Causes of Rust on Galvanized Steel Pipes and Anti-Corrosion Treatment Methods

1. Five Core Causes of Rust on Galvanized Steel Pipes

1. Mechanical Damage to the Zinc Coating (Most Common)

2. Accelerated Environmental Corrosion

• Coastal high salt spray, industrial acid-base gases, and humid water accumulation environments quickly consume the zinc coating.
• Groundwater or industrial water containing chloride ions penetrates the zinc oxide film and causes substrate corrosion.
• Unprotected buried pipes suffer accelerated corrosion from soil acid-base and microorganisms.

3. Substandard Zinc Coating Quality

• Insufficient zinc coating thickness (national standard for hot-dip galvanizing ≥85μm, electro-galvanizing ≤40μm).
• Missing plating, zinc nodules and poor adhesion form corrosion starting points.
• Lack of passivation treatment greatly reduces weather resistance.

4. Lack of Protection After Construction

5. Long Service Life and Lack of Maintenance

2. Graded Rust Removal and On-Site Repair Solutions

Light Corrosion (Local Rust Spots / White Rust)

1. Lightly polish with 120–240 grit sandpaper to remove floating rust and oxide layers.
2. Clean with a neutral degreaser, rinse with water and dry completely.
3. Apply cold galvanizing coating (zinc content ≥96%) with a dry film thickness ≥80μm, using two thin coats.

Moderate Corrosion (Continuous Rust Patches / Partial Zinc Loss)

1. Polish with an angle grinder and steel wire wheel until metallic luster is exposed.
2. Use phosphoric acid rust converter to treat residual rust, neutralize, rinse and dry.
3. Apply epoxy zinc-rich primer first, then cold galvanizing coating, and finally polyurethane topcoat.

Severe Corrosion (Large-Scale Peeling / Substrate Corrosion)

1. Conduct deep mechanical rust removal to eliminate loose corrosion layers.
2. After local welding repair, perform overall sandblasting treatment.
3. Apply thermal spray zinc or return to the factory for re-hot-dip galvanizing.
4. Add fluorocarbon topcoat for outdoor use; wrap with coal tar epoxy and glass fabric for buried pipes.

3. Long-Term Anti-Corrosion Preventive Measures (Eliminate Rust at the Source)

1. Material Selection and Inspection

• Prioritize national standard hot-dip galvanized pipes with zinc coating ≥85μm. Check for no peeling or falling off after bending tests.
• For coastal / chemical environments, select thickened zinc coating or galvanized plastic-lined composite pipes.

2. Construction Specifications

• Repair cuts with cold galvanizing immediately after cutting / drilling.
• Polish welds and threads before zinc repair and primer application.
• Avoid direct contact with dissimilar metals such as copper and stainless steel to prevent galvanic corrosion.

3. Environmental Protection

• Outdoor: Epoxy zinc-rich primer + acrylic polyurethane topcoat, total dry film thickness ≥120μm.
• Buried: Three-layer coal tar epoxy with two layers of glass fabric, or PE anti-corrosion sleeves.
• Indoor: Maintain ventilation and dryness to avoid long-term water and dust accumulation.

4. Regular Maintenance

• Inspect every 6 months and repair rust spots immediately.
• Reapply topcoat annually in highly corrosive environments to extend service life.

4. Key Construction Pitfalls to Avoid

1. Do not apply ordinary paint directly on a smooth zinc coating; epoxy zinc-rich primer is required to improve adhesion.
2. Ensure thorough drying after rust removal; construction in humid environments causes blistering and peeling.
3. Cold galvanizing repair must cover 5cm beyond damaged edges to prevent edge corrosion.
4. Buried pipes must not omit anti-corrosion layers, otherwise perforation will occur within 3–5 years.

5. Summary