Gummow, R. A.,"Using Coupons and Probes to Determine Cathodic Protection Levels", Materials Performance, August 1998.
Review: The two papers are essentially the same. They are carefully crafted papers containing a comprehensive review of relevant background material. Extensive citations document virtually all statements in the paper.
In the Materials Performance article, Gummow expresses the following opinion about cost effectiveness.
"The savings realized by not having to add more CP current could, in itself, pay for the cost of retrofitting CP coupons."Gummow gives the following clear explanation of accounting for 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."
Gummow lists the following attempts to account for IR drop.
- Subtraction from the voltmeter reading an amount based on the reference placed at remote earth,
- extrapolation of potential measurements taken with the reference placed at increasing distance from the pipe,
- extrapolation using a stepwise current reduction technique,
- moving the reference close to the interface where possible,
- interruption of the current with the measurement of an 'instant-off' potential.
Gummow points out that in some instances interruption of the current is impractical or even impossible. He further points out that, because most of the IR drop occurs very near a coating holiday, it is impractical to place the reference electrode close enough to a holiday to remove most of the IR drop.
His visualization lends credence to the use of coupons.
"However, if the holiday could be disconnected from the pipe, the reference electrode placement is not as critical."
The following are helpful quotations from Gummow's paper. Each pertains to the use of coupons to monitor cathodic protection.
- If a coupon is to simulate a holiday in a coated pipe then it would seem reasonable that the geometrical size and shape of the coupon must be analogous to a holiday.
- Since a coupon is a simulation for a coating defect, then the coupon should be placed in an environment that closely approximates the environment at the pipe surface otherwise the polarization characteristics of the coupon could be significantly different from the steel at a coating defect.
- If the structure being simulated by a cathodic protection coupon operates at a temperature higher than ambient soil conditions, then the coupon should be exposed to similar temperature conditions.
- Moghissi et al (Paper 1 in this report) … have shown … that with time larger coupons tended to polarize cathodically toward the potential of the smaller defects.
- 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.
- It follows therefore that the geometrical shape of the coupon must suit the reason for its intended use and if that use is to be a proxy for a coating holiday then the coupon should e sized to emulate a relatively large holiday.
- Greenwood concluded that "buried coupons provide the best method for accurate measurement of pipe/soil potentials on pipelines affected be stray current."
- Pipeline operators are finding that many pipelines having polarized potentials more positive than –850mV will exhibit more than 100 mV of cathodic polarization, thus eliminating the need for an up-grade in the cathodic protection system.
- Therefore when a coupon fails to meet the –850 mV criteria, the coupon can simply be disconnected from the pipe and the decaying polarized potential recorded over a period of time.