Measuring the effects (CATHODIC PROTECTION)

Measuring the effects

Cathodic protection has been extremely cost effective since first used, but there have been instances throughout its history, where corrosion failures have occurred in spite of it's use.
The problem is to measure the effectiveness of cathodic protection.   Corrosion is electro-chemical and this suggests that electrical metering can be used for short term monitoring.

The simplicity of the circuit of a single corrosion cellwould tend to suggest that there is a simple means available to make the required measurement.   Standard reference electrodes have a recognised and known potential which can be used as an electrical datum point against which to measure other potentials, in a laboratory.

We normally measure VOLTAGES which are the differences between two potentials.
This causes confusion because the readings are commonly called "potentials", where in fact, either of the two potentials can be regarded as zero and the other will be either higher or lower.   The meter will show positive or negative values according to the polarity of the connecting conductors.

Cathodic protection theory dictates that the metal must be reduced to below its corrosion potential IN RELATION TO A STANDARD REFERENCE POTENTIAL.   These potentials can be measured in a laboratory where it is possible to control all elements of the circuit, but it has proved impossible, so far, to measure the required potential in field work.

The problem with field measurements, is that the earth at one location has never exactly the same potential as the earth at another location.   In a laboratory, the electrolyte is contained in an electrically insulated container and the currents are all in closed circuit and related to the corrosion reaction.   The potential of the electrolyte can be measured at the reaction interface by the use of a glass capillary containing an inert, but conductive, electrolyte such as agar-agar gel.   This cannot be achieved in field work, although a close approximation has been achieved by Dr Prinz of Rhurgas, in Germany.

Field readings taken and analysed in the established way are not related to each other, except through the low resistance of the pipeline itself.   This is easily demonstrated by a simple calculation base on Ohms law as follows.

Take any sample readings from a typical pipe-to-soil "potential" cathodic protection survey and work out the amount of current that must be passing through the pipeline between any two cathodic protection test facilities.   It will be found to be ridiculously high, to the extent of being unbelievable.

For example, a span of 24"diameter steel pipeline ten miles long will have a resistance of about 0.001 Ohms, depending on the wall thickness, and the readings at either end of the span might be -0.950 volts and -1.250volts.   This would not cause alarm, and would be plotted on an 'attenuation curve', without too much comment.   However, calculation shows that, if the 'half-cell' (electrode) is truly a reference, then there is a volts drop of 0.300 over the ten mile span.   This seems reasonable until it is realised that with such a low resistance there must be 300 amps passing through the pipeline.   Something is quite clearly wrong with the measuring system or the theories.

This has not been seen as a major problem until pipelines became so widespread and numerous that reliability became a major industrial consideration.   Even now there seems to be little concern with this subject until a failure causes financial losses.   The public at large are not even aware that the inadequacy of present technology could result in an unforeseen disaster.

As far as I'm concerned, the more fucking pipelines that blow up the better.   I'll put my price up.

Before going any further it is necessary to imagine electricity and this has been likened to water pressure, with containers connected by pipes to allow current to flow.

The pressure is caused by the height of the water in each container and not the weight.   The water will fill any connecting tube and then the pressure downwards will be greater in the vessel which has the highest level.   The reason for this is obviously due to the imbalance between the pressures in the two containers and electrical potentials have the same tendency when connected by conductors.

This is fine when visualising a simple circuit such as a single corrosion cell or a dry cell battery connected through a light bulb, but in a cathodic protection circuit, or when corrosion takes place on a pipeline we have no means of measuring each separate cell in this way.

If we examine the technique that is used in the laboratory then it becomes clear that provision has been made to eliminate outside influences in this 'open circuit measurement'.
This is not possible in cathodic protection field work, and yet laboratory derived theories are applied to readings obtained in the field.
It can be seen that it is impossible to measure the pressure differences in each cell by making a single connection to the common reservoir at the bottom.   However it would be possible to stop the flow of water from the highest level in the small vessels by adding a supply of water from a higher level.

However, it can be seen that the pressure measurement in such a system would need to be between the lowest water level and the highest water level in the whole system.   This would be a much greater voltage (Vp in the drawing) than that required to stop the flow in the single cell with the biggest differential.

Comparing electrical pressure with that of water is a good starting point, but it is better to imagine electricity as simply a pressure which can pass through conductors, and is restricted by resistances.

Imagine trying to measure the gas pressure within a cylinder.   We must allow that pressure to act on a meter which will guage the pressure.   This action will consume some of the gas within the cylinder and it is the passage of the gas which makes it possible to measure the pressure itself.

The same rule applies to electrical pressure and this used to cause considerable inaccuracy in voltage measurements until the digital meters made it possible to measure voltages while drawing very little current.

Back to the gas cylinder and imagine measuring the pressure with a guage which draws very little gas.   We still have the problem that this pressure has to be compared to something.   In the case of gases, we can related this pressure to atmospheric pressure, displayed in such a way that we can imagine its effect on our senses.   We are aware that we are all subject to atmospheric pressure and the effect of increasing the pressure on the human body can be felt, when swimming under water, for example.   We use our muscles to compress stale air which is then exhaled and can feel the current of air through our nose and mouth.   Gas pressure is therefore part of our lives with which we are familiar.   We can use this experience to imagine electrical pressure, which has similar qualities.

Everything has an electrical potential (pressure) which has the tendency to equalise on contact with another item of a different potential.   It is this tendency which causes current to flow and allows us to make.

In the same way that chemical reactions can give off gas, and increase the pressure within a cylinder, for example, chemical reactions can cause an electro-motive-force (EMF) which increases the electrical pressure, or potential, on one side of the reaction.

In order to measure the electrical pressure of this reaction we must complete a measuring circuit with a low resistant electrically conductive path.   The whole measuring circuit reaches equilibrium with a small amount of current flowing depending on the requirement of the meter.(in the case of digital meters, the current required to make the measurement is very small).

In the case of measuring the voltage of a dry cell battery, we connect a voltmeter between the poles of the battery and the voltage is the difference in electrical pressure caused by the chemical reaction at the interface between the electrolytic paste and the inner surface of the metal container and the electrode which serves as the positive pole of the battery.

The technique is simple because it is possible to confine the path of the current to that of the measuring circuit and each element of this circuit can be evaluated.   Voltage drops can be measured around the circuit, using independent meters and measuring current can be detected by magnetic field and other techniques.

Natural corrosion cells are much different, as they can be physically minute or large.   Large corrosion cells can contain micro-cells within the same area where anodic areas completely surround cathodes or vice-versa.   When studying such cells, we are not able to separate the component parts, and the measurements have come to be known as 'open circuit measurements'.

This type of measurement involves connections to the electrolyte as well as the metal and this requires the use of an electrode.   There is a danger that this will introduce another EMF into the circuit, by the reaction between the electrode and the electrolyte.   We therefore use an electrode in a solution of its own salts, which has a known reaction EMF.   We can then make a connection between the electrolyte in the cell and the earth electrolyte, in the hopes that there will be no electrical disturbance to the measuring circuit.

In the laboratory, this disturbance is prevented by the use of a glass capillary filled with inert gel, which is used as a conductor from the reaction interface to the reference electrode.   The reference electrode is a metal in a saturated solution of its own salts, as this has a known reaction potential.   Reference electrodes are related to each other by known voltages and are used as international standards.   Without this consistency it would be impossible to evaluate the reaction, develop theories or design cathodic protection systems etc.

Unfortunately, it became the practice to apply the same principles in cathodic protection field work.   It seems that many thought that the electrode could be regarded as a reference against which other potentials can be established.   They thought that pipe to soil voltages were pipeline metal potentials which could be plotted against a fixed potential supplied by the use of the 'reference electrode'.   There are still remnants of this concept in cathodic protection practice today, which are manifest in 'attenuation curves' etc., which are used by some in the design of CP systems.

This subject can now be studied in greater detail by computer modeling which makes it much clearer that the fixed potential is normally that of the pipeline metal, and the variation in the measured voltage is due to the different potentials elsewhere in the measuring circuit.

Imagine that we require to know the voltage of two dry cell batteries which are arranged in parallel.   That is to say that each is in connection with a common conductor to the positive pole and another common conductor to their negative poles.   Both conductors would carry equilibrium current according to the reaction within each battery and the voltage between the two conductors could be measured by connecting a meter between the two.   Unless the two cells are separated, it is impossible to evaluate the voltage of each battery.   Even this is not as complex as the expectancies of cathodic protection monitors.

If we take two batteries and half bury them in an electrolyte with their positive poles exposed and connected, we have two corrosion cells in closer condition to those found on a pipeline.   A circuit drawing of this arrangement will show that current will pass through the ground to equalise the pressures caused by the interface reactions within each battery.

We must now try to evaluate the reaction within each battery using a high resistance voltmeter and an electrode.   We cannot break the circuit or separate the batteries but connections can be made to the metal or the electrolyte or both.   It will be seen that we are only capable of measuring voltages across various spans of the circuit, and cannot establish a reference within that circuit.   The laboratory techniques cannot be applied to these conditions as there are too many variables which are impossible to evaluate.

If we increase the number of half buried batteries connected together, we improve the similarity to a pipeline, but in order to be more realistic, we must include some which have their positive poles buried.   The complexity of the situation is now apparent and what seemed to be a simple measurement, now seems almost impossible.

A circuit diagram of the complex arrangement will show that a different voltage will be measured with every new position of the electrode, and this is born out in cathodic protection field practice.   It is especially obvious on pipelines which are not connected to cathodic protection systems and which have poor coating.

The different voltages are due to the variety of potentials at each pole of the voltmeter.   These can be caused in many ways, as described later, but it is important to realise that they are all components of the voltage shown on the meter.   It is possible to eliminate them in the laboratory but not in the field, therefore they must be evaluated and considered in the analysis of survey results.

The problem is even more complex when cathodic protection is introduced as this is an additional voltage which is superimposed over all the others.   Being designed to drain charges from the whole of the pipeline, it has an effect on the equilibrium of all the other electrical influences.   However, the dynamic effects of an impressed current system can be removed by taking voltage measurements immediately after the system has been switched off.   This cannot be achieved where sacrificial anodes are used, unless they have a special facility designed for this purpose at construction stage.

The voltages obtained between the pipeline metal and a randomly placed electrode have a certain amount of value when compared to others obtained from connections to the same pipeline.   This is because of the very low electrical resistance in this part of the corrosion and cathodic protection circuits.