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Cathodic Protection CP is a method used to protect metal structures from corrosion. Cathodic protection systems are most commonly used to protect steel water and fuel pipelines and tanks, steel pier piles, ships, and offshore platforms.
The first use of CP was in 1824, when Sir Humphry Davy, of the British Navy, attached chunks of iron to the external, below water line, hull of a copper clad ship. Iron has a stronger tendency to corrode then copper and when connected to the hull, the corrosion rate of the copper was dramatically reduced.
Today, galvanic anodes are made in various shapes using alloys of zinc, magnesium and aluminum. 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.
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 direct current power source ( ACAn alternating current AC is an electrical current, where the magnitude of the current varies in a cyclical form, as opposed to direct current, where the polarity of the current stays constant. The usual waveform of an AC circuit is generally that of a pe powered rectifierA rectifier is one or more diodes arranged for converting alternating current (AC) to direct current (DC). When just one diode is used to rectify AC (by omitting the negative portion of the waveform) the difference between the term diode and the term rect). Anodes for ICCP systems are tubular and solid rod shapes of various specialized materials. These include high siliconSilicon is the chemical element in the periodic table that has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon. It is the second most abundant element in the Earth's crust, making up 25 cast ironCast iron is a group of metals based on the three elements Iron, Carbon and Silicon. It is made by remelting pig iron, often along with substantial quantities of scrap iron, and taking various steps to remove undesirable contaminants such as phosphorus an, graphiteGraphite is one of the allotropes of carbon. See also allotropes of carbon. Unlike diamond, graphite is a conductor, and can be used, for instance, as the material in the electrodes of an electrical arc lamp. The pi orbital electrons delocalized across th, mixed metal oxide , platinumPlatinum is also a certification by the RIAA and other world recording industries, see: RIAA certification Platinum is a chemical element in the periodic table that has the symbol Pt and atomic number 78. A heavy, malleable, ductile, precious, gray-white and niobiumNiobium (or columbium is a chemical element in the periodic table that has the symbol Nb and atomic number 41. A rare, soft, gray, ductile transition metal, niobium is found in niobite and used in alloys. The most notable alloys are used to make special s coated wire and others.
A typical ICCP system for a pipeline would include an AC powered DCDirect current DC or continuous current ) is the continuous flow of electricity through a conductor such as a wire from high to low potential. In direct current, the electric charges flow always in the same direction, which distinguishes it from alternati 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 by a CP expert after conducting various potential measurements.
Protecting cars and trucks with CP is not technically feasible and devices marketed as such are scam products. Most modern cars have galvanized (zinc-coated) steel frames and panels. Unprotected steel forms a layer of iron oxide, which is permeable to air and water and allows corrosion to continue underneath. However, zinc oxide (produced on the surface of zinc-protected objects) is impermeable. As long as the zinc and zinc oxide layers are undisturbed (i.e. not scraped or sanded off), the steel underneath will not rust.
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