Potential Surveys

The most commonly used type of survey for determining the possibility of corrosion on a buried or submerged structure is the potential survey.  There are two types of potential measurements. A close reference electrode potential is the measurement of the potential (or voltage) difference between the structure and a reference electrode (normally a copper/copper sulfate electrode) contacting the earth surface directly above the structure.  The close reference electrode potential is useful in assessing corrosion activity and/or cathodic protection effectiveness of a structure. A remote reference electrode potential is the measurement of the potential difference between the structure and a reference electrode contacting the earth surface well away from any metallic structures which may have adverse effects on the reading.  The remote reference electrode potential is useful in determining electrical continuity between structures.
The general interpretation of a potential survey is as follows:
a) On an unprotected structure, the points on that structure which have the most negative potentials to a close and to a remote electrode are the ones where corrosion would be more active.
b) On a cathodically protected structure, an accepted criterion for satisfactory levels of cathodic protection is a structure that has a potential measurement (to a close copper/copper sulfate electrode) more negative than 0.85 volt.
c) On a cathodically protected structure, if the structure has a potential measurement (to a remote copper/copper sulfate electrode) more negative than 0.85 volt, the entire structure has its unshielded areas protected against long-line current corrosion.
Long-line currents are those caused by galvanic couples created by anodic and cathodic areas, which are far apart.  These could be caused by dissimilar soils, by earth magnetic currents, by foreign D.C. current sources, etc.  A potential more negative than 0.85 volt to a remote electrode does not preclude, however, the possibility of corrosion due to localized action especially in shielded areas where very little or no current at all can be picked up by the structure.