Galvanized sheet, the full name of hot-dip galvanized steel sheet (including cold-dip galvanized sheet), is a composite sheet that achieves anti-corrosion reinforcement by forming a zinc layer on the surface of cold-rolled or hot-rolled steel sheets. It combines the high strength and easy processability of steel plates with the excellent anti-corrosion properties of the zinc layer, and has outstanding cost performance. It has become one of the most widely used anti-corrosion plates in construction, home appliances, automobiles, packaging and other fields. From building roof enclosures to home appliance casings, from automobile parts to container bodies, the coating process and substrate material of galvanized sheets directly determine their service life and application suitability. This article will systematically analyze the coating technology, classification characteristics, selection points and application scenarios of galvanized sheets, providing professional reference for actual selection and quality control.
1. Core coating technology and anti-corrosion principles of galvanized sheets

The core anti-corrosion performance of galvanized sheets relies on the synergy between the zinc layer and the steel sheet matrix. Different coating processes determine the thickness, adhesion and uniformity of the zinc layer, thereby adapting to differentiated use environments and forming a diversified product system.

(1) Core anti-corrosion mechanism

The anti-corrosion effect of galvanized sheets comes from a dual protection mechanism to ensure their stable service in complex environments: First, physical isolation protection. The dense zinc layer completely covers the surface of the steel sheet, isolating air, moisture, corrosive media from contact with the substrate, and preventing the steel sheet from the source. Oxidation corrosion; the second is sacrificial anode protection. The electrode potential of zinc is lower than iron. When the zinc layer is scratched or damaged, zinc will preferentially undergo an oxidation reaction, providing cathodic protection for the steel plate matrix, delaying matrix corrosion, and maintaining anti-corrosion capabilities even if it is partially damaged. This dual protection feature makes the service life of galvanized sheets 4-6 times longer than that of ordinary steel sheets, and its performance is particularly outstanding in outdoor environments.

(2) Mainstream coating processes and characteristics

Industrial mass-produced galvanized sheets mainly use two processes: hot-dip galvanizing and cold-dip galvanizing. There are also high-end processes such as alloyed galvanizing to meet special needs. Different processes have significant differences in performance, cost, and applicable scenarios:

1. Hot-dip galvanizing process (GI): The pickled, degreased and dried steel plate is immersed in molten zinc liquid at about 450°C, so that a layer of zinc alloy layer is evenly attached to the surface of the steel plate, and a dense coating is formed after cooling. The coating thickness of this process is uniform (usually 60-180μm), with strong adhesion and excellent anti-corrosion properties. It can withstand outdoor moisture, soil corrosion, slight industrial pollution and other environments, and its service life can reach 15-25 years. Hot-dip galvanized sheet has mature production technology and high cost performance. It is currently the most widely used type and is suitable for building enclosures, outdoor steel structures, automobile chassis and other scenarios. The disadvantage is that the surface is relatively rough and the accuracy is slightly lower than that of cold-dip galvanized sheet.

2. Electro-galvanizing process (EG): Through electrolysis, zinc ions are deposited on the surface of the pre-treated steel plate to form a galvanized layer, also known as cold galvanizing. The coating thickness is thin (usually 5-30μm), the surface is smooth and smooth, the dimensional accuracy is high, the production cycle is short, and the cost is low. However, the adhesion and anti-corrosion properties are far inferior to hot-dip galvanized sheets. It is only suitable for indoor dry environments, temporary protection or decorative scenes, such as home appliance interior parts, indoor decorative panels, electrical control cabinet shells, etc. Coating shedding and matrix corrosion are prone to occur outdoors or in humid environments.

3. Alloying hot-dip galvanizing process (GA): Based on hot-dip galvanizing, a high-temperature annealing process is added to diffuse the zinc layer and the steel plate matrix to form a zinc-iron alloy layer (the main component is FeZn₁₃). The coating of this process has stronger adhesion and excellent wear resistance. Its welding performance and coating performance are better than ordinary hot-dip galvanized sheets, and its surface smoothness is higher. It is suitable for scenes such as automobile bodies and high-end home appliance casings that require high processing performance and appearance. The disadvantage is that the cost is higher than ordinary hot-dip galvanized sheets, and the anti-corrosion performance is slightly affected by alloying.

2. Classification system and core characteristics of galvanized sheets

Galvanized sheets can be classified according to coating process, substrate type, coating thickness and surface condition. There are obvious differences in the mechanical properties, processing adaptability and anti-corrosion capabilities of different types of products to adapt to the needs of different scenarios.

(1) Classification by substrate type

As the supporting foundation of the galvanized sheet, the substrate directly affects the mechanical properties and processing adaptability of the sheet. It is mainly divided into two categories: cold-rolled substrate and hot-rolled substrate:

1. Cold-rolled base plate galvanized plate: It is based on cold-rolled steel plate. It has a flat and smooth surface, high dimensional accuracy, uniform thickness, and excellent processing performance. It can realize complex processing such as bending, stamping, and stretching. It is suitable for scenes with high requirements on appearance and precision, such as home appliance casings, automotive interior parts, high-end decorative panels, etc. It can be adapted to various indoor and outdoor environments with different coating processes.

2. Hot-rolled base plate galvanized plate: It is based on hot-rolled steel plate. The surface is relatively rough and the thickness tolerance is large. However, it has high strength, low cost and excellent impact resistance. It is suitable for scenes that do not require high accuracy and focus on structural strength, such as building steel structures, outdoor fences, container floors, etc. The hot-dip galvanizing process is often used to improve anti-corrosion performance.

(2) Classification according to coating thickness and surface condition

1. Classification according to coating thickness: it can be divided into thin coating (≤50μm), medium coating (50-120μm), and thick coating (≥120μm). Thin coating is suitable for indoor dry environments, such as home appliance interiors; medium coating is suitable for ordinary outdoor environments, such as building walls; thick coating is suitable for strong corrosive environments such as coastal areas and industrial areas, such as offshore platform ancillary structures and chemical plant enclosures.

2. Classification by surface condition: The surface of hot-dip galvanized sheets can be divided into ordinary zinc flowers, small zinc flowers, and no zinc flowers. The surface gloss of ordinary zinc flowers is uneven, while the surfaces of small zinc flowers and no zinc flowers are smoother and suitable for high-end decoration scenes; the surface of electroplated galvanized sheets can be divided into bright type and matte type. The bright type is used for decorative parts and the matte type is used for concealed structural parts. Some high-end products can also be passivated and oiled to further improve the anti-corrosion performance and processing adaptability.

3. Key points for selection and quality control of galvanized sheets

The selection of galvanized sheets needs to be comprehensively judged based on the use environment, processing needs, appearance requirements and cost budget. At the same time, quality inspection is carried out to avoid hidden dangers caused by inferior products and ensure accurate matching of performance and demand.

(1) Scientific selection principles

1. Selection according to environment: For indoor dry environments (such as home appliances, interior decoration), electro-galvanized sheets or thin-coated hot-dip galvanized sheets can be used; for ordinary outdoor environments (such as inland buildings, outdoor fences), medium-coated hot-dip galvanized sheets can be used; in coastal areas, industrial areas, and high-humidity environments, thick-coated hot-dip galvanized sheets or alloyed galvanized sheets are preferred to enhance corrosion resistance.

2. Selection according to processing requirements: For complex processing parts that require bending, stamping, and stretching (such as auto parts, home appliance casings), cold-rolled substrate alloyed galvanized sheets or electro-galvanized sheets are selected to ensure that the coating does not fall off or crack during processing; for structural parts that do not require complex processing (such as steel structures, fences), hot-rolled substrate hot-dip galvanized sheets are selected to take into account strength and cost.

3. Selection based on appearance and cost: For high-end decoration scenes (such as high-end building curtain walls and home appliance exteriors), choose hot-dip galvanized sheets without zinc blooms or bright electro-galvanized sheets; for ordinary structural scenes, use ordinary zinc-splash hot-dip galvanized sheets to control costs; for temporary projects with limited budgets, thin-coated electro-galvanized sheets can be used.

(2) Key points of quality inspection

1. Appearance quality inspection: The surface coating of high-quality galvanized sheets is uniform, and there are no defects such as missing plating, bottom exposure, peeling, bulging, color difference, scratches and other defects. The hot-dip galvanized sheets have uniform zinc flakes (the surface of products without zinc flakes is smooth). The surface of the electro-galvanized panels is smooth and free of impurities; the edges of the plates have no burrs or deformations, the thickness is uniform, and the dimensional tolerances meet the standard requirements.

2. Coating performance testing: The coating thickness of the hot-dip galvanized plate must meet the selection requirements through the zinc layer thickness meter test. The zinc-iron alloy layer of the alloyed galvanized plate has a uniform thickness; the adhesion can be verified through the bending test and the cross-hatch test. The coating does not fall off or crack after bending 180° or cross-hatching; the corrosion resistance can be tested through the salt spray test. The thick-coated hot-dip galvanized plate has no obvious rust in the salt spray test for 72 hours.

3. Material and standard verification: Suppliers are required to provide material certificates and test reports to confirm that the substrate material (such as Q235, SPCC), coating process, and coating thickness comply with GB/T 2518-2019 "Continuous Hot-dip Galvanized and Zinc Alloy Coated Steel Sheets and Steel Strips", GB/T 15675-2008 "Continuous electroplated zinc, zinc-nickel alloy coated steel sheets and steel strips" and other national standards; products used in food contact and high-end home appliance scenarios need to provide additional environmental testing reports to ensure that no harmful impurities are precipitated.

4. Typical application areas and industry development trends of galvanized sheets

Galvanized sheets have long occupied a dominant position in many fields due to their high cost performance, reliable anti-corrosion and excellent processability. At the same time, with the upgrading of technological innovation and environmental protection requirements, they are gradually transforming into high-end, functional and green directions.

(1) Core application areas

1. Construction field: accounting for more than 40% of the usage of galvanized sheets, used for building roofs, wall enclosures, steel structure purlins, outdoor fences, balcony guardrails, municipal billboards, etc. Hot-dip galvanized sheets are often used for outdoor scenes, and zinc-free or alloyed galvanized sheets are used for high-end building curtain walls, taking into account both anti-corrosion and aesthetics.

2. Home appliance manufacturing field: The outer casings, inner pots, and brackets of household appliances such as refrigerators, washing machines, air conditioners, and microwave ovens require smooth surfaces and good processing performance. Cold-rolled substrates, electro-galvanized sheets or alloyed galvanized sheets are often used. Some outdoor appliances (such as air conditioner outdoor units) use hot-dip galvanized sheets to enhance anti-corrosion.

3. Automobile and transportation fields: Used in automobile bodies, chassis, door frames, truck compartments, container boxes, etc. Automobile bodies are mostly made of alloyed galvanized sheets to ensure welding performance and coating effects; chassis and compartments are made of hot-dip galvanized sheets to resist outdoor corrosion and vibration impacts.

4. Other fields: In the packaging field, it is used for galvanized steel drums and metal packaging containers; in the agricultural field, it is used for greenhouse frames and granary enclosures; in the industrial field, it is used for chemical plant partitions, equipment brackets, electrical control cabinets, etc., covering multiple scenes of production and life.

(2) Industry development trends

High-end upgrade: The demand for alloyed galvanized sheets and high-corrosion-resistant galvanized sheets (such as zinc-aluminum-magnesium alloy coatings) is growing, adapting to the high performance requirements of automobiles, high-end home appliances and other fields, and improving product service life and added value. Functional diversification: Develop functional products such as antibacterial galvanized sheets, self-cleaning galvanized sheets, and thermal insulation galvanized sheets, and expand applications in special scenarios such as medical care and high-end construction. Green and environmental protection: Promote cyanide-free galvanizing and low-energy hot-dip galvanizing processes to reduce pollutant emissions during the production process; increase the recycling rate of galvanized sheets and promote the development of a circular economy. Precision processing: Optimize the precision and coating process of cold-rolled substrates, improve the surface flatness and dimensional accuracy of galvanized sheets, and adapt to the complex processing needs of high-end manufacturing fields.

As the core representative of traditional anti-corrosion sheets, galvanized sheets play an irreplaceable role in industrial production and people's livelihood projects. In the future, with the continuous innovation of coating technology and processing technology, galvanized sheets will further break through the performance boundaries, consolidate their market position in differentiated competition, and provide more efficient and reliable anti-corrosion sheet solutions for various industries.