Galvanic Compatibility: Aluminum and Galvanized Steel

Compatibility Assessment

The zinc coating on galvanized steel is more anodic than aluminum, so the zinc will corrode first. Once the zinc is consumed, the underlying steel (0.85V) is very compatible with aluminum.

Compatibility by Environment

The acceptable voltage difference depends on the operating environment:

• Harsh (outdoor/marine): ❌ Not recommended (>0.15V)
• Normal (indoor/uncontrolled): ⚠️ Use caution (>0.25V)
• Controlled (HVAC): ✅ Compatible (≤0.50V)

Thresholds per MIL-STD-889 guidelines. Harsh = saltwater, industrial, outdoor. Normal = warehouse, uncontrolled indoor. Controlled = temperature/humidity regulated.

How Galvanic Corrosion Works

When two dissimilar metals are in electrical contact and exposed to an electrolyte (water, especially saltwater), an electrochemical cell forms:

1. The more anodic (active) metal — Galvanized Steel in this case — loses electrons and corrodes.
2. The more cathodic (noble) metal — Aluminum — is protected and may even corrode less than it would alone.
3. The corrosion rate increases with the voltage difference between the metals and the amount of moisture present.

The area ratio also matters critically: a small piece of anodic metal in contact with a large cathodic surface will corrode much faster than the reverse. For example, galvanized steel rivets in a aluminum sheet will fail quickly, while aluminum rivets in galvanized steel are less problematic.

Prevention Strategies

• Insulating gaskets/washers: Place non-conductive material between the metals to break the electrical path.
• Protective coatings: Apply paint, lacquer, or specialized coatings to one or both metals.
• Sacrificial anodes: In critical applications, add a more anodic (active) metal that corrodes preferentially.
• Design for drainage: Ensure water cannot pool at the junction of dissimilar metals.

Related Metal Pairings

• Aluminum + Stainless Steel — High Risk (0.4V)
• Aluminum + Steel — Low Risk (0.05V)
• Aluminum + Copper — Severe Risk (0.55V)
• Aluminum + Brass — High Risk (0.5V)
• Aluminum + Bronze — High Risk (0.5V)
• Aluminum + Zinc — Moderate Risk (0.35V)