EXPERIMENT HOW TO UNDERSTAND VOLTAGE OF CATHODIC PROTECTION SYSTEM

It is possible to make a model of electrical components to demonstrate further complications involved in DC electrical fields of this nature.

Two chains of randomly selected resistors are soldered together, linking two different potentials such as the poles of a dry cell battery.   The probes of the voltmeter can be placed in any position along either of the chains and a different voltage will be displayed.

 

The number of variations in voltage between the two nodes is equal to the square of the number of resistors.

If a grid of resistors is constructed with random values, and the probes of the voltmeter are placed at various locations in the grid, it would require a computer to predict the results.

Put a third dimension onto the grid, and a very powerful computer would have to be fed with every resistance value in order to predict the voltage resulting from the probes placed at surface nodes.

Two or more resistances of unspecified value included in the system would make the result impossible to predict by any computer.   (However it is true that given the same source voltage, the voltage between any two nodes would be constant.)

FIELD EXPERIMENTAL WORK

It is possible to apply this principle of an 'electrical picture' to field work by a simple experiment.

Choose a buried, insulated, cathodically protected pipeline running through areas of soil of different resistances.   Place a metal coupon in high resistance soil and another in low resistance soil, both connected to the pipeline.   Using two copper/copper-sulphate electrodes as probes, connected to the poles of a high resistance voltmeter

plot the potentials surrounding the coupons.   In the case of the high resistance soil, the 'potential gradient' will spread much further than that from the coupon in the low resistance soil.

The area of influence of the coupon can vary from a few millimeters in low resistance soils, to many meters in extremely high resistant circumstances such as fresh water, sand and gravel.  As a result, it is sometimes not possible to detect a corrosion cell or even a coating fault at ground level, if the resistance of the soil is too low, and the amount of current is small.

The effect of this can be seen in a drawing showing a single corrosion cell, where the back-fill is homogenous.

It is clear that the exact distance between each probe and the actual interface between the metal and the electrolyte is crucial to any calculation of the interface potential.

The most basic measurement in cathodic protection work is described in Procedure 1, and is known as the 'pipe to soil potential'.

The vast majority of existing cathodic protection records are composed of these readings which are in fact voltages.

A direct metalic connection is made to the steel of the pipeline  and the other pole of the voltmeter is connected to a copper rod suspended in a solution of copper sulphate in a container with a porous base.  This base is placed in contact with the ground and the resulting voltage is recorded.

It has been seen from the sand tray experiment and from the experiment with the steel coupon in contact with the pipeline, that the actual potential of the ground can vary.  When carrying out Procedure 1, it is often possible to obtain significantly differing readings by altering the position of the electrode within the radius of the connecting lead.  Errors as high as  30% or more are common for this reason and it can be imagined the voltages obtained by a left handed engineer would differ from those of a right handed engineer, under certain circumstances.

One example of where this was known to have happenned was at a pipeline cathodic protection post where the copper connecting lead to the pipeline, had faulty insulation just below ground level.  This would cause current to flow into the copper for two reasons.  Cathodic protection current would follow this path being the least line of resistance, and the less noble steel of the pipeline would tend to discharge current to the copper as the cathode of a bi-metalic corrosion cell.

Current flowing into the copper caused a considerable voltage gradient in the soil for about 1/2 meter, and this caused the meausred voltage to drop from -.950 volts, which was considered to be protected, to -.750 volts which was considered not to be protected.

Another example occured during an 'over the line close interval potential survey' where a depression in the ground voltage was caused by a galvanised fitting to a steel 'well-point tube' which had been abandonned during the construction of the pipeline through waterlogged soil.  In this case there was no connection between the pipeline and the scrap, and certainly no influence on the corrosion control of the pipeline, and yet the survey results showed the the pipeline was unprotected at this exact spot and adequately protected one meter distant.

It is crucial that cathodic protection engineers and technicians understand the nature of DC electricity and the significance of these voltage measurements.

It is no good understanding   thermo-dynamic theory and complex formulae if their components are grossly inaccurate.   Many measurements made in laboratory conditions cannot be made in field practice and it follows that an immeasurable criteria is about as useful as an elastic tape measure.