Review: This paper is a collection of opinions about the use of coupons as a tool for monitoring cathodic protection systems. The opinions expressed in this paper are a good summary of the findings and conclusions of several published studies on the subject. However, Didas does not reference these studies.
- The following three advantages of using coupons are very important:
- IR free potentials may be obtained without interrupting multiple CP power sources.
- IR free potentials may be obtained on buried structures with direct connected galvanic anodes.
- Depolarization testing may be performed without de-energizing the CP system.
Only one disadvantage is listed:
- The major disadvantage of coupons is the possible discrepancy between the true structure potentials and the coupon potentials.
Good engineering practice demands that a number of issues be considered when implementing the use of coupons to monitor cathodic protection systems. Didas briefly discusses seven such issues:
- number of coupons
- coating conductance
- coupon material
- method of construction
- surface preparation
- the voltmeter used for the potentials
- coupon placement in relation to any existing anodes
The design considerations listed in this paper are:
- Didas calls for a standard size to duplicate "typical" coating holidays and recommends 100 sq.cm.
- Didas recommends the use of older or aged steel of a grade similar to the structure's steel.
- Didas recommends that coupons manufactured from new steel remain disconnected from the structure and permitted to corrode freely for a period of one or two months.
- Didas recommends that the coupon be placed in backfill material similar to the material surrounding the structure and compacted to a similar density.
- Didas recommends that the coupon be installed at the same depth as the structure and about .33 meters (1 foot) from the structure.
- Didas recommends that the soil access tube be backfilled with well compacted native soil.