SPECIALIZED CORROSIONDOE-HDBK-1015/1-93CorrosionCH-02Rev. 0Page 28Figure 11 Differential Aeration Cell Pitting corrosionoccurs where the anodic site becomes fixed in a small area and the formationof holes (deep attack) in an otherwise unaffected area takes place. Crevice corrosion is a typeof pitting corrosion that occurs specifically within the low flow region of a crevice.To illustrate pitting attack, consider a special type of galvanic cell called a differential aerationcell such as the one illustrated in Figure 11. This particular differential aeration cell is showingcurrent flow as a result of depolarization of one electrode (cathode) by oxygen. In this type ofcell, two iron electrodes are exposed to a dilute solution of an electrolyte (NaCl, for example).Air (or oxygen) is bubbled around one electrode, and nitrogen is bubbled around the other. Acurrent flows through the wire connecting the two electrodes. The difference in potential is aresult of the difference in oxygen concentration at the two electrode surfaces. At the electrodeexposed to nitrogen, electrons are given up by the iron as it is oxidized. These electrons readilyflow through the external circuit to the electrode exposed to oxygen. At this depolarizedelectrode they can participate in a reduction reaction. As a result, oxidation occurs at theelectrode exposed to nitrogen and reduction occurs at the aerated electrode. Oxidation at oneelectrode and reduction at the other creates a potential and a flow of current through theconnecting wire. Note that loss of metal occurs at the electrode that is deficient in oxygen.
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