Galvanic Corrosion & Metal Compatibility
Galvanic corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte (like saltwater or humidity). The more anodic (active) metal will corrode to protect the more cathodic (noble) metal.
The Golden Rule of Dissimilar Metals
To prevent severe corrosion, metals in contact should have an Anodic Index difference of no more than 0.25 V for harsh environments (outdoors/marine) and no more than 0.50 V for controlled indoor environments.
Anodic Index Chart
| Metal / Alloy | Anodic Index (Voltage) | Electrochemical Behavior |
|---|---|---|
| Magnesium | -1.75 V | Most Anodic (Corrodes easily) |
| Zinc (Galvanized Steel) | -1.20 V | Highly Anodic (Sacrificial) |
| Aluminum Alloys | -0.90 V | Anodic |
| Carbon Steel / Iron | -0.85 V | Moderate |
| Lead | -0.50 V | Moderate |
| Brass / Bronze | -0.40 V | Cathodic |
| Copper | -0.35 V | Cathodic |
| Stainless Steel (304/316) | -0.05 V | Highly Cathodic (Noble) |
| Gold / Platinum | +0.15 V | Most Cathodic (Protected) |
Design Example: Stainless Steel and Aluminum
If you use a Stainless Steel 316 bolt (-0.05 V) to fasten an Aluminum plate (-0.90 V), the difference is 0.85 V. Because this exceeds 0.25 V, the Aluminum (Anodic) will rapidly corrode and disintegrate around the bolt hole if exposed to moisture.
Solution: Use dielectric isolators (nylon washers/sleeves) to prevent electrical contact, or select a fastener closer to Aluminum on the index (like Zinc-Plated Carbon Steel).