cathodic protection

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cathodic protection

  1. 1. Presented by*Abhishek mishra *Arunn kumar *Manish kumar Corrosion control methods Cathodic protection Sacrificial Impressed anode current Applications
  2. 2. What is Cathodic protection? 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. Cathodic protection can, in some cases, prevent stress corrosion cracking.
  3. 3. Principle involved The principle of cathodic protection is to prevent anodic sites occurring on the structure under protection by allowing the anodic reactions to occur on specially designed and installed anodes
  4. 4. HISTORY: Cathodic protection was first described by Sir Humphry Davy in a series of papers presented to the Royal Society in London in 1824. Thomas Edison experimented with impressed current cathodic protection on ships in 1890, but was unsuccessful due to the lack of a suitable current source and anode materials. In the USA by 1945 the use of CP was commonly applied to the rapidly expanding oil and natural gas industry. In the UK CP was applied from the 1950s onwards and Cathodic Protection Company Limited was established in this period, pioneering it’s use in the UK. CP is now well established on a large variety of immersed and buried metallic structures as well as reinforced concrete structures, and provides effective corrosion control.
  5. 5. Discription: The simplest method to apply CP is by connecting the metal to be protected with another more easily corroded "sacrificial metal" to act as the anode of the electrochemical cell. The sacrificial metal then corrodes instead of the protected metal. For structures where passive galvanic CP is not adequate, for example in long pipelines, an external DC electrical power source is sometimes used to provide current. Cathodic protection systems are used to protect a wide range of metallic structures in various environments. Common applications are; steel water or fuel pipelines and storage tanks such as home water heaters, steel pier piles; ship and boat hulls; offshore oil platforms and onshore oil well casings and metal reinforcement bars in concrete buildings and structures. Another common application is in galvanized steel, in which a sacrificial coating of zinc on steel parts protects them from rust. Another method of protection impresses a small direct current on a structure.
  6. 6. Why is it important? Corrosion costs money. Corrosion of metals costs the USA economy almost $300 billion per year and it is estimated that one third of this value could be saved with better selection of corrosion prevention techniques, including cathodic protection.o Our country has been losing around Rs 1.52 lakh crore every year owing to corrosion in various sectors, including infrastructure, utility services, production & manufacturing, and defence & nuclear waste.
  7. 7. Why does corrosion occur? The corrosion of metals, in particular steel in an aqueous environment (which can be either soil or water), occurs because the metal interacts with the local environment. In the case of steel, man has mined iron ore and processed it into steel. However due to certain characteristics of the steel, it is not ‘stable’ once in contact with an aqueous environment and interacts with the local environment in an attempt to return to its naturally occurring state. This process is corrosion.Place diagrams depicting corrosion
  8. 8. Basic Reactions of corrosion The basic process at an anodic site is the release of iron (Fe) from the steel surface into the environment and can be expressed as: Fe  Fe2+ + 2e- During the process two electrons (2e-) are generated which must be consumed by the environment (in aerated systems) and can be expressed as: 4H+ + O2 + 4e- 2H2O A summary of the above half reactions can be expressed as: 2Fe + 2H2O + O2 2Fe(OH)2 The term Fe(OH)2 is iron oxide which can be oxidized to form the red-brown Fe(OH)3 commonly referred to as rust.
  9. 9. How does it work? Simply CP works by preventing the anodic reaction of metal dissolution occurring on the structure
  10. 10. Presented by*Abhishek mishra *Arunn kumar *Manish kumar Corrosion control methods Cathodic protection Sacrificial Impressed anode current Applications
  11. 11. In the usual application, a galvanic anode, a piece of a more electrochemically"active" metal, is attached to the vulnerable metal surface where it is exposed to thecorrosive liquid. Galvanic anodes are designed and selected to have a more "active"voltage (more negative electrochemical potential) than the metal of the targetstructure (typically steel). For effective CP, the potential of the steel surface ispolarized (pushed) more negative until the surface has a uniform potential.Metals like (Zn,Al,Mg) are used for making anode because they have very lowelectrochemical potential as compared to steel hence more ‘active’.These metals act as anode and get corroded.For this purpose of increasing electrical contact, the active metal is placed in back fill(coal and NaCl).When it is consumed completely then replaced by a newer one.
  12. 12. Insulated wireGround level backfill Underground pipeline Sacrificial anode ( Zn, Al)
  13. 13. Cathodic anode
  14. 14. Presented by*Abhishek mishra *Arunn kumar *Manish kumar Corrosion control methods Cathodic protection Sacrificial Impressed anode current Applications
  15. 15. Impressed Current (ICCP) For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection. In this method,an impressed current is applied in opposite direction to nullify the corrosion current and convert the corroding metal from anode to cathode. ICCP systems use anodes connected to a DC power source. Usually this will be a cathodic protection rectifier, which converts an AC power supply to a DC output. In the absence of an AC supply, alternative power sources may be used, such as solar panels, wind power, etc. This current is given to insoluble anode like graphite, stainless steel or scrap iron burried in soil. The negative terminal of D.C. is connected to pipeline to be protected.The anode is kept in back-fill(composed of gypsum or coke breeze) to increase electrical contact with the surrounding soil.
  16. 16. Presented by*Abhishek mishra *Arunn kumar *Manish kumar Corrosion control methods Cathodic protection Sacrificial Impressed anode current Applications
  17. 17. Applications1.Piplines:Pipelines are routinely protected by a coating supplemented with cathodic protection.
  18. 18. 2.Ships: Cathodic protection on ships is often implemented by galvanic anodes attached to the hull, rather than using ICCP.
  19. 19. 3. Marine: Marine CP covers many areas, jetties, harbors, offshore structures.
  20. 20. 4.Internal CP: Vessels, pipelines and tanks which are used to store or transport liquids can also be protected from corrosion on their internal surfaces by the use of cathodic protection.
  21. 21. 5. Galvanized steel: Galvanizing generally refers to hot-dip galvanizing which is a way of coating steel with a layer of metallic zinc.
  22. 22. Problems arised due to cp: Production of hydrogen ions A side effect of improperly applied cathodic protection is the production of hydrogen ions, leading to its absorption in the protected metal and subsequent hydrogen embrittlementof welds and materials with high hardness. Cathodic Disbonding 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)o Cathodic Shielding Effectiveness of cathodic protection systems on steel pipelines can be impaired by the use of solid film backed dielectric coatings such as polyethylene tapes, shrinkable pipeline sleeves, and factory applied single or multiple solid film coatings. Protective electric current from the cathodic protection system is blocked or shielded from reaching the underlying metal by the highly resistive film backing.
  23. 23. o Safety Rectifier safety has recently become an industry concern for cathodic protection technicians and personnel who are responsible for rectifier operation, maintenance and repair. While rectifiers are manufactured according to national electrical codes and standards and inspected by authorities, the basic rectifier designs have not changed significantly over the past 25 years in regards to safety.

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