all about cathodic protection system, impress current cathodic protection system, sacrificial anode cathodic protection system, design calculation for cathodic protection system etccathodic protection basics
In order to monitor cathodic protection, we
must be able to contact the metal of the subject pipeline or structure.
There was a time when contact was achieved
by driving a steel rod into the ground from above the pipeline, and making
temporary contact by piercing the coating. The main reason for discontinuing
this practice was the physical damage that was possible to the pipe metal
itself, and that damage that was caused to the coating.
Most pipelines now have provision for
contact through electrical conductors connected to the subject metal in a
variety of ways.
The most common is a process known as
cadwelding, which can result in a low resistance, permanent, electrical bond
between the copper conductor and the steel of a pipeline.
The disadvantage of this type of connection
is that it needs considerable skill to achieve a good connection, in some field
conditions. The joint must be carefully
inspected and the integrity of the coating must be tested before backfilling.
We are all aware that copper and brass are
more noble than steel which will tend to disolve if coupled together in an
electrolyte. Cadwelding introduces such a ' bi-metalic coupling' to the surface
of the pipe and care must be taken that all metal is separated from the
electrolyte by a chemically impervious, electrically resistant coating. This coating must be compatible tho the
pipeline coating and to the insulation on the copper conductor cable.
The use of copper conductor cables also
introduces the possibility of a bi metalic coupling if its insulation is not
perfect. It should be remembered that
the voltage that we are measuring is between the potential of copper in a
saturated solution of its own salts, and steel in the local environmental
If the conductor to the pipe is severed,
then the voltage that we measure will be that between copper in a saturated
solutionof its own salts and copper in the salts that are present in the local
environment. This voltage will be very low, as the difference between the
potentials, will be small.
Readings can be very confusing however as
they are sometimes affected by the cathodic protection current. Charges will be passing onto the broken
copper tail which is still attached to the steel of the pipe, due to the
galvanic activity, and this will cause a variation in the potential of the
ground in the immediate vicinity. The extent of this area of influence depends
on the area of contact between the copper and the electrolyte, and the
resistance of that electrolyte.
If the conductor is not completely severed,
it will definitely draw currentfrom the ground, and this will have a
significant effect on the measured voltage if the insulation damage is close to
the electrode position.
The contact between the conductor and the
subject metal must have a low electrical resistance, as it may be used for
The best test facility is direct contact
with the pipeline at a riser but there are many sections of pipeline which are
buried with no riser.
There was a period when some test posts
were connected to two conductors which contacted the pipeat two locations
exactly 100m apart. The purpose was to
enable current direction readings to be taken, but although I saw several
attempts to do this, I never saw it done successfully, or was never able to
obtain meaningful readings myself. I
read and understand the theory behind this type of measurement, but the
application seems impractical in field work.
The electrical resistance of the pipeline
itself is extremely low, for example a 4" dia. steel pipeline is 0.141
ohms per mile and a 24" dia. pipeline is an incredibly low 0.0161 ohms
per mile.(Peabodies) If we are dealing
with other structures such as storage tanks we can never consider the
resistance of the metal itself, as a significant feature in cathodic protection
It therefore follows that the POTENTIAL of
the pipe metal does not vary significantly, over a 2km section of continuous
welded steel pipeline, with the diameters quoted. This matter was debated during the application
of over-the-pipeline potential surveys, conducted in the UK, on high pressure,
welded steel, gas mains.
I was part of a team that carried out the
field test which resolved this matter, on a 2km section of 24" dia. welded
steel, coal tar enamel coated, buried pipeline.
This pipeline was protected by impressed current cathodic protection
which was switched on continuously during these tests.
The negative pole of a high resistance
voltmeter was connected to the test post conductor terminal at the top of a
test post at location A.
A standard copper/copper-sulphate electrode
was placed in a fixed position at location A.
A reel of armature wire was used to connect
the electrode to the positive pole of the high resistance voltmeter and the
reading was noted.
The wire was reeled off the spool and used
to make contact with a standard copper/copper-sulphate electrode at location B,
which was 2km distance from location A.
A changed voltage was noted on the meter,
which was still connected to the test post at location A.
The change of standard electrode positions
had significantly altered the recorded voltage.
The armature wire was then used to connect
the negative pole of the voltmeter to the distant test post at location B.
The positive pole of the voltmeter was then
reconnected to the electrode at location A.
The voltage on the meter was identical to
the first reading.
Altering the position of contact to the
pipeline, by a distance of 2 km, had no detectable influence on the voltage
The meter was taken to location B and
connected between that test post terminal and electrode B.
The voltage recorded was identical to the
second voltage of the test, confirming that the location of the electrode is
the only significant feature.
The positive pole of the meter, at location
B, was then connected to the armature wire which was connected to the electrode
in the fixed position at location A.
The voltage recorded was identical to the
first voltage recorded confirming, once more that the point of contact to the
pipeline has no detectable effect on the recorded voltages.
The discussions culminating in this test,
resulted in a re-appraisal of test post locating within the operating
company. It was decided that fewer test
posts were needed, and that the priority importance was access to the test post
The best form of test post, for a steel
pipeline consists of a steel bar welded directly onto the pipeline metal and
protruding through the surface of the ground directly above. This is protected
by encasement in a concrete block, which includes a vertical 4" dia.pipe
filled with the local ground material.
The standard electrode would always be placed in the top of 4" pipe
for the purposes of periodic voltage measurements.
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
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
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
As far as I'm concerned, the more fucking pipelines that
blow up the better. I'll put my price
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
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
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
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.
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