Differential aeration cells

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. 

Figure 2.6 Differential aeration cell at a waterline. Water contains sodium chloride.

Concentration cells have two similar electrodes in contact with a solution of differing composition. The two kinds of concentration cells are salt concentration cells and differential aeration cells [186]. 

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]. 

Figure 4-434. Differential aeration cell illustrated by waterline corrosion. (From Ref. [165].)...

The deposit partially shields the steel surface, creating a differential aeration cell (Figure 4-435). 

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. 

Figure 53.4 Crevice corrosion driven by (a) a differential aeration cell and (b) a differential metal ion concentration cell...

Fig. 1.47 Early form of the Evans differential aeration cell . (Courtesy U. R. Evans)...

It is evident from this description of the operation of the differential aeration cell that other factors must be involved if the differential aeration... 

Fig. 1.48 Examples of differential aeration cells (a) and (b) Differential aeration cells formed by the geometry of a drop of NaCl solution on a steel surface (c) differential aeration cells formed by the geometry of a vertical steel plate partly immersed in a NaCl solution. Increasing concentrations of Na2 CO3 decrease the anodic area (d) until at a sufficient concentration attack is confined to the water line (e) (/) shows the membrane of corrosion products formed at water...

At first sight the mechanism of crevice corrosion appears to be simply the formation of a differential aeration cell in which the freely exposed metal outside the crevice is predominantly cathodic whilst the metal within the crevice is predominantly or solely anodic the large cathode current acts on the small anodic area thus resulting in intense attack. However, although differential aeration plays an important role in the mechanism, the situation in reality is far more complex, owing to the formation of acid within the crevice. 

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 )...

Saraby-Reintjes, A., Differential Aeration Cell , 7. Electro. Chem. Interfacial Eiectrochem., 37. 357 (1972) C.A., 77, 55484f... 

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). 

Both iron- and copper-based alloys are corroded more easily on either side of the neutral pH band. In low pH conditions e.g. due to carbon dioxide, the acidic environments attack the alloys readily, causing damage both at the points of initial corrosion and perhaps, consequentially, further along the system, by screening the surface with corrosion products and permitting the development of differential aeration cells. 

Soil corrosion does not lend itself readily to direct study in the laboratory. However, indirect methods involving the action of differential aeration cells have yielded valuable information in comparing the probable corrosivities of different soils towards steel. The details of this technique were described by Denison , Ewing , Schwerdtfeger, and by Logan, Ewing and Denison ... 

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. 

Stainless steel also is susceptible to crevice corrosion and deposit attack. Differential aeration cells, formed between stagnant and well-aerated areas on the metal surface, may promote rapid attack. 

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). 

Discontinuities in the corrosion product film result in differential aeration cells, leading to pitting corrosion. A pitting rate of 0.25-0.38 mm/yr on bare steel and 0.5 mm/yr on steel with mill scale has been observed. Assuming the average corrosion rate of 0.125 mm/yr, the pitting factor works out to be 2 to 3 for... 

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... 

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