ASTM A780 – Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings

Hot-dip galvanized coatings provide excellent corrosion protection for structural steel and hardware. However, coatings may become damaged due to welding, cutting, transport, erection, or handling. ASTM A780 – Standard Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings provides approved repair methods to restore corrosion protection.

This guide explains ASTM A780 repair procedures in practical terms for coating inspectors, QA/QC engineers, fabricators, and site supervisors.

1️⃣ Scope of ASTM A780

ASTM A780 covers repair of:

• Damaged galvanized coatings after fabrication
• Weld-burned areas
• Flame-cut edges
• Handling damage
• Uncoated areas after galvanizing

Repair extent and method must be agreed upon between contracting parties.

2️⃣ Approved Repair Methods (Three Options)

ASTM A780 allows three main repair methods:

🔹 1. Zinc-Based Alloy Repair (Solder Method)

Description:

Low-melting zinc alloy rods or powders are applied to the damaged area.

Process Summary:

• Clean surface (wire brush, light grinding, or blast cleaning)
• Remove weld flux and spatter
• Preheat surface to minimum 600°F (315°C)
• Do not exceed 750°F (400°C)
• Apply zinc alloy rod or powder
• Remove flux residue
• Measure thickness

Key Advantages:

• Metallurgical bond
• Good durability
• Suitable for shop repairs

🔹 2. Zinc-Rich Paint (Most Common Field Method)

These paints contain high levels of zinc dust (typically 65–92% in dry film).

Surface Preparation:

• Clean, dry, oil-free surface
• For immersion service → SSPC-SP10 (Near White Blast)
• For normal exposure → SSPC-SP11 (Power Tool Clean)
• Minimum acceptable → SSPC-SP2 (Hand Tool Clean)

Application:

• Spray or brush apply
• Follow manufacturer instructions
• Apply in single coat with multiple passes
• Verify dry film thickness (SSPC-PA2)

Advantages:

• Easy field application
• No heating required
• Most commonly used repair method

ASTM A123/A123M Hot-Dip Galvanizing Standard: Coating Thickness, Inspection & Acceptance

🔹 3. Sprayed Zinc (Metallizing)

Process:

• Blast clean to SSPC-SP5 (White Metal)
• Spray molten zinc using wire or powder
• Apply immediately after blasting
• Achieve agreed coating thickness

Advantages:

• Strong adhesion
• High-performance repair
• Ideal for large repair areas

3️⃣ Repair Coating Requirements

Repair material must:

✔ Provide minimum 2.0 mils (50 µm) thickness
✔ Offer barrier protection
✔ Preferably remain anodic to steel
✔ Be applicable in shop or field

Thickness must be verified using:

• Magnetic gauge
• Electromagnetic gauge
• Eddy-current gauge

4️⃣ Surface Preparation is Critical

For all repair methods:

• Remove oil and grease
• Remove weld flux residue
• Remove corrosion products
• Extend surface prep into surrounding galvanized coating

Improper surface preparation reduces repair durability.

5️⃣ Important Field Notes for Inspectors

• Repair area size must comply with the governing galvanizing specification (such as ASTM A123).
• Ensure minimum 2 mil thickness.
• Measure DFT after repair.
• Avoid overheating surrounding galvanized areas during solder repair.
• Follow SSPC standards where applicable.

ASTM A780 vs ASTM A123 (Quick Comparison)

Practical Conclusion

ASTM A780 ensures that damaged galvanized coatings can be effectively restored using:

• Zinc alloy repair sticks
• Zinc-rich paint systems
• Thermal spray zinc

When performed correctly, these repairs maintain corrosion protection and structural integrity.