Cathodic protection wikipedia
Excerpt from Wikipedia, the free encyclopedia
Cathodic protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. The simplest method to apply CP is by connecting the metal to be protected with a piece of another more easily corroded "sacrificial metal" to act as the anode of the electrochemical cell. The sacrificial metal then corrodes instead of the protected metal. For structures where passive galvanic CP is not adequate, for example in long pipelines, an external DC electrical power source is sometimes used to provide current. Cathodic protection systems are used to protect a wide range of metallic structures in various environments. Common applications are; steel water or fuel pipelines and storage tanks such as home water heaters, steel pier piles; ship and boat hulls; offshore oil platforms and onshore oil well casings and metal reinforcement bars in concrete buildings and structures. Another common application is in galvanized steel, in which a sacrificial coating of zinc on steel parts protects them from rust.
Cathodic protection can, in some cases, prevent stress corrosion cracking.
Aluminium sacrificial anodes (light colored rectangular bars) mounted on a steel jacket structure.
Zinc sacrificial anode (rounded object) screwed to the underside of the hull of a small boat.
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.
Davy was assisted in his experiments by his pupil Michael Faraday, who continued his research after Davy's death. In 1834, Faraday discovered the quantitative connection between corrosion weight loss and electric current and thus laid the foundation for the future application of cathodic protection.
Thomas Edison experimented with impressed current cathodic protection on ships in 1890, but was unsuccessful due to the lack of a suitable current source and anode materials. It would be 100 years after Davy's experiment before cathodic protection was used widely on oil pipelines in the United States.
CP was applied to steel gas pipelines beginning in 1928 and more widely in the 1930s.
In the usual application, a galvanic anode, a piece of a more electrochemically "active" metal, is attached to the vulnerable metal surface where it is exposed to the corrosive liquid. Galvanic anodes are designed and selected to have a more "active" voltage (more negative electrochemical potential) than the metal of the target 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 with the protected surface is removed. The galvanic anode continues to corrode, consuming the anode material until eventually it must be replaced. Polarization of the target structure is caused by the electron flow from the anode to the cathode, so the two metals must have a good electrically conductive contact. The driving force for the CP current is the difference in electrochemical potential between the anode and the cathode.
Galvanic sacrificial anode attached to the hull of a ship, showing corrosion.
Galvanic or sacrificial anodes are made in various shapes and sizes using alloys of zinc, magnesium and aluminium. ASTM International publishes standards on the composition and manufacturing of galvanic anodes.
In order for galvanic cathodic protection to work, the anode must possess a lower (that is, more negative) electrochemical potential than that of the cathode (the target structure to be protected). The table below shows a simplified galvanic series which is used to select the anode metal. The anode must be chosen from a material that is lower on the list than the material to be protected.
Impressed current systems
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. Usually this will be a cathodic protection rectifier, which converts an AC power supply to a DC output. In the absence of an AC supply, alternative power sources may be used, such as solar panels, wind power or gas powered thermoelectric generators. For example, all telephone lines are biased to -36 to -60 volts are compared to earth, to reduce galvanic corrosion.
Anodes for ICCP systems are available in a variety of shapes and sizes. Common anodes are tubular and solid rod shapes or continuous ribbons of various materials. These include high silicon, cast iron, graphite, mixed metal oxide, platinum and niobium coated wire and other materials.
For pipelines, anodes are arranged in groundbeds either distributed or in a deep vertical holes depending on several design and field condition factors including current distribution requirements.
Simple impressed current cathodic protection system. A source of DC electric current is used to help drive the protective electrochemical reaction.