Crevice corrosion anodic polarization curves

The anodic polarization curve for a specimen with an active crevice will be in principle as shown in Figure 7.17. In this case a very small free external surface is assumed, and any internal hydrogen reduction is disregarded. is the potential as measured with the reference electrode positioned outside the crevice. As explained above, the real potential in the crevice, Ei , is more negative. The lower limit for corrosion in an active crevice is the protection (or repassivation) potential Epr. However, the critical potential that must be exceeded for initiation of the ereviee corrosion process, the crevice corrosion initiation potential, is higher than the protection potential. 

The basic electrochemical method for evaluating the susceptibihty of metals to pitting and crevice corrosion is covered by ASTM G 61 [30]. An anodic polarization curve is measured by a cyclic potentiodynamic polarization method using a specific scanning rate, from which breakdown potential and protection potential can be determined. See Fig. 11. 

It is important to study the potential distribution inside the crevice and its relation to the polarization curve in order to obtain a better understanding of the mechanism by which the crevice corrosion occurs. Factors that influence both the potential distribution and the polarization curve include the metal environment reaction products and the crevice geometry as well as the well-known promoters of crevice corrosion acidification and chloride ion build-up within the crevice. When excess oxygen entered the crevice via flushing air-saturated solution through it, the corrosion rate decreased which was consistent with the proposed need to keep the cathodic reaction outside the crevice, separate from the anodic reaction, for a stable crevice corrosion to occur [151]. 

Polarization curves are shown for four titanium materials in boiling 1 Msodium chioride with 1 iWhydrochioric add. The corrosion potentials of both 71-0.15 Pd and PdO/nOj-Ti, where no anodic peaks occur, are more noble than those of CP titanium and Ti-0.3Mo-0.8Ni. Source B. Satoh eted., The Crevice Corrosion Resistance of Some Titanium Materials, Plat. Met. Rev., Vol 31,1987, p 115-121... 

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