SPECIALIZED CORROSIONDOE-HDBK-1015/1-93CorrosionCH-02Rev. 0Page 30Figure 13 Pit in Metal Surface Promoted by DepolarizationThe presence of oxygen can also promote pitting at areas on the metal surface that are initiallyanodic with respect to an adjacent area. For example, suppose that adjacent areas on a metalsurface exhibit slightly different oxidation potentials. Oxidation, or loss of metal, proceeds atthe region of higher potential. Corrosion in the region of higher potential leads to formation(at least initially) of a porous oxide film. The thickness of the film formed on the adjacentcathodic region will be much less. Oxygen in the bulk of solution can reach the cathodic surface(with the thin film) more readily than it can the nearby anodic surface region (with the thickeroxide film). Depolarization of the cathodic region (thin film) by oxygen tends to maintain thisregion cathodic, while a deficiency of oxygen under the thicker porous corrosion film assistsin maintaining an anodic condition in this region. The overall result is corrosion, or wastingaway, of the metal in the anodic region under the thicker film. Thus, a pit in the metal surfaceis formed under the mound of surface oxide, as illustrated in Figure 13. Pitting of this type iscommon in both low temperature and high temperature iron-water systems if precautions arenot taken to remove the oxygen from the water within the system.It is also found that certain ions, notably chloride ions, cause pitting of iron and steel. The exactmechanism by which this occurs is not clear, but in some way chloride ions cause defects in thepassivating oxide layer on the metal surface. The defects are highly localized and aresurrounded by large passive areas that tend to be cathodic. Thus, a small anodic (oxidation)site is surrounded by a large cathodic (reduction) area. The current density will then be verylarge at the anodic site, and attack on the metal will be rapid. In some test cases, deep pits havebeen observed within a few hours.
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