Corrosion phenomenon Passivation

Furthermore, the advantage of the Pourbaix diagram as shown in Figure 3.6a is that it predicts the electrochemical state for immunity, corrosion, and passivation, and it is related to the polarization curve shown in Figure 3.6b. However, only the polarization curve predicts the corrosion rate through the current density icorr for metal oxidation and the passivation rate through the passive current density for metaUic cation reduction to form an oxide protective film on the electrode surface. The passivation phenomenon is discussed in details in Chapter 6. 

Corrosion, especially pitting corrosion, is a typical heterogeneous reaction composed of several processes. Usually, it is reduced to each elemental phenomenon, such as breakdown of passive film and substrate dissolution, which are treated separately to establish the theoretical and experimental bases of corrosion. 

Sometimes it happens that incursion of oxoanion reduction in place of hydrogen evolution as the cathodic reaction in the corrosion of iron leads, not to an increased rate of corrosion, but to a drastic retardation. This is because strongly oxidizing conditions (e.g., in concentrated nitric acid) can force the immediate oxidation of iron to iron(III), rather than via the persistent iron(II) intermediate (as described in Sections 16.1 and 16.2), so that an insoluble iron (III) oxide layer forms at once on the anodic and cathodic surfaces alike and the iron becomes passivated (Section 16.3). Michael Faraday s demonstration of this phenomenon is instructive ... 

Considerable progress has been made during the past decade toward a better insight into the basic concepts and mechanism involved in metallic dissolution and corrosion. More emphasis has been placed on the "fundamental particles (metallic ions, electrons, and electron acceptors) and on the use of current-voltage characteristics. The wide recognition of dissolution and corrosion as electrode processes, and the idea of a polyelectrode exhibiting a mixed potential, have augmented the use of electrochemical techniques in the study and interpretation of corrosion phenomena. There is even some evidence that the phenomenon of passivity may soon be clarified. 

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