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Cathodic Protection

 
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Cathodic protection (CP) is a technique to control the corrosion
Corrosion

Corrosion means the breaking down of essential properties in a material due to chemical reactions with its surroundings. In the most common use of the word, this means a loss of electrons of metals reacting with water and oxygen....
 of a metal surface by making it work as a cathode
Cathode

A cathode is an electrode through which electric charge flows out of a polarized electrical device. Mnemonic: CCD .From an electrochemical point of view, positively charged ion invariably move toward the cathode and/or negatively charged ion move away from it to balance the electrons arriving from external circuitry....
 of an electrochemical cell
Electrochemical cell

An electrochemical cell is a device used for generating an electromotive force and current from electrochemistry, or the reverse, inducing a chemical reaction by a flow of current....
. This is achieved by placing in contact with the metal to be protected another more easily corroded metal to act as the anode of the electrochemical cell
Electrochemical cell

An electrochemical cell is a device used for generating an electromotive force and current from electrochemistry, or the reverse, inducing a chemical reaction by a flow of current....
.






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Cathodic protection (CP) is a technique to control the corrosion
Corrosion

Corrosion means the breaking down of essential properties in a material due to chemical reactions with its surroundings. In the most common use of the word, this means a loss of electrons of metals reacting with water and oxygen....
 of a metal surface by making it work as a cathode
Cathode

A cathode is an electrode through which electric charge flows out of a polarized electrical device. Mnemonic: CCD .From an electrochemical point of view, positively charged ion invariably move toward the cathode and/or negatively charged ion move away from it to balance the electrons arriving from external circuitry....
 of an electrochemical cell
Electrochemical cell

An electrochemical cell is a device used for generating an electromotive force and current from electrochemistry, or the reverse, inducing a chemical reaction by a flow of current....
. This is achieved by placing in contact with the metal to be protected another more easily corroded metal to act as the anode of the electrochemical cell
Electrochemical cell

An electrochemical cell is a device used for generating an electromotive force and current from electrochemistry, or the reverse, inducing a chemical reaction by a flow of current....
. Cathodic protection systems are most commonly used to protect steel
Steel

Steel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.14% by weight , depending on grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten....
, water or fuel pipelines
Pipeline transport

Pipeline transport is the transportation of goods through a Pipe . Most commonly, liquid and gases are sent, but pneumatic tubes that transport solid capsules using compressed air have also been used....
 and storage tank
Storage tank

A storage tank is a container, usually for holding liquids, sometimes for compressed gases . The term can be used for reservoirs , and for manufactured containers....
s, steel pier piles, ships, offshore oil platform
Oil platform

An offshore platform, often referred to as an oil platform or oil rig, is a large structure used to house workers and machinery needed to drill wells in the ocean bed, extract Petroleum and/or natural gas, process the produced fluids, and ship them to shore....
s and onshore oil well
Oil well

An oil well is a general term for any boring through the Earth's surface designed to find and produce petroleum Petroleum hydrocarbons. Usually some natural gas is produced along with the oil, and a well designed to produce mainly or only gas may be termed a gas well....
 casings.

Cathodic protection can be, in some cases, an effective method of preventing stress corrosion cracking
Stress corrosion cracking

Stress corrosion cracking is the unexpected sudden failure of normally ductile metals or tough thermoplastics subjected to a tensile stress in a corrosion environment, especially at elevated temperature in the case of metals....
.

History

Cathodic protection was first described by Sir Humphry Davy
Humphry Davy

Sir Humphry Davy, 1st Baronet Fellow of the Royal Society Royal Irish Academy was a Cornish chemist and inventor. He is probably best remembered today for his discoveries of several alkali metal and alkaline earth metals, as well as contributions to the discoveries of the elemental nature of chlorine and iodine....
 in a series of papers presented to the Royal Society
Royal Society

The Royal Society of London for the Improvement of Natural Knowledge, known simply as the Royal Society, or even the Royal, is a learned society for science that was founded in 1660 and is considered by most to be the oldest such society still in existence....
 in London in 1824. After a series of tests, the first application was to the HMS Samarang in 1824. Sacrificial anode
Sacrificial anode

A sacrificial anode, or sacrificial rod, is a metallic anode used in cathodic protection where it is intended to be dissolved to protect other metallic components....
s made from iron
Iron

Iron is a chemical element with the symbol Fe and atomic number 26. Iron is a Group 8 element and period 4 element. Iron is lustrous and silvery in color....
 were attached to the copper
Copper

Copper is a chemical element with the symbol Cu and atomic number 29.It is a ductile metal with very high thermal and electrical conductivity....
 sheath of the hull below the waterline and dramatically reduced the corrosion rate of the copper. However, a side effect of the CP was to increase marine growth. Copper, when corroding, releases copper ions which have an anti-fouling effect. Since excess marine growth affected the performance of the ship, the Royal Navy
Royal Navy

The Royal Navy of the United Kingdom is the oldest of the British Armed Forces . From the mid-18th century until well into the 20th century, it was the most powerful navy in the world, playing a key part in establishing the British Empire as the dominant world power from 1815 until the early 1940s....
 decided that it was better to allow the copper to corrode and have the benefit of reduced marine growth, so CP was not used further.

Galvanic CP

Today, galvanic
Galvanic anode

A galvanic anode, a type of sacrificial anode, is one of the main components of a galvanic cathodic protection system used to protect metals from corrosion, by the use of a metal electrode which is itself consumed instead in an anodic oxidation reaction....
 or sacrificial anode
Sacrificial anode

A sacrificial anode, or sacrificial rod, is a metallic anode used in cathodic protection where it is intended to be dissolved to protect other metallic components....
s are made in various shapes using alloy
Alloy

An alloy is a partial or complete solid solution of one or more chemical element in a metallic matrix. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may be homogeneous in distribution depending on thermal history....
s of zinc
Zinc

Zinc is a metallic chemical element with the symbol Zn and atomic number 30. It is a first-row transition metal of the group 12 element of the periodic table....
, magnesium
Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, atomic weight 24.3050 and common oxidation number +2.Magnesium, an alkaline earth metal, is the ninth most abundance of the chemical elements in the universe by mass....
 and aluminium
Aluminium

Aluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13....
. The electrochemical potential
Electrochemical potential

In electrochemistry, the electrochemical potential, , sometimes confusingly abbreviated to ECP, is a thermodynamic measure that combines the concepts of energy stored in the form of chemical potential and electric charge....
, current capacity, and consumption rate of these alloys are superior for CP than iron.

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
Steel

Steel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.14% by weight , depending on grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten....
). 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
Anode

An anode is an electrode through which electric charge flows into a polarized electrical device. Mnemonic: ACID . Electrons flow in the opposite direction to the positive electric current....
 to the cathode. The driving force for the CP current flow is the difference in electrochemical potential between the anode and the cathode.

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
Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as battery , thermocouples, solar cells, and commutator-type electric machines of the dynamo type....
 power source (a cathodic protection rectifier
Cathodic protection rectifier

Cathodic protection rectifiers are Alternating current-powered electrical equipment that provide direct current for cathodic protection#Impressed Current CP systems....
). Anodes for ICCP systems are tubular and solid rod shapes or continuous ribbons of various specialized materials. These include high silicon
Silicon

Silicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. The atomic mass is 28.0855....
 cast iron
Cast iron

Cast iron usually refers to Gray iron, but also identifies a large group of ferrous alloys, which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy....
, graphite
Graphite

The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek language ??afe?? : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead....
, mixed metal oxide
Mixed metal oxide

Mixed metal oxide electrodes are devices with useful properties for chemical electrolysis. The term refers to a mixture of platinum-group metals used to form the surface of the electrode, which provides efficient electrolytic action while resisting corrosion from highly reactive products such as chlorine....
, platinum
Platinum

Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is in Group 10 of the periodic table of elements....
 and niobium
Niobium

Niobium , or columbium , is a chemical element with symbol Nb and atomic number 41. A rare, soft, grey, ductile transition metal, niobium is found in the minerals pyrochlore and columbite....
 coated wire and others.

Cathodic Protection Rectifier
A typical ICCP system for a pipeline would include an AC powered rectifier with a maximum rated DC output of between 10 and 50 ampere
Ampere

The ampere is the International System of Units unit of electric current. The ampere, in practice often shortened to amp, is an SI base unit, and is named after Andr?-Marie Amp?re, one of the main discoverers of electromagnetism....
s and 50 volt
Volt

The volt is the SI SI derived unit of electric potential difference or electromotive force, commonly known as voltage. It is named in honor of the Lombard physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery ....
s. The positive DC output terminal is connected via cable
Cable

A cable is a large fiber or metal rope, used for hauling, lifting, or towing, or an assembly of two or more insulated electrical conductors, laid up together as an assembly....
s to the array of anodes buried in the ground (the anode groundbed
Groundbed

A groundbed is an array of electrodes installed in the ground to provide a low Electrical resistance electricity path to Ground or earth. A groundbed is a component in an earthing system....
). For many applications the anodes are installed in a 60 m
Metre

The metre or meter is a Unit of measurement of length. It is the SI base unit of length in the metric system and in the International System of Units , used around the world for general and scientific purposes....
 (200 foot) deep, 25 cm (10-inch) diameter vertical hole and backfilled with conductive coke
Coke (fuel)

Cokes are the solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. Cokes from coal are grey, hard, and porous....
 (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 after conducting various tests including measurements of electrochemical potential
Electrochemical potential

In electrochemistry, the electrochemical potential, , sometimes confusingly abbreviated to ECP, is a thermodynamic measure that combines the concepts of energy stored in the form of chemical potential and electric charge....
.

Testing

Electrochemical potential
Electrochemical potential

In electrochemistry, the electrochemical potential, , sometimes confusingly abbreviated to ECP, is a thermodynamic measure that combines the concepts of energy stored in the form of chemical potential and electric charge....
 is measured with reference electrode
Reference electrode

A Reference electrode is an electrode which has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant concentrations of each participants of the redox reaction....
s. Copper-copper(II) sulfate electrode
Copper-copper(II) sulfate electrode

The Copper-copper sulfate electrode is a type of reference electrode, based on the redox reaction with participation of the metal and its salt - copper sulfate....
s are used for structures in contact with soil
Soil

Soil is the naturally occurring, unconsolidated or loose covering on the Earth's surface. Soil is composed of particles of broken rock that have been altered by chemical and environmental processes including weathering and erosion....
 or fresh water. Silver chloride electrode
Silver chloride electrode

A silver chloride electrode is a type of reference electrode, commonly used in Electrochemistry measurements. For example, it is usually the internal reference electrode in pH meters....
s are used for seawater
Seawater

Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5%, or 35 parts per thousand . This means that every 1 kg of seawater has approximately 35 grams of sea salt ....
 applications.

Galvanized steel

Galvanizing (or galvanising, outside of the USA) generally refers to hot-dip galvanizing
Hot-dip galvanizing

Hot-dip galvanizing is a form of galvanization. It is the process of coating iron or steel with a thin zinc layer, by passing the steel through a molten bath of zinc at a temperature of around 860 ?F ....
 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
Coating

Coating is a covering that is applied to an object. The aim of applying coatings is to improve surface properties of a bulk material usually referred to as a Substrate ....
 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
Sacrificial anode

A sacrificial anode, or sacrificial rod, is a metallic anode used in cathodic protection where it is intended to be dissolved to protect other metallic components....
.

Potential problems


Production of hydrogen ions

A side effect of improperly performed cathodic protection may be production of hydrogen
Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the chemical symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly combustion and explosive Diatomic molecule gas with the molecular formula H2....
 ions, leading to its absorption in the protected metal and subsequent hydrogen embrittlement
Hydrogen embrittlement

Hydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and crack following exposure to hydrogen....
 of welds and materials with high hardness. Under normal conditions, the ionic hydrogen will combine at the metal surface to create hydrogen gas, which cannot penetrate the metal. Hydrogen ions, however, are small enough to pass through the crystalline steel structure, and can in some cases lead to hydrogen embrittlement
Hydrogen embrittlement

Hydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and crack following exposure to hydrogen....
.

Cathodic Shielding

Effectiveness of cathodic protection systems on steel pipelines is often nullified by the use of solid film backed corrosion coatings such as polyethylene tapes, shrinkable pipeline sleeves, and factory applied single or multiple solid film coatings. This phenomenon occurs because of the property of high electrical resistivity which solid film backings exhibit. Protective electrical current from the cathodic protection system is blocked by the highly resistive film backing, and cannot reach the steel pipeline surface. Cathodic shielding was first defined in the 1980s as being a problem,and today solid film backed coatings are largely prohibited in North America.

A 1999 report concerning a 20,600 bbl. spill from a Saskatchewan crude oil line contains an excellent definition of the cathodic shielding problem:

"The triple situation of disbondment of the (corrosion) coating, the dielectric nature of the coating and the unique electrochemical environment established under the exterior coating, which acts as a shield to the electrical CP current, is referred to as CP shielding. 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. 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. This produces an area on the pipeline of insufficient CP defence against metal loss aggravated by an exterior corrosive environment." (See "Analysis" section of report).

Cathodic shielding is referenced in a number of the standards listed below. For example, the NACE SP0169:2007 standard defines shielding in section 2, cautions against the use of materials that create electrical shielding in section 4.2.3, cautions against use of external coatings that create electrical shielding in section 5.1.2.3, and instructs readers to take 'appropriate action' when the effects of electrical shielding of CP current are detected on an operating pipeline in section 10.9.

Standards

  • DNV-RP-B401 - Cathodic Protection Design - Det Norske Veritas
  • 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 SP0169:2007 - Control of External Corrosion on Underground or Submerged Metallic Piping Systems
  • NACE TM 0497 - Measurement Techniques Related to Criteria for Cathodic Protection on Underground or Submerged Metallic Piping Systems


External links

  • (formerly the National Association of Corrosion Engineers) - largest professional association of CP experts
  • US Army Corps of Engineers, "", Engineering manual 1110-2-2704, 12 July 2004
  • - Cathodic Protection Theory and useful documents on Cathodic Protection
  • - a helpful basic guide to cathodic protection as it pertains to the Masonry Clad Steel Frame Building (complete with Links to Published Articles)
  • - manual pdf free download
  • - pdf free download
  • - a beginner's guide to cathodic protection offshore
  • - a graphic explanation of cathodic protection shielding