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 Cathodic Protection
History
Cathodic protection was first described by Sir Humphry Davy in a series of papers presented to the Royal Society[1] in London in 1824. After a series of tests, the first application was to the HMS Samarang[2] in 1824.Sacrificial anodes made from iron were attached to the copper sheath of the hull below the waterline and dramatically reduced the corrosion rate of the copper.
The electrochemical series shows the difference in the ‘energy’ levels of copper and iron. It is this difference that allows one metal to behave sacrificially to another.
Cathodic protection is the most widely applied electrochemical corrosion control technique. This is accomplished by applying a direct current to the structure which causes the structure potential to change from the natural corrosion potential (Ecorr) to a protective potential in the immunity region.
The required cathodic protection current is supplied by sacrificial anode materials or by an impressed current system. Most metals in contact with an aqueous environment having a near neutral pH can be cathodically protected.


There are types of cathodic protection:

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.
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 electron 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.
Impressed current CP

For larger structures or when long term corrosion protection is required, 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.

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 embrittlement 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 lead in some cases to hydrogen embrittlement.

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