Pitting corrosion rate-determining step

Whether the total corrosion process is determined by the kinetics of anodic metal dissolution or the cathodic process depends on the size of the cathode and the kinetics of the partial electrode processes. The slowest reaction is the rate-determining step, as is usual in kinetics. In the case of a well-passivated valve metal, this is most probably the cathodic reaction, whereas for metals with semiconducting oxides, the rate-determining step win he anodic metal dissolution. In order to study the partial reactions of pitting corrosion separately, potentiostatic experiments are preferred. The cathodic process is replaced hy the electronic circuit of the potentiostat to investigate the anodic metal... 

The accumulation of corrosion products within the pits suggest that a high concentration of chloride is a necessary condition for a stable growth in their early stage of development. As a consequence, the kinetics of repassivation of small pits may be related to the transport of accumulated aggressive anions from the pit to the bulk electrolyte [19, 29]. If this transport is the rate-determining step, one expects the repassivation time to increase with the depth of a corrosion pit and thus to the distance the chloride has to travel by diffusion. If we simply apply the relation of Einstein-Smoluchowski for the transport time fr out of a pit of radius r (Eq. 14), and if the radius r is given by the local current density ic,p and the lifetime fp of the pit by Eq. (15), we obtain Eq. (16) for the repassivation time fr. 

One may therefore conclude that the rate-determining step for the repassivation is the transport of accumulated aggressive anions out of small pits. This result coincides well with the explanation that localized corrosion is stabilized at its initial stage by the accumulation of aggressive anions, which prevent the formation of a passive layer at the active pit surface. 

Potentiodynamic polarization (intrusive). This method is best known for its fundamental role in electrochemistry in the measurement of Evans diagrams. A three-electrode corrosion probe is used to polarize the electrode of interest. The current response is measured as the potential is shifted away from the free corrosion potential. The basic difference from the LPR technique is that the apphed potentials for polarization are normally stepped up to levels of several hundred millivolts. These polarization levels facihtate the determination of kinetic parameters, such as the general corrosion rate and the Tafel constants. The formation of passive films and the onset of pitting corrosion can also be identified at characteristic potentials, which can assist in assessing the overall corrosion risk. 

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