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.

TN 7 Meaning of Design Life of Reference Electrodes

Field grade reference electrodes contain a saturated salt solution in a gypsum-bentonite gel.  CuSO4 is the salt in copper/copper sulphate electrodes; KCl is the salt in silver/silver chloride electrodes.   The accuracy of a reference electrode depends upon this salt solution remaining saturated.  During use, salt will diffuse out from the reference electrode which can affect the concentration in the gel.  The design life of a reference electrode is an estimate of the time based on testing it would take for enough salt to diffuse out from the inner core to lower the salt concentration to below saturation.  At EDI, we use several techniques to extend this time as much as possible.  One of these techniques is to increase the amount of salt reserve contained in the gel.  This is one reason why longer life electrodes have physically bigger housings.  Download our paper Factors Affecting the Accuracy of Reference Electrodes from the Technical section of our website to learn more.

TN 3 Element Selection – Gelled Silver/Silver Chloride

Gelled silver/silver chloride elements are most often used in environments with more than 500 ppm chloride or other halides although they can also be used in chloride free environments.  They consist of 99.99% pure silver coated with silver chloride and immersed in a saturated potassium chloride solution. Reference electrodes intended for long term service will contain a gelling agent and do not require any periodic maintenance.  Portable Ag/AgCl electrodes which contain a liquid rather than a gelled electrolyte are limited to laboratory use.

Silver/silver chloride elements can be used in portable, immersion or underground units.  Use of these elements in electrolytes with other halides (iodides or bromides) or in electrolytes with any sulfides present will contaminate the element causing its reference potential to drift.  The reference potential of Ag/AgCl/sat. KCl elements is 105 mV negative to that of a saturated Cu/CuSO4 reference electrode.  Use of sodium chloride rather than potassium chloride electrolytes can cause a junction potential error.