Pitting corrosion local current density

In the previous analysis, homogeneous current distribution has been assumed but, on many occasions, corrosion occurs with localized attack, pitting, crevice, stress corrosion cracking, etc., due to heterogeneities at the electrode surface and failure of the passivating films to protect the metal. In these types of corrosion processes with very high local current densities in small areas of attack, anodic and cathodic reactions may occur in different areas of disparate dimensions. 

Experimental studies usually yield good agreement between the rates of corrosion obtained from polarization resistance measurements and those derived from weight-loss data, particularly if we recall that the Tafel slopes for the anodic and the cathodic processes may not be known very accurately. It cannot be overemphasized, however, that both methods yield the average rate of corrosion of the sample, which may not be the most critical aspect when localized corrosion occurs. In particular it should be noted that at the open-circuit corrosion potential, the total anodic and cathodic currents must be equal, while the local current densities on the surface can be quite different. This could be a serious problem when most of the surface acts as the cathode and small spots (e.g., pits or crevices) act as the anodic regions. The rate of anodic dissolution inside a pit can, under these circumstances, be hundreds or even thousands of times faster than the average corrosion rate obtained from micro polarization or weight-loss measurements. 

The local current density within corrosion pits may be extremely large. If the precipitation of corrosion products does not occur, the metal dissolution is controlled by charge transfer and ohmic effects, and hence the corrosion process is potential dependent. This situation requires a sufficiently acidic solution to avoid the precipitation of insoluble oxides or a still not saturated or supersaturated pit electrolyte with no formation of a salt layer. Pitting at potentials close to the critical value Ep occurs usually with moderately small local current densities / c,p that, however, may increase with potential to extremely... 

An interesting alternative is pitting on thin vapor-deposited metal films such as A1 or Ni-20 Fe [23-25]. In this case, pitting leads to a rapid perforation of the film and a further circular growth. These two-dimensional pits lead to simpler relations for the accumulation of corrosion products in comparison to the hemispherical situation, i x is radius independent and proportional to /c,p instead of to Also in these cases, local current densities of up to (c,p = 100 A cm have been measured. 

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. 

It should be mentioned that passive layers are not protective in all environments. In the presence of so-called aggressive anions, passive layers may break down locally, which leads to the formation of corrosion pits. They grow with a high local dissolution current density into the metal substrate with a serious damage of the metal within very short time. In this sense halides and some pseudo halides like SCN are effective. Chloride is of particular interest due to its presence in many environments. Pitting corrosion starts usually above a critical potential, the so-called pitting potential /i]>j. In the presence of inhibitors an upper limit, the inhibition potential Ej is observed for some metals. Both critical potentials define the potential range in which passivity may break down due to localized corrosion as indicated in Fig. 1. 

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