TN 21 Maintaining Liquid Filled References

Portable, liquid filled, reference electrodes must be cleaned and refilled regularly.  The copper sulfate solution contains dissolved oxygen.  Oxygen gradually reacts with the copper to form copper oxide which shifts the potential of the reference.  The more copper oxide the greater the shift.  Potential shifts of up to 10 mV in a week’s time are possible.  This problem can be prevented by cleaning and refilling the reference electrode regularly (preferably weekly but at least monthly).  The newer gelled filled portable reference electrodes do not have this problem since the element and gel have minimal contact with the atmosphere.

Another problem occurs on reference electrodes that have a ceramic tip.  The insulating tip is porous so that the copper sulfate solution will leak through and allow conductance.  If the tip dries out, the holes can become partially plugged with copper sulfate salt which increases the electrical resistance through the reference.  The process is progressive, with the resistance increasing a bit more with each dry-down until, eventually, the tip becomes fully insulating (totally resistive).  Boiling the ceramic tip in distilled water for an hour or two will restore it.  A similar problem to this occurs when the ceramic tip becomes plugged with either dirt or oil.  When this happens, the ceramic tip should be replaced.

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TN 20 Potential Errors in Concrete

A common source of error encountered when making potential measurements in or through concrete is junction potential; this topic has been discussed in Technical Note 19.  There are additional sources of error in concrete potential measurements.  The mobility of all ions in concrete is retarded due to the material’s cellular microstructure.  Consequently, concrete has high electrolyte resistivity so the presence of any internal current flowing through it will be marked by relatively high IR drops.  Sources of these internal currents are often corrosion currents from corrosion of rebars.  The associated IR drops become incorporated into corrosion potential measurements and can result in a several hundred millivolt error.

Errors of the same magnitude have been documented for measurements made through concrete, such as to a structure buried beneath a concrete slab.  In a detailed study on this topic1, potential measurements were made on buried tanks at nine different service stations in the northeast.  When the measurement was made with the reference electrode contacting the concrete slab, the potential was from 20 to 260 mV more negative than when the reference electrode was directly contacting the soil through an access hole.  The rectifier was off during these measurements to eliminate CP currents as a possible error source.  In the same study, potential measurements were made on a pipe located beneath an airport runway.  Measurements made through the concrete near the edge of the runway were about 200 mV more negative than the same measurement made through the grass adjacent to the slab.  Wetting both the concrete and the grass did not significantly change the measured values.  Clearly, measurements of buried structures should never be made through concrete without using a soil contact access port.

  1. B. Husock, “Techniques for Cathodic Protection Testing Over Airfield Pavements,” US Air Force Report CEEDO-TR-78-31, Tyndall AFB, FL, July 1978.

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TN 19 Junction Potentials

Junction potentials are a common error source encountered when making potential measurements in or through concrete.  Current is carried through an electrolyte by means of ions which physically move through the electrolyte.  In a potential field, anions move in one direction and cations in the opposite direction.  If the mobilities of the ions are unequal, a balancing potential builds up due to separation of the charges.  This potential, termed a junction potential, becomes incorporated into the measurement as an error.  In concrete, it is quite common to have areas of different electrolyte compositions.  For example, sodium chloride (NaCl) is often spread on the surface for deicing; sodium and chloride ions have very different ionic mobilities.  Another example is carbonation of concrete, the reaction of the material with atmospheric carbon dioxide, which proceeds inward from an exposed surface and causes a change in both the chemical composition and pH of concrete.  Each of these can contribute to a junction potential error in concrete measurements.

A junction potential can also form within a silver-silver chloride reference electrode if sodium chloride is used for the filling solution.  The different ionic mobilities will cause the potential to build up across the membrane or frit separating the filling solution from the external environment.   Potassium chloride should be used for the filling solution for silver-silver chloride reference electrodes since the mobility of potassium and chloride ions is similar thus minimizing any junction potential across the membrane.

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TN 18 How Fast is Instant Off

The instant off method of removing external IR drop in potential measurements involves interrupting the cathodic protection current.  This action produces an instantaneous voltage drop which is considered to be the external IR drop.  The potential measured immediately after this instantaneous drop is considered to be the “IR drop free” potential of the structure.  Clearly, this method only works with an impressed current cathodic protection system where all the rectifiers on that system can be interrupted simultaneously and there are no other sources of current flowing through the electrolyte.

An issue which should be considered when using current interruption for instant-off measurements is:   What is meant by instantaneous?  The answer is not simple since it depends upon the structure, the electrolyte and the method of interrupting the current.  Putting the answer in electrical terms, it depends upon the capacitance and the inductance of the circuit.  IR drop free measurements can be made microseconds after current interruption on small uncoated specimens in a low resistance electrolyte.  Large coated structures, such as pipelines, or high resistance electrolytes, such as concrete, usually require several hundred milliseconds or more for IR-drop free measurements.  Interrupting current on the AC side rather than on the DC side of the rectifier will increase the time delay because the circuit inductance is higher.

For situations where current interruption cannot be reliably used to minimize external IR drop error in potential measurements, cathodic protection coupons are frequently used.  These are small pieces of metal similar to the structure which are electrically bonded to the structure through a switch.  Measurements are made as above except that the coupon rather than the rectifier is momentarily disconnected.  These measurements are termed instant disconnect measurements in order to distinguish them from instant off current interruption measurements.

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TN 17 Using Reference Electrodes in Oil-Water Mixtures

Making potential measurements in an oil water mixture can be very difficult.  The surface energy of oils is much lower than that of water. When they are mixed, the two liquids will separate into distinct phases rather than dissolving into each other. While the addition of surfactants can overcome this somewhat, their use would defeat the purpose of oil-water separators where these mixes are encountered in industry. The lower surface energy of oil will make it preferentially wet any solid surfaces in contact with an oil-water mix. Since relative wettability is a property of the liquids rather than the solids, there are no materials which will preferably be wet by water rather than oil.

When installing a cathodic protection system in the water zone of oil water separators, oil can coat the membrane of the reference electrode during the initial filling of the vessel.  The oil film increases the resistance of the measurement circuit making measurements difficult. A work-around which can be used is to coat the membrane end of the reference electrode with clay prior to installing it. As the vessel is refilled and the oil phase rises up past the reference electrode, it will coat the clay on the end.  Once the vessel is filled so the reference electrode is in the water phase, turbulence will remove enough clay so that measurements are possible.

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TN 16 Interrupting CP Coupon Current

The most common method to reduce voltage drop error in potential measurements is to use a CP coupon.  These 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, voltage drop error in the measurement.

Voltage drop error can be further reduced by interrupting CP current flowing to the coupon and measuring its potential immediately before it depolarizes.  This measurement is often referred to as instant-disconnect potential.  The potential between the reference and coupon is measured as the connection between the coupon and structure is disconnected.  This task is much easier if the connection is made through an EDI Model SM Magnetic Switch.  This is a sealed reed switch for use in above and below ground test stations. The switch is activated by holding a magnet next to the color band.  Green bands denote normally closed switches which are momentarily opened with the magnet. These are most often used for instant-disconnect cathodic protection coupon measurements.  Red bands denote normally open switches which are momentarily closed with the magnet. These can be used to electrically isolate a reference electrode in test stations which may become submerged.  For best results, the use of model SM-MAG magnets is recommended.

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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.

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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.

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