Differential aeration oxygen

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. 

As discussed above, deposits can cause accelerated localized corrosion by creating differential aeration cells. This same phenomenon occurs with a biofilm. The nonuniform nature of biofilm formation creates an inherent differential, which is enhanced by the oxygen consumption of organisms in the biofilm. 

Differential Aeration-the stimulation of corrosion at a localized area by differences in oxygen concentration in the electrolytic solution that is in contact with the metal surface. 

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. 

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

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 )...

Gladysheva, V. P. and Shatalov, A. Ya., Corrosion of Metals During Differential Aeration , Part 2 Work of Differential Aeration Pairs During Varied Oxygen Concentration Proportions at the Cathode and Anode , Izv. Vyssh. Ucheb. Zaved., Khim. i Khim. Tekhnol., 16, 1663 (1973) C.A., 80, 55218J... 

The rate of water flow is also most important. This determines the supply of oxygen to the rusting surface, and may remove corrosion products that would otherwise stifle further rusting. A plentiful oxygen supply to the cathodic areas will stimulate corrosion, but so may smaller supplies at a slow rate of flow, if this leads to the formation of differential aeration cells (see Section 1.6). 

White rust If a fresh zinc surface is allowed to stand with large drops of dew on it, as may easily happen if it is stored in a closed place in which the temperature varies periodically, it is attacked by the oxygen dissolved in the water, owing to differential aeration between the edges and the centres of the drops. A porous form of zinc oxide builds up away from the surface and quickly takes up carbon dioxide from the air to form the basic carbonate known as white rust or wet storage stain. 

Differential Aeration Differences in oxygen concentration in the electrolyte solution in contact with a metal. Differential aeration stimulates corrosion of that area where the oxygen concentration is lower, which becomes the anodic site. 

Oxygen Concentration Cell see under Differential Aeration. 

Oxygen corrosion usually takes the form of deep pitting and involves both tuberculation and differential aeration corrosion mechanisms. The BW commonly is brown and murky, and chemical treatment reserves usually are very low or absent. The source of the oxygen is either MU water dissolved oxygen (DO) or air in-leakage. 

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). 

One can provide several practical examples of localized corrosion occurring by differential aeration. Crevice attack is a common phenomenon (Fig. 12.27), or, one may mention the corrosion of partially immersed metals in sea water (Fig. 12.28). The region near the waterline provides easy access to oxygen and thus becomes an electron-source area for the lower part of the metal, which becomes an election sink because of its relative oxygen starvation. 

The differential-aeration principle can also be exemplified by the underground corrosion of an iron pipe that runs partly through sand with high oxygen permeability and partly through clay soil with low oxygen permeability (Fig. 12.30) the portion of the pipe in the clay corrodes more heavily than the portion in the sand. 

Another example of a differential-aeration corrosion cell is an iron sheet with a drop of moisture on it (Fig. 12.31). The central region of the drop is oxygen starved compared with the peripheral regions, which therefore become electron-source areas, and corrosion is observed at the central electron-sink section. 

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... 

Production of sulfides. This may involve the production of FeS, Fe (OH)2 etc. and an aggressive chemical agent such as hydrogen sulfide (H2S) or acidity. Micro-organisms may also consume chemical species that are important in corrosion reactions (e.g., oxygen or nitrite inhibitors). Alternatively, their physical presence may form a slime or poultice, which leads to differential aeration cell attack or crevice corrosion. They may also break down the desirable physical properties of lubricating oils or protective coatings. (Stott)5... 

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