Many customers’ first reaction upon discovering rust on their galvanized steel duct is to replace it. However, in most cases, it’s not that serious—with proper handling, the duct can still be used normally. More importantly, the vast majority of rust problems can be completely avoided through correct preventative measures. BYFO helps you understand when to address the issue, when to replace it, and how to solve the problem once and for all.
I. Three Levels of Rust: At What Level Can It Still Be Used?
When rust appears, don’t rush to scrap it. First, determine the severity level, then decide on the treatment:
✔️ Level 1 (Slight) – Surface White Rust (Basic Zinc Carbonate)
• Appears as a grayish-white powder, corroding only the zinc layer and not damaging the steel substrate.
• Treatment: Brush off the surface rust. It doesn’t affect the lifespan and doesn’t require pipe replacement.
⚠️ Level 2 (Moderate) – Red Rust Appears
• The zinc layer is completely consumed, and the steel substrate begins to oxidize (turning reddish-brown).
• Treatment: Grind to remove rust → Apply anti-rust paint/cold galvanizing paint → Can continue to be used (non-critical parts)
✖️ Level 3 (Severe) – Rust Through-hole
• The steel plate is completely rusted through; pressing it by hand will leave a dent.
• Treatment: Must be replaced; there is no value in repairing it.
In short: Don’t worry about white rust, treat red rust immediately, and replace if it’s perforated.
II. Four Major Causes of Rust (with Prevention Solutions)
| Causes | Why causes rusting | How to prevent |
| Improper storage | The galvanized layer was intact when it left the factory, but it rusted after 3 months of storage on site—the most common causes are ground moisture and exposure to rain. | Elevate it at least 15cm off the ground, cover it with a rainproof tarpaulin, and maintain ventilation. |
| Processing damage | Scratches in the zinc layer during lock forming, cutting, and handling expose the steel plate, making it the starting point for rust. | Smooth the lock forming machine roller surface, apply anti-rust paint to the cut edges after cutting, and use slings instead of steel cables during handling. |
| Environmental corrosion | Salt spray at the beach, industrial acid and alkaline gases, kitchen fumes, and high humidity all accelerate corrosion. | For coastal /corrosive environments, choose Z275 high-galvanized steel or 304 stainless steel; do not use Z100. |
| Insufficient material grade | If Z100 (low zinc coating) was selected during procurement, it will fail in 1-2 years in humid environments. | For general environments, choose Z180; for humid/coastal environments, choose Z275, and require the supplier to provide material certification. |
III. On-site Treatment Methods for Three Common Types of Rust
Depending on the location and nature of the rust, different treatment plans are adopted:
1. Surface White Rust (Basic Zinc Carbonate)
• Phenomenon: Grayish-white powder, rough surface
• Treatment: Remove surface rust with a copper wire brush or scouring pad → Wipe clean with a dry cloth → Apply rust-preventive oil or passivating agent for protection
• Caution: Do not use a wire brush too forcefully, as this will damage the unrusted zinc layer.
2. Localized Red Rust (Dotted or Striped)
• Phenomenon: Reddish-brown rust spots, uneven surface
• Treatment: Grind with an angle grinder and a grinding wheel to remove rust until the metallic luster is exposed → Clean the surface → Apply cold galvanizing paint (zinc content ≥80%) 2-3 times, with an interval of 15-20 minutes between each coat.
• Caution: The grinding area should extend 5-10cm beyond the edge of the rusted area to ensure that the surrounding zinc layer transition zone is also covered.
3. Rust on the Seam
• Phenomenon: Linear rust along the seam line.
• Treatment: Apply polyurethane sealant or epoxy resin coating along the seam to completely seal the gap and isolate moisture and air.
• Prevention: Use an internal bite process during production, folding the bite seam towards the outside of the duct to reduce the possibility of internal water accumulation.
4. Rust on Flange Holes and Cut Edges
• Phenomenon: Red rust appears around the flange bolt holes and on the edges of the duct cut.
• Treatment: Remove rust with fine sandpaper → apply two coats of zinc-rich primer → apply one coat of topcoat.
• Prevention: Apply anti-rust paint to the cut edges promptly during processing; do not wait until the installation site.
IV. BYFO’s Prevention Approach
BYFO believes that the most effective solution to rust problems is prevention, not remediation. Therefore, we implement the following in our equipment and processes:
| Link | What has BYFO done |
| Processing damage control | The lock forming machine roller surface is finely polished to maximize the protection of the galvanized layer from scratches. |
| Bite design | Offers an internal bite seam option, ensuring the seam is not exposed on the inner wall of the duct or on the external water surface. |
| Material recommendation | Recommend the appropriate zinc coating grade based on the customer’s environment to avoid “buying the wrong grade”. |
| Incision protection guidance | A rust-preventive coating guide for the cut surfaces is provided with the equipment to help customers establish standard procedures. |
| Material certification | All coils are traceable to the steel mill brand and galvanization amount, eliminating false labeling of grades. |
In short: Prevention is key; duct lasts 20 years. Even the best remedial work only extends it by 5 years. Taking rust prevention into account at every stage, from material selection to processing to installation, is far more effective than post-installation rust removal and painting.
V. Frequently Asked Questions
Q1: What if rust is found after the duct is installed?
Dry environment inside the ceiling → Local treatment followed by application of anti-rust paint; it can continue to be used. If it has rusted through → The entire section of duct must be replaced; online repair is not possible.
Q2: What is the lifespan difference between Z100 and Z275?
General indoor environment: Z100 starts to rust in about 5-8 years, while Z275 can last 15-20 years. Coastal or industrial areas: Z100 lasts about 2-3 years, while Z275 lasts about 8-12 years.
Q3: Is stainless steel guaranteed to never rust?
Stainless steel can also experience pitting corrosion in chloride ion environments (coastal areas, disinfectant solutions). Grade 316 should be selected. 304 stainless steel is sufficient for general indoor environments, but it is not suitable for continuous exposure to salt water or acidic gases.
VI. Conclusion
Rusting of galvanized steel duct is not necessarily a product quality issue—90% of rust stems from improper storage, processing, or material selection. By selecting the correct grade during procurement, protecting the zinc layer during processing, and ensuring proper moisture control during storage, the lifespan of the duct can easily reach or even double the expected lifespan.
| Your approach | Duct lifespan |
| Select Z100, open-air storage, do not coat the cut edges | 3-5 years |
| Select Z180, indoor storage, apply cut paint | 10-15 years |
| Select Z275, indoor storage + internal bite + full-process protection | Over 20 years |
BYFO can provide recommendations for sheet metal with different zinc plating grades and processing protection solutions. For specific advice, please contact us and specify your environment type (coastal/industrial/general indoor). We will respond within 24 hours.
Email: byfo@byfomachine.com
Whatsapp: (0086)18255536645
