Differential aeration

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.

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. 

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...

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

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...

A similar situation arises when a vertical metal plate is partly immersed in an electrolyte solution (Fig. 1.48c), and owing to differential aeration the upper area of the plate will become cathodic and the lower area anodic. With time the anodic area extends upwards owing to the mixing of the anolyte and catholyte by convection and by the neutralisation of the alkali by absorption of atmospheric carbon dioxide. 

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 )...

Gladysheva, V. P. and Shatalov, A. Ya., Effect of Hydrogen-ion Concentration on the Performance of Differentially Aerated Couples , Izv. Vysshikh. Uchebn. Zavedenii, Khim. i Khim. Tekhnoi., 9, 48 (1966) C.A., 65, 5002a... 

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

Richard, A. and Nicolaides, G., Differential Aeration Corrosion of Passivating Metal Under a Moist Film of Locally Variable Thickness , J. Electrochem. Soc., 121, 183 (1974)... 

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). 

Deposit attack and pitting When water speeds are low and deposits settle on the surface (particularly at water speeds below about 1 m/s), pitting of copper and copper alloys is liable to occur by differential aeration effects. 

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. 

The US Bureau of Mines found the chemical and galvanic corrosion behaviour of both the TZM and Mo-30W alloy to be generally equal or superior to that of unalloyed molybdenum in many aqueous solutions of acids, bases and salts. Notable exceptions occurred in 6-1 % nitric acid where both alloys corroded appreciably faster than molybdenum. In mercuric chloride solutions the TZM alloy was susceptible to a type of crevice corrosion which was not due to differential aeration. The alloys were usually not adversely affected by contact with dissimilar metals in galvanic couple experiments, but the dissimilar metals sometimes corroded galvanically. Both alloys were resistant to synthetic sea water spray at 60°C. 

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