Review: This paper is an extension of NACE Paper No. 572 [DID]. The two papers and their reviews should be considered together.
In this article, Didas makes the following very important point:
"The polarized potential of the coupon will not necessarily duplicate the structure polarized potential at the same location. This is due to the coupon representing a holiday of similar size on the pipeline and not actually being located at the holiday location. The structure potential would be representative of the nearest or most influencing holiday, while the coupon potential would be at the coupon, therefore when comparing potentials taken at the same location, there should be some difference."
Didas makes the following recommendations for developing a coupon program:
- Perform a close interval survey (CIS) over the entire pipeline segment being evaluated.
- Evaluate the CIS data and determine your areas that do not meet your criteria (anomaly areas). The evaluation of anomalies should include:
Didas lists three factors which determine coupon spacing:
- The lowest potential or lowest polarization.
- The lowest soil resistivity.
- The locations with ineffective and/or disbonded coating.
- The locations with no coating (installed bare).
- The locations with the most corrosive soil or corrosive conditions.
Remediate the anomalies as required, then determine the locations/quantities of coupons necessary to effectively evaluate the structure's CP system. The locations should be in the anomaly areas, at locations with a high IR drop, at critical locations as determined by the responsible corrosion control personnel, and when possible, at easily accessible locations.
"The spacing of coupons along a buried pipeline is determined by several factors, primarily coating condition, type of cathodic protection (CP) system and economics."
A rule of thumb for coupon spacing is provided for five combinations of coating conditions and CP system.
In a section on troubleshooting, the author addresses problems which might arise as the result of shielding, air pockets in the soil access tube, reverse polarization, and premature failure of permanent reference electrodes.