TN 15 How Concentric Coupons Work

A measured potential is the sum of the voltage drops occurring in the measurement circuit and those occurring in the electrolyte.  Most of the individual measurement circuit voltage drops are negligible except for the one at the structure electrolyte interface which is the potential of interest.  Other components of the measurement circuit voltage drop are discussed further in EDI Technical Note TN8 Measurement Circuit IR Drop.

Voltage drops occurring in the electrolyte represent an error in the measurement.  These voltage drops are due to external current flowing through the electrolyte.  The current can be the structure’s own CP current as well as telluric currents, foreign structure CP systems or mass transit systems.  Eliminating the voltage drop error from the structure’s own CP system can be done by interrupting that current.  Other stray currents are not easily interrupted so different methods are used to eliminate their error.

The most common method is CP coupons which are small pieces of metal electrically bonded to the structure so they come to the same potential as the structure.  They are placed within a few centimeters of a reference electrode.  When the coupon potential is measured, the short distance between the reference and the coupon reduces, but does not eliminate, the voltage drop error in the measurement.  In a concentric CP coupon, the sensing port is located in the center of the coupon which reduces the electrolyte path to about a millimeter.  This extremely short distance virtually eliminates electrolyte voltage drop error.

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TN 14 Use of Zinc Electrodes with Concentric CP Coupons

Cathodic protection (CP) coupons are most effective when the coupon is placed within a couple centimeters of the reference electrode membrane.  This reduces the length of the electrolyte path thus reducing the amount of voltage drop error incorporated in the potential measurement.  Concentric CP coupons are a special type of CP coupon in which the reference electrode sensing port is located in the center of the CP coupon.  This reduces the electrolyte path length to about a millimeter which, for all practical purposes, eliminates voltage drop error in the measurement.

All reference electrodes allow ions to diffuse through the membrane.  It is the diffusion of these ions which allows the measurement circuit current to pass through the membrane.  The amount of material being leached from the electrode is extremely small and it will rapidly diffuse into the surrounding environment.  However, when the reference electrode membrane is located within a couple millimeters of a steel coupon surface, the ions do not move away quickly enough which can alter the corrosion behavior of the steel coupon.

There are three types of reference electrodes commonly used for cathodic protection measurements:  copper/copper sulfate, silver/silver chloride and zinc/zinc sulfate.  Any of these electrodes can be used with CP coupons where there is a couple centimeter gap between the electrode sensing port and the coupon surface.  The only type of reference which can be successfully used with concentric CP coupons is the zinc/zinc sulfate reference as nothing leaching from it will affect the steel corrosion behavior.   Chloride ions leaching from silver/silver chloride reference electrodes changes the type of corrosion product formed on steel and hence the potential.  Copper ions leaching from a copper/copper sulfate reference electrode will spontaneously plate out on the steel surface creating a strong galvanic cell which alters the potential.  This phenomenon, known as cementation, is further discussed in our Technical Note TN 13 Copper Deposition on Steel.