Selecting the right galvanized steel for a construction project can be daunting, especially when confronted with technical terms like "hot-dipped galvanized," "electrolytic galvanized," "G90," or "A60." Choosing incorrectly can compromise project quality or even create safety hazards. This article examines the principles, characteristics, applications, and selection criteria for these two primary galvanizing methods.

Galvanizing is a widely used anti-corrosion treatment for steel surfaces. By forming a zinc coating, it isolates the steel from environmental exposure, delaying corrosion and extending service life. Although the term "galvanizing" originates from Luigi Galvani's research on bioelectricity, it remains the standard designation for this process.

Hot-dipped galvanizing (HDG) involves immersing steel in molten zinc to form a zinc-iron alloy layer and pure zinc coating. This method offers superior corrosion resistance, strong adhesion, and a thick protective layer, making it ideal for demanding applications.

The HDG process typically includes:

1. Base Material Preparation: Cold-rolled steel (or occasionally hot-rolled pickled steel) is used as the substrate, fed into the production line as coiled strips.
2. Continuous Annealing: Some production lines include annealing furnaces to enhance mechanical properties and coating quality. Alternatively, pre-annealed substrates may be used.
3. Zinc Immersion: The steel strip passes through a molten zinc bath, forming the protective coating.
4. Cooling and Post-Treatment: The coated strip undergoes cooling, passivation, or oiling to improve corrosion resistance and surface quality.

HDG coatings are categorized into two main types:

• Pure Zinc Coating (Free Zinc Coating): Composed primarily of zinc, this coating features a bright spangled surface and excellent corrosion resistance. Designated with a "G" prefix (e.g., G90, G60), the number indicates zinc weight in ounces per square foot—higher values denote greater protection (e.g., G90 outperforms G30).
• Zinc-Iron Alloy Coating: Contains iron, presenting a matte gray appearance without spangling. While easier to spot-weld, its corrosion resistance is slightly inferior. Denoted by an "A" prefix (e.g., A60, A40).

• Chemical Passivation: Forms an oxidation-inhibiting film on the zinc layer.
• Oiling: Applies a thin oil layer to prevent rust during storage/transport.
• Painting: Adds aesthetic and protective layers, commonly used for garage and entry doors.

• Construction: Structural frames, roofing, walls, fencing.
• Transportation: Highway barriers, bridges, vehicle components.
• Utilities: Transmission towers, substation equipment.
• Agriculture: Greenhouse frames, irrigation systems.
• Appliances: Housings, brackets.

HDG production follows ASTM A653 ("General Requirements for Steel Sheet Zinc Coated by the Hot-Dip Process").

Electrolytic galvanizing (EG), or cold galvanizing, deposits a thin zinc layer via electrolysis. Compared to HDG, EG offers lower cost, smoother surfaces, and aesthetic appeal but reduced corrosion resistance.

1. Base Material Preparation: Cold-rolled steel serves as the substrate.
2. Pre-Treatment: Cleaning and pickling remove contaminants to improve coating adhesion.
3. Electroplating: Steel is immersed in a zinc-salt electrolyte, depositing zinc ions onto its surface.
4. Post-Treatment: Coated steel undergoes washing, passivation, or phosphating to enhance durability and paintability.

EG coatings are classified as A, B, or C based on thickness—higher grades provide better protection. EG coatings are generally thinner than HDG.

• Chemical Passivation: Improves oxidation resistance.
• Phosphating: Creates a phosphate layer to boost paint adhesion and corrosion resistance.

• Appliances: Refrigerator, washer, and air conditioner housings.
• Automotive: Interior parts, body panels.
• Electronics: TV, computer, and audio equipment casings.
• Furniture: Office and household steel fixtures.

EG production adheres to ASTM A591 ("Electrolytic Zinc Steel Sheets").

• Corrosive Environments: Prioritize HDG for outdoor, humid, or chemical-exposed settings.
• Budget Constraints: EG suffices for indoor/dry applications.
• Aesthetics: EG offers smoother finishes for visible components.
• Welding Needs: Opt for HDG zinc-iron alloy coatings.
• Painting Requirements: EG typically requires painting; HDG can also be painted for added protection.

When ordering galvanized steel, specify:

• Type: HDG or EG.
• Dimensions: Thickness, width, length.
• Coating Grade: G90, A60, C-grade, etc.
• Surface Treatment: Passivation, oiling, phosphating.
• Standard: ASTM A653 or A591.

Inspect delivered materials for:

• Surface Quality: Uniformity, absence of bubbles, peeling, or scratches.
• Coating Thickness: Compliance with specifications.
• Adhesion: Strong bond between coating and substrate.
• Mechanical Properties: Verified strength and hardness.

Understanding these distinctions ensures optimal material selection, balancing performance, cost, and project requirements.