Galvanized sheet: "Anti-corrosion Guardian" in industry and life

Among metal materials, galvanized sheet, with its exceptional corrosion resistance, excellent comprehensive mechanical properties, and wide adaptability, has become a key material bridging industrial production and daily life. From the skeletons of large steel structures to the housings of home appliances and automotive parts, galvanized sheet, with its "steel body" clad in a "zinc armor," safeguards the material's stability in complex environments and drives the efficient development of various industries.

I. Definition and Structure: The "Perfect Synergy" of Steel and Zinc
Galvanized sheet is essentially a composite sheet made by coating the surface of cold-rolled or hot-rolled steel with a layer of metallic zinc through a specific process. Its core value lies in the synergistic effect of "zinc" and "steel": the steel substrate provides the required strength, rigidity, and processability, while the surface zinc layer acts as a dense barrier, physically isolating and electrochemically protecting the steel substrate from air, moisture, and corrosive media, significantly extending the sheet's service life.

From a microstructural perspective, the coating of high-quality galvanized steel sheets is not a single zinc layer, but rather a multi-layered, coordinated protective system. For example, hot-dip galvanized steel sheets typically comprise the following components, from the inside out:

Steel substrate: The core load-bearing layer, typically made of low-carbon or low-alloy steel. Depending on the application, its thickness can range from 0.1mm (for ultra-thin appliances) to 20mm (for heavy-duty structural applications), and determines the fundamental mechanical properties of the galvanized sheet.
Zinc-iron alloy layer: Located between the steel substrate and the pure zinc layer, this transition layer (primarily composed of FeZn₇ and Fe₂Zn₁₀) forms during the hot-dip galvanizing process. Its thickness and uniformity directly impact the bonding strength between the zinc layer and the substrate, preventing the zinc layer from falling off during subsequent processing. Pure zinc layer: The outer protective layer, typically 5μm-100μm thick (this can be controlled by the zinc coating dosage, such as 80g/m² or 120g/m²). Zinc itself has excellent corrosion resistance and, even with minor damage, can continue to provide protection through the "sacrificial anode" effect (zinc corrodes preferentially, protecting the steel substrate).

Passivation film (optional): Some galvanized sheets undergo a chromate or chromium-free passivation treatment on the surface of the pure zinc layer, forming an extremely thin oxide film (typically less than 1μm thick). This further improves fingerprint resistance and resistance to white rust (a white corrosion product formed when the zinc layer is exposed to moisture), while also providing better adhesion for subsequent coatings (such as plastic spraying and painting).

Oil film (optional): Used as a temporary protection during storage and transportation, typically with a light mineral oil or anti-rust oil, it prevents premature corrosion of the galvanized sheet caused by moisture in storage. It can be removed by washing before use. II. Classification: Precisely Matching Requirements Based on Process and Characteristics
Galvanized steel sheets can be classified in various ways, with core dimensions including "galvanizing process," "zinc layer type," and "application." Different types of galvanized steel sheets vary significantly in performance, cost, and application scenarios, meeting diverse needs from general civilian applications to high-end industrial applications.
(I) Classification by Galvanizing Process: The "Technical Difference" Between Hot-Dip and Cold-Dip
This is the most core classification method for galvanized steel sheets, directly determining the structure, bonding strength, and corrosion resistance of the zinc layer.

Hot-Dip Galvanized Steel Sheet (HDG)
The process involves immersing a cold-rolled or hot-rolled steel substrate in molten zinc (approximately 450°C). The zinc layer forms through a chemical reaction and physical adhesion between the zinc and steel. Advantages: Thick zinc layer (typically ≥50g/m², thicker than 300g/m² upon request), strong zinc-iron alloy layer bonding, strong corrosion resistance (service life of 15-30 years in outdoor environments), and relatively manageable cost.
Disadvantages: Surface flatness is slightly inferior to cold-dip galvanized sheet, and the thicker zinc layer makes subsequent precision stamping slightly more difficult.
Typical Applications: Outdoor steel structures (such as factory frames, bridge guardrails, and photovoltaic mounting systems), automotive chassis components, and large appliance housings (such as washing machine drums).

Electro-Galvanized Steel Sheet (EG): The process involves depositing zinc ions onto the steel substrate through electrolysis (similar to electroplating). The zinc layer thickness is precisely controlled by the current intensity and electrolysis time. Advantages: Thin and uniform zinc layer (typically 5-20g/m²), high surface finish (can achieve a mirror effect), high dimensional accuracy, suitable for precision stamping (such as ultra-thin mobile phone and computer casings);
Disadvantages: Poor corrosion resistance (no zinc-iron alloy layer, thin zinc layer, prone to rust in outdoor use), higher cost than hot-dip galvanized sheet of the same thickness;
Typical Applications: Internal components of home appliances (such as refrigerator evaporators, air conditioner heat sinks), electronic device casings (such as laptop chassis), and decorative parts (such as furniture hardware). (II) Classification by Zinc Coating Type: "Performance Upgrade" from Pure Zinc to Alloys
To further optimize corrosion resistance or specific properties, some galvanized sheets utilize a zinc alloy coating rather than a pure zinc coating:

Zinc-aluminum alloy sheet (e.g., Galvalume®, a 55% zinc-aluminum alloy): With 55% aluminum added to the zinc layer, it offers corrosion resistance 2-3 times that of standard hot-dip galvanized sheet. It is also heat-resistant (withstanding temperatures exceeding 600°C), making it suitable for high-temperature environments (e.g., oven liners and automotive exhaust heat shields).

Zinc-magnesium alloy sheet (e.g., Magnelis®, a zinc-magnesium-aluminum alloy): With the addition of magnesium and aluminum, it offers corrosion resistance 5-10 times that of standard hot-dip galvanized sheet and excellent weldability. It is commonly used in high-end automotive bodies (e.g., battery housings for new energy vehicles) and steel structures in marine environments.

Alu-zinc sheet (Galvalume): With an aluminum content of approximately 55% and a zinc content of approximately 10%. 43.5%, combining the high-temperature resistance of aluminum with the sacrificial anodic protection of zinc. Its silver-gray surface is suitable for outdoor decoration (such as building curtain walls and roof color steel sheets).
(III) Classification by Application: Precisely Adapted to Industry Needs
Structural Galvanized Sheet: 3-20mm thick, with a thick zinc layer and high strength (such as Q355 material), used for factory steel structures, bridge supports, and container frames;
Home Appliance Galvanized Sheet: 0.3-1.5mm thick, with a smooth surface and fingerprint resistance. It is divided into inner panels (such as refrigerator linings, primarily electroplated galvanized) and outer panels (such as washing machine casings, primarily hot-dip galvanized or zinc-aluminum alloy);
Automotive Galvanized Sheet: It is divided into outer body panels (thin, electroplated galvanized or zinc-magnesium alloy, requiring flatness and corrosion resistance) and chassis components (thick, hot-dip galvanized, requiring strength). The proportion of zinc-magnesium alloy sheet in new energy vehicles has increased significantly;
Decorative Galvanized Sheet: Surface treated with brushed, printed, or colored coatings, used in furniture and interior decoration (such as suspended ceilings and partitions), emphasizing appearance and texture. III. Core Performance: Why is it the "First Choice for Corrosion Protection"?
The widespread use of galvanized sheet steel stems from its comprehensive advantages in corrosion resistance, mechanical properties, and processability. These properties work synergistically to meet the stringent requirements of various scenarios.
(I) Excellent Corrosion Resistance: A "Dual Protection" Mechanism
Galvanized sheet steel's corrosion resistance stems from two protective effects of zinc:

Physical Isolation: A dense zinc layer covers the steel substrate, acting like armor to isolate oxygen, moisture, and chloride ions (such as seawater and acid rain), preventing oxidative corrosion of the steel substrate.

Electrochemical Protection (Sacrificial Anode): When the zinc layer is slightly damaged by scratches or wear, the zinc electrode potential (-0.76V) is lower than that of iron (-0.44V). It preferentially corrodes as an anode, releasing electrons to protect the steel substrate from oxidation. This "sacrificing oneself to protect others" property allows galvanized sheet steel to continue to provide protection even with localized damage.
Data shows that in normal outdoor environments (such as urban air), the annual corrosion rate of hot-dip galvanized steel sheets is only 0.5-2μm, with a service life of over 20 years. In contrast, the annual corrosion rate of ungalvanized cold-rolled steel sheets can reach 10-50μm, and noticeable rust will appear in 1-2 years. (II) Excellent Mechanical Properties: Balanced Strength and Toughness
The mechanical properties of galvanized sheet are primarily determined by the steel substrate, while the zinc layer does not affect the core properties of the substrate:

Strength: Galvanized sheet on a low-carbon steel substrate can achieve a yield strength of 235MPa-550MPa (such as the Q235 and Q355 series), capable of withstanding the loads of building structures and the impact of automobile driving.
Toughness: At room temperature, the elongation of galvanized sheet can reach 15%-30%, offering excellent bending and stamping properties. It can be processed into complex shapes such as U-shaped channels and corrugated sheets, and the zinc layer is resistant to cracking.
Wear Resistance: The zinc layer itself has a low hardness (HV approximately 70), but this can be increased to HV above 150 through alloying (such as zinc-magnesium alloys), making it suitable for applications requiring wear resistance (such as automotive chassis components). (3) Excellent Processing and Adaptability: Eases Subsequent Manufacturing
Galvanized sheet's processing properties are compatible with most metalworking processes, reducing production complexity for downstream enterprises:

Stamping Performance: Electro-galvanized sheet boasts a smooth surface and a thin zinc layer, making it suitable for deep drawing (such as the complex curves of appliance housings). Hot-dip galvanized sheet can also meet moderate stamping requirements by controlling the zinc layer thickness and annealing process.

Welding Performance: Galvanized sheet can be welded using conventional methods such as spot welding and arc welding. However, proper ventilation is required. Some zinc-magnesium alloys also minimize weld spatter.

Painting Performance: Passivated galvanized sheet has a moderate surface roughness and can be directly sprayed or painted. Coating adhesion can reach level 0 (the highest level), eliminating the need for additional rust removal or priming, reducing painting costs. (IV) Economic and Environmental Advantages: Cost-Effectiveness and Sustainability
Cost Advantage: Compared to stainless steel (costing approximately 3-5 times that of galvanized sheet) or aluminum alloy (costing approximately 2-3 times that of galvanized sheet), galvanized sheet achieves comparable corrosion protection at a lower cost, offering an extremely high cost-effectiveness.
Recyclability: Both the steel substrate and the zinc layer of galvanized sheet are 100% recyclable. During the recycling process, the zinc is separated through high-temperature smelting and reused in galvanizing production, aligning with the concept of a circular economy and reducing resource waste.
IV. Production Process: A "Precision Journey" from Coil to Finished Product
Galvanized sheet production is a continuous, high-precision process. Taking mainstream hot-dip galvanized sheet as an example, the core steps include:

Substrate Pretreatment: Removing scale and oil from the steel coil surface is key to ensuring the adhesion of the zinc layer. Steps: Uncoiling → Shearing and butt welding (joining multiple coils into a continuous strip) → Alkaline degreasing (removing surface oil and dirt) → Pickling and rust removal (using hydrochloric acid or sulfuric acid to remove scale) → Water washing → Drying → Annealing (heating the coil to 700-800°C to improve the toughness of the substrate and further remove residual scale).
Hot-dip galvanizing: The pre-treated coil is immersed in molten zinc to form a zinc layer.
Steps: The strip enters the zinc bath (zinc bath temperature around 450°C, zinc purity ≥99.9%) → Zinc deposition (forming a zinc layer on the strip surface) → Air knife control (using high-speed nitrogen injection to adjust the zinc layer thickness and precisely control the zinc coating amount, such as 80g/m² or 120g/m²) → Cooling (using water mist or air cooling to solidify the zinc layer).
Post-treatment: Improves the surface properties and storage stability of the galvanized sheet. Steps: Passivation (chromate or chromium-free passivation to form a passivation film) → Oiling (spraying anti-rust oil to prevent storage rust) → Leveling (using a leveler to adjust the strip shape and improve surface flatness) → Straightening (correcting camber in the strip to ensure dimensional accuracy) → Coiling or Shearing (cutting into cut-to-length steel sheets or coils according to customer requirements) → Inspection (testing zinc coating thickness, adhesion, corrosion resistance, and other indicators) → Packaging and storage.

The electrogalvanized sheet process differs in the "galvanizing step": an electrolytic cell replaces the zinc pot, with the steel strip serving as the cathode and the zinc sheet serving as the anode. Zinc ions are deposited on the strip surface through electrolysis. Other pre-treatment and post-treatment steps are similar to those for hot-dip galvanized sheet, but the surface cleanliness requirements are higher (all oil and scale must be removed to avoid affecting the electrolysis effect). 5. Application Areas: Penetrating Every Aspect of Industry and Life
Galvanized steel sheets are used in nearly every area related to steel corrosion protection, from macro-scale infrastructure projects to micro-scale electronic components. (I) Construction and Infrastructure: Guardians of the Outdoor Environment
The construction sector is the largest application for galvanized steel sheets, with a core requirement being "outdoor weather resistance":

Steel structures: Factory frames, warehouse columns, prefabricated housing frames, etc., are often made of hot-dip galvanized steel sheets (galvanizing content 120-275g/m²), which resist rain and UV damage and reduce ongoing maintenance costs.
Municipal engineering: Bridge guardrails, streetlight poles, traffic sign poles, and underground pipe gallery supports. The long lifespan of hot-dip galvanized steel sheets (over 20 years) eliminates frequent replacement and reduces municipal maintenance burdens.
Architectural decoration: Roofing color-coated steel sheets (mostly galvanized steel sheets or zinc-aluminum alloy sheets), curtain wall keels, and suspended ceiling keels combine corrosion resistance with aesthetics. For example, large shopping mall roofs often use galvanized steel sheets, which can be customized in color and have a service life of up to 30 years. (II) Home Appliance Industry: Balancing Performance and Appearance
Home appliance requirements for galvanized sheet metal are "smooth surface, fingerprint resistance, and easy processing":

White appliances: refrigerator housings (hot-dip galvanized sheet, fingerprint resistance required), washing machine drums (hot-dip galvanized sheet, water corrosion resistance), air conditioner outdoor unit housings (hot-dip galvanized sheet, outdoor rain resistance), oven liners (zinc-aluminum alloy sheet, high temperature resistance);
Small appliances: microwave housings, rice cooker liners (electrogalvanized sheet, smooth surface), soymilk maker bodies. The high precision and aesthetics of electrogalvanized sheet metal enhance the product's quality. (III) Automotive Industry: The Dual Demand for Lightweight and Corrosion Resistance
The demand for galvanized sheet metal in automotive manufacturing has evolved from "corrosion protection" to "lightweight + high corrosion resistance":

Body parts: Exterior panels (such as doors and hoods, typically made of electrogalvanized sheet metal or zinc-magnesium alloy sheet metal, 0.6-1.0mm thick, requiring flatness and dent resistance); Chassis components (such as crossmembers and longitudinal beams, using hot-dip galvanized sheet metal, 1.5-3.0mm thick, for impact and corrosion resistance);
New energy vehicles: Battery casings (zinc-magnesium alloy sheet metal, resistant to electrolyte corrosion and high strength); Motor casings (hot-dip galvanized sheet metal, resistant to high and low temperature cycles). The high corrosion resistance of zinc-magnesium alloy sheet metal can extend battery life. (IV) Other Sectors: Niche but Critical Applications
Agriculture: Greenhouse frames (hot-dip galvanized steel, resistant to moisture and pesticide corrosion), agricultural machinery components (such as tractor chassis, hot-dip galvanized steel, resistant to mud and abrasion);
Electronics: Computer cases, mobile phone charger casings (electrogalvanized steel, smooth surface, shielding against electromagnetic interference);
Packaging: Galvanized steel drums (used for storing chemical raw materials and lubricants, hot-dip galvanized steel, resistant to chemical corrosion). VI. Development Trends: Technological Innovation Drives "Higher Performance, More Environmentally Friendly"

With the increasing demand for "green, efficient, and high-performance" in downstream industries, the galvanized sheet industry is developing in three directions:

(I) Environmentally Friendly: Chromium-Free Passivation and Low-Energy Processes

Chromium-Free Passivation Alternatives: Traditional chromate passivation, which contains hexavalent chromium (toxic and environmentally polluting), is being gradually replaced by chromium-free passivations (such as silane passivation and titanium-zirconium passivation). The corrosion resistance of chromate-free passivation films approaches that of chromate films and complies with environmental standards such as EU RoHS and China GB/T 24429.

Low-Energy Production: Hot-dip galvanizing can reduce production energy consumption by over 30% by optimizing the zinc pot heating method (such as using induction heating instead of gas heating) and recovering waste heat (using annealing furnace waste heat to preheat air). Electrogalvanizing, on the other hand, improves current efficiency by improving electrolytic cell design.

High-performance: "Iterative Upgrades" of Zinc Alloy Coatings

Popularization of Zinc-Magnesium Alloy Sheets: Zinc-magnesium alloy sheets offer 5-10 times the corrosion resistance of conventional hot-dip galvanized sheet and possess higher strength (yield strength exceeding 500 MPa). Currently in mass production in new energy vehicles and high-end home appliances, they are expected to replace some conventional hot-dip galvanized sheet in the future.

Customized Ultra-Thin/Ultra-Thick Zinc Coatings: Targeting the electronics industry, we have developed ultra-thin electro-galvanized sheet with a zinc layer thickness of only 3-5μm. Targeting marine engineering applications, we have developed ultra-thick hot-dip galvanized sheet with a zinc coating weight of 300-500g/m², offering a seawater corrosion resistance lifespan exceeding 50 years. (III) Intelligence: Digital Transformation of Production and Inspection
Intelligent Production: Utilizes IoT technology to monitor parameters such as zinc pot temperature, air knife pressure, and electrolysis current in real time. AI algorithms are used to optimize process parameters and reduce product defects (such as uneven zinc coating and missed plating).
Intelligent Inspection: Utilizes machine vision to detect surface defects (such as scratches and zinc particles) and uses X-ray thickness gauges to measure zinc coating thickness in real time, with an accuracy of ±1μm, ensuring consistent product quality.
VII. Summary: An Indispensable "Cornerstone of Corrosion Prevention"
From the simple hot-dip galvanizing process of the Industrial Revolution to today's zinc-magnesium alloys, chromium-free passivation, and intelligent production, galvanized sheet has consistently leveraged its core "steel + zinc" combination to continuously address the core requirement of corrosion prevention. It serves as the "steel armor" that protects infrastructure projects from the elements, the "invisible guardian" that enhances quality in home appliances and automobiles, and a crucial material for promoting green manufacturing and a circular economy.

With the continuous innovation of technology, galvanized steel sheets will play a role in a wider range of fields (such as deep-sea engineering and lightweight aerospace components), and become an indispensable "anti-corrosion cornerstone" in the modern industrial system with higher corrosion resistance, lower cost and more environmentally friendly process.