Pitting corrosion cyclic anodic polarization scans

Figure 24 Schematic Evans diagram and polarization curve illustrating the origin of the negative hysteresis observed upon cyclic polarization for materials that do not pit. Line a represents the (unchanging) cathodic Evans line. Line b represents the anodic Evans line during the anodically directed polarization, while line c represents the anodic Evans line for the material after its passive film has thickened because of the anodic polarization. The higher corrosion potential observed for the return scan (E (back)) is due to the slowing of the anodic dissolution kinetics.

The most common electrochemical test for localized corrosion susceptibility is cyclic potentiodynamic polarization. As was discussed briefly in the section on the electrochemical phenomenology of localized corrosion, this test involves polarizing the material from its open circuit potential (or slightly below) anodically until a predetermined current density (known as the vertex current density) is achieved, at which point the potential is scanned back until the current reverses polarity, as shown in Fig. 42. The curve is generally analyzed in terms of the breakdown (Ebi) and repassivation potentials (Elf). Very often, metastable pits are apparent by transient bursts of anodic current. The peaks in current shown in Fig. 42 for a potentiodynamic scan are due to the same processes as those shown in Fig. 25 for a potentiostatic hold. 

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. 

The cychc polarization method is a standardized traditional electrochemical method to determine relative loealized eorrosion susceptibility. This method involves anodic polarization of a specimen until localized corrosion initiates as indicated by alaige increase in the apphed current. An indicationofthe susceptibility to initiation of pitting corrosion in this test method is given by the potential at which the anodie current increases rapidly, that is the breakdown potential. The nobler this potential, obtained at a fixed sean rate in this test, the less susceptible is the alloy to the initiation of loealized eorrosioa Conventional understanding is that the breakdown potential is the potential above which pits are initiated, whereas the repassivation potential obtained at reverse sean is the potential below which pits repassivate. In cyeUc polarization measurements, scatters in the breakdown potential and its dependence on scan rate are often experienced. It should also be noted that results from a cyclic polarization test are not intended to correlate in a quantitative manner with the rate of localized corrosion. 

The principles of cyclic polarization are similar to that of cyclic voltammetry (CV). The cyclic polarization technique is used to qualitatively measure pitting tendencies of a metallic sample in a corrosive solution environment. ASTM Standard G6I (35] describes the experimental procedures typically used in this study. The beginning potential scan is toward the anode from a potential in the vicinity of Ecorr- When the measured cur-... 

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