the zinc layer, effectively enhancing the corrosion resistance of the zinc layer and reducing the possibility of white rust formation in humid environments. Oil coating is to uniformly apply a thin layer of oil on the surface of the galvanized coil, which can isolate air and moisture during transportation and storage, further protecting the zinc layer. Paint sealing is to apply a transparent organic coating film on the surface of the galvanized coil, which not only improves the fingerprint resistance but also improves the lubricity during subsequent processing and shaping, and provides a good adhesion base for subsequent coatings. Phosphating treatment forms a phosphate conversion film on the surface of the galvanized coil, improving the adhesion and corrosion resistance of the coating, reducing the risk of corrosion during storage and transportation, and can significantly improve the forming performance of the galvanized coil when used with suitable lubricants in combination.

III. Classification of Galvanized Coils

By galvanizing method:

Hot-dip galvanized coils: Hot-dip galvanized coils have many advantages. The coating is uniform, and the zinc layer forms a zinc-iron alloy layer with the sheet, with extremely strong adhesion. This makes hot-dip galvanized coils maintain good corrosion resistance in harsh environments and have a longer service life. According to the thickness of the galvanized layer and the different elements added in the galvanizing solution, hot-dip galvanized coils can be further classified. Ordinary hot-dip galvanized coils have a galvanized layer thickness generally between 20 - 60g/m², which can meet the routine corrosion resistance requirements of general buildings, furniture, and household appliances. Tin-added hot-dip galvanized coils add a certain amount of tin in the galvanizing pot, which further improves the corrosion resistance of the galvanized layer, making it suitable for fields with higher corrosion resistance requirements, such as the manufacturing of some chemical equipment. Alloy hot-dip galvanized coils add alloy elements such as aluminum and magnesium to the galvanizing solution, which not only enhance the mechanical strength of the coating, making it less prone to damage during processing, but also further improve the corrosion resistance, suitable for applications with more stringent material performance requirements, such as the manufacturing of the shells of high-end electronic products, providing more reliable protection and better appearance effects for the products.

Electro-galvanized coils: Electro-galvanized coils have a unique appearance, presenting a silver-white and shiny color. The coating is uniform and detailed, and it also has good corrosion resistance. According to the thickness difference of the galvanized layer, electro-galvanized coils can be divided into general electro-galvanized coils and high-performance electro-galvanized coils. General electro-galvanized coils have a galvanized layer thickness generally between 8 - 20g/m², which can meet the appearance and basic corrosion resistance requirements of materials in general building, automotive, and furniture applications. High-performance electro-galvanized coils have a galvanized layer thickness of up to 30 - 70g/m², which makes them have better corrosion resistance and mechanical properties, suitable for some fields with more specific material performance requirements, such as the manufacturing of the shells of high-end electronic products, providing more reliable protection and better appearance effects for the products.

By substrate type:

Cold-rolled base plate galvanized coils: Galvanized coils produced from cold-rolled sheets have good surface quality and high dimensional accuracy. Due to the cold rolling process being carried out at room temperature, no iron oxide scale is produced, making the surface of the cold-rolled base plate relatively smooth and flat. After galvanizing treatment, this type of galvanized coil performs well in processing performance, capable of meeting the requirements of some applications with high processing accuracy and surface quality, such as the stamping forming of automotive parts and the precise manufacturing of household appliance shells.

Hot-rolled acid-washed base plate galvanized coils: Hot-rolled acid-washed coil plates are used as the base plate for galvanized coils. This type of base plate-produced galvanized coils have good strength and toughness, suitable for some fields with higher material strength requirements, such as the manufacturing of building structural components and large equipment. Although its surface quality is slightly inferior to that of cold-rolled base plates, it still has certain advantages in some applications where surface quality is not overly demanding but material strength and cost are more important.

According to the surface structure of the zinc layer:

Normal zinc flower coating: The normal zinc flower coating is formed when the zinc layer solidifies normally, with zinc grains freely growing to form a coating with distinct zinc flower morphology. This coating has a moderate size of zinc flowers and is widely used in some occasions where the appearance requirements are not particularly high but the corrosion resistance is required, such as the roofs and walls of ordinary buildings.

Small zinc flower coating: The small zinc flower coating is formed during the solidification of the zinc layer through artificial control measures to limit the growth of zinc grains, resulting in an extremely fine zinc flower coating. Its surface is relatively smooth and has a better appearance effect than the normal zinc flower coating, suitable for some fields with certain appearance requirements, such as the shells of household appliances and furniture decoration.

Zinc-free coating: The zinc-free coating is obtained by adjusting the chemical composition of the plating solution and other special processes. Its surface does not have a visible zinc flower morphology and presents a uniform and consistent appearance. This coating has unique advantages in some fields with extremely high requirements for surface flatness and appearance, such as the shells of high-end electronic products and precision instrument and meter shells, and is widely used in the manufacturing of automotive bodies and some industrial equipment that require painting.

Zinc-iron alloy coating: The zinc-iron alloy coating is obtained by heat treatment of the steel strip after passing through the zinc plating bath, forming an alloy layer of zinc and iron throughout the coating. This coating has a dark gray appearance and no metallic luster, but it is not prone to peeling during intense forming processes, and is suitable for applications where surface flatness and appearance are extremely high, such as high-end electronic product shells and high-end electronic product shells, providing excellent appearance effects for the products.

According to surface quality classification:

Normal surface (FA): The surface of the galvanized coil with normal surface quality allows for some minor defects that do not affect the performance, such as slight scratches and zinc particles. This type of surface quality galvanized coil is suitable for general industrial uses with not overly strict surface quality requirements, such as some ordinary building components and general container manufacturing.

Higher-grade surface (FB): The surface quality of the galvanized coil with a higher grade is relatively better, with fewer defects and higher surface flatness and smoothness. It can be applied in fields with certain surface quality requirements, such as household appliance manufacturing and furniture production, and can meet the basic appearance requirements of these fields.

Advanced surface (FC): The surface quality of the galvanized coil with an advanced grade is excellent, with almost no obvious defects, a smooth and flat surface, and good decorative properties. It is often used in the manufacturing of high-end products with extremely high surface quality requirements, such as high-end car bodies and high-end electronic product shells, providing excellent appearance effects for the products.

According to surface treatment classification:

Passivation: As mentioned earlier, the galvanized layer is treated by passivation, forming an oxide film on its surface, reducing the possibility of rusting (white rust) under humid storage conditions. However, the corrosion resistance of this chemical treatment is limited, and to a certain extent, it will hinder the adhesion of most coatings. Generally, except for smooth surfaces, production plants usually perform passivation treatment on other types of galvanized layers to improve the stability of the product during transportation and storage.

Oil coating: Oil coating is a simple and effective protective method, which can reduce rusting of the steel plate under humid storage conditions. After passivation treatment, the steel plate and steel strip are coated with oil, further enhancing the protective effect. The oil layer should have good adhesion and can be removed without damaging the zinc layer using degreasing agents to facilitate subsequent surface treatment. Paint sealing: The paint sealing treatment involves applying a very thin transparent organic coating film on the surface of the galvanized coil, providing an additional anti-corrosion effect for the product. It particularly excels in the fingerprint resistance aspect. During the forming process, the paint sealing layer can improve lubricity, making the processing smoother, and serving as a good adhesion base for subsequent coatings, helping to enhance the adhesion and durability of the subsequent coatings.

Phosphating: The phosphating treatment forms a phosphate conversion film on the surface of the galvanized coil, improving the adhesion and anti-corrosion performance of the coating. After phosphating treatment, galvanized coils can undergo coating operations without further processing except for normal cleaning. At the same time, phosphating treatment can reduce the risk of corrosion of the product during storage and transportation, and when used with suitable lubricants, it can significantly improve the forming performance of the product. Therefore, it is widely used in some fields that require complex forming processing and coating.

No treatment: Only when the customer explicitly requests no treatment and assumes responsibility for it, the galvanized coils supplied according to the standard can be not subjected to surface treatments such as passivation, oiling, paint sealing, or phosphating. This situation is relatively rare and usually occurs only in specific application scenarios or when customers have special requirements.

IV. Characteristics of Galvanized Coils

Excellent corrosion resistance:

One of the most remarkable characteristics of galvanized coils is their excellent corrosion resistance. The zinc layer, as a relatively active metal, reacts with oxygen in the air to form a dense oxide zinc film. This film acts as a solid barrier, preventing the direct contact of the steel plate substrate with corrosive media such as oxygen and moisture, effectively preventing rust and corrosion of the steel plate. Compared to ordinary steel plates, galvanized coils have a lifespan that can be extended by several times or even dozens of times under the same environmental conditions, giving them a significant advantage in various outdoor facilities, building structures, and humid environments.

Good processing performance:

Galvanized coils inherit the good processing performance of steel plates. They can be easily processed through cutting, bending, stamping, welding, etc. During cutting, the galvanized layer does not cause significant damage to the cutting equipment, and the cut surface is neat without zinc layer detachment problems. During bending and stamping processing, the galvanized layer can deform together with the steel plate substrate, maintaining good adhesion, and avoiding problems such as zinc layer cracking or detachment. In welding, although the galvanized layer has some impact on the welding process to a certain extent, by selecting appropriate welding processes and parameters, good welding results can still be achieved, ensuring the strength and sealing of the welding joint. This good processing performance enables galvanized coils to meet the diverse material processing and forming requirements of different industries and products.

High strength and toughness:

The base material used in galvanized coils usually has certain strength and toughness. After galvanizing treatment, the overall strength and toughness of the coil will not be significantly weakened. In some scenarios that require bearing large pressure, tension, or impact forces, galvanized coils can reliably perform their functions due to their own strength and toughness. For example, in building structures, galvanized coils used as steel beams and columns need to withstand the self-weight of the building and wind loads, earthquake loads, etc., and their high strength and toughness can ensure the safety and stability of the building under various conditions. In the automotive manufacturing field, galvanized coils are used to manufacture body structure components and chassis parts, requiring sufficient strength and toughness to ensure the safety and reliability of the vehicle during operation.

Aesthetically pleasing surface:

Galvanized coils have an aesthetically pleasing surface. Through different surface treatments and process control, they can present diverse appearance effects. Whether it is a hot-dip galvanized sheet with distinct zinc flakes, an electro-galvanized sheet with a uniformly shiny surface, or a zinc-free or small-zinc-flake coating that has undergone special treatment, all of these can provide a unique appearance texture for the products. This attractive surface not only enhances the overall image of the product but also gives it strong competitiveness in some fields that have high requirements for appearance, such as home appliance shells, furniture decoration, and architectural decoration. For example, in home appliances, the galvanized sheets used to make the shells of refrigerators and washing machines, with their smooth and shiny surface, can enhance the sense of class of the product and attract consumers' attention. In architectural decoration, galvanized sheets can be used to make metal curtain walls and decorative lines, adding modernity and beauty to the buildings.

V. Application Areas of Galvanized SheetsConstruction Industry:In the construction industry, galvanized sheets are widely used.From the foundation construction of buildings to the decoration of their exteriors, galvanized sheets are always present.In terms of building structures, galvanized sheets are often used to manufacture lightweight steel thin-walled structures, steel beams, steel columns, and other load-bearing components.Due to their excellent corrosion resistance and high strength, they can maintain the stability and safety of the structure in various complex environmental conditions, ensuring the lifespan of the building.In terms of roofing and wall materials, galvanized sheets can be made into metal roof tiles, sandwich panels, and exterior wall panels.Metal roof tiles have excellent waterproof performance and durability, effectively resisting the erosion of rain and snow.Sandwich panels have multiple functions such as insulation, sound insulation, and heat insulation, and are widely used in industrial plants, warehouses, and temporary buildings for construction.Exterior wall panels not only serve to protect the walls but also add aesthetic appeal to the building through different color and design options.Additionally, galvanized sheets can be used to manufacture the frames of building doors and windows, balcony railings, and stair handrails, which not only need to have certain strength and corrosion resistance but also have certain appearance requirements.Galvanized sheets precisely meet these needs.Automotive Industry:The automotive industry is also an important application area for galvanized sheets.In the automotive manufacturing process, galvanized sheets are mainly used to manufacture the body structure components and interior and exterior parts of vehicles.Body structure components such as body frames, door inner panels, engine compartment covers, etc., need to have high strength and good corrosion resistance to ensure the safety and durability of the vehicle during operation.Galvanized sheets can provide reliable protection for these components, preventing their strength from decreasing due to rust over time.At the same time, the excellent processing performance of galvanized sheets enables them to be precisely processed into various complex-shaped body components through stamping, welding, etc.In the automotive interior, galvanized sheets can be used to manufacture instrument panel frames, seat frames, etc., which not only need to ensure strength and safety but also have certain aesthetic requirements.Galvanized sheets' surface quality and processing performance can well meet these requirements.Moreover, as the automotive industry increasingly demands lightweight and energy-saving, galvanized sheets, while meeting strength and corrosion resistance requirements, can achieve lightweighting of components through optimized design and manufacturing processes, helping to reduce the overall weight of the vehicle and improving fuel economy.Appliance Industry:The demand for galvanized sheets in the appliance industry is also considerable.Almost all large appliances, such as refrigerators, washing machines, air conditioners, televisions, etc., use galvanized sheets for their outer shells.Appliances' outer shells need to have good corrosion resistance to prevent rusting due to contact with moisture, dust, etc., which would affect the appearance and lifespan of the product.The excellent corrosion resistance of galvanized sheets can provide long-term reliable protection for appliance shells.At the same time, appliances' outer shells have high requirements for surface quality and appearance, and galvanized sheets, after surface treatment, can present a smooth, flat, and uniform surface, meeting the high appearance requirements of appliance products.Moreover, the excellent processing performance of galvanized sheets enables them to be processed into various complex-shaped and precisely-sized appliance shell components through stamping, bending, etc., improving production efficiency and product quality.In some small appliance products, such as microwave ovens, electric ovens, vacuum cleaners, etc., galvanized sheets are also used to manufacture internal structure components and shells, providing guarantees for the quality and performance of these products.Container Manufacturing Industry:In the container manufacturing field, galvanized sheets are widely used to manufacture various types of containers and tanks.Whether it is for storing liquids in oil tanks, water tanks, or for storing gases in gas tanks, gas storage cabinets, etc., they need to have good corrosion resistance and sealing performance.Galvanized sheets can effectively prevent the containers from being corroded by the substances they store during long-term storage, ensuring the safety and reliability of the containers.