1. What is galvanized steel sheet?
Galvanized steel sheet is a metal sheet coated with a layer of zinc on the surface of a carbon or low-alloy structural steel substrate through a specific process. Its core function is to utilize the sacrificial anodic protective properties of zinc to isolate the substrate from air and moisture, significantly improving the steel's corrosion resistance and extending its service life.
What are the main classifications of galvanized steel sheet?
Common classification methods include galvanizing process, zinc layer thickness/weight, substrate material, and surface condition. They are as follows:
Galvanizing process: Hot-dip galvanized sheet (GI), electrogalvanized sheet (EG), alloyed hot-dip galvanized sheet (GA), continuous galvanized sheet, etc.
Zinc layer weight: Light zinc layer (e.g., below 40g/m²), medium zinc layer (40-120g/m²), heavy zinc layer (above 120g/m²)
Substrate material: Mild steel galvanized sheet (most commonly used), low-alloy high-strength galvanized sheet (e.g., Q355 substrate), high-carbon steel galvanized sheet
Surface condition: Ordinary zinc spangled sheet, small zinc spangled sheet, no zinc spangled sheet, zinc-iron alloy sheet, color-coated galvanized sheet (color-coated sheet)
What is the core difference between hot-dip galvanized sheet (GI) and electrogalvanized sheet (EG)? The two differ significantly in process, performance, and cost. The core differences are as follows:
Different processes: GI immerses the substrate in molten zinc (approximately 450°C), forming a zinc layer through chemical reaction and physical adhesion; EG deposits a zinc layer on the substrate surface through electrolysis (room/low-temperature process).
Different zinc layer properties: GI zinc layers are thicker (typically 50-200 g/m²), have strong adhesion, and may exhibit spangles; EG zinc layers are thinner (typically 10-50 g/m²), are highly uniform, and have a smoother surface without noticeable spangles.
Different corrosion resistance: GI has stronger corrosion resistance (especially in outdoor environments), while EG has weaker corrosion resistance and is more suitable for dry indoor environments.
Different costs and applications: GI is less expensive and is used in construction and outdoor installations; EG is more expensive and is used for appliance housings and precision parts.
4. What is special about alloyed hot-dip galvanized steel (GA)?
GA undergoes an additional "alloying treatment" after hot-dip galvanizing (heating the galvanized sheet to 550-560°C to allow the zinc layer to diffuse with the iron atoms in the substrate to form a Zn-Fe alloy layer). Its core features include:
The zinc layer is an alloy structure (no pure zinc layer), free of spangles, a dark gray surface, and excellent weldability (no "zinc explosion" issues);
The coating has strong adhesion (suitable for subsequent painting and lamination), and better scratch resistance than GI;
The corrosion resistance is slightly lower than GI with the same zinc layer weight, but much higher than EG. It is primarily used in automotive bodies and high-end home appliances.
5. What is spangle? Does spangle size affect the performance of galvanized sheet?
Spangle is a hexagonal pattern formed when the zinc solution cools and crystallizes on the substrate surface during hot-dip galvanizing. It is essentially the growth pattern of zinc crystals. Spangle size is controlled by the cooling rate and zinc melt composition (such as antimony and lead additions): slow cooling results in larger spangles, fast cooling results in smaller spangles.
Performance Impact: Spangle size does not affect corrosion resistance or mechanical properties, only its appearance. Large spangles are commonly used in general building panels (for lower cost), while small or no spangles are commonly used in home appliance and automotive panels (for a neat appearance and easier processing).