Galvanic Compatibility: Aluminum and Steel

Compatibility Assessment

Despite common perception, aluminum and plain carbon steel are actually very close on the galvanic series. The 0.05V difference makes this a low-risk pairing. However, the steel may rust and stain the aluminum surface.

Compatibility by Environment

The acceptable voltage difference depends on the operating environment:

• Harsh (outdoor/marine): ✅ Compatible (≤0.15V)
• Normal (indoor/uncontrolled): ✅ Compatible (≤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 — Aluminum in this case — loses electrons and corrodes.
2. The more cathodic (noble) metal — Steel — 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, aluminum rivets in a steel sheet will fail quickly, while steel rivets in aluminum are less problematic.

Prevention Strategies

• 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 + Copper — Severe Risk (0.55V)
• Aluminum + Brass — High Risk (0.5V)
• Aluminum + Bronze — High Risk (0.5V)
• Aluminum + Zinc — Moderate Risk (0.35V)
• Aluminum + Titanium — Severe Risk (0.6V)