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.
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.
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 to iron. Polyaniline may also be effective in improving the efficiency of these materials or can be used alone.
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.
Sacrificial anode systems depend on the differences in corrosion potential that are established by the corrosion reactions that occur on different metals or alloys.
For example, the natural corrosion potential of iron is about -0.550 volts in seawater. The natural corrosion potential of zinc in seawater is about -1.2 volts. Thus if the two metals are electrically connected, the corrosion of the zinc becomes a source of negative charge which prevents corrosion of the iron.
The materials used for sacrificial anodes are either relatively pure active metals, such as zinc or magnesium, or are magnesium or aluminum alloys that have been specifically developed for use as sacrificial anodes. In applications where the anodes are buried, a special backfill material surrounds the anode in order to insure that the anode will produce the desired output.
Sacrificial anodes are normally supplied with either lead wires or cast-m straps to facilitate their connection to the structure being protected. The lead wires may be attached to the structure by welding or mechanical connections. These should have a low resistance and should be insulated to prevent increased resistance or damage due to corrosion.
When anodes with cast-in straps are used, the straps can either be welded directly to the structure or the straps can be used as locations for attachment.
A low resistance mechanically adequate attachment is required for good protection and resistance to mechanical damage. In the process of providing electrons for the cathodic protection of a less active metal the more active metal corrodes.
The more active metal (anode) is sacrificed to protect the less active metal (cathode). The amount of corrosion depends on the metal being used as an anode but is directly proportional to the amount of current supplied.
The anodes in sacrificial anode cathodic protection systems must be periodically inspected and replaced when consumed.
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.
As in sacrificial anode systems, impressed current systems depend on a supply of high energy electrons to stifle anodic reactions on a metal surface. In the case of an impressed current system these high energy electrons are supplied by a rectifier.
Low energy electrons that are picked up at a non-reactive anode bed are given additional energy by the action of a rectifier to be more energetic than the electrons that would be produced in the corrosion reaction.
The energy for the “electron energy pump” action of the rectifier is provided by ordinary alternating current. The effect of these electrons at the structure being protected is the same as that derived from the sacrificial anode type of cathodic protection system. However, the anode material serves only as a source of electrons and need not be consumed in providing protective current.
The materials used for impressed current cathodic protection can the pass a current into the environment without being consumed at a high rate. Graphite and high silicon cast iron are the most commonly used impressed current cathodic protection anode materials, however, other materials (such as magnetite, platinum, and newly developed ceramic materials) have been successfully used.
For buried anodes, a backfill of carbonaceous material is used to surround the anode to decrease the electrical resistance of the anode, to provide a uniform, low resistivity environment surrounding the anode and to allow gasses produced at the anode surface to vent. In practice, materials such as graphite are used for impressed current cathodic protection system anodes that are slowly consumed.
Anodes in impressed current systems must be inspected and replaced if consumed or otherwise damaged. As is the case for any electrical equipment, rectifiers used for impressed current cathodic protection systems require preventative maintenance to insure proper operation.
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.
When Cathodic Protection Should be Considered
Cathodic protection should be considered wherever the system requiring protection is exposed to an aggressive environment in such a manner that cathodic protection is technically feasible. Cathodic protection is technically feasible when the surfaces to be protected are buried or submerged.
Structures That Are Commonly Protected
External surfaces of buried metallic structures, surfaces of metal waterfront structures, such as sheet pilings or bearing piles, and the internal surfaces of tanks containing electrolytes, such as water, are applications where cathodic protection is usually technically feasible and cathodic protection is used in protecting such structures. Internal surfaces of small diameter pipelines and other areas where ion flow in the electrolyte is restricted by electrolyte resistance, cathodic protection has limited applicability.
Determining the Need for Protection
When construction of a new buried or submerged system is being planned, the corrosivity of the environment should be considered as one of the factors in the design of the system. If experience with similar systems in the vicinity of the construction site has shown that the site conditions are aggressive based on leak and failure records, cathodic protection should be considered as a means of controlling corrosion on the new system. Cathodic protection is one of the few methods of corrosion control that can be effectively used to control corrosion of existing buried of submerged metal surfaces. Thus, if leak records on an existing system show that corrosion is occurring, cathodic protection can be applied to stop the corrosion damage from increasing. Cathodic protection can, however, only stop further corrosion from occurring and cannot restore the material already lost due to corrosion.
When Protection Is Required
In some cases, cathodic protection is required by policy or regulation. Regulations by the Department of Transportation have established standards for transporting certain liquids and compressed gas by pipelines in order to establish minimum levels of safety. These regulations require that these pipelines be protected by cathodic protection combined with other means of corrosion control, such as protective coatings and electrical insulation. These regulations provide excellent guidelines for the application of cathodic protection to buried and submerged pipelines. In addition to these regulations, primarily due to the safety and environmental consequences of system failure, there are an increasing number of federal, state and local governmental regulations regarding the storage and transportation of certain materials that require corrosion control. Many of these regulations either specify cathodic protection as a primary means of corrosion control or allow its use as an alternative method of controlling corrosion.