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Cathodic protection prevents metal corrosion by transforming the metal structure into a cathode using an electrochemical cell with a more electropositive metal as the anode. This method can be used on underground pipes, above-ground structures, and partially submerged structures. Galvanic protection and impressed current cathodic protection are two methods used to prevent corrosion. Galvanic protection uses a sacrificial anode, while impressed current cathodic protection uses a power supply to generate an electric current.
Cathodic protection is a method of protecting metal structures from corrosion. The metals these structures are made of – commonly steel – are subject to corrosion through an oxidation reaction when in frequent contact with water. The reaction involves the metal giving up electrons and is promoted by trace amounts of salts dissolved in the water, causing the water to act as an electrolyte. Corrosion can therefore be seen as an electrochemical process. Cathodic protection transforms the metal structure into a cathode, a positively charged electrode, by installing an electrochemical cell using a more electropositive metal as the anode, so that the structure does not lose electrons to its surroundings.
This method of protection can be used on underground pipes and tanks; above-ground structures, such as electricity pylons; and partially submerged structures, such as ships and drilling rigs. It can also be used to protect steel rods in reinforced concrete. Metals that are more resistant to corrosion tend to be more expensive than steel and may not have the strength needed, so corrosion protected steel is usually the best option, although other metals that can corrode can also be protected in this way.
Steel consists mainly of iron, which has a redox potential of -0.41 volts. This means that it will tend to lose electrons in an environment that has a less negative redox potential, such as water, which can come into contact with this metal in the form of rain, condensation or the surrounding moist soil. The drops of water in contact with the iron form an electrochemical cell in which the iron is oxidized by the reaction Fe -> Fe2+ + 2e-. Iron II ions (Fe2+) go into solution in water, while electrons flow through the metal, and at the water’s edge, an interaction of electrons, oxygen and water produces hydroxide ions (OH-) by the reaction: O2 + 2H2O + 4e- -> 4OH-. Negative hydroxide ions react with positive iron II ions in water, forming insoluble iron II hydroxide (Fe(OH)2), which is then oxidized to iron III oxide (Fe2O3), better known as rust.
There are two main methods of cathodic protection which seek to prevent this corrosion by providing an alternative source of electrons. In galvanic protection, a metal with a more negative redox potential than the metal to be protected is connected to the structure via an insulated wire, forming an anode. Magnesium, with a redox potential of -2.38 volts is often used for this purpose – other commonly used metals are aluminum and zinc. This procedure sets up an electric cell with a current flowing from the anode to the structure, which serves as the cathode. The anode loses electrons and is corroded; for this reason it is known as a “sacrificial anode”.
One problem with galvanic cathodic protection is that, eventually, the anode will be corroded to the point where it no longer provides protection and must be replaced. An alternative cathodic protection system is impressed current cathodic protection (ICCP). This is similar to the galvanic method, except that a power supply is used to generate an electric current from the anode to the structure to be protected. Direct current (DC) is required, as opposed to alternating current (AC), so a rectifier is used to convert AC to DC. This method provides much longer lasting protection as the current is supplied from the outside instead of being generated by the reaction of the anode with its surroundings, so the life of the anode is greatly increased.
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