Galvanic corrosion occurs when two dissimilar metals in electrical contact are exposed to a common electrolyte (e.g. seawater, condensation, process fluid). The more active metal (anode) corrodes preferentially while the more noble metal (cathode) is protected. The driving force is the galvanic potential difference between the two metals in the specific electrolyte. This calculator ranks galvanic risk using the standard galvanic series in seawater and provides engineering guidance on area ratio effects.
Galvanic Corrosion Risk Estimator
Galvanic series position in seawater — relative ranking only. Actual potential difference depends on specific alloy composition, surface condition, temperature, oxygen content, and electrolyte chemistry. Always measure open-circuit potentials in the actual service environment for critical applications per ASTM G71 or ASTM G82.
Galvanic Series in Seawater (Simplified)
| Position | Metal / Alloy | Approx. Potential vs SCE (mV) | Role in Galvanic Pair |
|---|---|---|---|
| Most active (anodic) | Magnesium alloys | −1,500 to −1,700 | Anode — corrodes |
| Zinc (galvanised steel) | −980 to −1,030 | Anode — sacrificial protection of steel | |
| Aluminium 1xxx/5xxx | −740 to −800 | Anode when coupled to steel or Cu | |
| Carbon steel / cast iron | −500 to −600 | Anode to Cu, SS; cathode to Zn, Mg, Al | |
| 304/316 stainless (active) | −400 to −500 | Anode when sensitised | |
| Lead, tin | −250 to −350 | Moderate position | |
| Copper, brass, bronze | −100 to −250 | Cathode to steel and Al; anode to SS | |
| 304/316 SS (passive) | −50 to +50 | Cathode to steel, Cu alloys | |
| Duplex 2205 (passive) | 0 to +100 | Cathode to most structural metals | |
| Titanium alloys | +100 to +200 | Noble; cathode to nearly all metals | |
| Most noble (cathodic) | Platinum, gold | >+200 | Pure cathode — never corrodes |
Key Engineering Rules for Galvanic Corrosion Control
- Select compatible metals: Keep galvanic series separation to ≤200 mV in seawater; larger separations require isolation or protection measures.
- Maximise anode-to-cathode area ratio: Large anode / small cathode = low current density on the anode = acceptable corrosion rate. The worst case is a small steel bolt connecting large copper bus bars.
- Break the electrical circuit: PTFE or nylon isolating gaskets, sleeves, and washers between flange faces and bolt holes prevent metallic contact between flanges of different materials.
- Coat the cathode: If only one surface can be coated, coat the more noble (cathodic) metal — a holiday in cathode coating concentrates galvanic current at a tiny area of exposed cathode rather than at a tiny area of exposed anode, which would be devastating.
- Cathodic protection: For submerged or buried carbon steel anodic to copper or passive stainless, impressed current or sacrificial zinc/aluminium anodes provide reliable protection.
References
- ASTM G82-98(2014) Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance.
- ASTM G71-81(2019) Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes.
- Hack, H.P. (ed.), Galvanic Corrosion. ASTM STP 978, 1988.
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