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General

A CP coupon may be used to determine the level of CP of a buried or submerged metallic structure. CP coupons are installed in the electrolyte near the structure and are then connected to it through a test station. This allows the CP coupon to be connected to the CP system on the structure, thus simulating a similar-sized bare area of the structure's surface, such as at a holiday in the coating. The CP coupon may be disconnected from the circuit during periodic testing, and its instant-disconnect potential measured. The potential of the CP coupon may then continue to be monitored and the depolarization calculated. These measurements represent the polarized and depolarized potentials of the structure in the vicinity of the CP coupon. They also allow the IR drop in the electrolyte to be calculated for use in conventional potential measurements made from grade level. A second, freely corroding (native) coupon may be installed at the same location as the CP coupon to measure the free-corrosion potential of the structure in open-circuit conditions.

NACE SP01691 includes criteria for determining the CP status of a buried or submerged structure. For voltage measurements that are made when CP current is applied, IR drops other than those across the structure-to-electrolyte boundary must be considered. NACE SP0169 includes a number of ways this may be done and NACE Standard TM04972 includes a number of test methods used for these criteria. CP coupons may also be used to evaluate compliance with CP criteria, including considering the IR drop. The practices described in this standard must be followed with careful evaluation of the specific situation in which the coupons are to be used.

CP coupons have several advantages. Structure-to-electrolyte potentials that have the IR drop considerably reduced or eliminated may be obtained without interrupting multiple CP sources. CP coupons may also be used on buried structures with direct-connected galvanic anodes, which must not be interrupted. Using CP coupons, depolarization testing may be performed in most cases without de-energizing the CP system. An additional advantage is the ability to record accurate structure-to-electrolyte potentials on structures affected by stray currents.

When CP coupons are used, there may be differences between polarized potentials of the CP coupon and the structure. This is because the polarized structure-to-electrolyte potential measured at grade is usually the combined potential of the structure over a rather large area, including holidays in the coating and locations where the electrolyte or other conditions that affect the potential of a structure may vary. Errors caused by these variations are included in a potential measured at any given point along a structure and may be significant. These errors generally do not occur with coupons because of their small size and uniform conditions. Coupons located in areas where these variables are different can provide a good representation of the CP effectiveness on a structure.

A typical problem in measuring a structure-to-electrolyte potential is the effect of IR drops from uninterruptible current sources. By design, CP coupons may 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, long-line 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.