Stray currents generating structure corrosion are usually caused by earth return currents from large DC systems. Currents that normally should be contained in the structure, take an alternative path through the electrolyte, causing corrosion damage to the original structure.
Examples are the return currents of welding posts or the protection current flowing through a metal pipeline that is cathodically protected at the inner side.
Earth currents generally result from the leakage of the return current from large DC systems that are earthed or that have a bad earth-insulated return path. Common examples are electric rail- and tramways and other areas where large DC currents are employed, such as, for instance, electric welding and cathodic protection. Metallic structures in the neighborhood of those earth current sources, can suffer from earth current corrosion.
The duration and the amplitude of the involved processes are important for the resulting metallic corrosion. For relatively short action such as welding, simple precautions are often sufficient. Cathodic protection systems and railways on the other hand, rely for their function on large, persistent earth currents or suffer from periodic traffic, consuming substantial currents. Here, the protection techniques are more complicated, as will become clear in what follows.
Study and Evaluation of Stray Current Influences on Cathodic Protection Systems of Buried Pipelines, L. Bortels, ELSYCA - Kranenberg 6 - 1731 - BELGIUM, ELSYCA