Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making it work as a cathode of an electrochemical cell. 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. Cathodic protection systems are most commonly used to protect steel, water or fuel pipelines and storage tanks, steel pier piles, ships, offshore oil platforms and onshore oil well casings.

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

History

Cathodic protection was first described by Sir Humphry Davy in a series of papers presented to the Royal Society in London in 1824. After a series of tests, the first application was to the HMS Samarang 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. 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 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 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 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.

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.

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 after conducting various tests including measurements of electrochemical potential.

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.

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 localized cathodic protection - the zinc acts as a sacrificial anode.

Potential problems

A side effect of improperly performed cathodic protection may be 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 can in some cases lead 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

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 - Cathodic Protection Theory and useful documents on Cathodic Protection
• 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
• Cathodic Protection 101 - a beginner's guide to cathodic protection offshore
• IMP-UK - specialists in Cathodic Protection and Electronic AntiFouling

References