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Cathodic protection

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Aluminium anodes mounted on a steel jacket structure.

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[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. 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.

Types

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 less 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, 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 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 after conducting various tests including measurements of electrochemical potential.

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.

Combinable metals

In order for cathodic protection to work, the anode must possess a lower potential (more negative) than that of the protected structure (cathode). The table below shows which metals can thus be combined.[3]

metal voltage
type 316 stainless steel (inactive) -0.05
monel -0.08
type 304 stainless steel (inactive) -0.08
silver -0.13
titanium -0.15
inconel alloy 600 -0.17
type 316 stainless stainless steel (active) -0.18
silicon bronze -0.18
nickel 200 -0.20
copper -0.24
cupronickel 70/30 -0.25
manganese bronze -0.27
brass -0.29
copper -0.36
type 304 stainless steel (active) -0.53
lead -0.55
grey cast iron -0.61
carbon steel -0.61
aluminum -0.75
cadmium -0.80
aluminum 3003 -0.94
zinc -1.03
galvanized iron -1.05
magnesium alloy -1.6

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 or Saturated calomel electrodes (SCE) are used for seawater applications. Electrodes used in open circuit, such as pipeline cathodic protection monitoring, are not reference cells. This is because they cannot be placed at the interface between the anode of the corrosion cell and the electrolyte, as is done in laboratory work. Its widely recognised that there are errors in pipeline cathodic protection measurements and attempts to remove these errors are included in all cathodic protection surveys.

Problems

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.

Effectiveness

Effectiveness of cathodic protection systems on steel pipelines is often nullified by the use of solid film backed anti-corrosion coatings such as polyethylene tapes, shrinkable pipeline sleeves, and factory applied single or multiple solid film coatings. This phenomenon occurs because of the high electrical resistivity of these film backings. 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." <Transportation Safety Board of Canada, Report Number P99H0021, 1999>[1](See "Analysis" section of report).

Cathodic shielding is referenced in a number of the standards listed below. Newly issued USDOT regulation Title 49 CFR 192.112, in the section for Additional design requirements for steel pipe using alternative maximum allowable operating pressure requires that "The pipe must be protected against external corrosion by a non-shielding coating". Also, 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

  • sderw* ASME B31Q 0001-0191
  • 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

References

  1. ^ Davy, H., Phil. Trans. Roy. Soc., 114,151,242 and 328 (1824)
  2. ^ Ashworth V., Corrosion Vol. 2, 3rd Ed., 1994, 10:3
  3. ^ Het handboek voor de zeiler by H.C. Herreshoff

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