Examples of Corrosion Cells

A difference in potential will be observed if electrically connected specimens of the same metal are immersed in solutions having different concentrations of ions of the metals. This is one common type of concentration cell. Another type is the oxygen concentration or differential aeration cell. 

Local cells may be produced by differences between small nearby areas on the metal surface. They may also be the result from differences 

A second phase constituent will have a different corrosion potential compared to that of an adjacent soHd solution. The difference may be in the thickness of a surface film at adjacent sites which in turn may reflect metal differences in the substrate. 

Any two or more of the cells described in the following sections can operate at the same time. Sometimes they will work in synergy, sometimes they will be opposed. For example, in a well casing, there is usually an oxygen cell and a temperature cell, both tending to make the deep pipe act as an anode. At the same time, there may be two or more ordinary concentration cells, as the casing passes through different strata of soil. 

The corrosion of guyed tower anchors such as shown in Fig. 7.13 and Fig. 7.14 and which is becoming an issue of critical importance will be used in the next sections to illustrate some of these corrosion cells. In many cases, the loss of one anchor to corrosion (Fig. 7.15) has resulted in the catastrophic loss of the supported communication tower (Fig. 7.16). 

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Chapter 7 has provided many examples of corrosion cells that may happen in a soil environment. The following sections illustrate some effects these corrosion cells may have on buried structures with a particular focus on pipelines which are, by far, the greater portion of... 

Figure 5.39. Example of corrosion cell in a drop of water. During corrosion, dissolved oxygen acts as an electron acceptor. The cathode area is formed along the edge of the drop with the easiest access of 02- The anode area is formed in the middle of the drop. During the anode process, Fe is oxidized into Fe " and is dissolved. The liberated electrons are included in the cathode process where dissolved oxygen O2 is reduced and forms hydroxide ions OH with the water. The product of the reaction is the well-known "rust which is hydrates of Fe(OH)2-...

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