Repair Galvanized Steel: Standards And Methods

Galvanized steel is valued for its durability and corrosion resistance, making it essential in various industries. However, it is not entirely free from defects. From bare spots and peeling to mechanical damage, understanding these common defects is critical for ensuring long-term performance.

This article discusses how standards like ISO 1461 and ASTM A123/A123M set guidelines for defect tolerances and proper maintenance practices. It also examines common defects in galvanized steel and effective repair methods.

Common Defects in Galvanized Steel

Bare Spots

Uncoated areas on the steel surface expose it to corrosion, often caused by poor surface preparation or uneven zinc application.

Nguyên nhân: Poor surface preparation, contaminants on the steel surface, or insufficient immersion time during the galvanizing process.

Peeling or Flaking

The zinc layer detaches from the steel, leaving it vulnerable to rust, typically due to improper adhesion or excessive stress.

Nguyên nhân: Poor adhesion due to improper surface cleaning, rapid cooling, or excessive stresses during handling.

White Rust

A powdery white residue forms from moisture exposure during storage, which can weaken the zinc’s protective properties over time.

Nguyên nhân: Exposure to moisture during storage or transport, especially in poorly ventilated conditions.

Blisters

Raised bubbles on the coating surface, caused by trapped hydrogen or impurities, compromise the uniformity and durability of the coating.

Nguyên nhân: Trapped hydrogen during the pickling process or impurities in the molten zinc bath.

Excess Zinc Accumulation

Drips or lumps of zinc create an uneven surface, which may affect aesthetics or functionality, often due to excess immersion time.

Nguyên nhân: Over-immersion in the zinc bath or uneven withdrawal speeds.

Cracking

Small cracks in the coating, typically along bends or edges, expose the steel to rust and reduce its overall durability.

Nguyên nhân: High stress in the steel during galvanization or inadequate flexibility of the zinc layer.

Spangle Irregularities

Irregular patterns in the zinc’s crystalline finish can affect appearance, though functionality usually remains intact.

Nguyên nhân: Cooling rate inconsistencies or variations in the alloy composition of the molten zinc.

How To Repair Galvanized Steel？

Zinc-Rich Paint

Zinc-rich paint is cost-effective and easy to use, making it ideal for small, localized defects. It restores corrosion resistance by forming a sacrificial zinc layer. However, its longevity is slightly less than other methods in aggressive environments.

Method:

1. Clean: Remove rust, dirt, and grease from the damaged area using a wire brush or cleaner.
2. Apply Paint: Brush or spray zinc-rich paint (≥90% zinc in dry film) evenly over the defect.
3. Verify: Check the coating thickness using a gauge to meet the required standard.

Thermal Spraying (Zinc Metal Spray)

Thermal spraying creates a highly durable coating, suitable for large defects and extreme conditions. While it offers excellent performance, it requires specialized equipment and expertise.

Method:

1. Prepare Surface: Blast the area with abrasive grit to clean and roughen the surface for adhesion.
2. Spray Zinc: Use a thermal spray gun to apply molten zinc evenly.
3. Check Thickness: Ensure the coating thickness matches the original specification using a gauge.

Zinc-Alloy Soldering

Zinc-alloy soldering is precise and effective for small, intricate repairs, such as edges or corners. It provides strong adhesion but is time-intensive for larger areas.

Method:

1. Prepare Area: Clean the defect and preheat if necessary for better adhesion.
2. Apply Solder: Melt the zinc-alloy rod with a torch and evenly spread it over the damage.
3. Inspect: Let the area cool and check for a uniform coating.

Phần kết luận

These methods offer tailored solutions for different repair scenarios: zinc-rich paint for small areas, thermal spraying for extensive damage, and zinc-alloy soldering for precision work.

For detailed requirements on defect tolerances and repair standards, refer to the following section.

Standards for Repairing Galvanized Steel

Several recognized standards, including ISO 1461, ASTM A123, ASTM A153, Và ASTM A767, outline detailed guidelines for repairing defects in galvanized steel products. These standards specify acceptable repair area sizes, coating thickness after repair, and recommended methods to ensure the durability and performance of the material.

The table below provides a quick reference to the repair requirements for galvanized steel, helping you understand the key parameters for compliance and quality assurance.

ISO 1461

Defect Tolerances

• Repair Area Size: Maximum of 1.55 in² (10 cm²) per defect.
• Total Repair Area: Cannot exceed 0.5% of the total surface area.
• Bare Spots: Not allowed; full coverage is mandatory.
• Adhesion: No peeling or flaking permitted under normal handling.

Repair Methods

• Zinc-Rich Paint: Must contain at least 90% zinc in the dry film.
• Phun nhiệt: Application of molten zinc to damaged areas.
• Zinc-Alloy Soldering: For small repairs use zinc-based materials.

ASTM A123

Defect Tolerances

• Repair Area Size: Limited to areas ≤ 1 in (25.4 mm) in the longest dimension.
• Bare Spots: Not permitted; uniform coverage is required.
• Chất lượng bề mặt: Minor imperfections (e.g., zinc runs) are allowed if they do not impair functionality.

Repair Methods

• Zinc-Rich Paint: Dry film must contain at least 65% zinc.
• Phun nhiệt: Used for localized repairs to restore coating integrity.

ASTM A153

Defect Tolerances

• Repair Area Size: No specific size limits, but defects must be localized.
• Bare Spots: Must be repaired; no exposed steel allowed.
• Adhesion: No cracks, peeling, or flaking that affect performance.

Repair Methods

• Zinc-Rich Paint: For bolts, nuts, and small components with at least 65% zinc content in the dry film.
• Zinc-Alloy Soldering: Suitable for small hardware repairs.

ASTM A767

Defect Tolerances

• Repair Area Size: Only localized damage (e.g., during transport or installation) is allowed.
• Bare Spots: Not permitted; must be repaired to prevent corrosion.
• Adhesion: Coating must not peel or crack under normal handling.
• Surface Area Limit: Repairs should not exceed 1% of the rebar surface.

Repair Methods

• Zinc-Rich Paint: Must contain at least 94% zinc in the dry film.
• Phun nhiệt: To restore uniform coating.
• Zinc-Alloy Soldering: Allowed for smaller damage areas.

Flawless Galvanized Steel for Lasting Performance

Steel Pro Group delivers products engineered to meet ISO and ASTM standards. Our rigorous quality control minimizes defects like bare spots and peeling, ensuring consistent performance and compliance. For steel that stands strong and resists flaws, trust Steel Pro Group to protect your investment.