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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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.