L51640 Improved Pipe-to-Soil Potential Survey Methods

Pipe-to-soil potential measurements are the primary means for monitoring the effectiveness of cathodic protection (CP) systems. All criteria for cathodic protection employ, in one form or another, a potential measurement of the pipe with respect to a reference electrode. Although the pipeline industry obviously depends on pipe-to-soil potential measurements for monitoring CP systems, very little is known concerning what portion of the pipe is being sampled by the potential measurement. Prior to performing this project, many questions remained unanswered, such as: (1) what is the length of pipe being sampled by aboveground measurements, (2) is the potential sampled an average value around the circumference of the pipe or do above-ground measurements sample only the potential at the top of the pipe, and (3) how far away can a holiday or potential anomaly be detected.

The overall objective of this research program was to improve the ability to perform and interpret close interval on-potential and off-potential surveys. The following general conclusion can be made based on the results of this study. Ground level potential measurements provide average potential values, which are weighed based on pipe diameter, pipe depth, coating versus bare pipe, and other factors. Thus, close interval surveys should be considered as another tool to provide information on the condition of the pipe but should not be interpreted as providing definitive information on that condition.

Benefit: This research was a three-year program conducted for the Corrosion Supervisory Committee of PRCI with the main focus directed at establishing the area of pipe sampled during a pipe-to-soil potential measurement. The program was divided into two parts: bare pipelines and coated pipelines. The work examining bare pipelines was performed during 1988 and 1989, and the work examining coated pipelines was performed during 1990. The overall objective of this program was to improve the ability to perform and interpret close-interval potential surveys.