Oxygen differential aeration cell

Table 6.4 Oxygen Partial Pressure and Cell Potential of Oxygen Differential Aeration Cell...

Electrochemical kinetics of the oxygen differential aeration cell The differential aeration cell shown in Fig. 6.11 consists of identical iron electrodes placed in a two-compartment cell separated by a membrane. Two electrically connected iron electrodes of equal area are... 

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 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 )...

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). 

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... 

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... 

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)... 

OXYGEN CONCENTRATION CELL - (see differential aeration cell). 

If a differential aeration cell develops, oxygen is mainly reduced at the outer region of the droplet and then delamination can occur via the cathodic process of oxygen or proton reduction. However, if this cathodic process is hindered underneath the coating due to... 

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. 

Oxygen is reduced at the actively corroding site or at the filament head. The mechanism is the same as crevice corrosion. A differential aeration cell is established between a deaerated acidified anode at the filament head and an alkaline cathode at the filament tail, saturated with water and oxygen [112]. The metal oxidation is followed by hydrolysis and acidification down to pH 1 [113]. Fe(OH)3, shown in Fig. 7.22, is formed from Fe reacting with the aerated solution in the tail and diffuses through the microcracks in the coating. 

FIGURE 4.11 Differential aeration cell and ion concentration ceU mechanisms as operative on a biofihn-metal system. Relative size difference of the arrows represents the corresponding oxygen partial pressures. When via cathodic reactions, atomic metal (M) is oxidized, it will liberate enongh positively charged ions (cations, M ) to shift the charge balance to yield an ion concentration difference. In either case, pitting under the biofilm is inevitable. 

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