Galvanized Steel Sheet: Characteristics, Classification and Diverse Applications

Galvanized steel refers to cold-rolled or hot-rolled steel coated with a zinc layer through processes such as hot-dip galvanizing or electroplating. This zinc layer acts as a "cathodic protection" to isolate air and moisture from the steel, significantly improving its corrosion resistance while preserving the strength and workability of the base material. It is a leading corrosion protection material in industries such as industry, construction, and home appliances, finding widespread application in everything from automotive exteriors to building roofs, appliance panels, and municipal guardrails.

I. Core Characteristics: Why is galvanized steel the preferred corrosion protection option?

Excellent Corrosion Resistance: The zinc layer forms a dense protective layer on the steel surface. Even if scratched, the zinc preferentially corrodes (cathodic protection), preventing rust. For example, hot-dip galvanized steel used outdoors in a non-polluted environment can last 15-20 years, 5-8 times longer than ordinary steel. In humid indoor environments, electroplated galvanized steel resists rust 3-5 times longer than cold-rolled steel. Balancing strength and processability: The base material retains the mechanical properties of cold-rolled/hot-rolled steel (tensile strength of 300-600 MPa), while the zinc layer possesses a certain degree of ductility, enabling complex processing such as stamping, bending, and shearing. For example, the automotive industry uses galvanized sheet to stamp car doors and hoods. Bend 180° without cracking the zinc layer, meeting the forming requirements of automotive parts.

High cost-effectiveness: Compared to stainless steel, galvanized sheet costs 40%-60% less. Compared to the "standard steel plate + post-painting" process, galvanized sheet requires no additional coating, reduces processing costs by 25%, and offers longer-lasting corrosion protection. For small and medium-sized outdoor components (such as guardrails and brackets), galvanized sheet is the optimal choice for both cost and performance. Highly controllable surface finish: By adjusting the galvanizing process and post-treatment, various surface finishes (such as spangle-free, spangle-free, and color-passivated) can be achieved. This not only meets the practical needs of industrial substrates but also meets the aesthetic requirements of decorative applications, making it suitable for a far wider range of applications than ordinary steel sheets.

II. Scientific Classification: Selecting the Right Sheet Based on Core Dimensions

(I) Classification by Galvanizing Process: Determines the Performance and Application of the Zinc Layer

Hot-dip galvanized sheet (GI): Steel sheets are immersed in molten zinc (temperature 440-460°C), forming a zinc layer with a thickness of 50-200g/㎡. The zinc layer adheres tightly to the steel sheet, providing strong corrosion resistance. GI accounts for over 70% of total galvanized sheet production.

Advantages: Thick zinc layer with strong adhesion, suitable for long-term outdoor use.

Specifications: Thickness 0.3-6.0mm, Width 600-1800mm.

Applications: Building roof panels, municipal guardrails, container side panels, and heavy equipment bases. Electrogalvanized Sheet (EG): A thin, uniform zinc layer (10-50g/㎡) is deposited on the steel surface through electrolysis, resulting in a smooth surface suitable for indoor fine processing.
Advantages: Smooth surface and high precision, suitable for direct use as home appliance panels.
Specifications: Thickness 0.2-2.0mm, Width 600-1250mm.
Applications: Appliance housings (such as refrigerator side panels and washing machine back panels), food packaging containers, and precision electronic components.
Galvanized Alloy Sheet (GA): After hot-dip galvanizing, the zinc layer and steel sheet are heat-treated to form a zinc-iron alloy (70%-90% zinc content). This results in a high surface hardness (HV 200-250), excellent weldability, and superior wear and high-temperature resistance compared to standard galvanized sheet.
Advantages: Scratch resistance and high-temperature resistance (no discoloration after short-term use at 200°C).
Applications: Automotive body panels (such as doors and roofs), and housings for high-temperature equipment (such as oven linings). (II) Classification by Substrate Type: Matching Strength and Processing Requirements
Cold-Rolled Galvanized Sheet: Based on cold-rolled steel, it features a smooth, highly flat surface (thickness tolerance ≤ ±0.02mm), making it suitable for fine processing.
Applications: Electro-galvanized sheet is often used in this category, and is used for home appliance panels and precision stampings. Alloyed hot-dip galvanized sheet is also commonly used in cold-rolled substrates for automotive bodies.
Hot-Rolled Galvanized Sheet: Based on hot-rolled steel, it features a slightly oxidized surface and is thicker (1.5-6.0mm). It offers high strength and is 15%-20% cheaper than cold-rolled sheet.
Applications: Hot-Dip Galvanized Sheet is often used in this category, and is used for building steel structures, heavy equipment supports, and container floors.
(III) Classification by Surface Condition: Matching Appearance and Functionality
Ordinary Zinc Spangle Sheet: Hot-Dip galvanizing produces naturally formed zinc spangles (1-5mm in diameter) with a metallic luster. No additional treatment is required, making it suitable for industrial substrates (such as roofing panels and guardrails) and offering the lowest cost. Non-spangled sheet: By adjusting the galvanizing process to suppress spangle formation, the surface is smooth and pattern-free, approaching the smoothness of electro-galvanized sheet. This makes it suitable for applications requiring subsequent painting or color film application (such as appliance housings and decorative panels).
Passivated sheet: After galvanizing, chromate passivation is applied to form a transparent or colored passivation film (1-3μm thick), improving corrosion resistance by 30% and making the surface less susceptible to fingerprints.
Transparent passivation: Used for appliance panels and electronic components, preserving the metallic color of the zinc layer.
Colored passivation (blue, yellow, and black): Used for decorative pieces (such as furniture brackets and display stands), combining aesthetics with corrosion resistance.
Oil-coated sheet: After galvanizing, a thin layer of anti-rust oil is applied to the surface to further enhance short-term rust protection. This sheet is suitable for corrosion protection during long-term storage or transportation (such as galvanized coils not immediately processed). Cleaning and degreasing are required before use.

III. Diverse Applications: Covering Scenarios from Construction to Public Life

(I) Construction: Core Material for Outdoor Corrosion Protection

Roofs and Walls: 0.3-0.8mm thick hot-dip galvanized sheet (zinc layer 120-180g/㎡), processed into profiled roof or wall panels, are used in factories and logistics warehouses. They offer wind resistance of 1.2kPa (capable of withstanding a Category 10 typhoon) and require no painting, allowing for direct use after installation. For buildings in coastal areas, hot-dip galvanized sheet with a zinc layer of 200g/㎡ is used, offering resistance to salt spray corrosion and a service life of over 20 years. Municipal and outdoor components: Municipal guardrails utilize 1.0-2.0mm thick hot-dip galvanized sheet (150g/㎡ zinc coating), welded into a grid pattern and then spray-coated for both corrosion resistance and aesthetics, offering 10 years of service with no visible rust. Outdoor billboard brackets utilize 0.8-1.5mm thick hot-dip galvanized sheet, bent into a frame structure, which requires no maintenance and withstands rain and UV damage.
Steel structural auxiliary components: Purlins and supports for factory steel structures utilize 1.5-3.0mm thick hot-rolled base galvanized sheet with a zinc coating of 100-120g/㎡. These connect the main beams to the roof, preventing rust in humid environments and reducing future maintenance costs. (II) Home Appliances and Consumer Electronics: Ideal for Precision Processing

White Appliances: Refrigerator side panels and washing machine back panels use 0.3-0.5mm thick electrogalvanized sheet (zinc layer 20-30g/㎡). This surface is free of spangles and transparently passivated, with a flatness tolerance of ≤0.5mm, ensuring precise assembly with other components. Air conditioner exterior casings use 0.5-0.8mm thick hot-dip galvanized sheet (free of spangles). After spraying, it withstands outdoor rain and lasts 4-6 years longer than ordinary steel sheet casings.

Small Appliances and Electronics: Microwave oven casings and electric oven interiors use 0.2-0.4mm thick alloyed hot-dip galvanized sheet (GA). This sheet is resistant to high temperatures (200°C) and scratches, and resists rust even after long-term exposure to high temperatures and moisture. Mobile phone holders and charger housings use 0.2-0.3mm thick electrogalvanized sheet (color passivation), combining compactness and corrosion resistance, meeting the lightweight requirements of consumer electronics. (III) Automotive and Transportation: Ensuring Safety and Durability

Automotive bodies: Car doors and hoods are made of 0.6-1.0mm thick alloyed hot-dip galvanized sheet (GA, zinc layer 40-60g/㎡). Formed through stamping, the zinc-iron alloy layer enhances collision resistance and offers excellent weldability, meeting the high-strength requirements of automobile bodies. Truck side panels are made of 1.5-2.5mm thick hot-dip galvanized sheet (zinc layer 80-100g/㎡), which withstands cargo friction and outdoor rain, and has a service life 5-7 years longer than ordinary steel sheet.

Transportation equipment: Bus shelter roofs are made of 1.0-1.2mm thick hot-dip galvanized sheet (no spangles), with a plastic-sprayed surface for protection from rain, sun, and rust. Highway noise barriers are made of 1.2-1.5mm thick hot-dip galvanized sheet, bent into a honeycomb structure, for both sound insulation and outdoor corrosion resistance. (IV) Other Sectors: Adapting to Diverse Needs
Packaging Industry: 0.15-0.2mm thick electro-galvanized sheet (zinc layer 10-20g/㎡) is used for food packaging. Its clean, oil-free surface meets food hygiene standards and can be made into cans and biscuit boxes to prevent food spoilage. 0.3-0.5mm thick hot-dip galvanized sheet is used for industrial packaging to make metal turnover boxes. It withstands cargo impact and humid environments, and can be reused 3-4 times more often than ordinary steel boxes.

Machinery and Agricultural Machinery: 0.8-1.2mm thick hot-dip galvanized sheet (zinc layer 80-100g/㎡) is used for the casings of small mechanical equipment. It can withstand oil and humidity in the workshop without painting. 1.5-2.0mm thick hot-dip galvanized sheet with hot-rolled base is used for agricultural machinery parts (such as tractor fenders). It has high strength and resists corrosion from muddy fields, reducing the frequency of maintenance.

IV. Purchasing Tips: Avoid Mistakes and Choose the Right Sheet Material

Select the process and zinc coating based on the application environment:

For long-term outdoor use (roofs, guardrails): Choose hot-dip galvanized sheet with a zinc coating ≥ 120g/m2 (≥ 180g/m2 for coastal areas);

For indoor fine processing (home appliances, electronics): Choose electrogalvanized sheet with a zinc coating of 10-50g/m2, preferably spangle-free and passivated;

For high temperature/high wear (automotive bodies, ovens): Choose alloyed hot-dip galvanized sheet (GA) with a zinc coating of 40-80g/m2.

Select the base material and surface based on the processing requirements:

For stamping (e.g., automotive parts, home appliance housings): Choose cold-rolled galvanized sheet with a smooth surface and good ductility;

For thick-walled load-bearing applications (e.g., brackets, steel structures): Choose hot-dip galvanized sheet with a hot-rolled base for high strength and low cost;

For subsequent painting/film coating: Choose spangle-free galvanized sheet with a smooth, non-streak surface to avoid affecting coating adhesion. Check key parameters to assess quality:
Zinc layer thickness: Measured using a zinc thickness gauge, ≥50g/m2 for hot-dip galvanized sheets and ≥10g/m2 for electro-galvanized sheets. Zinc layer uniformity error ≤10% (to prevent rust from occurring due to thinner areas of the zinc layer).
Adhesion: No zinc layer peeling after 180° bending (hot-dip galvanized sheets), no cracking after stamping (electro-galvanized sheets). Tape can be used to test for no zinc layer peeling.
Surface quality: No missing plating, zinc bumps, or scratches. No yellowing or base exposure for passivated sheets. Even oil film for oiled sheets (no lumps or missing coating). Choose the form and storage based on your needs:

Continuous production lines (such as home appliance factories): Choose galvanized coils (5-30 tons) to reduce restocking frequency.

Small-batch processing (such as hardware): Choose galvanized flat sheets (sizes of 1220×2440mm, etc.) to reduce cutting losses.

Storage requirements: Electro-galvanized sheet should be kept away from high temperatures and humidity (which can easily lead to white rust). Hot-dip galvanized sheet should be covered with a rainproof sheet for long-term storage. Oiled sheet should be cleaned and degreased before use.

Choose a reputable manufacturer: Prioritize products from large steel companies (such as Baosteel, Ansteel, and Shougang). Check the material report (including zinc layer thickness, substrate material, and corrosion resistance test data) to avoid purchasing "non-standard" galvanized sheet (with insufficient zinc layer or severe plating defects). These sheets are prone to rusting quickly, increasing rework costs.