Galvanized Defects: Causes and Solutions for Bare Spots on Galvanized Pipes

In this guide, we’ll explore the five main causes of galvanized defects and provide actionable solutions to prevent them.

5 Common Causes of Galvanized Defects (Bare Spots)

1. Oil or Grease on the Base Pipe

· Problem: Oil residue increases surface tension, preventing proper acid cleaning before galvanizing. Rust or protective salt layers may form, inhibiting zinc adhesion.

· Result: In the molten zinc bath, oxidation occurs rapidly, and zinc fails to bond with the steel. After withdrawal, zinc flows unevenly, leaving thin spots with visible black residue.

· Solution: Ensure thorough degreasing and acid pickling before galvanizing.

2. Severe Corrosion on the Base Pipe

· Problem: If rust (iron oxide) remains after cleaning, the zinc cannot react properly with the steel surface.

· Result: Bare spots appear, often with pitting corrosion.

· Solution: Use abrasive blasting or intensive pickling to remove all oxidation before galvanizing.

3. Thick Oil Stains & Contaminants

· Problem: Heavy grease or stubborn stains lead to inevitable galvanized defects, making full cleaning difficult.

· Result: Small, continuous bare spots with smoother edges.

· Solution: Apply high-pressure washing and chemical cleaning to eliminate all residues.

4. Scratches & Surface Roughness

· Problem: Deep scratches trap contaminants that acid/alkali cleaning can’t remove.

· Result: Long, thin bare streaks (a rare but critical defect).

· Solution: Inspect pipes for surface damage before galvanizing; grind or polish deep scratches.

5. Low Zinc Bath Temperature

· Problem: If the molten zinc temperature is too low, the coating may not form properly.

· Result: Weak adhesion, leading to peeling or flaking.

· Solution: Maintain optimal bath temperature (typically 445–465°C / 833–869°F).

How to Fix & Prevent Galvanized Defects

To minimize galvanized defects, follow these best practices:
✅ Pre-Treatment: Remove oil, rust, and scale via degreasing, pickling, and fluxing.
✅ Quality Control: Inspect pipes before galvanizing for scratches or contamination.
✅ Process Optimization: Ensure proper zinc bath temperature and immersion time.
✅ Post-Galvanizing Check: Use a zinc thickness gauge or copper sulfate test to detect bare spots.

Conclusion

Galvanized defects—especially bare spots—can weaken corrosion resistance and reduce pipe lifespan. By identifying the root causes (oil, rust, scratches, or improper temperatures) and implementing strict quality control, manufacturers can ensure high-performance galvanized pipes.

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