Galvanic Corrosion Risk Calculator
Predict galvanic corrosion risk between dissimilar metals from standard reduction potentials.
Why Dissimilar Metals Corrode
Connect two metals in an electrolyte and you have built a battery. The less noble metal becomes the anode and dissolves; the more noble one becomes the cathode and is protected. The driving force is the difference in their electrode potentials.
| Metal | Position | Behaviour when coupled |
|---|---|---|
| Magnesium | Most active | Corrodes readily — used as sacrificial anode |
| Zinc | Active | Sacrificial — the basis of galvanising |
| Aluminium | Active | Corrodes against copper or steel |
| Carbon steel | Intermediate | Anodic to copper, cathodic to zinc |
| Copper / brass | Noble | Protected in most couples |
| Titanium, graphite | Most noble | Strongly cathodic — drives corrosion of partners |
Area Ratio Matters More Than Potential
This is the part most often missed. Corrosion rate depends on current density at the anode, so the geometry is decisive:
| Configuration | Result | Example |
|---|---|---|
| Large anode, small cathode | Safe — current spread over a wide area | Steel plate with copper rivets |
| Small anode, large cathode | Severe — current concentrated | Steel bolts in a copper plate |
Steel bolts holding a copper sheet will fail rapidly; copper bolts in a steel sheet will not. Same metals, same potential difference, opposite outcome — because the small anode carries all the current.
Carbon fibre composites are a modern trap: graphite is highly noble, so aluminium or steel fasteners in a carbon panel corrode aggressively. Titanium fasteners or an insulating layer are the standard fix.
Worked Examples
Common Mistakes
Area ratio matters as much. A small anode against a large cathode concentrates current density and can fail in months even at modest potential difference.
Carbon fibre composites behave as a strongly cathodic metal. Aluminium and steel fasteners in carbon panels corrode rapidly.
Coating the anode is worse than useless — a small pinhole becomes a tiny anode against a huge cathode. Coat the cathode instead, or insulate the joint.
No electrolyte means no galvanic cell. Dry indoor assemblies are far less at risk than marine or de-iced road environments.
Frequently Asked Questions
Formula Explorer connections
Interpretation: This formula links electron transfer, charge, potential, current or ionic transport in an electrochemical system. Assumption: Balance electron count and half-reactions, preserve sign conventions, and use consistent concentration, temperature and electrical units. Real cells include losses and overpotential.