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Corrosion is a kind of serious waste to energy. Electrochemical corrosion is much more common than corrosion of other types, extremely detrimental to metals. The corrosion of metal can significantly affect metal performance. The harm is not limited to metal itself, but to the structure of metal. If metal is protected through corrosion and anti-corrosion knowledge, many losses can be avoided.

There are a variety of corrosion principles of metal, and electrochemical corrosion is the most extensive one. When metal is placed in water solution or humid air, a kind of micro cell, also called corrosive cell (its electrodes are usually called cathode and anode rather than positive electrode and negative electrode), will form on metal surface. Oxidizing reaction takes place on anode, thus making anode dissolve. Reduction reaction takes place on cathode, usually playing the role of transmitting electrons. The major reason for the formation of corrosive cell is that metal surface absorbs moisture in air, which forms a layer of water film, so CO2, SO2 and NO2 in the air dissolves in the layer of water film, thus forming electrolyte solution. In addition, metal soaked in the layer of solution is impure, such as industrial steel and iron. It is actually alloy, which means that apart from iron, it also includes graphite, Fe3C and other metals and foreign matter. Most of them are not as active as iron. In such a way, the anode of corrosive battery is iron while the cathode is foreign matter. Furthermore, iron and foreign matter are in close contact with each other, so corrosion continues to happen.

Sacrificial anode protection method

The method refers to the fact that active metal (zinc or zinc alloy) is connected with the protected metal. In the case of electrochemical corrosion, such active metal, as cathode, goes through oxidizing reaction, thus reducing or avoiding the corrosion of the protected metal. Such method is often used for protecting steel piles and sea-craft shells in water, such as steel gates. It is common that ship shell under water line or rudder close to propeller is welded with several zinc blocks to avoid corrosion of ship shell.

External current protection method

The protected metal is connected with the cathode of power, and another conductive inert material is selected to connect with anode of power. After the electrification, negative charge (electron) accumulates on metal surface, thus restraining the loss of electron and further achieving the purpose of protection. The method is mainly used for avoiding the corrosion of metal equipment in soil, seawater and river water. Another method of electrochemical protection is called anode protection method. Through external voltage, anode may encounter passivation within the given potential scope, which can effectively reduce or avoid the corrosion of metal equipment in acid, alkali and salt.