What is the process of hot-dip galvanizing?
The hot-dip galvanizing (HDG) process is a sophisticated metallurgical procedure designed to provide steel with a durable, corrosion-resistant coating that lasts for decades. Unlike paint, it creates a permanent bond between zinc and the base metal. Here is a detailed breakdown of the nine-step process:
I. Surface Preparation: The Foundation of Success
Before the steel can react with molten zinc, its surface must be chemically pure. Any organic or inorganic contaminants will prevent a proper metallurgical bond.
1. Degreasing: Fabricated steel is immersed in an alkaline bath to dissolve drawing oils, shop grease, fingerprints, and dust. Heavy paint or coatings must be mechanically removed beforehand, as the galvanizing process cannot burn through thick paint.
2. Rinsing: A clean water rinse follows degreasing to prevent alkaline chemicals from contaminating the subsequent acid bath. This step is critical for maintaining bath life and preventing coating defects.
3. Pickling (Scale Removal): The steel is submerged in diluted hydrochloric or sulfuric acid to dissolve mill scale and rust, exposing the pure metallic iron. Modern facilities often favor hydrochloric acid because it reacts faster and offers better control.
4. Rinsing after Pickling: A second rinse stops the acid reaction and cleanses the surface, ensuring it is neutral and ready for fluxing.
II. Surface Activation and Drying
Once the steel is clean, it must be protected from flash rusting before it enters the molten zinc.
5. Pre-Fluxing: The steel is dipped in a zinc ammonium chloride solution. This prevents fresh oxidation and promotes "wetting," allowing the molten zinc to flow and react evenly across the surface.
5.5 Drying: Fluxed parts are dried or pre-heated. This is a safety-critical step; any trapped moisture would instantly flash into steam in the kettle, causing dangerous splashes or resulting in bare spots on the finish.
III. The Metallurgical Core: The Zinc Bath
6. The Zinc Bath: The clean, activated steel is immersed into molten zinc at temperatures typically between $445\text{ ℃}$ and $455\text{ ℃}$ with a purity level of at least 98%. Within seconds, a metallurgical reaction begins where zinc and iron form a series of alloy layers permanently bonded to the steel. This creates a new composite material rather than just a surface coating.
IV. Final Treatment and Quality Control
7. Cooling and Quenching: After withdrawal, excess zinc drains off and the coating solidifies. Parts are air-cooled or water-quenched, often with a mild passivation treatment to prevent "white rust" during storage.
8. Inspection: Quality control involves using magnetic gauges to verify coating thickness and uniformity against standards like ASTM A123 or ISO 1461. Visual checks ensure there are no bare areas or ash inclusions.
9. Finishing: Minor zinc runs or spikes are smoothed out, ventilation plugs are removed, and the pieces are prepared for safe handling and assembly.
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