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Cathodic protection is a method of protecting metal structures from corrosion. The metals from which these structures are made — commonly steel — are prone to corrosion through an oxidation reaction when they are in frequent contact with water. The reaction involves the metal giving up electrons and is promoted by traces of salts dissolved in the water, causing the water to act as an electrolyte. Corrosion can thus be seen as an electrochemical process. Cathodic protection turns the metal structure into a cathode — a positively charged electrode — by setting up 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 the steel rods in reinforced concrete. Metals that are more resistant to corrosion tend to be more expensive than steel and may lack the necessary strength, so corrosion-protected steel is usually the best option, although other metals that can corrode may 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 may come into contact with this metal in the form of rain, condensation or moist, surrounding soil. Water droplets in contact with iron form an electrochemical cell in which iron is oxidized by the reaction Fe -> Fe2+ + 2e-. The iron II (Fe2+) ions go into solution in the water, while the electrons flow through the metal, and at the edge of the water, an interaction of the electrons, oxygen and water produces hydroxide (OH-) ions by the reaction: O2 + 2H2O + 4e- -> 4OH-. The negative hydroxide ions react with the positive iron II ions in the 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 that seek to prevent this corrosion by providing an alternative source of electrons. In galvanic protection, a metal with a more negative redox potential potential than the metal to be protected is connected to the structure by 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 electrical cell with a current flowing from the anode to the structure, which acts as the cathode. The anode loses electrons and is corroded; for this reason, it is known as a “sacrificial anode.”
A problem with galvanic cathodic protection is that, eventually, the anode will be corroded to the point where it no longer provides protection and needs to 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 electrical current from the anode to the structure to be protected. A direct current (DC), as opposed to alternating current (AC), is required, so a rectifier is used to convert AC to DC. This method provides much more lasting protection as the current is supplied externally instead of being generated by the reaction of the anode with its surroundings, so that the lifespan of the anode is greatly increased.