Galvanic Compatibility: Stainless Steel + Galvanized Steel
Severe Risk — 0.7V Anodic Index Difference
Zinc coating (then steel) will corrode preferentially in this pairing.
Compatibility Assessment
A common mistake in construction. Stainless steel bolts in galvanized steel beams will strip the zinc coating rapidly. The galvanic cell drives the zinc to corrode first, then the underlying steel follows.
| Property | Stainless Steel | Galvanized Steel |
|---|---|---|
| Anodic Index | 0.5V | 1.2V |
| Galvanic Role | Cathode (protected) | Anode (corrodes) |
| Voltage Difference | 0.7V — Severe Risk | |
Compatibility by Environment
The acceptable voltage difference depends on the operating environment:
- Harsh (outdoor/marine): ❌ Not compatible (0.7V > 0.15V)
- Normal (indoor/uncontrolled): ❌ Not compatible (0.7V > 0.25V)
- Controlled (HVAC): ❌ Not compatible (0.7V > 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:
- The more anodic (active) metal — Galvanized Steel in this case — loses electrons and corrodes.
- The more cathodic (noble) metal — Stainless Steel — is protected and may even corrode less than it would alone.
- 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 stainless steel sheet will fail quickly, while stainless steel 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.
- Use compatible fasteners: Choose fasteners from the same galvanic group as the base material.
- Apply sealant: Marine-grade sealants (like 3M 5200) can isolate metals while maintaining structural integrity.
- 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 + Galvanized Steel — Moderate Risk (0.3V)
- Steel + Stainless Steel — High Risk (0.35V)
- Stainless Steel + Copper — Low Risk (0.15V)
- Stainless Steel + Brass — Low Risk (0.1V)
- Stainless Steel + Bronze — Low Risk (0.1V)
Check any metal combination
Use our Interactive Galvanic Compatibility Checker to test any two metals for galvanic corrosion risk — including anodic index, environment ratings, and prevention recommendations.
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