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Encyclopedia > Cathodic protection
Aluminium anodes mounted on a steel jacket structure

Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell. Image File history File links Anodes-on-jacket. ... Image File history File links Anodes-on-jacket. ... Aluminum redirects here. ... For other uses, see Steel (disambiguation). ... For the hazard, see corrosive. ... Diagram of a copper cathode in a Daniells cell. ... A demonstration electrochemical cell setup resembling the Daniell cell. ...


It is a method used to protect metal structures from corrosion. Cathodic protection systems are most commonly used to protect steel, water/fuel pipelines and storage tanks; steel pier piles, ships, offshore oil platforms and onshore oil well casings. For other uses, see Steel (disambiguation). ... // A tank is a container, usually for liquids, sometimes for gases. ... A pile is one type of building foundation. ... Offshore has three principal meanings: Physical - in the sea away from the shore; not on the shoreline but out to sea. ... The Hibernia platform is the worlds largest oil platform. ... An oil well is seen in Texas. ...


A side effect of improperly performed cathodic protection may be production of molecular hydrogen, leading to its absorption in the protected metal and subsequent hydrogen embrittlement. This article is about the chemistry of hydrogen. ... Hydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and crack following exposure to hydrogen. ...


Cathodic protection is an effective method of preventing stress corrosion cracking. Stress corrosion cracking (SCC) is the unexpected sudden failure of normally ductile metals subjected to a constant tensile stress in a corrosive environment, especially at elevated temperature. ...

Contents

Origins

The first use of CP was in 1824, when Sir Humphry Davy, of the British Navy, attached chunks of iron to the external, below water line, hull of a copper clad ship. Iron has a stronger tendency to corrode (rust) than copper and when connected to the hull, the corrosion rate of the copper was dramatically reduced. 1824 was a leap year starting on Thursday (see link for calendar). ... Sir Humphry Davy, 1st Baronet, FRS (17 December 1778 – 29 May 1829) was a British chemist and physicist. ... The Royal Navy is the navy of the United Kingdom. ... For other uses, see Iron (disambiguation). ... A hull is the body or frame of a ship or boat. ... For other uses, see Copper (disambiguation). ... For the hazard, see corrosive. ... For other uses, see Rust (disambiguation). ...


Galvanic CP

Today, galvanic or sacrificial anodes are made in various shapes using alloys of zinc, magnesium and aluminium. The electrochemical potential, current capacity, and consumption rate of these alloys are superior for CP than iron. A galvanic anode, also known as a sacrificial anode, is used to protect metals from galvanic corrosion, by the use of a metal electrode which is itself consumed instead in an anodic oxidation reaction. ... A sacrificial anode, or sacrificial rod, is a metallic anode used in an electrochemical process where it is intended to be dissolved to protect other metallic components. ... An alloy is a homogeneous hybrid of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. ... General Name, symbol, number zinc, Zn, 30 Chemical series transition metals Group, period, block 12, 4, d Appearance bluish pale gray Standard atomic weight 65. ... General Name, symbol, number magnesium, Mg, 12 Chemical series alkaline earth metals Group, period, block 2, 3, s Appearance silvery white solid at room temp Standard atomic weight 24. ... Aluminum redirects here. ... Electrochemical potential is a thermodynamic measure that reflects energy from entropy and electrostatics and is typically invoked in molecular processes that involve diffusion. ...


Galvanic anodes are designed and selected to have a more "active" voltage (technically a more negative electrochemical potential) than the metal of the structure (typically steel). For effective CP, the potential of the steel surface is polarized (pushed) more negative until the surface has a uniform potential. At that stage, the driving force for the corrosion reaction is halted. The galvanic anode continues to corrode, consuming the anode material until eventually it must be replaced. The polarization is caused by the current flow from the anode to the cathode. The driving force for the CP current flow is the difference in electrochemical potential between the anode and the cathode. For other uses, see Steel (disambiguation). ... Diagram of a zinc anode in a galvanic cell. ...


Impressed Current CP

For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection. Impressed Current Cathodic Protection (ICCP) systems use anodes connected to a DC power source (a cathodic protection rectifier). Anodes for ICCP systems are tubular and solid rod shapes or continuous ribbons of various specialized materials. These include high silicon cast iron, graphite, mixed metal oxide, platinum and niobium coated wire and others. Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ... Cathodic Protection Rectifiers are AC powered electrical equipment that provide direct current for impressed current cathodic protection systems. ... Not to be confused with Silicone. ... Cast iron usually refers to grey cast iron, but can mean any of a group of iron-based alloys containing more than 2% carbon (alloys with less carbon are carbon steel by definition). ... For other uses, see Graphite (disambiguation). ... This article is about metallic materials. ... An oxide is a chemical compound containing at least one oxygen atom and other elements. ... General Name, Symbol, Number platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10, 6, d Appearance grayish white Standard atomic weight 195. ... General Name, Symbol, Number niobium, Nb, 41 Chemical series transition metals Group, Period, Block 5, 5, d Appearance gray metallic Standard atomic weight 92. ...

A cathodic protection rectifier connected to a pipeline

A typical ICCP system for a pipeline would include an AC powered rectifier with a maximum rated DC output of between 10 and 50 amperes and 50 volts. The positive DC output terminal is connected via cables to the array of anodes buried in the ground (the anode groundbed). For many applications the anodes are installed in a 60 m (200 foot) deep, 25 cm (10-inch) diameter vertical hole and backfilled with conductive coke (a material that improves the performance and life of the anodes). A cable rated for the expected current output connects the negative terminal of the rectifier to the pipeline. The operating output of the rectifier is adjusted to the optimum level by a CP expert after conducting various tests including measurements of electrochemical potential. Image File history File links Download high resolution version (1024x768, 74 KB) Summary A Cathodic Protection Rectifier. ... Image File history File links Download high resolution version (1024x768, 74 KB) Summary A Cathodic Protection Rectifier. ... Cathodic Protection Rectifiers are AC powered electrical equipment that provide direct current for impressed current cathodic protection systems. ... For other uses, see Ampere (disambiguation). ... Josephson junction array chip developed by NIST as a standard volt. ... For other uses, see Cable (disambiguation). ... A groundbed is a array of electrodes installed in the ground to provide a low resistance electrical path to ground or earth. ... This article is about the unit of length. ... A foot (plural: feet or foot;[1] symbol or abbreviation: ft or, sometimes, ′ – a prime) is a unit of length, in a number of different systems, including English units, Imperial units, and United States customary units. ... This article is being considered for deletion in accordance with Wikipedias deletion policy. ... Coke Coke is a solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. ... Electrochemical potential is a thermodynamic measure that reflects energy from entropy and electrostatics and is typically invoked in molecular processes that involve diffusion. ...


Telephone wiring uses a form of cathodic protection. A circuit consists of a pair of wires, with forty-eight volts across them when the line is idle. The more positive wire is grounded, so that the wires are at 0 V and -48 V with respect to earth ground. The 0 V wire is at the same potential as the surrounding earth, so it corrodes no faster or slower than if it were not connected electrically. The -48 V wire is cathodically protected. This means that in the event of minor damage to the insulation on a buried cable, both copper conductors will be unaffected, and unless the two wires short together, service will not be interrupted.


If instead the polarity were switched, so that the wires were at 0 V and +48 V with respect to the surrounding earth, then the 0 V wire would be unaffected as before, but the +48 V wire would quickly be destroyed if it came into contact with wet earth. The electrochemical action would plate metal off the +48 V wire, reducing its thickness to the point that it would eventually break, interrupting telephone service. This choice of polarity was not accidental; corrosion problems in some of the earliest telegraphy systems pointed the way. Telegraph and Telegram redirect here. ...


Testing

Electrochemical potential is measured with reference electrodes. Copper-copper(II) sulfate electrodes are used for structures in contact with soil or fresh water. Silver chloride electrodes are used for seawater applications. Electrochemical potential is a thermodynamic measure that reflects energy from entropy and electrostatics and is typically invoked in molecular processes that involve diffusion. ... Reference electrode is an electrode which has a stable and well-known electrode potential. ... Diagram of an electrode used in the field The Copper-copper(II) sulfate electrode is a type of reference electrode, based on the redox reaction with participation of the metal (copper) and its salt - copper(II) sulfate. ... Loess field in Germany Surface-water-gley developed in glacial till, Northern Ireland For the American hard rock band, see SOiL. For the System of a Down song, see Soil (song). ... A silver chloride electrode is a type of reference electrode, used for measuring electrochemical potential. ... Annual mean sea surface salinity for the World Ocean. ...


Galvanized Steel

Galvanizing (or galvanising, outside of the USA) generally refers to hot-dip galvanizing which is a way of coating steel with a layer of metallic zinc. 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 localised cathodic protection - the zinc acts as a sacrificial anode. Galvanization, named after the Italian scientist Luigi Galvani, was originally the administration of electric shocks (in the 19th century also termed Faradism, after Michael Faraday). ... Hot-dip galvanizing is a form of galvanization. ... A coating is a covering that is applied to an object to protect it or change its appearance. ... A sacrificial anode, or sacrificial rod, is a metallic anode used in an electrochemical process where it is intended to be dissolved to protect other metallic components. ...


Standards

  • EN 12068:1999 - Cathodic protection. External organic coatings for the corrosion protection of buried or immersed steel pipelines used in conjunction with cathodic protection. Tapes and shrinkable materials
  • EN 12473:2000 - General principles of cathodic protection in sea water
  • EN 12474:2001 - Cathodic protection for submarine pipelines
  • EN 12495:2000 - Cathodic protection for fixed steel offshore structures
  • EN 12499:2003 - Internal cathodic protection of metallic structures
  • EN 12696:2000 - Cathodic protection of steel in concrete
  • EN 12954:2001 - Cathodic protection of buried or immersed metallic structures. General principles and application for pipelines
  • EN 13173:2001 - Cathodic protection for steel offshore floating structures
  • EN 13174:2001 - Cathodic protection for harbour installations
  • EN 13509:2003 - Cathodic protection measurement techniques
  • EN 13636:2004 - Cathodic protection of buried metallic tanks and related piping
  • EN 14505:2005 - Cathodic protection of complex structures
  • EN 15112:2006 - External cathodic protection of well casing
  • EN 50162:2004 - Protection against corrosion by stray current from direct current systems
  • BS 7361-1:1991 - Cathodic Protection
  • NACE RP0169:2002 - Control of External Corrosion on Underground orSubmerged Metallic Piping Systems
  • NACE TM 0497 - Measurement Techniques Related to Criteria for Cathodic Protection on Underground or Submerged Metallic Piping Systems

External links

  • NACE International (formerly the National Association of Corrosion Engineers) - largest professional association of CP experts
  • US Army Corps of Engineers, "Engineering and Design - Cathodic Protection Systems for Civil Works Structures", Engineering manual 1110-2-2704, 12 July 2004
  • Cathodic Protection 101 - a helpful basic guide to cathodic protection as it pertains to the offshore oil and gas market (complete with diagrams and chemical equations)
  • LIDA(R) products - datasheets of Titanium MMO anodes for Cathodic Protection
  • Cathodic Protection Papers - a library of technical articles and papers about offshore cathodic protection (provided by Deepwater Corrosion Services)
  • Cathodic Protection Masonry Clad Steel Frame Buildings - a helpful basic guide to cathodic protection as it pertains to the Masonry Clad Steel Frame Building (complete with Links to Published Articles)
  • Cathodic Protection - manual pdf free download
  • Electrical Engineering Cathodic Protection - pdf free download

  Results from FactBites:
 
Cathodic protection (412 words)
Cathodic protection is often applied to coated structures, with the coating providing the primary form of corrosion protection.
Cathodic protection has probably become the most widely used method for preventing the corrosion deterioration of metallic structures in contact with any forms of electrolytically conducting environments, i.e.
Cathodic protection basically reduces the corrosion rate of a metallic structure by reducing its corrosion potential, bringing the metal closer to an immune state.
SESCO, Inc. :: Cathodic Protection Engineering (3887 words)
The zinc is being corroded, and the steel pipe is being polarized and protected from corrosion.
Cathodic Protection can be defined as…the control of electrolytic corrosion…by the application of direct current in such a way that the structure to be protected is made to act as the cathode of an electrolyte cell.
This is a basic criterion for the cathodic protection of a buried structure.
  More results at FactBites »

 
 

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