Galvanic Compatibility: Magnesium + Steel
Severe Risk — 0.9V Anodic Index Difference
Magnesium will corrode preferentially in this pairing.
Compatibility Assessment
Magnesium is one of the most anodic common engineering metals. Contact with steel causes extremely rapid corrosion of the magnesium. Magnesium alloy parts require complete isolation from steel fasteners.
| Property | Magnesium | Steel |
|---|---|---|
| Anodic Index | 1.75V | 0.85V |
| Galvanic Role | Anode (corrodes) | Cathode (protected) |
| Voltage Difference | 0.9V — Severe Risk | |
Compatibility by Environment
The acceptable voltage difference depends on the operating environment:
- Harsh (outdoor/marine): ❌ Not compatible (0.9V > 0.15V)
- Normal (indoor/uncontrolled): ❌ Not compatible (0.9V > 0.25V)
- Controlled (HVAC): ❌ Not compatible (0.9V > 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 — Magnesium in this case — loses electrons and corrodes.
- The more cathodic (noble) metal — 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, magnesium rivets in a steel sheet will fail quickly, while steel rivets in magnesium 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 + Steel — Low Risk (0.05V)
- Steel + Stainless Steel — High Risk (0.35V)
- Steel + Copper — High Risk (0.5V)
- Steel + Brass — High Risk (0.45V)
- Steel + Bronze — High Risk (0.45V)
- Steel + Zinc — Moderate (intentional) Risk (0.4V)
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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