PDF | Cathodic protection (CP) is a method of preventing corrosion of a metal surface by making it the cathode of an electrical circuit. Without. Cathodic protection system (CPS) is used to reduce corrosion by minimizing the difference in potential between anode and cathode. The main objective of this work is to discuss and compares the two methods of applying cathodic protections by using an impressed current system and. is called a “cathodic” protection system.” The cathodic protection anodes are installed to become the anode in this larger corrosion cell and provide the location.
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The reference cell is placed as close as possible to the structure under test (near structure). ➢ A structure-to-soil potential is read and recorded. ➢ The reference. Principles governing cathodic protection are illustrated in Fig. 16.2 below. As per mixed potential theory, the zero current criterion is shown. An equilibrium is. Cathodic protection is a proven corrosion control method for protection of underground Cathodic protection is now widely applied in the protection of oil.
The Potential Criterion The polarized potential of the protected structure is to be equal to or more negative than -850 millivolts mV with respect to CSE. Note: These criteria are for carbon steel; criteria may differ for various metal types.
Types of Cathodic Protection There are two types of cathodic protection: 1. CP current is created by the potential difference between sacrificial anodes and the protected structure.
The type of anode used depends on electrolyte resistivity and the chemical compositions of the electrolyte to which the substrate is exposed. The Basics of Cathodic Protection Page 2 of 2 2.
The anode bed is a series of buried anodes that are electrically connected and surrounded by certain backfill to reduce their resistance to the earth.
The anode bed should be placed remotely from the protected structure. Three types of anodes are used: Soluble anodes aluminum and steel , semi-soluble anodes graphite and high silicon cast iron HSCI , and non-soluble anodes platinum, mixed metal oxide, and polymer The main component of this type of CP is the TR, which forces the current to flow from the anodes to the protected structure cathode.
Application of Cathodic Protection Systems on Buried Pipelines Pipelines are used for transporting water, petroleum products, natural gas, and other utilities. Theres a huge network of piping systems used in every country all over the world. Pipelines may be onshore or offshore, and are subject to corrosion in both cases. If corrosion isn't mitigated, dangerous and expensive damage can be the result. There are several corrosion control techniques used on pipelines; cathodic protection is one of them.
It can be applied either to coated pipelines to mitigate the corrosion attack on areas where coating quality may be poor. It is also used on bare pipelines. Both types of CP can be applied to buried pipelines. CP aims to polarize a pipeline to a minimum potential of -850 mv, for carbon steel and for adequate CP.
The polarized potential is to be measured through test stations, which are to be installed at the following locations along the route of pipeline: At frequent intervals e.
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.
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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.
The basic cathodic protection.pdf
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 13509:2003 and NACE TM0497 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.Cathodic Protection Criteria In order to achieve adequate CP, the protected structure must be polarized to a certain value.
The function of cathodic protection is to reduce the potential difference between anodes and cathodes to a neglected value. Cathodic protection markers over a gas pipeline in Leeds , West Yorkshire , England.
Current flow onto any metal will shift its normal potential in the negative direction. The CP shielding phenomenon induces changes in the potential gradient of the CP system across the exterior coating, which are further pronounced in areas of insufficient or sub-standard CP current emanating from the pipeline's CP system. Since ships are regularly removed from the water for inspections and maintenance, it is a simple task to replace the galvanic anodes.
Two dissimilar metals 2.