What is Galvanic Corrosion?
When two dissimilar metals are in electrical contact in the presence of an electrolyte (water, salt spray, humidity), the more reactive metal corrodes faster than it would on its own. This is galvanic corrosion — and it’s one of the most common (and preventable) failure modes in mechanical design.
The further apart two metals are on the galvanic series, the worse the corrosion. The more anodic (reactive) metal sacrifices itself to protect the more cathodic (noble) metal.
Anodic Index Table
The anodic index is a measure of a metal’s electrochemical voltage potential relative to gold. Use it to check compatibility:
| Metal | Anodic Index (V) |
|---|---|
| Gold (Au) | 0.00 |
| Silver (Ag) | 0.15 |
| Stainless Steel (passive, 304/316) | 0.50 |
| Nickel (Ni) | 0.30 |
| Copper (Cu) | 0.35 |
| Brass | 0.40 |
| Tin (Sn) | 0.65 |
| Lead (Pb) | 0.70 |
| Steel / Iron (mild) | 0.85 |
| Aluminum 2024 | 0.75 |
| Aluminum 6061 | 0.90 |
| Zinc (Zn) | 1.25 |
| Magnesium (Mg) | 1.75 |
Compatibility Guidelines
Calculate the difference in anodic index between your two metals:
| Environment | Max Acceptable Difference |
|---|---|
| Controlled (indoor, dry) | 0.50V |
| Normal (indoor, some humidity) | 0.25V |
| Harsh (outdoor, marine, industrial) | 0.15V |
Example
Aluminum 6061 (0.90V) bolted to stainless steel 316 (0.50V) = 0.40V difference. Fine for indoor use, but a problem outdoors or in a marine environment. The aluminum will corrode at the interface.
How to Prevent Galvanic Corrosion
- Choose compatible metals — keep the anodic index difference within the limits above
- Isolate the metals — use nylon washers, rubber gaskets, or paint between contact surfaces
- Apply protective coatings — anodize aluminum, plate steel, or use paint/powder coat
- Use sacrificial anodes — intentionally add a more reactive metal (zinc anodes on boat hulls)
- Design for drainage — don’t trap water at dissimilar metal joints
Common Problem Pairs
- Aluminum + Stainless Steel — very common in architecture/construction; use isolation washers
- Aluminum + Copper — never run copper pipe directly into an aluminum fitting
- Steel + Stainless Steel — the mild steel corrodes; not as bad as aluminum + SS
- Magnesium + anything — magnesium is extremely anodic; always isolate it
For detailed material compatibility in your application, reach out and I’ll help you work through it.