Galvanic Compatibility: Stainless Steel and Copper

Compatibility Assessment

Stainless steel and copper are relatively compatible, with only a 0.15V difference. This is acceptable even in normal environments, though the stainless may see minor surface effects.

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 — Stainless Steel in this case — loses electrons and corrodes.
2. The more cathodic (noble) metal — Copper — 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, stainless steel rivets in a copper sheet will fail quickly, while copper rivets in stainless steel 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)
• Steel + Stainless Steel — High Risk (0.35V)
• Steel + Copper — High Risk (0.5V)
• Stainless Steel + Brass — Low Risk (0.1V)
• Stainless Steel + Bronze — Low Risk (0.1V)