Three conditions, all required
A galvanic couple needs all three of the following at once:
- Two metals with different corrosion potentials in the environment concerned
- An electronic path between them — direct contact, a bolt, a shared support or a common ground
- An electrolyte wetting both surfaces — seawater, cooling water, condensate, wet soil or a thin moisture film
Remove any one and the cell stops. Every mitigation method works by breaking one of these three conditions.
What sets the rate
| Factor | Effect |
|---|---|
| Potential difference | The farther apart the two metals sit in the galvanic series for that electrolyte, the greater the driving force |
| Area ratio | A small anode connected to a large cathode concentrates the current on the anode — the worst case |
| Electrolyte conductivity | Seawater and brines carry current over a wide area; thin moisture films confine attack close to the junction |
| Polarization and surface films | Some cathodes polarize or film over and deliver less current than the potential gap suggests |
The ranking comes from the galvanic series table, measured in a stated electrolyte; ASTM G82 is the guide for developing and using such a series, and ASTM G71 covers galvanic corrosion testing in electrolytes. Metals described as galvanically compatible are pairs whose potentials are close enough for the service. For fasteners, platings and enclosures, the quick screening tool is the anodic index.
Where it shows up in plants
Carbon steel water boxes and tubesheets coupled to copper-alloy or titanium tubes, carbon steel piping flanged to stainless steel, stainless fasteners in aluminum, and copper piping joined to galvanized steel. In each case the carbon steel, aluminum or zinc is the member that pays.
What to do about it
- Select galvanically compatible materials for the electrolyte actually present
- Break the electrical path with insulating gasket kits, sleeves and washers, then verify that the isolation holds
- Coat the cathode, or both metals; never coat only the anode, because every holiday becomes a small anode facing a large cathode
- Keep the anode large relative to the cathode: noble fasteners in active plate, not the reverse
- Add cathodic protection or sacrificial anodes where the couple cannot be avoided