Corrosion differential oxygenation type

Although each form of concentration cell may be considered a discrete form of corrosion, in practice, more than one type may occur simultaneously. These forms of corrosion are all characterized by localized differences in concentration of hydrogen, oxygen, chloride, sulfate, and other minerals, but especially oxygen (producing the so-called differential oxygen concentration cell, or differential-aeration cell). The basic mechanisms surrounding each of these specific forms of concentration cell corrosion are discussed next. 

An example of differential aeration type corrosion is lead-sheathed cable in partially silt-flUed ducts. The area of the lead covered with silt is in contact with an environment that has a relatively low concentration of dissolved oxygen, while the area in water is in contact with an environment that has a relatively high concentration of dissolved oxygen. The... 

Under-deposit corrosion is a particular type of corrosion caused by differential aeration. If sparingly soluble salts, loosely adherent corrosion products, algal, or other fouling, is deposited on a metal surface, then these areas become depleted in oxygen. Unfouled or less fouled areas have a greater supply of oxygen and hence become cathodic to the fouled areas. Thus, the anodic under-deposit areas will corrode preferentially. 

Identical metals in contact with different concentrations In this case, the metal immersed in a dilute solution is dissolved from the electrode and deposited on the electrode immersed in a more concentrated solution. The other type of electrochemical concentration ceU is known as a differential aeration cell. In this case, the electrode potential difference occurs when the electrode is immersed in the same electrolyte with different oxygen partial pressures. Differential aeration initiates crevice corrosion in aluminum or stainless steel when exposed to a chloride environment. 

Therefore, such a type of corrosion is also called a differential concentration cell. However, not only oxygen, but other elements dissolved in the solution can bring about the concentration ceU. 

Oxygen is essential for corrosion to occur in the pH range commonly found in soils. Soils such as clays restrict oxygen movement therefore, these soils are prone to anaerobic conditions. Often, however, soils such as clays tend to promote galvanic corrosion cells that are caused by differences in oxygen concentration (differential concentration cell corrosion). Bacterial corrosion often occurs in clay soils because anaerobic conditions and organic matter promote bacterial growth. A common type of bacteria that causes... 

Differential aeration cells can be caused by crevices, lap joints, dirt and debris, and moist insulation. Under these conditions, the oxygen starved areas are anodic, while the areas with free access to oxygen are cathodic. Common terms for this type of corrosion include crevice corrosion, oxygen screening, and poultice action. 

In the above cell, HCl is in two different concentrations. The activity (molality x activity coefficient) ai is greater than activity 02 fli > <12-Several types of concentration cells are encountered in corrosion. For example, a concentration cell is formed if one end of a pipe is exposed to soil and the other end to air. The end of the pipe in air is exposed to a high concentration of oxygen than the end of the pipe in the soil. The formation of a concentration cell leads to differential aeration corrosion in buried structures in the soil. 

Filiform corrosion may be considered as a specific type of differential aeration cell corrosion that occurs from defects where bare metal is exposed on painted or coated Al surfaces. It is worth of mention here, since as recently as the 1970s and 1980s, filiform corrosion was not considered as a damage mode upon Al. Much like crevice corrosion, filiform attack is driven by a differential aeration cell with an anodic head growing under a coating and a cathodic tail where oxygen is reduced. The filiform filaments are filled with corrosion... 

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