Cathodic Protection Interactive Experiment/Demonstration.
Bench experiments that show that the 'half-cell' is NOT a reference electrode.
Copper in a saturated solution of copper-sulphate is a complete cell. The term 'half-cell' means half of the components that are required to constitute a cell.
The reaction potential of this cell is known when refered to a standard reference electrode in closed circuit, in a laboratory, under controlled conditions.
This demonstration shows conditions similar to those found in cathodic protection field work.
The conditions replicated cannot be controlled but the electrical equilibrium of the corrosion systems can.
We control the electrical equilibrium in field work using a transformer rectifier similar to that used in these experiments.
A model pipeline is constructed for this purpose.
The model pipeline is steel coated with electrical insulation tape.
A coating fault can be seen in the centre of the pipeline span.
The pipe is backfilled leaving the 'test post' exposed for connection.
A croc clip is attached to the pipe for convenience.
You can see that no part of the connection touches the electrolyte.
This represents a normal buried pipeline with a test post.
A number of 'half-cells have been made in a variety of shapes and sizes.
The first reading 0.647mv, is made with the meter set on the 2 volt range.
These measurements are between the pipe metal and the electrode. 0.642mv .. 5mv variation.
You will notice that this measurement is 0.646v ... 4mv variation from the last position.
This is because the 'ground potential' at the point of contact is different.
There is an 8mv variation across the span of these 4 electrodes.
These readings can be regarded as 'natural potentials' but are in reality voltages between the whole of the pipe metal and each of the electrodes.
In the next series of measurements the meter is connected between two electrodes and the voltages are due to the difference in potential of the points of contact with the 'ground'.
You will notice that the polarity of the reading on the meter is due to the way that each of its poles are connected. There is no absolute zero and no 'positive or 'negative'. All voltages are relative in cathodic protection measurements.
These pictures show the variety of connections resulting in different voltages.
It should be noted that the pipe is not connected to anything in this series.
These voltages are prely between the electrodes in their relative contact positions.
The voltages span 13 mv.
In DCVG 'off' mode there is a pretence that these measurements can be related to the corrosion activity at the interface between the anode and the electrolyte. This claim is unfounded.
It can be seen in this picture that the reading between the far two electrodes remains at -0.006v as it was in the first picture in this series.
The pipe is now connected to the transformer rectifier that is NOT switched on. It is not energised but it's presence in the circuit results in a change in the electrical equilibrium between each measured pair of the electrodes.
These next measurements are equivalent to those obtained in a DCVG survey in the 'off' mode.
You will notice that the voltages are not consistant but there is electrical 'flux' in the whole system.
The mathmatics of these measurements does not add up in the way that it does in the previous series that did not include the transformer rectifier in the measured system.
This could be due to the capacitance within the electronic devices or due to 'polarisation currents' in the corrosion events. This can be investigated using the osciloscope that can be seen attached to the fromt of the tray in these pictures.
This next picture shows a reading of 0.065v on the 2 volt range
In this next series the transformer/rectifier is energised at a setting of 3.5 volts. The meter is connected between the pipe metal and the electrodes which makes it similar to the 'on' voltages obtained during a Close Interval Potential Survey (CIPS)
It can be seen that the resistance of the ground to the cathodic protection current causes a potential profile to be detectable at the surface.
The effect of this current can be seen at electrode position 3 where the measurement is reduced from 3.21 volts to 2.46 volts
The next electrode is over good coating and therefore less current is flowing to the pipe with the result thet the potential difference between the ground above the pipe and the pipe metal is 3.11 volts.
Now we must look at how these electrodes compare with each other.
The meter is on the 200mv range and the readings are 2.6mv and 0.4mv
This is well within the range of half-cells that have been 'calibrated' by authorised NACE approved laboratories.
Such Laboratories do not 'calibrate' half-cells as they cannot be calibrated.... they can be validated by comparison to a standard hydrogen cell but this is totally unnecessary for cathodic protection field work.
This experiment shows that there can be potential differences between two copper/copper-sulphate electrodes of up to 3.2 volts which would represent a 8,000% error if you are trying to justify the 'pipe-to-soil' potential measurements in volts.
There is therefore no way that the conventional use of the copper/copper-suphate ground contact electrode can be regarded as a reference potential in cathodic protection field work and this is why I cannot have confidence in the many NACE publications and scientific papers that refer to this assumption.
I am continuing to offer my time in discussion of this subject with NACE and Institute professionals but they either refuse to see the demonstrations or immediately agree with my conclusions and refuse to comment in writing.
This demonstration raises doubts about the credibility of advice given by NACE and the Institutions relating to 'attenuation' curves, pipe-to-soil potentials, and CIPS surveys.