Soil, lead through dissolution

When a very long, unprotected or poorly protected embedded structure such as a pipeline crosses different types of soil, the dissolution potentials of the metal with respect to the soil are not consistently the same. This leads to the circulation of currents, which results in localised corrosion at the exit zones into the soil. This is also observed with immersed or semi-immersed structures such as ship hulls. For this reason, the return current should not flow through the hull, as one would be inclined to do on a small craft with battery-powered electric equipment. One conductor for each polarity is required if the system is distributing direct current, and one conductor per phase (plus one for the neutral, if required) for alternating current. 

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