Reference: Lawson, K. M., Thompson, N. G.,"The Use of Coupons for Monitoring the Cathodic protection of Buried Structures", Corrosion '98, NACE Paper No. 672.


Review: This paper presents the results of an ongoing field study focused on coupon and pipe off-potentials and the coupon currents.

In the Introduction the authors list all the problems associated with the commonly used instant-off potential method of correcting for IR-drop. They point out that the purpose of coupons is to provide an alternative method to correct for IR-drop. They point out that:

"The polarized potential of the coupon does not mirror the pipe polarized potential."

"It is theorized that the polarized potential of the coupon simulates the polarization of a holiday of similar size on the pipe."

Because this is an ongoing study, the authors claim not to make definitive conclusions but do present statements to summarize the findings to date.

"In this study, potential differences as large as 100 mV were observed circumferentially around the pipe."

"In general, the coupon off-potential does not represent the off-potential of the pipe at the coupon location."

"The work performed to date tends to substantiate the theory that the polarization behavior of the coupon represents that of a holiday of similar size on the pipe."

"Coupons respond to changes in the CP level in a similar manner as the pipe."

" there is a slight trend for coupons on the bottom of the pipe to be polarized more than coupons located on the top of the pipe."

"There is a trend for the standard (small) coupon to be polarized more than the large coupon. Although this effect is not large, this trend would be expected if the coupons simulate a holiday on the pipe."