TN 1 How to measure tube sheet potentials

The potential across a tubesheet under cathodic  protection can show large variations from one location to another. Different waterboxes of the same apparent design can produce different potential distributions. Areas with excessively electronegative potentials can cause hydrogen damage to titanium or ferritic stainless steel tubes. Other areas may have potentials insufficiently negative to adequately protect the copper alloy tubesheet. These potential gradients cannot be detected by a reference electrode mounted on the side wall of the waterbox.

EDI’s Model TE Tubesheet Mounted Reference Electrode is designed to mount any place on the face of a tubesheet. They are shipped with a double tube plug which is inserted into the end of a condenser tube and tightened. Tube plugs are available for common tube sizes between 5/8 inch and 1-1/4 inch. The electrode’s lead wire terminates in a waterproof connector which plugs into a mating connector on the Model TW wire which has been affixed to the tube sheet face.  This attachment system allows the electrode to be easily removed during scheduled outages.  For additional information, visit EDI website.

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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 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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August Featured Product Model FH

Through-wall reference electrodes are used for measuring corrosion potential on the inside of condenser waterboxes, circulating pipes, tanks and vessels. These electrodes are installed by threading into a tapped hole on the wall; a junction box is typically attached to the other end to protect the wiring connections. Through-wall reference electrodes can be ordered with any commonly used sensing element.

The Heavy Duty Probe (Model FH) has a glass reinforced epoxy (G-10 GRE) extension tube and a 316L stainless steel nipple. It can be used at pressures up to 75 psi (0.5 MPa) and intermittent temperatures up to 210ºF (98ºC). Model FH is available in three size variations:  Model FH10 is threaded into a 1 inch NPT hole, Model FH7 is threaded into a 3/4 inch NPT hole, Model FH5 is threaded into a 1/2 inch NPT hole; all three variations have a 1 inch NPT thread on the termination side.

The temperature limits stated are those for the wetted materials of construction. Through-wall reference electrodes should generally not be continuously used at temperatures exceeding 110ºF (45ºC) because the reference potential will be significantly different from its value at ambient temperature and the electrode service life will be drastically shortened. The product will survive occasional brief temperature excursions up to the limits stated in the preceding paragraph. For applications involving continuous exposure to temperatures over 110ºF (45ºC), our Model FE Process Vessel Reference Electrode is recommended.

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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TN 8 Measurement Circuit IR Drop

The components used for making potential measurements and the equivalent electrical schematic are shown below.  A reference electrode located close to the structure is connected to the meter by a test lead.  A second lead wire connects the structure to the meter.  In this simple DC circuit, the driving voltage is the potential that exists between the reference electrode and the structure.  When a measurement is being made, current will flow through the circuit as a result of this potential. The magnitude of the current flow follows Ohm’s law, I = E/R.  The current is proportional to the driving voltage and inversely proportional to the sum of all resistances in the circuit.  For example, if the circuit potential is one volt and the sum of the resistances is ten mega-ohms (MW), a tenth of a micro-amp will flow through the measurement circuit.

Voltage drops occur across each of the resistive elements in the measurement circuit.  These voltage drops are separate and distinct from the more commonly discussed voltage drops, or IR drops, which are due to external current flowing through the electrolyte.  In the figures, the external current is shown as ie.  Both measurement circuit voltage drops and external voltage drops become incorporated into potential measurements causing errors.  Different methods must be employed to minimize errors caused by each type.  Download our paper  Effect of Measurement and Instrumentation Errors on Potential Readings from the Technical section of our website to learn more.

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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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