1. Bimetallic Contact

Bimetallic corrosion is serious and it occurs when two materials differing in potential are joined together. The galvanic series is a practical guide for engineers. The position of two metals or alloys in galvanic series dictates the extent of bimetallic corrosion. Consider, for instance, aluminum and copper. It can be observed from the table that aluminum is active to copper in the galvanic series and hence it would act as anode to copper which become the cathode due to its relatively more noble position. Joining the two would, therefore, give rise to bimetallic corrosion. The joining of two metals and alloys close to each other in galvanic series would not cause bimetallic corrosion. The more close together are the materials in galvanic series, the less the potential difference (the driving force for corrosion) and the further apart the materials in the galvanic series the greater would be the potential difference and a greater driving force for corrosion. The following are the factors affecting bimetallic corrosion:

   a)      Difference of potential.

   b)      A small anodic area to a large cathodic area.

   c)      Conduction of electrolyte.

   d)      Deposition of impurities and deposits, such as hygroscopic particles, sand or salt particles.

   e)      Contact with insulation materials.