?GENERAL CORROSIONDOE-HDBK-1015/1-93CorrosionCH-02Rev. 0Page 14Figure 6 Representation of Cathodic Depolarization by Oxygen The overall reaction can be obtained by combining Equations (2-4) and (2-11).The controlling step is believed to bediffusion of O to the metal surface2where it can react directly with iron orwith FeO.(2-12)(2-13)Oxygen, therefore, has two effects: itremoves the polarizing layer of atomichydrogen, and it can react directly withthe metal or metal oxide; thus, thecorrosion rate increases. Substances,such as O in this case, that remove the2absorbed atomic hydrogen are calleddepolarizers. The depolarizing effect ofO is illustrated in Figure 6.2The effect of the pH of water to whichiron or steel is exposed is influenced bytemperature in the following manner.The potential of hydrogen or symbol(pH) is defined as the negative logarithmof the hydrogen concentration,represented as [H ] in moles/liter.+pH = -log [H ]+The pH value is used to represent theacidity of a solution.
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