Does Corrugated Metal Rust?

Corrugated metal is a popular and versatile building material used for roofing, siding, and fencing applications. Its wavy, ridged profile provides strength and durability. However, the material’s composition makes it susceptible to corrugated metal rust over time when exposed to moisture and the elements. Understanding what factors cause corrugated metal to rust can help you prevent or control corrosion and extend its usable lifespan.

What Is Corrugated Metal?

Corrugated metal gets its name from its corrugated shape. It consists of wavy ridges and grooves pressed into the sheet metal to add strength and flexibility. Galvanized steel and aluminum are the most common metals used to manufacture corrugated sheets.

Steel goes through a galvanization process to apply a protective zinc coating to prevent rusting. Aluminum has inherent corrosion resistance and does not require a coating. The corrugated shape adds stiffness and impact resistance compared to flat sheets, making corrugated panels suitable for exterior applications and durable enough to span gaps as roofing and siding.

Common uses for corrugated metal include:

• Roofing – corrugated steel or aluminum roofing panels are lightweight, easy to install, and can span large distances.
• Siding – durable and economical exterior wall covering.
• Fencing – corrugated sheets make sturdy and affordable corrugated metal fencing.
• Agricultural and industrial buildings – often use corrugated steel for walls, roofs, and lean-tos.
• Signs and billboards – corrugated aluminum sheets are popular for road signs, billboards, and store signage.

Rusted Corrugated Metal 

Corrugated steel will eventually start to rust as the galvanized coating wears off from exposure to the elements. Aluminum corrodes but does not rust. Rust specifically refers to the oxidation process of iron. Let’s look closer at how each type of metal holds up:

Corrugated Steel

• The galvanization (zinc coating) protects the underlying steel and acts as a sacrificial barrier against rust. It corrodes first before the steel starts to rust.
• Over time, the zinc coating becomes exhausted through normal weathering. Once the steel is exposed, it will start to oxidize and form rust.
• Rusting progresses quicker in coastal areas with saltwater exposure or highly humid, industrial environments with acid rain.
• Unpainted corrugated steel typically starts showing signs of rust in 5-10 years. Painted steel lasts longer before rusting.

Corrugated Aluminum

• Pure aluminum naturally forms a thin oxide film that protects the surface from further corrosion. This makes it highly resistant to rusting.
• In most environments, uncoated aluminum maintains its shiny appearance for decades with little degradation.
• Aluminum corrodes in acidic or high salt conditions but will not rust. It forms white, powdery aluminum oxide rather than red rust.
• Painted aluminum lasts even longer before showing any signs of corrosion.

So in summary, corrugated steel will eventually rust, while aluminum is highly corrosion-resistant. But there are steps you can take to maximize the lifespan of corrugated metal roofing, siding, or fencing before any rust damage occurs.

Preventing Rust on Corrugated Metal

Here are some tips for preventing or delaying rust on corrugated steel panels:

Select a Thicker Galvanized Coating

• Specify a G90 zinc coating or heavier steel corrugated panels. The higher the ‘G’ number, the thicker the galvanization.
• G90 is suitable for most corrosive environments. For coastal areas or heavy industrial pollution, use G140 or higher.

Use Aluminum Instead of Steel

• Aluminum corrugated sheets will never rust. This comes at a higher initial cost but pays off with zero maintenance and decades of use before replacement is needed.

Apply Protective Paint or Coatings

• For long-term rust prevention on steel panels, have them pre-painted before installation. Use exterior-grade acrylic paint.
• Clear polyurethane, epoxy or powder coatings also protect against corrosion on aluminum or steel sheets.

Perform Regular Maintenance & Inspections

• Wash roof and siding annually with a pressure washer to remove grime and debris.
• Check for scratches or coating damage and touch up immediately with repair paint.
• Look for rust spots around fasteners, seams, and edges, and sand the area to bare metal before re-painting.

Ensure Proper Installation

• Overlap panels correctly, use appropriate fasteners, and add sealing washers to prevent moisture seepage behind panels. This avoids hidden corrosion.
• Pitch roofing at least 1/4 inch per foot minimum slope for drainage.
• Leave ventilation space and use drip edges overhangs. Condensation buildup accelerates corrosion.

Control Environmental Factors

• Cut back overhanging tree branches to avoid damp leaves and debris piling up on the metal roof.
• Shelter the structure from excessive wind, rain, and salt spray with windbreaks or roof slope orientation.
• In industrial settings, advocate for pollution controls to reduce acid rain deposition on roofs.

Treating Rusted Corrugated Metal

If rust is already present on corrugated panels, prompt action can stop further spread. Here are some effective remedies:

Remove Surface Rust

Use a wire brush or sandpaper to scrub off any loose rust flakes down to bare metal. This prevents further oxidation.

Apply Rust Converter Primer

Special primers containing tannic or phosphoric acid convert existing rust into an inert polymer while sealing out moisture to halt the rusting process.

Spot Paint Affected Areas

For small rust spots, sand the area smoothly and apply a rust-inhibiting primer followed by an enamel topcoat to match the panel color.

Replace Severely Rusted Sections

Badly corroded panels or extensive rusting may require replacing whole sections of siding or roofing. This prevents leaks or weakening of the structure.

Increase Ventilation

Adding more vents or leaving gaps in overlapping panels promotes airflow to keep condensation from forming behind the metal surface and accelerating hidden rust damage.

Re-Galvanize the Surface

For salvaging heavily rusted corrugated steel, have the panels hot-dip galvanized again to apply a fresh protective zinc coating across the entire surface.

With prompt action to address any rust issues, corrugated steel panels can still last upwards of 20-25 years in most environments. Keeping them well-maintained and taking steps to prevent corrosion in the first place will maximize their lifespan. Corrugated aluminum provides even longer-lasting performance if longevity is a priority.

Corrugated metal is a versatile and economical building material, but the steel version is prone to rusting when its protective galvanized coating wears off over time. Aluminum panels are highly corrosion-resistant and will not rust. Maintaining the finish, controlling moisture, and preventing scratches or coating damage can significantly delay the onset of rust on corrugated steel.

But ultimately, the material will show signs of corrosion after 5-10 years unless recoated. Taking prompt action to fix early rust spots, seal out moisture, and re-paint damaged sections can add many extra years of service life. With proper care, corrugated metal can continue providing outstanding performance and value as a roofing, siding, or fencing solution.

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