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The use of coupons has been discussed and utilized for some time and is more prevalent in Europe than in the United States. Externally buried coupons have been utilized for different reasons including corrosion rate measurements, but their primary use has been for monitoring the effectiveness of a CP system. A detailed review of coupon studies is presented below. Renewed interest in the coupon technology was prompted by several studies performed by CC Technologies for PRCI that examined the use of off-potential measurements for evaluating the effectiveness of the CP system. These studies examined possible errors in the off-potential measurements including: Potential transients following the interruption of the CP system (spiking phenomenon). Interference due to multiple pipelines in the same right-of-way. Errors caused by long-line currents. Errors in monitoring local pipe conditions due to averaging large areas of pipe when making ground level pipe-to-soil potential measurements. In addition to the above studies which show conditions for which the off-potential measurements may not accurately represent the conditions locally on the pipe surface, a relatively common condition is the inability to interrupt all sources of the CP current on pipelines in congested areas or those structures with sacrificial anode cathodic protection directly bonded to the structure. Interpretation of pipe-to-soil potentials is made more difficult in areas of stray and/or telluric current activity. Therefore, there is a desire to utilize coupons as a more general tool for evaluating the level of CP. The areas targeted in this study are those areas for which the off-potential measurements are either difficult to perform or to interpret and for which a better means is needed for monitoring the CP level of the structure. Beginning in 1992, PRCI funded a program for the development, proof of concept, and evaluation of the coupon technology as applied to monitoring cathodic protection effectiveness on underground pipelines. The objectives for this project were: To provide a proof of concept for the use of coupons as a method to monitor the effectiveness of CP without interruption of the CP System. (Phase I). To establish guidelines for the use of coupons as a monitoring methodology for determining the level of protection on a pipeline. (Phase II). To provide a better understanding of the relationship between the coupon and the surface condition of the pipe. (Phase III). To quantify the technology such that it can be used as an alternative to conventional monitoring practices, specifically as a means of applying an acceptable criteria for CP. (Phase III). The scope of work included laboratory tests involving large soil boxes containing simulated coated-pipe segments with full size coupon test stations, finite element modeling of coupons near the pipe, full scale pipe tests at the Sugar Grove Test Facility and CC Technologies' Dublin, Ohio Pipe Test Facility, and test sites on operating pipelines throughout the United States.

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As pipeline coating and associated cathodic protection (CP) systems age, areas along the pipeline inevitably develop that fall below a prescribed CP criterion. In efforts to meet an adequate CP criterion, engineers often resort to supplementing their existing CP system with magnesium anodes at these "low" potential areas resulting in a "hybrid" cathodic protection system consisting of an impressed current CP system (ICCP) supplemented with magnesium (Mg) anodes. This often achieves the desired result i.e. the potential measured over the pipe becomes more negative. However, there remain several unanswered questions concerning the real benefits to the polarization level of the pipe and the overall effect on the impressed current cathodic protection system. The primary objective of this PRCI project was to develop a better understanding of the relationship between ICCP systems and Mg anodes installed as hot-spot protection. This understanding will assist CP engineers in the design and operation of effective, economic CP systems.

No abstract