Section I: Summary of All Papers Reviewed

There is significant agreement among the published papers related to the use of coupons to monitor cathodic protection systems. This in itself is not remarkable because there are relatively few papers and several of those are summaries of other papers.

IR Drop

"The polarity of IR drop when the current direction is toward the structure, as is the case with a cathodic protection current, is additive thereby making the measured potential numerically greater than the polarized potential. This makes it appear that the structure is better protected than it really is. Conversely, when the current direction is away from the structure, as might be the case with a stray current, the polarity of the IR drop subtracts from the polarized potential and makes the structure appear more poorly protected than it is. Accordingly, when the IR drop component is significant, as would be the case for large current or high soil resistivity, then substantial measurement error arises unless corrective action is taken." [GUM]

Historical Perspective of the Use of Coupons

The earliest reference to the use of coupons to monitor cathodic protection systems appears to be a brief mention in the famous 1967 book by Peabody. [PEA]

The often cited GERG study indicates that coupons have been used to monitor cathodic protection systems in Europe since 1960. Most of these coupons were installed since about 1975. [GER]

As early as 1979 researchers at Tokyo Gas built on holiday simulation work by J. Vrable in 1967 to conclude that coupons permit accurate measurement of IR free pipe-to-soil potential and current density in the presence of stray current. [KAS]

As early as 1986, R. Greenwood recommended to British Gas that buried coupons should be used for routine measurement of pipe-to-soil potentials on pipelines affected by stray currents. He also suggests that the use of coupons in this manner was common in 1986. [GRE]

In North America, the largest application of coupons to monitor cathodic protection systems is the on-going test on the Trans Alaska Pipeline. [MOG][STE][STE2]

The most extensive published study of the use of coupons to monitor cathodic protection systems is the on-going AGA sponsored study being conducted by CC Technologies. [THO1][THO2][THO3]

In 1994 B. A. Martin reported on the use of coupons to monitor the impressed current cathodic protection system at river crossings on a pipeline (in Papua, New Guinea) exposed to both Telluric currents and changing resistivity of the surrounding electrolyte. [MAR]

Incentive for Using Coupons

"The increased interest in buried coupons for cathodic protection monitoring purposes seems to have been stimulated by the 1992 revision of the criteria in NACE Standard RP 0169-92 which emphasized consideration of IR drop for valid compliance with the 850 mV potential criteria. [GUM]

CP coupons are installed to:

Theory to Justify the Use of Coupons

Moghissi et al [MOG] have shown that with time larger coupons tended to polarize cathodically toward the potential of the smaller defects. [GUM]

Barlo and Fessler demonstrated that the 'instant OFF' potential measured on a coated pipe having numerous holidays of different surface areas approximate the true potential of the largest holiday and that the largest holiday has the least negative potential.[GUM]

General Conclusions about the Use of Coupons

"Buried coupons provide the best method available for accurate measurement of pipe/soil potentials on pipelines affected by stray current." [GRE]

"When it is impractical to correct for voltage drops in pipeline potential measurements by disconnecting all current sources, CP coupons may be used to insure that adequate cathodic protection has been achieved."[MOG]

"CP coupons assess the adequacy of the cathodic protection system at the coupon monitoring station location and may need to be used in conjunction with other monitoring tools to insure pipeline integrity." [STE]

"It is theorized that the polarized potential of the coupon simulates the polarization of a holiday of similar size on the pipe." [THO4]

Advantages of Using Coupons

"IR free potentials may be obtained without interrupting multiple CP power sources." [DID]

"IR free potentials may be obtained on buried structures with direct connected galvanic anodes." [DID]

"Depolarization testing may be performed without de-energizing the CP system." [DID]

Sound Engineering Practices for the Use of Coupons

"Size coupons to represent a large coating defect."[MOG]

"Place the coupon in the same environment as the pipeline."[MOG]

"Avoid preferential distribution of current to the CP coupon." [MOG]

"Install a sufficient number of coupons to adequately monitor the cathodic protection system." [MOG]

Use older or aged steel of a grade similar to the structure's steel. [DID]

Coupons manufactured from new steel should remain disconnected from the structure and permitted to corrode freely for a period of one or two months.[DID]

Install the coupon at the same depth as the structure and about .33 meters (1 foot) from the structure. [DID]

The coupon material, mill scale, polished steel all determine the coupon's potential and could cause distortion in the readings. [DID]

Coupons must be placed outside the gradient of any existing anodes. [DID]

Unresolved Issues

Determining the coating conductance becomes very important when implementing a coupon program. [DID]

"The major disadvantage of coupons is the possible discrepancy between the true structure potentials and the coupon potentials." [DID]

"The polarized potential of the coupon will not necessarily duplicate the structure polarized potential at the same location. This is due to the coupon representing a holiday of similar size on the pipeline and not actually being located at the holiday location. The structure potential would be representative of the nearest or most influencing holiday, while the coupon potential would be at the coupon, therefore when comparing potentials taken at the same location, there should be some difference." [DID2]

A small percentage of the sites being studied by CC Technologies exhibit discrepancies between coupon potentials and pipe potentials. It is conjectured that long-line currents are responsible. They suggest that due to long-line currents the polarized potential of the pipe is more positive than the off-potential measured. The coupon off-potential is unaffected by the long-line current. [THO]

There is no standard with respect to coupon size, geometry, or orientation. There is no standard with respect to criteria or interpretation of data collected from the coupons. [GER]