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 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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July Featured Product Model AT

The Model AT Linear Modular Anode System is an impressed current linear anode system which permits maximum design flexibility. It consists of a copper cored titanium flat wire coated with mixed metal oxide and attached to a #14 AWG HMW/PE insulated bus wire. The wire pair is surrounded by a flexible plastic mesh. Individual anodes are 50 ft. long and are easily connected in the field to form a hermetically sealed joint. This flexibility allows the design engineer to choose a layout which is best suited for the application.

Perhaps the most significant feature of the Model AT is its unique connector system. Each anode is supplied with a pin connector on one end and a socket connector on the opposite end which can be mated either to another anode section to form a string or to a power feed cable. Specially developed “tee” connectors allow intermediate current feeds on long strings. No splicing or welding is required in the field so installation can be completed in substantially less time than any other system. For example, it takes less than 6 man-hr. to install this anode system in a 60 ft diameter tank.

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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 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.

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TN 6 Importance of Backfill in Underground References

The backfill surrounding a bagged underground reference electrode is a mix of gypsum and bentonite. The primary purpose of backfill is to retain water which ensures that a low contact resistance between the electrode and the surrounding earth is maintained.  Additionally, backfill usually prevents the inner core of the electrode which contains a saturated salt gel from drying out.  However, during severely dry conditions, the electrode may still dry out.  The backfill will eventually rewet with local groundwater, and the electrode should re-activate.  However, local ground water will have many other chemicals dissolved in it that can affect the accuracy of the electrode.  If this situation is suspected, the electrode should be calibrated against a reference electrode of known accuracy to determine whether replacement is necessary.

When installing underground reference electrodes in areas known to have extreme seasonal dry periods, a good practice is to place additional gypsum-bentonite backfill around the reference bag.  This backfill is commonly known as driller’s mud.  This extra backfill will hold additional water around the reference electrode and extend the time before it dries out.

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TN 5 Element Selection Zinc

Zinc elements consist of high purity metallic zinc rod.  When these elements are used in our underground reference electrodes, the zinc element is encased in a gypsum-bentonite backfill. Reference potential of a zinc element encapsulated in backfill is about 1,100 mV negative to that of a saturated Cu/CuSO4 reference electrode.  The presence of halides in the environment will not affect the reference potential of an encapsulated zinc electrode.

In through-wall and immersion reference electrodes, the zinc element is directly wetted by the electrolyte.  The reference potential of zinc directly exposed to an electrolyte depends on the composition of the electrolyte.  The potential of zinc is also affected by temperature and can approach that of steel at around 60°C.  Bare zinc electrodes will perform best when their use is limited to clean full-strength seawater at ambient temperature.

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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.

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TN 2 Element Selection – Copper/Copper Sulfate

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.

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