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Zinc sacrificial anode (rounded object) screwed to the underside of the hull of a small boat. Cathodic protection ( CP ; / k æ ˈ θ ɒ d ɪ k / ⓘ ) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell . [ 1 ]
The terms anode and cathode are not defined by the voltage polarity of electrodes, but are usually defined by the direction of current through the electrode. An anode usually is the electrode of a device through which conventional current (positive charge) flows into the device from an external circuit, while a cathode usually is the electrode through which conventional current flows out of ...
In brief, corrosion is a chemical reaction occurring by an electrochemical mechanism (a redox reaction). [1] During corrosion of iron or steel there are two reactions, oxidation (equation 1), where electrons leave the metal (and the metal dissolves, i.e. actual loss of metal results) and reduction, where the electrons are used to convert oxygen and water to hydroxide ions (equation 2): [2]
Another example is the cathodic protection of buried or submerged structures as well as hot water storage tanks. In this case, sacrificial anodes work as part of a galvanic couple, promoting corrosion of the anode, while protecting the cathode metal.
Appliances that provide a continual supply of hot water are called water heaters, hot water heaters, hot water tanks, boilers, heat exchangers, geysers (Southern Africa and the Arab world), or calorifiers. These names depend on region, and whether they heat potable or non-potable water, are in domestic or industrial use, and their energy source.
If DC voltage is used, electrolysis of water occurs, decomposing water into its elements H 2 at the cathode (negative electrode) and O 2 at the anode (positive electrode). The electrode boiler is 99.9% efficient with almost all the energy consumed producing steam. [1] Losses are radiant heat from the vessel only.
When the anode is made of the metal that is intended for coating onto the cathode, the opposite reaction may occur at the anode, turning it into dissolved cations. For example, copper would be oxidized at the anode to Cu 2+ by losing two electrons. In this case, the rate at which the anode is dissolved will equal the rate at which the cathode ...
Heaters require a low voltage, high current source of power. Miniature receiving tubes for line-operated equipment use on the order of 0.5 to 4 watts for heater power; high power tubes such as rectifiers or output tubes use on the order of 10 to 20 watts, and broadcast transmitter tubes might need a kilowatt or more to heat the cathode. [14]
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