Review: This paper is a companion to NACE Paper No. 564.
This paper presents the results of a four-year laboratory study designed for application to the Trans Alaska Pipeline. They evaluated the effect of four factors (exposed steel area, geometry, distance from CP anodes, and temperature) on the correlation between the potential of a pipeline coating defect and a cathodic protection coupon buried in the same environment. The investigators are employees of companies with strong interest in Trans Alaska Pipeline.
In this study they used a minimum of -850 mV polarized potential and/or 100 mV polarization to indicate adequate cathodic protection.
The following quotation defines appropriate application of CP coupons to monitor cathodic protection on underground pipelines.
"When it is impractical to correct for voltage drops in pipeline potential measurements by disconnecting all current sources, CP coupons may be used to ensure that adequate cathodic protection has been achieved."
CP coupons are installed to: 1) assess adequacy of a cathodic protection system, 2) determine compliance with federal regulations, and 3) determine compliance with industry standards.
The fact that a CP coupon permits monitoring without retrieval and inspection is an important distinction between conventional corrosion coupons and CP coupons.
The study did not actually consider coupon geometry. The investigators present a logical argument based on physics and economics to exclude disk, rod, and spherical anodes. They chose to conduct the study using a cylindrical anode constructed of small diameter pipe. Uniformity in current distribution and ease of construction are cited as the primary reasons for the choice.
The four year laboratory program consisted of four procedures:
1. 48 inch (122 cm)diameter pipe in a water filled pit,
2. Long term polarization measurements in soil and water,
Disconnecting coupon only compared to simultaneously disconnecting coupon and anodes produced an insignificant 10 mV difference.
They examine two size coupons placed at least 28 in. from an anode and concluded in both cases that coupon and defect of like size polarized to similar potentials. Actual differences in polarized potentials were 54 mV and 69 mV. Actual distance from anode to coupon is not specified.
They examined the affect of three different electrolyte resistivities and concluded in all three cases that coupons and defects of similar sizes polarized to similar potentials. Actual differences in potential were 33 mV, 70 mV, and 87 mV.
In 4 kWcm water and 100 kWcm water neither exposed area nor coupon location demonstrated any effect. In 18 kWcm the coupon polarized to a more negative potential than a larger sized pipe defect and to a more positive potential than a smaller sized pipe defect.
This five month procedure determined no affect of temperature on long term polarization of steel in soil and water. Some other observations, particularly important to Trans Alaska Pipeline, are recorded.
3. Buried 128 ft (39 m) test pipe,
Four coupon assemblies placed near four different size coating defects on a 128 ft. section of coated 8 in. buried pipe were used. No difference between coupon and defect potentials were identified. There is no indication of the size of the coupons nor of a comparison of coupon size with defect size.4. Model to determine effect of clustered defects.
A computer model with numerous assumptions was used to conclude that defect spacing may affect short term polarization but has minimal affect on long term polarization.
The following quotations from the conclusions of the paper are important.
"Disconnecting coupons from the pipe removes IR-drop and mixed potential errors from measurements without having to interrupt the CP system or sources of interference."
"The results of this work showed that when it is impractical to disconnect all current sources to correct for voltage drops in the pipe-electrolyte potential measurement, CP coupons may be used to ensure that adequate CP has been achieved."