Cathodic protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathodic side of an electrochemical cell. The simplest method to apply CP is by connecting the metal to be protected with another more easily corroded metal to act as the anode of the electrochemical cell.
Cathodic protection can, in principle, be applied to any metallic structure in contact with a bulk electrolyte although in practice its main use is to protect steel structures buried in soil or immersed in water.
Cathodic protection systems are used to protect a wide range of metallic structures in various environments. The most common applications include:
CP use on Pipelines
Cathodic protection is an important method of preventing corrosion on buried metal pipelines. Every pipeline operator must carry out regular measurements of CP – at transformer rectifiers and tests points (in impressed current systems) and at sacrificial anodes (in galvanic systems).
Collecting and analysing these CP measurements is labour-intensive, very expensive and (more importantly) they can only be reactive – CP problems can lie undetected for long periods, during which the pipeline is insufficiently protected.
Cathodic Protection History
The first practical application of cathodic protection is generally attributed to Sir Humphry Davy who, in the 19th century, improved the resistance of copper-clad ships to seawater corrosion by the attachment of small quantities of iron, zinc or tin.
In the 20th century, particularly in the United States, the method was developed and, by 1945, had become the standard procedure for protection of metal pipelines as the oil and gas industries expanded rapidly.
The costs of laying a metal pipeline, determined by its specification, wall thickness and installation in the ground, are very high. Degradation of the material of the pipe is very expensive to correct and, at worst, can lead to a pipe failure with unpredictable consequences.
Today, with its proven track record of maintaining pipelines over many decades, CP is well established. It is used on pipelines, other immersed or buried metal structures and in reinforced concrete to enhance resistance to corrosion. It enables thinner metal sheets or pipes to be used, thereby reducing costs.
Cathodic Protection Principles
Corrosion is the action of a metal that has been extracted from ore reverting to its primary state when exposed to oxygen and water. The most common example is the rusting of steel. Corrosion is an electrochemical process, normally occurring at the anode but not the cathode.
The principle of cathodic protection is to connect an external anode to the metal to be protected and to pass a DC current between them so that the metal becomes cathodic and does not corrode.
In a pipeline system there are two ways of doing this:
Using an external galvanic anode, where the DC current arises from the natural difference in potential between the metals of the anode (eg Zn, Al or Mg) and the pipe (eg carbon steel). The anode is electrically connected to the pipeline, causing a positive current to flow from the anode to the pipe so that the whole surface of the steel becomes more negatively charged, i.e the cathode.
Using an external DC power source (rectified AC) to impress a current through an external anode (usually inert) onto the surface of the pipe, which becomes the cathode.
Galvanic systems are easy to install, have low operating costs and minimal maintenance requirements, do not need an external power supply and rarely interfere with foreign structures. However, they offer limited protection of large structures and are therefore used for quite localised CP applications.
Impressed current systems are more frequently used to protect pipelines and underground storage tanks. Their high current output is capable of protecting large underground metal structures economically, is flexible to deal with varying conditions and less susceptible to soil resistivity. However, they rely on the continuity of their AC power source and can interfere with other nearby buried structures.
The level of CP current that is applied from impressed current systems is important. Too little current will lead to corrosion damage; excessive current can lead to disbanding of the coating and hydrogen embrittlement. For these reasons impressed current systems require regular monitoring.
Cathodic Protection Measurements
The main standard measurements of cathodic protection are as follows:
Pipe-to-Soil Potential (ON Potential) - The potential of a pipeline at a given location is commonly referred to as the pipe-to-soil potential. It results from the corrosive electrolytic reaction between the buried pipe and its surrounding soil (the electrolyte). It is actually measured between the pipeline and a reference electrode (most commonly copper sulphate), placed in the soil directly over the pipeline. It is also known as the ON potential because the measurement is made while the CP system is energised.
Instant OFF Potential - When a pipe-to-soil measurement is made, the pipeline potential will appear to be more negative then its true potential, due to IR drop errors. The instant OFF measurement corrects for these errors; the CP current is briefly interrupted to produce a "true" pipe-to-soil potential, free from undesirable IR drop effects and before any appreciable depolarisation has occurred. This is a truer measure of the level of protection afforded to the pipeline. If it is not possible to disconnect the CP momentarily then an alternative approach is the use of a corrosion coupon (see below).
Coupon Current - Corrosion coupons connected to cathodically-protected structures can be used to monitor the effectiveness of the CP system. A coupon is a representative sample of the pipeline material, buried close to the pipe so that it is subjected to the same environment. Connected to the pipeline via a test post, it simulates how the pipeline would react if there were a defect (often referred to as a "holiday") in its coating. It is especially useful when it is not possible to interrupt the CP system, since instant OFF potentials can conveniently be measured by interrupting the CP connection to the coupon. The measurement of current flow to/from the coupon can also be determined by measuring the voltage across a shunt. The surface area of the coupon allows the current density to be calculated.
These measurements may be taken at the Transformer Rectifier or, in the field, at CP test posts/stations. However, they are only representative of the pipeline at that point – and for a short length either side.
Close Interval Potential Survey (CIPS)
CIPS fills in the “gap” between measurements taken at test points. A direct connection is made to the pipeline and this trailing wire is unwound from a spool as the technician walks along its length. As he goes, the TR current output is interrupted to enable the technician to take a pipe-to-soil OFF potential measurement at approximately 1m intervals. On pipelines with multiple TRs, all the outputs (or at least those that influence the potential measurement at that point) have to be interrupted synchronously. Interruption cycle times vary but the selected "on" period is longer than the "off" period to limit depolarisation of the pipeline during the survey.
Direct Current Voltage Gradient (DCVG)
DCVG is used for locating and sizing defects in the coating of the pipeline. A coating “holiday” on a pipeline with an impressed current CP system will give rise to a voltage gradient – with the highest gradient closest to the defect. Measurement of the voltage gradient at the surface above the pipeline enables even small flaws to be detected and positioned accurately.
Both CIPS and DCVG techniques are increasingly used – but can be time-consuming to set up in the field because of the requirement to synchronise transformer rectifier outputs.
Our MERLIN Transformer Rectifier Monitor has a remote synchronisation option, using GPS technology, designed to facilitate these surveys. Rectifiers can be configured to synchronise and interrupt their output simply by sending a message from a cellphone.
The MERLIN Interrupter TX is designed for precise solid state interruption of CP rectifiers or solar stations connected to a buried pipeline. Used in conjunction with specific MERLIN Transformer Rectifier Monitors, it enables interruption (switching of the current output) at a rectifier or solar station to be controlled remotely.
The Interrupter fails safe and switches the rectifier output loads encountered under the temperature, environmental and electrical conditions experienced in a rectifier cabinet. The solid state circuitry overcomes the limitations of electromechanical relays.
Compatible with industry standard interruption patterns, the MERLIN Interrupter may be switched on and off, or the cycle changed, from Abriox’s CPSM or iCPSM software. Interruption and cycle time management can also be controlled from a cell/mobile phone when in the field.
Cathodic Protection Monitoring
All pipeline operators use CP extensively on their transmission pipelines. The big advantage of CP over other forms of corrosion treatment is that it is applied very simply by maintaining a DC circuit and its effectiveness can be monitored continuously.
Because of the importance of CP in protecting the pipe, operators are required to take and report regular measurements of CP data, both of the levels of protection applied to the pipe (at source) and the in situ levels measured along the pipe itself.
In an impressed current system, measurements are taken at transformer rectifiers and test posts.
In a galvanic system, measurements are taken at the sacrificial anode.
The frequency of measurements at the various points is generally in compliance with NACE guidelines.
Pipeline operators are responsible for providing their national regulatory body with evidence that their monitoring is adequate to demonstrate effective management of their CP systems.
The data is gathered in the field by technicians. However, the cost of this activity is significant and there are other disadvantages of manual data collection.
The MERLIN system has been designed, in discussion with CP professionals and pipeline operators, specifically for automatic monitoring of CP data. It enables operators to:
Reduce operational monitoring costs significantly
Monitor pipeline CP levels on a daily basis automatically
Respond immediately to a potential corrosion hazard
Deploy skilled labour more effectively
Reduce lone working and the need to access remote sites
Demonstrate best practice to Regulators
Reduce environmental emissions / carbon footprint from survey vehicles
Cathodic Protection Design
Because it inhibits corrosion, CP allows the use of thinner metal thicknesses and can therefore be extremely cost-effective over the operating life of an underground asset.
When designing a new CP system, a survey is usually made and an economic justification of the project produced. This takes account of:
National and international guidelines
Negotiation with landowners, public bodies or other interested parties
Type (galvanic or impressed current)
Current demand and electrical supply requirements
Number and location of anodes or transformer rectifiers
We work across the globe with CP design consultants and companies to assist with their CP monitoring requirements. Contact us for more information on how Abriox remote monitoring systems can improve your network management.