Instant off Potentials for Cathodic Protection
of Buried Steel Structures
Including postal code and
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
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
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.