Role of Overpotential in Corrosion

Consider first the polarization curve (i.e., Tafel plot) for the anodic halfreaction occurring in corrosion of stainless steels (Fig. 16.8). The diagram for the active region is much the same as has been seen for other anodes (Figs. 15.4 to 15.7). As E i is increased to a certain specific value, however, a sudden and dramatic drop in the anodic current density i occurs, corresponding to formation of an oxide film. At higher E, i remains constant at a very low level (the horizontal scale in Fig. 16.8 is logarithmic), and the metal has become passive, that is, effectively immune from corrosion. 

The passive range of typical stainless steels conveniently spans most of the E t stability field of neutral water. This can be appreciated by examination of the E° or E values for reactions 16.20 to 16.25, with the caveat that these refer to pure iron and chromium metals rather than to stainless steels and that the conditions are standard ones rather than, for example, the very low [Cr +] in equilibrium with the FeCr204 film. The formation 

A diagram similar to Fig. 16.8 could be constructed for ordinary iron or steel, but the onset of passivity, corresponding to the formation of an Fe203 film directly over the entire surface of the metal, would occur at a much higher value than for stainless steel [E° for Fe + (aq) /Fe (aq) is +0.77 V]. The transpassive region, corresponding to anodic oxidation of the 

Fe20s film to soluble Fe04, requires a high impressed EMF and is only feasible in alkaline conditions (cf. the Pourbaix diagram of the iron-water system, Fig. 15.3). 

In Fig. 16.9, the operationally significant parts of qualitative polarization curves for a typical steel and a stainless steel are superimposed. It is seen that, for a given Eh value E in the active range of the stainless steel, the current density will be higher for dissolution of the stainless steel than for corrosion of the iron. It is therefore very important that stainless steels be prevented from becoming active in service, because, if they do, they corrode rapidly, more than ordinary iron would. 

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