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 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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