galvanic corrosion

GALVANIC: When dissimilar metals are connected in the presence of an electrolyte a galvanic corrosion reaction occurs. Both the presence of an electrolyte solution and a bi-metallic coupling is required for this type of corrosion to occur.

Electrolytes are electrically conductive solutions such as sea water or road salt spray which contain chloride or rain, mist or dew containing sulfur or nitrogen compounds. The most common sources of chloride contamination are marine and coastal environments as well as winter road salting spray. Sulfur and nitrogen compounds are more ubiquitous being the product of air pollution and industrial fallout.

When two dissimilar metals are connected electrically through an electrolyte or salt bridge one metal will become anodic with reference to the other and the potential difference will create a corrosion current causing the more anodic metal to corrode.