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.

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.

TN 11 Potentiometric Voltmeters

Most general purpose digital meters used today for corrosion measurements have 10 mega-ohm input resistance.  While this may seem high, it may not be adequate in some circumstances.  Consider a structure with a 900mV potential and a circuit resistance external to the meter of 10 kilo-ohms.  Ten kilo-ohms is 0.1% of 10 mega-ohms.  Thus 0.1% of the voltage will be dropped in the circuit external to the meter and 99.9% will be measured by the meter.  Now let’s consider the case where the structure is in concrete, rock or dry soil.  The external resistance in this situation could be 1 mega-ohm or higher.   The total circuit resistance would now be 11 mega-ohm with 90% being in the meter and 10% external to the meter.  The voltage drop in this case would be similarly divided with 810mV across the meter and 90mV external.  Measurement circuit IR drop errors always result in a lower apparent potential reading.  This can result in unnecessary and costly up-grades and/or replacements of CP systems.

A preferred way of making potential measurements in high resistance circuits is with a potentiometric-voltmeter.  This type of meter was the standard field meter for corrosion personal up until the mid-1970s.  An internal battery in this meter applies a voltage with opposite polarity to that being measured. The applied voltage is adjusted to exactly balance the potential being measured.  The applied battery voltage is then read.  With no current flowing through the circuit, there is no measurement circuit IR drop.  Potentiometric-voltmeters may cost significantly more than a general purpose digital meter so it is easy to understand why they are not as widely used.  It is possible to convert an ordinary voltmeter to a potentiometric-voltmeter with the addition of a simple inexpensive converter circuit.  Two such approaches are shown above.  The circuit using a single meter requires that the potential read on the meter be reversed to give the actual potential.  When using the circuit with two meters, the potential can be read directly.  These are basic conceptual circuits; it may be necessary to adjust the values of some components to suit particular circumstances.

TN 10 Data Logger and RMU Errors

Special precautions should be taken when using data loggers and remote monitoring units (RMUs) with reference electrodes.  The amount of current flowing through a reference electrode is inversely proportional to the circuit impedance.  This current causes the reference to polarize, i.e. shift potential, and the amount of shift is proportional to the amount of current flowing.  If the shift is small, or of short duration, the reference will usually recover.  When the shift is large and/or of long duration, the reference may be permanently damaged.

Most modern potential measuring devices have an input impedance of at least 10 MΩ in order to minimize IR drop error due to current flowing through the circuit.  These high input impedances are used during the measurement cycle to minimize errors from voltage drops in the measuring circuit while the measurement is actually being made.  When the unit is in stand-by or off mode, the impedance may be significantly lower (sometimes only a couple thousand ohms) depending upon the individual components used and the overall circuitry.  A drop in impedance during stand-by or off mode will not affected many transducers but a reference electrode will polarize if this occurs.  If a data logger or RMU is to be used with reference electrodes, the input impedance must be a minimum of 10 MΩ at all times including during active measurements, standing by between measurements, shut down with power connected or shut down with power disconnected.

Testing for changes in input impedance of a data logger or RMU as it goes through its cycles is relatively simple.  A 1½ volt battery is connected to the input terminals through a 10 MΩ resistor (see figure).  The voltage across this resistor is monitored with a portable voltmeter as the unit is put through its cycles.  This voltage should be about 0.15 volts for units claiming 100 MΩ input impedance during measurements.  If the measured voltage increases to greater than ¾ volt during standby or off cycles, then the unit is capable of damaging a reference electrode connected to it and should not be used.  Many RMUs have multiple independent input channels.  Each channel must be separately tested through all operating modes to be assured that all of the channels are suitable for reference electrodes.


TN 9 Effect of Meter Impedance

Some meters designed for corrosion potential measurements have selectable input impedance with the highest value typically being 250 MW.  However, general-purpose digital meters with a fixed 10 MW input impedance are frequently used for corrosion measurements.  While this may seem high, it is not adequate in many situations.  Electrolyte resistance is highly variable ranging from quite low for potential measurements in seawater to very high for potential measurements in dry soils.  The proper strategy is to select a meter whose internal resistance (input impedance) is several orders of magnitude higher than any other resistance in the circuit so that voltage drop across the meter will, for practical purposes, represent the entire voltage drop in the circuit.

For example, consider a structure with a 900mV potential and a circuit resistance external to the meter of 1 MW which is typical in concrete, rock or dry soil.  The total circuit resistance when using a 10 MW meter would be about 11 MW with 90% being in the meter and 10% external to the meter.  The voltage drop in this case would be similarly divided with 810mV across the meter and 90mV external to it.  If a 100 MW meter were used, the circuit resistance would be 101 MW so that almost all of the voltage drop would occur in the meter.  Measurement circuit IR drop errors result in a more positive apparent potential reading.

When using a meter with selectable input impedance, successive readings can be made, each time increasing the input impedance.  When two successive readings are the same, the measurement can be presumed to be free of measurement circuit IR drop error.  If it is not possible to obtain two successive readings that are the same, there are two methods that may be used to eliminate this error.  A potentiometric-voltmeter will eliminate these potential measurements errors.   Alternatively, a correction factor can be calculated from measurements made with a meter that has variable input resistance.  Download our paper  Effect of Measurement and Instrumentation Errors on Potential Readings from the Technical section of our website to learn more.

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.


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.