Position of Reference Electrode

The potential of a cathodically protected stmcture is determined ideally by placing the reference electrode as close as possible to the structure to avoid an error caused by IR drop through the electrolyte. Any IR drop through corrosion-product films or insulating coatings will persist, of course, even where adequate precautions are exercised otherwise, tending to make the measured potential more active than the actual potential at the metal surface. In practice, for buried pipelines, a compromise position is chosen at the soil surface located directly over the buried pipe. This position is chosen because cathodic protection currents flow mostly to the lower surface and are minimal to the upper surface of a pipe buried a few feet below the soil surface. 

The reference electrode is sometimes located at a position remote from the pipeline, recommended because currents do not penetrate remote areas, and hence, IR drop effects are avoided. Actually, the potential measured at a remote position is a compromise potential at some value between that of the polarized structure and the polarized auxiliary or sacrificial anode. These potentials differ by the IR drop through the soil and through coatings. The potential measured at a remote location, therefore, tends to be more active than the true potential of the structure, resulting in a structure that may be underprotected. 

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Fig. 9.3 Illustration of the electrical potential distribution calculated for the flat-plate cell with ideally symmetrical working and counter electrodes, according to [25]. Solid lines correspond to the equipotential surfaces. Possible positions of reference electrodes are indicated...

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