Instant off Potentials
for Cathodic Protection
of Buried Steel Structures
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Steel pipelines transport the world’s energy
supplies from their natural reservoirs to the consumers. These pipelines pass
under ground which is sometimes densely populated or where the public would be
threatened by an explosive failure.
The integrity of these pipelines is
therefore of huge financial and public safety consequence, and yet there is no
international standard criterion for cathodic protection of these
pipelines. . This is the problem that is
addressed by this paper.
Steel naturally corrodes when in contact
with an electrolyte, such as the ground in which it is buried or the water in
which it is submerged. The first line of defense against corrosion is a coating
of inert material which chemically separates and electrically
isolates the pipeline metal from its environment.
However, a perfect coating is impossible in
practice and the electro-chemical reaction is concentrated at coating faults.
Cathodic protection has been developed as an extremely cost effective method of
extending the life of metal structures by an order of magnitude. Unfortunately
it has proved very difficult to monitor the effects of cathodic protection and
there are frequent disastrous corrosion failures at locations where it was
thought that corrosion had been prevented.
In 1974 two oil pipelines in
Cathodic Protection requires scientific
knowledge and engineering discipline to apply. In 1978 a wrong connection at an
oilfield installation caused 20 pipelines to leak within 3 weeks due to the
accelerated corrosion caused by the direct electric current leaving the pipe
metal.
Despite cathodic protection being required
by law in most countries there is no internationally agreed criterion at
present.
Many failures of cathodic protection
demonstrate the need for a better understanding and a clear benchmark for
pipeline operators in respect of this method of corrosion control.
The most definite method of monitoring the
performance of corrosion control is by physical examination by excavation. The
development of the 'intelligent pig' has allowed excavations to be carried out
at locations suspected of damage before the pipeline actually fails but this
method does not indicate the measures that must be taken to stop the progress
of any damage that is found.
The next most certain way of measuring the
actual progress of corrosion is to install a series of “weight loss coupons” which will
reflect the performance of the pipeline metal, but this is a long-term exercise
with practical difficulties. It requires expert and highly trained technicians
to apply the technique and is not commonly practiced.
The electrical element of corrosion can be
controlled and it would seem that a simple electrical measurement would suffice
to establish when corrosion has stopped. After all Faraday determined that the
amount of metal that goes into solution in the corrosion reaction is directly
proportional to the amount of DC current which results from that reaction.
The problems in making field measurements
are discussed later in this paper but until the early 1970's it was generally
believed that a standard reference electrode could be used in cathodic
protection field work and the instructions were that it should be placed as
close as possible to the pipeline or structure.
It was believed that if the pipeline became
negative relative to a copper/copper-sulfate reference electrode in excess of
0.85 volts with the CP system operating, then corrosion had been halted. The
academics supported this notion and Codes of Practice have been published. (CP
1021 B.S.I. Code of Practice for Cathodic Protection of Pipelines)
Fig. 1.
The standard method of monitoring cathodic protection.
The 'Pourbaix' diagram was published in the
1950's and was hailed as validation of the -0.850v criterion in relation to a
copper/copper sulfate electrode.
Fig. 2. The Pourbaix
diagram of Iron in water.
Pipelines have continued to leak, even when
this criterion has been reached, and new criteria have been proposed, but none
have been adopted internationally.
In 1982 the paper ' New Developments in
Measuring the Effectiveness of Cathodic Protection' was presented to the
Conference of the Australasian Corrosion Association,
devices have achieved international acceptance, and there is still no reliable
criterion for cathodic protection.
The papers rely on the reader being
conversant with details of corrosion science whereas the aim is to find a
criterion, which is easy to understand, and achievable with instruments that
are rugged enough for field use.
In the late 1970's worldwide experience was
showing that the methods of ascertaining the corrosion status of buried, steel
pipelines were inadequate as pipelines were subject to corrosion failure at
locations where they were thought to be cathodically
protected.
The traditional method of monitoring a CP
system was to make a voltage measurement between the pipeline metal and an
electrode placed as close as possible to the pipeline in the ground in which it
was buried (or in the water in which it was submerged).
Field experience had revealed that such CP
voltages were subject to an “IR drop in the soil'” which was of indeterminate
value. The “IR” drop in the soil is
another way of expressing the potential difference (voltage) between two places
in the soil. The effect of these variations in voltage is to alter the reading
shown on the meter.
A laboratory in Holland had shown that this error
could be removed by using an analogue recording voltmeter while the current was
switched off to show a 'kick' in the downward curve at a value at which it was
thought the metal was 'polarized' by the cathodic protection system.
Fig 4. Kick in the downward curve.
The theory is that the 'IR drop' is caused
by the flow of the current from the cathodic protection anodes to the pipeline
metal, and that this current could be switched off to eliminate the voltage
drop. It is known that the pipeline holds its voltage due to
“polarization,” and
it was thought that it would be possible to measure the “polarized potential'”
immediately after the current was switched off.
Trials were carried out for an international
oil company in
The 'two half-cell' survey was developed and
used successfully in
Close
Interval Potential Survey (Switching) (CIPS)
In the early 1980's a survey was devised to
take 'Instant off' voltage readings at close intervals over all pipelines in a
major network that included urban and rural, high-pressure gas mains.
The author was a participant of this most
significant CIPS survey that was carried out between 1980 and 1984 in the
following way.
Disconnecting the cathodic protection
“bonds” at insulation joints and isolation flanges isolated sections of the
pipeline network.
The position of the pipeline was marked
using “Pearson type” instruments.
The transformer/rectifier for this section
was switched on for 5 seconds and off for 2 seconds.
The pipeline metal was contacted through CP
test points and a trailing wire was connected to the negative pole of a digital
voltmeter, carried by the technician. The positive pole of the voltmeter was
connected to a copper rod suspended in a saturated solution of copper sulfate
that was put in contact with the ground.
Fig 6. Close interval potential survey.
The technician noted the highest and the
lowest readings on the meter as the switching occurred at each point of ground
contact. This was repeated at 10-meter intervals over the pipeline.
Fig 7. The form on which the technician
entered the voltage readings.
Millions of 'on' and 'off' readings were fed
into the computer during a four-year period by four teams of technicians and
two engineers engaged on the survey. The computer plotted the 'on readings',
the 'off readings' and the difference between the two, which was anticipated to
show the results of corrosive activity. The third plot was abandoned after a
few months as it was then thought to be insignificant.
Fig 8. Computer generated plot of readings supplied
by technician.
During the four-year condition audit,
several 'Pearson type' techniques were used to identify the position of the
pipeline and to pinpoint coating faults.
A method was devised to record the 'Pearson'
signal and make a permanent, graduated record that could be compared to
physical examination, following excavation.
Fig.
9 Permanent graduated record of a 'Pearson Style' survey.
The 'two-half-cell' method of survey [ref 2]
was used extensively to identify exact locations for excavation, and the
Alexander Cell was field tested with permission of the pipeline operating
company.
Fig 10 One example of a 'two-half-cell' plot showing potential gradients
in the soil.
Further refinements were made to the original survey and the format entirely changed to determine the locations to excavate for physical examination.
Fig. 11. The final report format
for excavation at locations suspected as having
active
corrosion.
At
total of 102 excavations showed less than 10% success in predicting the
condition of the coating and pipeline metal by the CIPS survey.
The operating organization developed 'the
intelligent pig' concurrently with this over line audit, but cathodic
protection monitoring has the advantage of indicating the cause of corrosion
damage as well as suggesting remedial measures by adjusting the cathodic
protection system. It is also possible to monitor the success of remedial
measures inexpensively.
The survey did not meet the requirements of
the laboratory recommendations for making the 'polarized potential' measurement
that should be made on an analogue recording voltmeter capable of showing the
'kick' in the downward curve.
Attempts were made to find a satisfactory
way to conduct a recorded CIPS and they clearly demonstrated that the
“polarized” potential could not be readily identified as required by the
laboratory test and that other influences would have to be evaluated to obtain
a true potential for the purposes of applying the principles of the Pourbaix
Diagram.
Fig 12. Analog recording voltmeter with stepped electrode showing the difficulty of identifying the 'kick' in the downward curve.
Attempts were made to correct readings using
analog recording voltmeters with static electrodes to evaluate the voltage
differences over a period of time at a single location. This would also be
necessary in order to apply the principles of Pourbaix and Faraday.
Fig 13 Recording voltmeter with static electrode showing how the on and off potentials varying during a period of several minutes.
All cathodic protection systems work in an environment
influenced by other cathodic protection systems. By isolating one section we
alter the equilibrium, which we wish to measure, in the act of making the
measurement. (See also later note)
Synchronized switching of TR's was attempted and found to be impossible to achieve.
Several techniques have been suggested since this audit but successful results
do not seem widely available despite the marketing of sophisticated and
expensive
instrumentation.
No attempt was made to switch the
sacrificial anodes that were influencing the sections under test although some
of these were temporarily disconnected.
The exact position of each reading would
have to be recorded and repeatable for the survey to have any value.
Comments
on this type of survey.
Cathodic protection systems are switched on
while in service, and the current flowing from the
cathodic protection anode is a significant part of the electrical equilibrium.
It is a powerful electro-motive force within the integrated circuits of both
the corrosion reactions and the measuring system and to remove this force from
the corrosion reaction is to falsify the results of any measurement.
Removing the 'IR drop' in the measuring
circuit can easily and cheaply be achieved using an “isopotential cell” as
suggested by many reputable and leading scientists. There are two versions
patented and a simple unpatented version of the same principle can easily and
cheaply be made by anyone.
“Instant off” and CIPS readings cannot be
analyzed using either Ohms Law or Kirchoff's Laws but are traditionally
interpreted by the informed guesswork of specialists.
We should aim to determine whether corrosion
has stopped by non-intrusive techniques that can be confirmed by physical
inspection at coating faults that are detected to be corroding.