Differential aeration examples

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. 1.48 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...

Differential aeration can play an important role in producing accelerated attack on metals in sea water—for example, at crevices. These crevices may be formed by loosely attached marine organisms, by debris, or by close-fitting surfaces to which sea water has restricted access (Figure 3,6). 

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. 

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. 

Differential aeration in buried structures. A clear example of macrocell action was documented in diaphragm walls in Berlin, illustrated in Figure 8.1 [6]. In this case, anodic areas had formed at the lower, non-aerated parts of the reinforcement at the ground side, while steel on the free side and higher up acted as cathode. Large amounts of corrosion products were found inside the concrete at various distances from the anodes and in the soil, suggesting that relatively soluble iron(II) oxides had formed that were able to move away from the anodes. Chlorides originated... 

These with, or without, potential monitoring and sometimes referred to as zero resistance ammetry find their most common application for the assessment of galvanic coupling between dissimilar materials or possibly between areas of the same material but exposed respectively to the free medium and to occluded areas, as for example, in crevice corrosion and differential aeration conditions. 

Many hazardous corrosion situations arise because of differential aeration. To understand differential aeration one should first consider two pieces of the same metal placed in media identical except for their oxygen concentration. For example, as in Fig. 9.8, one is placed in unstirred solution below an air atmosphere and the other is placed in a solution saturated with oxygen by a stream of gas. The metal samples will take up different potentials and corrode at different rates that in the oxygen-saturated solution must be more positive and the corrosion current will be... 

There are many examples of the way in which differential aeration combines with passivation to produce difficult corrosion problems. Figure 9.9 illustrates the case of crevice corrosion. The outer flat surface receives a plentiful supply of oxygen and hence passivates to produce a large protected area where oxygen reduction can... 

Filiform corrosion occurs independent of light, metallurgical factors in the steel, and bacteria. Although threads are visible only under clear lacquers or varnishes, they probably also occur under opaque paint films. They have been observed under various types of paint vehicles and on various metals, including steel, zinc, aluminum, magnesium, and chromium-plated nickel. This type of corrosion takes place on steel only in air of high relative humidity (e.g., 65-95%). At 100% relative humidity, the threads may broaden to form blisters. They may not form at all if the film is relatively impermeable to water, as is stated to be the case for paraffin [17]. The mechanism appears to be a straightforward example of a differential aeration cell. 

Differential aeration cells are frequently encountered in practice. They form in particular on carbon steel in neutral environments, whenever oxygen accesses certain parts of the structure with more difficulty. Figure 7.19 gives a few examples (a) presence of a crevice between a poorly tightened nut and a plate (b) accumulation of muddy residue on a steel plate (c) a steel structure partially immersed in water (d) a buried pipe that is not entirely surrounded by aerated backfill. 

An example of differential aeration type corrosion is lead-sheathed cable in partially silt-flUed ducts. The area of the lead covered with silt is in contact with an environment that has a relatively low concentration of dissolved oxygen, while the area in water is in contact with an environment that has a relatively high concentration of dissolved oxygen. The... 

Such differential aeration cells are also very common on buried pipes. For example, a pipe usually rests on undisturbed soil at the bottom of a ditch. Around the sides and on top of the pipe is relatively loose backfill that has been replaced in the ditch. A corrosion cell is... 

Corrosion cells not only are formed when two different metals are in contact with each other but also can be the result of differences in the corrosive environment. A typical example is differential aeration cells, which are due to differences in accessibility of oxygen to the surface of a metal. Because the value of the corrosion potential depends on the kinetics of the anodic as well as the cathodic partial reactions, a different accessibility of oxygen results in a difference in corrosion potential and hence in the establishment of a corrosion cell. 

In the above cell, HCl is in two different concentrations. The activity (molality x activity coefficient) ai is greater than activity 02 fli > <12-Several types of concentration cells are encountered in corrosion. For example, a concentration cell is formed if one end of a pipe is exposed to soil and the other end to air. The end of the pipe in air is exposed to a high concentration of oxygen than the end of the pipe in the soil. The formation of a concentration cell leads to differential aeration corrosion in buried structures in the soil. 

If two areas of a component in close proximity differ in the amount of reactive constituent available, the reaction in one of the areas is speeded up. An example of this is crevice corrosion, which occurs when oxygen cannot penetrate a crevice and a differential aeration cell is set up. Corrosion occurs rapidly in the area with less oxygen. The potential for crevice corrosion can be reduced by ... 

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