General

A CP coupon is used to simulate coating defects (holidays) on the structure being evaluated. The CP coupon is installed near the structure and then connected to it through an aboveground test station. This allows the CP coupon to be connected to the CP system on the structure, thus simulating a holiday in the coating. The CP coupon can then be disconnected from the circuit during periodic testing, and an instant-off potential measured. The CP coupon can then continue to be monitored and the depolarization measured. These measurements approximate the polarized and the depolarized potential of the structure in the vicinity of the CP coupon and allow the operator to calculate the IR drop. A second "free-corroding" native coupon has sometimes been installed at the same location as the CP coupon to measure the native potential of the coupon.

NACE Standard RP01691 states, "Voltage (IR) drops other than those across the structure-to-electrolyte boundary must be considered. . ." in order to assess the effectiveness of a CP system using fixed potential measurements. Several methods are used to evaluate the IR drop value and thus the effectiveness of CP systems:

• Minimizing the distance between the reference electrode and the surface of the structure;

• Measuring the potential when the current flow is interrupted (the "instant-off" potential); or

• Installing CP coupons in the vicinity of the structure to replicate a coating holiday.

All of these methods have their advantages and disadvantages. NACE Standard TM04972 includes some of the methods used to satisfy NACE Standard RP0169. CP coupons have several advantages. IR-drop errorreduced potentials can be obtained without interrupting multiple CP sources. CP coupons can also be used on buried structures with direct-connected galvanic anodes, which cannot be interrupted. Using CP coupons, depolarization testing can be performed without deenergizing the CP system. An additional advantage is the ability to record interference-reduced structure potentials on structures affected by stray currents.

A disadvantage of using CP coupons is the possibility of discrepancy between the polarized potential of the structure and that of the CP coupon. Because the CP coupon represents a holiday of similar size on the structure, the polarized potential of the CP coupon does not always duplicate the polarized potential of the structure at the same location. The holiday location (as observed by the polarized potential of the structure measured at grade) is not necessarily at the CP coupon location, and can be at a location some distance away.

An additional complication in measuring a structure potential is the effect of IR drops from uninterruptable current sources. By design, CP coupons can be disconnected from the structure and CP system, thereby eliminating the IR drop attributable to these current sources. Even when all current sources have been interrupted, longline currents can still affect the structure-to-electrolyte potential readings measured at grade on a pipeline. Because the effective reference point of a CP coupon is very close to the CP coupon surface, IR drops caused by long-line currents are minimized.