This paper presents measurements on probes consisting of a salt bridge and steel coupons. The probe allows consideration of voltage drops in pipeline potential measurements. Potential distribution on polarized coupons, size of the coupons, and salt bridge backfills are also discussed.
The cathodic protection criteria for corrosion control of buried and submerged metal structures have been established by NAC!E International in RP0169-92 1 . All criteria require Consideration or elimination of voltage drops in the soil. To achieve voltage-drop-free measurements, different techniques have been developed and tested. This paper is focused on use of coupons for consideration of voltage drops in soil or voltage-drop-free measurements as one of the practical and promising measurement techniques.
USE OF COUPONS
Pairs of pre-weight coupons have been commonly installed in a pipeline trench to determine the effectiveness of the cathodic protection system. One coupon was permanently connected to the pipeline and the other was buried isolated as a control. After excavation, comparison of the coupon metal loss provided an indication of cathodic protection effectiveness. Voltage gradients in soils and water at the coupons simulating coating holidays have been investigated by several authors . Different techniques to determine or eliminate the voltage drops in soils have been tested in the field. Several authors discussed possibility of errors when the instant off technique is used. The authors determined that voltage spikes and equalizing (circulating) currents, caused by differences in polarization levels along the pipeline after the current interruption, could introduce errors in the potential measurements. Coupons have been used to evaluate pipeline potential measurements. Description and use of an integrated salt bridge and steel coupon assembly, and the advantages and limitations of this technique have been discussed in a previous paper.
