Pitting potential described

After a constant potential step beyond the pitting potential is applied to a nickel electrode in NaCl solution, the current transient shown in Fig. 39 is observed. The J vs. 1/VT plot according to Eq. (104) is shown in Fig. 40. From the linear portion corresponding to Eq. (104), the slope of the plot can be described as a function of the surface coverage 6 of the passive film in the following... 

The process of pitting may be divided into initiation and growth. Initiation of pitting may be caused by setting up of local cathodes due to impurities as described above. This causes the local cell potential to rise above the pitting potential. Thus, local weak spots in the oxide film break down and a local anodic area develops. Oxide films may be weakened by the... 

Critical pitting potentials of 0.38 V (S.H.E.) in IV NaCl and 0.45 V in 0.1 V NaCl [6] indicate that the metal is vulnerable to pitting in seawater. It undergoes intergranular S.C.C. in anhydrous methyl or ethyl alcohol containing HCl, but not when a small amount of water is added [7]. This behavior, similar to that of commercial titanium, suggests that stress may not be necessary and that the failure is perhaps better described as intergranular corrosion. 

Whatever the origin of defects in a passive film, a local loss of passivity can only occur when the exposed metal surface does not immediately repassivatc. Indeed, it has been observed that already well below the critical pitting potential depassivation and repassivation events may occur. These can be seen particularly well when working with electrodes of small surface area (microeleetrodes), because they contain relatively few defects that lead to breakdown events. Individual events therefore can be studied more easily. The results of Figure 6.41 illustrate the described behavior. It presents potentiostatic transients observed in the passive potential region on an iron-chromium alloy in NaCl using a microelectrode [40]. Each individual current peak represents a... 

Because pitting exhibits probabilistic behavior, the induction time T (the time for the occurrence of the first pit) in potentiostatic conditions is a probabilistic value and here the model described above does not hold. In the same way, the pitting potential measured in potentiokinetic conditions is probabilistic. 

Let us note that this description of the probabilistic behavior for the appearance of observable pits does not imply, at this stage, any mechanistic assumption about the mechanisms responsible for this behavior. It is only a convenient way to describe the experimental results. Shibata and Takeyama suggested another way to analyze the probabilistic behavior. They proposed a stochastic model including a pit initiation A.(F) frequency and a pit repassivation p(F) frequency, leading to a conventional pitting potential defined by = i(Fpjj). 

Electrochemical corrosion is understood to include all corrosion processes that can be influenced electrically. This is the case for all the types of corrosion described in this handbook and means that data on corrosion velocities (e.g., removal rate, penetration rate in pitting corrosion, or rate of pit formation, time to failure of stressed specimens in stress corrosion) are dependent on the potential U [5]. Potential can be altered by chemical action (influence of a redox system) or by electrical factors (electric currents), thereby reducing or enhancing the corrosion. Thus exact knowledge of the dependence of corrosion on potential is the basic hypothesis for the concept of electrochemical corrosion protection processes. 

The second procedure is different from the previous one in several aspects. First, the metallic substrate employed is Au, which does not show a remarkable dissolution under the experimental conditions chosen, so that no faradaic processes are involved at either the substrate or the tip. Second, the tip is polarized negatively with respect to the surface. Third, the potential bias between the tip and the substrate must be extremely small (e.g., -2 mV) otherwise, no nanocavity formation is observed. Fourth, the potential of the substrate must be in a region where reconstruction of the Au(lll) surface occurs. Thus, when the bias potential is stepped from a significant positive value (typically, 200 mV) to a small negative value and kept there for a period of several seconds, individual pits of about 40 nm result, with a depth of two to four atomic layers. According to the authors, this nanostructuring procedure is initiated by an important electronic (but not mechanical) contact between tip and substrate. As a consequence of this interaction, and stimulated by an enhanced local reconstruction of the surface, some Au atoms are mobilized from the Au surface to the tip, where they are adhered. When the tip is pulled out of the surface, a pit with a mound beside it is left on the surface. The formation of the connecting neck between the tip and surface is similar to the TILMD technique described above but with a different hnal result a hole instead of a cluster on the surface (Chi et al., 2000). 

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