What Causes Galvanic Corrosion Between Aluminum and Steel?

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Galvanic corrosion happens when two dissimilar metals touch in the presence of moisture, creating an electrical cell in which the less noble metal, usually aluminum in contact with steel, corrodes faster. In a fence, this is prevented by separating the metals with insulating materials, using compatible fixings, and keeping water from bridging the joint.

How galvanic corrosion works

Every metal has a position in the galvanic series, which ranks how readily it gives up electrons. When two metals with different positions are electrically connected and an electrolyte such as rainwater, condensation or salty air bridges them, current flows. The more active metal becomes the anode and slowly dissolves, while the nobler metal is protected.

Aluminum and steel are a common pairing in fences and gates, and the aluminum is the one that suffers. The reaction needs three things at once: two different metals, electrical contact between them, and an electrolyte. Remove any one of these and the process largely stops.

Why aluminum and steel are a risky pair

Aluminum sits well below steel in the galvanic series, so when they are joined, aluminum becomes the sacrificial metal. Hot-dip galvanized steel is zinc-coated, which narrows the difference, but cut edges, drilled holes and scratched coatings still expose bare steel.

Aluminum posts bolted directly to steel frames
Steel brackets fixed to aluminum rails without insulation
Stainless steel screws in aluminum profiles

Where the damage usually appears

Corrosion tends to concentrate at the contact point, not across the whole fence. You may see white or grey powdery deposits, swelling at the joint, or pitting around the fixing. Areas near the coast, under overhanging eaves that trap moisture, and at ground level where splashing occurs are the most vulnerable.

Because the damage is localized, it can look minor at first while the structural connection weakens underneath. Regular visual checks of joints are the simplest early warning.

How to prevent it during installation

The most reliable prevention is electrical isolation. Place non-conductive gaskets, such as EPDM or nylon washers and sleeves, between the metals, and use insulating bushings in bolt holes. Sealants that remain flexible can also keep water out of the contact zone.

Fixings should be compatible with the lighter metal. Use aluminum or stainless steel fasteners on aluminum components, and make sure the coating system on steel parts is intact before assembly. Avoid leaving bare steel cut edges exposed near aluminum.

Coatings and finishing

Surface treatment reduces the risk further. Hot-dip galvanizing protects steel by sacrificing zinc, and powder coating adds a barrier layer that keeps electrolytes away from both metals. Anodized or powder-coated aluminum resists corrosion better than raw metal.

Coatings are only effective when undamaged, so touch up scratches and cut ends after fabrication and on site. The exact specification for coating thickness and standards should be confirmed with the project engineer.

Design and drainage tips

Good design limits how long moisture stays on the metal. Specify drainage holes at the bottom of hollow sections, avoid horizontal ledges where water pools, and keep fence bases clear of standing water. Where possible, design joints so that water runs away from contact points rather than into them.

Clear spacing between the metals and adequate ventilation also help the joint dry out after rain, which slows the electrochemical reaction considerably.

Frequently asked questions

Can aluminum and steel ever be bolted together safely?

Yes, provided the metals are electrically isolated with insulating washers, sleeves or gaskets and the fixings are suitable. Without isolation, galvanic corrosion is likely over time.

Does galvanized steel still cause corrosion on aluminum?

It can. Galvanizing protects the steel, but the aluminum remains the more active metal at a contact point, so insulation is still recommended.

Is stainless steel safe to use with aluminum?

Stainless steel is still more noble than aluminum, so it can drive corrosion of the aluminum if they touch. Use insulating barriers even with stainless fixings.

How can I tell if galvanic corrosion has started?

Look for white or grey powder, bubbling paint, swelling or pitting around the joint. If you notice these signs, have the connection inspected before it weakens.

Common mistakes to avoid

Installers often bolt aluminum directly to steel or use bare steel screws in aluminum profiles, assuming the coating alone is enough. Skipping insulating washers and leaving cut steel edges exposed are the most frequent causes of early failure.

Our recommendation

Always isolate aluminum from steel with non-conductive washers, sleeves and gaskets, use aluminum or stainless fasteners on aluminum parts, and touch up every cut or scratched coating before the fence leaves the workshop.

Summary

Galvanic corrosion between aluminum and steel is an electrochemical reaction that attacks the less noble metal at the contact point, and it is prevented by isolation, compatible fixings, coatings and good drainage.

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