Figure 6 Representation of Cathodic Depolarization by Oxygen

Factors Affecting General Corrosion Rate

Figure 7 Effect of pH on the Corrosion Rate of Iron in Water

Chemistry Volume 1 of 2
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? GENERAL CORROSION DOE-HDBK-1015/1-93 Corrosion CH-02 Rev. 0 Page 14 Figure 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 be diffusion of O to the metal surface 2 where it can react directly with iron or with FeO. (2-12) (2-13) Oxygen, therefore, has two effects: it removes the polarizing layer of atomic hydrogen, and it can react directly with the metal or metal oxide; thus, the corrosion rate increases. Substances, such as O in this case, that remove the 2 absorbed atomic hydrogen are called depolarizers. The depolarizing effect of O is illustrated in Figure 6. 2 The effect of the pH of water to which iron or steel is exposed is influenced by temperature in the following manner. The potential of hydrogen or symbol (pH) is defined as the negative logarithm of the hydrogen concentration, represented as [H ] in moles/liter. + pH = -log [H ]⁺ The pH value is used to represent the acidity of a solution.

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