Cathodic protection CP is a method of corrosion control that can be applied to buried and submerged metallic structures. It is normally used in conjunction with coatings and can be considered as a secondary corrosion control technique. Why is it Important? The Principles of Cathodic Protection Useful metals are often extracted from their naturally existing states as ores to be commerically and industrially viable. This process is often known as corrosion in layman terms and the most common example notable in everyday life is the rusting of iron and its various alloys, an example being steel. Advantages and Uses of Cathodic Protection The primary benefit Cathodic Protection offers over other anti-corrosion systems is its ability to be deployed merely by the mantainenece of a Direct Current Circuit.
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Cathodic protection markers over a gas pipeline in Leeds , West Yorkshire , England. Hazardous product pipelines are routinely protected by a coating supplemented with cathodic protection.
An impressed current cathodic protection system ICCP for a pipeline consists of a DC power source, often an AC powered transformer rectifier and an anode, or array of anodes buried in the ground the anode groundbed.
The DC power source would typically have a DC output of up to 50 amperes and 50 volts , but this depends on several factors, such as the size of the pipeline and coating quality. The positive DC output terminal would be connected via cables to the anode array, while another cable would connect the negative terminal of the rectifier to the pipeline, preferably through junction boxes to allow measurements to be taken.
The choice of groundbed type and size depends on the application, location and soil resistivity. It is sometimes more economically viable to protect a pipeline using galvanic sacrificial anodes.
This is often the case on smaller diameter pipelines of limited length. Water pipelines of various pipe materials are also provided with cathodic protection where owners determine the cost is reasonable for the expected pipeline service life extension attributed to the application of cathodic protection. Cathodic protection on ships is often implemented by galvanic anodes attached to the hull and ICCP for larger vessels. Since ships are regularly removed from the water for inspections and maintenance, it is a simple task to replace the galvanic anodes.
As with all galvanic cathodic protection, this application relies on a solid electrical connection between the anode and the item to be protected. For ICCP on ships, the anodes are usually constructed of a relatively inert material such as platinised titanium. A DC power supply is provided within the ship and the anodes mounted on the outside of the hull.
The anode cables are introduced into the ship via a compression seal fitting and routed to the DC power source. The negative cable from the power supply is simply attached to the hull to complete the circuit. The current density required for protection is a function of velocity and considered when selecting the current capacity and location of anode placement on the hull.
Some ships may require specialist treatment, for example aluminium hulls with steel fixtures will create an electrochemical cell where the aluminium hull can act as a galvanic anode and corrosion is enhanced. In cases like this, aluminium or zinc galvanic anodes can be used to offset the potential difference between the aluminium hull and the steel fixture.
Marine[ edit ] Marine cathodic protection covers many areas, jetties , harbors , offshore structures. The variety of different types of structure leads to a variety of systems to provide protection.
Galvanic anodes are favored,  but ICCP can also often be used. Because of the wide variety of structure geometry, composition, and architecture, specialized firms are often required to engineer structure-specific cathodic protection systems. Sometimes marine structures require retroactive modification to be effectively protected  Steel in concrete[ edit ] The application to concrete reinforcement is slightly different in that the anodes and reference electrodes are usually embedded in the concrete at the time of construction when the concrete is being poured.
The usual technique for concrete buildings, bridges and similar structures is to use ICCP,  but there are systems available that use the principle of galvanic cathodic protection as well,    although in the UK at least, the use of galvanic anodes for atmospherically exposed reinforced concrete structures is considered experimental.
However, in a typical atmospherically exposed concrete structure such as a bridge, there will be many more anodes distributed through the structure as opposed to an array of anodes as used on a pipeline. This makes for a more complicated system and usually an automatically controlled DC power source is used, possibly with an option for remote monitoring and operation.
Galvanic systems offer the advantage of being easier to retrofit and do not need any control systems as ICCP does. For pipelines constructed from pre-stressed concrete cylinder pipe PCCP , the techniques used for cathodic protection are generally as for steel pipelines except that the applied potential must be limited to prevent damage to the prestressing wire.
An additional problem is that any excessive hydrogen ions as a result of an excessively negative potential can cause hydrogen embrittlement of the wire, also resulting in failure. The failure of too many wires will result in catastrophic failure of the PCCP. A simpler option is to use galvanic anodes, which are self-limiting and need no control.
Galvanized steel[ edit ] Galvanizing generally refers to hot-dip galvanizing which is a way of coating steel with a layer of metallic zinc or tin. Galvanized coatings are quite durable in most environments because they combine the barrier properties of a coating with some of the benefits of cathodic protection.
If the zinc coating is scratched or otherwise locally damaged and steel is exposed, the surrounding areas of zinc coating form a galvanic cell with the exposed steel and protect it from corrosion. This is a form of localized cathodic protection - the zinc acts as a sacrificial anode. Galvanizing, while using the electrochemical principle of cathodic protection, is not actually cathodic protection.
Cathodic protection requires the anode to be separate from the metal surface to be protected, with an ionic connection through the electrolyte and an electron connection through a connecting cable, bolt or similar. This means that any area of the protected structure within the electrolyte can be protected, whereas in the case of galvanizing, only areas very close to the zinc are protected. Hence, a larger area of bare steel would only be protected around the edges.
Automobiles[ edit ] Several companies market electronic devices claiming to mitigate corrosion for automobiles and trucks. Copper-copper sulphate electrodes are used for structures in contact with soil or fresh water. The methods are described in EN and NACE TM along with the sources of error  in the voltage that appears on the display of the meter.
Interpretation of electrode potential measurements to determine the potential at the interface between the anode of the corrosion cell and the electrolyte requires training  and cannot be expected to match the accuracy of measurements done in laboratory work. Problems[ edit ] Production of hydrogen[ edit ] A side effect of improperly applied cathodic protection is the production of atomic hydrogen ,  leading to its absorption in the protected metal and subsequent hydrogen embrittlement of welds and materials with high hardness.
Under normal conditions, the atomic hydrogen will combine at the metal surface to create hydrogen gas, which cannot penetrate the metal. Hydrogen atoms, however, are small enough to pass through the crystalline steel structure, and lead in some cases to hydrogen embrittlement.
Cathodic disbonding[ edit ] This is a process of disbondment of protective coatings from the protected structure cathode due to the formation of hydrogen ions over the surface of the protected material cathode. Cathodic disbonding occurs rapidly in pipelines that contain hot fluids because the process is accelerated by heat flow.
This phenomenon occurs because of the high electrical resistivity of these film backings. Cathodic shielding was first defined in the s as being a problem, and technical papers on the subject have been regularly published since then.
The combination of tenting and disbondment permits a corrosive environment around the outside of the pipe to enter into the void between the exterior coating and the pipe surface. With the development of this CP shielding phenomenon, impressed current from the CP system cannot access exposed metal under the exterior coating to protect the pipe surface from the consequences of an aggressive corrosive environment.
This produces an area on the pipeline of insufficient CP defense against metal loss aggravated by an exterior corrosive environment. Also, the NACE SP standard defines shielding in section 2, cautions against the use of materials that create electrical shielding in section 4. Standards[ edit ] 49 CFR External organic coatings for the corrosion protection of buried or immersed steel pipelines used in conjunction with cathodic protection.
Tapes and shrinkable materials EN - General principles of cathodic protection in sea water EN - Cathodic protection for submarine pipelines EN - Cathodic protection for fixed steel offshore structures EN - Internal cathodic protection of metallic structures EN - Cathodic protection of steel in concrete EN - Cathodic protection of buried or immersed metallic structures. General principles and application for pipelines EN - Cathodic protection for steel offshore floating structures EN - Cathodic protection for "Harbour Installations".
EN - Cathodic protection measurement techniques EN - Cathodic protection of buried metallic tanks and related piping EN - Cathodic protection of complex structures EN - External cathodic protection of well casing EN - Evaluation of a.
Whilst the requirements and recommendations are general, this document contains advice on how amendments can be made to include project specific requirements. Some of the design recommendations and methods in Sec. This RP is primarily intended for CP of permanently installed offshore structures associated with the production of oil and gas. Mobile installations for oil and gas production like semi-submersibles, jack-ups and mono-hull vessels are not included in the scope of this document. However, to the discretion of the user, relevant parts of this RP may be used for galvanic anode CP of such structures as well. Detailed design of anode fastening devices for structural integrity is not included in the scope of this RP. Considerations related to safety and environmental hazards associated with galvanic anode manufacture and installation are also beyond its scope Compared to the edition of DNV-RP-B, design considerations for impressed current CP have been deleted from the scope of the revision whilst the sections on anode manufacture and installation are made more comprehensive.