Bimetallic or galvanic corrosion is defined as the accelerated corrosion of the metal because of an electrical contact with a nobler or a non metallic conductor in a corrosive electrolyte.
The less corrosion resistant or the “active” member of the couple experiences accelerated corrosion while the more corrosion resistant or the “noble” member of the couple experiences reduced corrosion due to the “cathodic protection” effect.
The most severe attack occurs at the joint between the two dissimilar metals. Further away from the bimetallic joint, the degree of accelerated attack is reduced.
Bimetallic corrosion mechanism
Different metals and alloys have different electrochemical potentials is the same electrolyte. When the corrosion potential of various metals and alloys are measured in common electrolyte and when listed in orderly tabulated format, a galvanic series is created. Hence it is emphasized that corrosion potentials must be measured for all metals & alloys in same electrolyte under same environmental conditions.
How to prevent Bimetallic corrosions
- Select metals/alloys as close together as possible in the galvanic series.
- Avoid unfavorable area effect of a small anode and large cathode.
- Insulate dissimilar metals wherever practical
- Apply coatings with caution. Paint the cathode (or both) and keep the coatings in good repair on the anode.
- Avoid threaded joints for materials far apart in the galvanic series.