Overview of Galvanizing Industry Knowledge
1. What is Galvanizing?
Galvanizing refers to the process of coating a layer of metallic zinc onto the surface of steel or iron. It utilizes the physical barrier effect and sacrificial anode protection (electrochemical protection) of zinc to slow down the corrosion and rusting of the steel substrate. Because zinc is more chemically active than iron, even if the coating is scratched, the zinc will oxidize preferentially, thereby protecting the internal steel.
2. Classification of Common Galvanizing Processes
|
Process Name |
Abbreviation |
Key Characteristics |
Typical Applications |
|---|---|---|---|
|
Hot-Dip Galvanizing |
HDG |
Immersing workpieces into molten zinc (~450°C). Features a thick coating (typically 45–600μm), strong corrosion resistance, and low cost. |
Power transmission towers, highway guardrails, steel pipes, structural steel sections |
|
Electro-Galvanizing |
EG |
Zinc deposited via electrolysis. Features a thin coating (5–30μm), smooth/bright surface, and good dimensional accuracy, but weaker corrosion resistance than HDG. |
Automotive body panels, home appliance panels, precision hardware |
|
Galvannealed |
GA |
Post-heating of hot-dipped coating to form a Zinc-Iron alloy layer. Dull matte finish with excellent weldability and paint adhesion. |
Automotive outer body panels |
|
Mechanical Galvanizing / Sherardizing |
- |
Room-temperature mechanical plating or powder diffusion. Minimal deformation; suitable for small parts. |
Bolts, nuts, fasteners, standard parts |
📌 Industry Practice: In steel trading, "galvanized coils/sheets" usually refer to Continuous Hot-Dip Galvanized Steel (HDG) or Electro-Galvanized Steel (EG); "galvanized pipes/angles" typically refer to Hot-Dip Galvanizing.
3. Principles of Galvanized Coating Protection
Physical Barrier: The zinc layer isolates the steel from contact with air and moisture.
Sacrificial Anode: When exposed to corrosive environments, zinc acts as the anode and dissolves preferentially (zinc has a more negative electrochemical potential than iron), protecting the cathodic steel substrate.
Passivation Film (Common in Electro-Galvanizing): Chromate passivation treatment further improves resistance to "white rust" (currently being phased out in favor of chrome-free passivation).
4. Key Quality Indicators (Industry Focus)
Coating Weight / Thickness: e.g., Z80, Z120, Z275 (g/m² double-sided). Higher numbers indicate thicker coatings and stronger corrosion resistance.
Surface Structure: Zero spangle, minimized spangle, regular spangle, large spangle.
Passivation Method: Chromated (Cr⁶⁺) vs. Chrome-free (required by RoHS/REACH regulations).
Yield Strength & Elongation: Particularly important for high-strength galvanized sheets used in automotive and home appliances.
5. Major Application Fields
Construction / Infrastructure: Substrate for pre-painted steel, roofing, floor decks, C/Z purlins, guardrail panels.
Automotive Manufacturing: Body panels (GA/EG), chassis components (HDG).
Home Appliances: Refrigerator/washing machine casings (EG/Pre-coated steel).
Power & Telecommunications: Transmission towers, communication masts, cable trays.
Agriculture / Municipal: Greenhouse frames, street light poles, fencing.
6. Common Industry Issues
White Rust: Alkali zinc carbonate corrosion occurring on fresh galvanized parts under humid, poorly ventilated conditions. This is a storage/transport issue that can be avoided through passivation, drying, and ventilation.
Zinc Pimples / Dross: Common surface defects on hot-dip galvanized sheets that affect subsequent painting.
Environmental Pressure: Waste acid treatment from pickling, fume/dust from zinc pots, and the phase-out of hexavalent chromium passivation are key areas for plant upgrades in recent years.