Long-line currents

Those interested in long-line currents are referred to the publications of Gish , Gill and Rogers and Mudd". 

Although iron pipes suffer from the same corrosion risk as steel pipelines, associated with the generation of a galvanic cell with a small anode and a large cathode, the risk is mitigated for iron pipelines because the electrical continuity is broken at every pipe joint. For this reason long-line currents are uncommon in iron lines and cathodic protection is rarely necessary. It also accounts for the ability to protect iron lines by the application of nonadherent polyethylene sleeving . 

Along the length of the pipe, variations in soil resistivity may stimulate localized metal corrosion and can cause long line currents in the area of lower resistivity soil the buried metal surface is usually anodic, with a more negative potential, compared to that in the soil of higher resistivity. 

The difference in activity coefficient between the aqueous and the non-aqueous media is enough to confer some potential on the electrodes and cause corrosion of one of them. This sort of effect is observed in metal pipes that run a considerable distance and pass from one kind of soil into another. The resulting corrosion is then due to what is called long line currents . When it is possible to put the pipes into the same type of soil all the way, e.g. by filling the trenches with sand or gravel before laying the pipes, this corrosion is prevented. 

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