Differential Aeration Oxygen Concentration Cells

The oxygen content of any solution ranks high on the list of factors influencing the corrosion of iron and numerous other metals. Elimination of oxygen by deaeration is a practical means of reducing corrosion, as in the case of steam boilers which are operated with deaerated feed water. 

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. 

Oxygen not only enables a corrosion reaction by maintaining a cathodic reaction, but it can promote one. This occurs where there is a difference in the concentration of dissolved oxygen (DO) between two points of the same metal surface. Since the direction of the reaction is toward equilibrium, the only way that equilibrium can be approached by corrosion will be to reduce the concentration of oxygen where it is highest. Such reduction can be done by consuming the oxygen as shown in Eq. (7.1). 

The end result is that whenever there is a difference in the concentration of DO at two points on a metal surface, the surfaces in 

It is easy to demonstrate an oxygen concentration cell with an experimental setup using two containers (Fig. 7.25). In this experiment, pieces of steel electrically connected are immersed in a sodium-chloride solution in the two containers. The solution in one container is saturated with oxygen and the solution in the other container is saturated with nitrogen. This establishes a difference in the concentration of DO in contact with the two pieces of steel. The high concentration of DO in one container makes the steel surface cathodic to the steel in the other container. 

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Common exanples of stagnation include nondtain-ing stmctures, dead ends, badly located components, and poor assembly or maintenance practices (Fig. I). General problems include localized corrosion associated with differential aeration (oxygen concentration cells), crevice corrosion, and deposit corrosion. 

At first sight it might appear that differential aeration could be explained in terms of a reversible oxygen concentration cell, for which... 

Oxygen Concentration Cell see under Differential Aeration. 

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. 

These forms of corrosion are all characterized by localized differences in concentration of hydrogen, oxygen, chloride, sulfate, etc., but especially oxygen (producing the so-called differential oxygen concentration cell or differential aeration cell). 

Production of differential aeration cell. A scatter of individual barnacles on a stainless steel surface creates oxygen concentration cells. The formation of biofilm generates several critical conditions for corrosion initiation. Uncovered areas will have free access to oxygen and act as cathodes, while the covered zones act as anodes. Underdeposit corrosion (crevice corrosion) or pitting can occur. Depending on the oxidizing capacity of the bacteria and the chloride ion concentration, the corrosion rate can be accelerated. However, the presence of a biofilm does not necessarily mean that there will always be a significant effect on corrosion. (Dexter)5... 

OXYGEN CONCENTRATION CELL - (see differential aeration cell). 

An oxygen concentration cell an electrolytic cell resulting from differences in dissolved oxygen at two points. See also differential aeration cell. 

Differential Aeration Cells. This type of concentration cell is more important in practice than is the salt concentration cell. The cell may be made from two electrodes of the same metal (i.e., iron), immersed completely in dilute sodium chloride solution (Figure 4-433). The electrolyte around one electrode (cathode) is thoroughly aerated by bubbling air. Simultaneously the electrolyte around the other electrode is deaerated by bubbling nitrogen. The difference in oxygen concentration causes a difference in potential. This, in turn, initiates the flow of current. This type of cell exists in several forms. Some of them are as follows [188]. 

Air—Water Interface. This is another good example of a differential aeration cell (Figure 4-436). Here the water at the surface contains more oxygen than the water slightly below the surface. This difference in concentration can cause preferential attack just below the waterline. 

Fig. 1.52 Mechanism of filiform corrosion showing how atmospheric oxygen and watCT enter the active head through the film (lacquer) and how water leaves through the inactive tail. This results in a high concentration of oxygen at the V -shaped interface between the tail and the head, and to a differential aeration cell (after Uhlig )...

Differential aeration galvanic cell. Distilled water is an important medium since it is commonly used. Evans differential aeration cell (a galvanic cell created by a difference in oxygen concentration) for pitting has been shown to be important or essential for cracking in distilled water. (Miller)24... 

DIFFERENTIAL AERATION CELL - An electrolytic cell, the electromagnetic force of which is due to a difference in air (oxygen) concentration at one electrode as compared with that at another electrode of the same material, (see concentration cell)... 

A differential aeration galvanic cell consists of differences in oxygen concentration, leading to pitting and cracking in distilled water (96). 

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. 

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