Hot-dip galvanizing precautions

Hot-dip galvanizing (HDG) is a highly effective corrosion protection method for steel, but proper handling before, during, and after the process is crucial. Below are essential precautions to ensure quality, safety, and performance.

 

Author: Anna

 

Hot-dip galvanizing (HDG) is a highly effective corrosion protection method for steel, but proper handling before, during, and after the process is crucial. Below are essential precautions to ensure quality, safety, and performance.

 

 

1. Pre-Galvanizing Precautions

 

A. Surface Preparation

• Proper Cleaning is Critical:

  • Remove oil, grease, and dirt using alkaline cleaning or degreasing.

  • Eliminate mill scale, rust, and oxides via abrasive blasting or acid pickling (HCl or H₂SO₄).

  • Fluxing (zinc ammonium chloride) ensures proper zinc adhesion.

• Avoid Contamination: Any residues (paint, weld slag, grease) can cause uncoated spots or poor adhesion.

 

B. Design & Fabrication Considerations

• Avoid Trapped Air or Liquids:

  • Provide vent/drain holes in hollow structures (e.g., tubes, tanks).

  • Weld fully sealed seams to prevent zinc leakage.

• Smooth Welds & Edges:

  • Grind rough welds to prevent excess zinc buildup or dripping.

  • Avoid sharp edges, which may lead to thin coating or peeling.

• Proper Venting of Assemblies:

  • Closed structures must have vents ≥6mm diameter to allow zinc flow and air escape.

 

 

2. During Galvanizing Process

 

A. Temperature & Immersion Control

• Preheat Steel Before Dipping to avoid thermal shock (causing warping).

• Optimal Zinc Bath Temp: ~450°C (840°F). Too hot can lead to excessive zinc-iron alloy layers (brittle coating).

• Dip Time: Typically 3–10 minutes (varies by steel thickness). Over-dipping can cause thick, uneven coatings.

 

B. Handling & Fixturing

• Use Proper Hanging Rigs:

  • Avoid thin wires that may burn off, causing parts to fall into the bath.

  • Ensure parts are fully submerged without touching tank walls.

• Prevent Distortion:

  • Heavy/long parts should be dipped vertically to minimize warping.

  • Symmetrical designs reduce uneven cooling stresses.

 

 

3. Post-Galvanizing Precautions

 

A. Cooling & Handling

• Air Cooling Preferred: Quenching in water can cause thermal cracking.

• Avoid Mechanical Damage: Freshly galvanized coatings are softer until fully cooled.

 

B. Inspection & Quality Control

• Check for Defects:

  • Bare spots (from poor cleaning or fluxing) – Must be repaired.

  • Dross inclusions (zinc impurities) – Grind and touch up.

  • Excess zinc (icicles, runs) – Remove for safety and aesthetics.

• Coating Thickness Test: Use a magnetic gauge (minimum 70–100 µm for standard HDG).

 

C. Storage & Transportation

• Avoid Stacking Freshly Galvanized Parts – Use spacers to prevent coating damage.

• Store Indoors or Under Cover – Prolonged exposure to rain can cause wet storage stain (white rust).

 

 

4. Safety Precautions

 

• Worker Protection:

  • Wear heat-resistant gloves, face shields, and aprons (molten zinc splashes cause severe burns).

  • Ensure proper ventilation (zinc fumes can cause metal fume fever).

• Environmental Controls:

  • Treat pickling acid waste to prevent pollution.

  • Recycle zinc skimmings and dross for sustainability.

 

 

5. Common Mistakes to Avoid

 

❌ Skipping Surface Prep → Poor adhesion, flaking.
❌ Inadequate Venting → Explosive zinc ejection.
❌ Overheating the Bath → Brittle, spangled coating.
❌ Improper Handling Post-Dip → Coating damage before curing.

 

 

Conclusion

Hot-dip galvanizing provides long-lasting corrosion protection, but success depends on:
✅ Proper cleaning & design before dipping.
✅ Controlled bath temperature & immersion during the process.
✅ Careful cooling, inspection, and storage afterward.