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.
When ions of noble metals such as copper come into contact with more active metals such as steel or aluminum, the noble metal will spontaneously plate out on the active metal surface. The active metal is oxidized and the noble metal is reduced in accordance with the following chemical reaction:
Cu ++ + Fe (s) → Cu (s) + Fe++
This process is quite useful in the mining industry where it is known as cementation. It was first used in China a thousand years ago to extract copper from mine water1. It is still used in the copper mining industry today where copper is leached from low grade ores and the solution is then trickled over scrap iron to recover the copper. The same process is also used by high school science teachers to dazzle students by dipping a steel nail into a copper sulfate solution where copper will plate out on all wetted surfaces of the nail. The process happens quickly enough to hold the student’s attention.
There is a less useful side to the cementation process. When water passes over a copper surface, it will pick up enough copper so that when it subsequently passes over aluminum (or other active metal) surface, copper will plate on the active metal surface. This can occur even when copper concentration is in the parts per million range. A galvanic cell is formed which leads to pitting corrosion of the active metal. This process is sometimes referred to as deposition corrosion.
Copper/copper sulfate reference electrodes will leach very minute amounts of copper and sulfate ions 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 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 surface, some of the copper ions will deposit on the steel. This creates a local galvanic cell which alters the corrosion behavior of the steel.
1 The history of copper cementation on iron – The world’s first hydrometallurgical process from medieval china. T. N. Lung; Hydrometallurgy, Vol. 17, No. 1; Nov. 1986, P 113 – 129.
Copper/copper sulfate elements are typically used in environments with less than 500 ppm chloride or other halides. They consist of high purity metallic copper immersed in a saturated copper sulfate (CuSO4) solution. Cu/CuSO4 elements can be used in portable, immersion or underground units. Reference electrodes intended for long term service will contain a gelling agent and do not require any periodic maintenance. Portable Cu/CuSO4 electrodes which contain a liquid rather than a gelled electrolyte will require periodic cleaning and electrolyte renewal. Use of Cu/CuSO4 elements in electrolytes with higher halide levels or in electrolytes with any sulfides present will contaminate the element causing its reference potential to drift. Because of their widespread use, Cu/CuSO4 electrodes are the ones upon which many cathodic protection criteria are based.