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 12 Splicing Ref Wires
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.
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.