Repassivation potential anodic current

Figure II. Schematic anodic polarization curves at a fixed temperature. Determination of either transpassive potential ( ,) or pitting potential ( p and repassivation potential ( ,) at the critical current density (/ ). t rcvis the current density at which the scan is reversed. ...

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. 

FIGURE 22.29 Schematic polarization diagrams (a) for the repassivation of pitting dissolution of metals and (b) for transformation from the electropolishing mode of pitting to the active mode of localized dissolution EP = passivation potential in the solution bulk, p = passivation potential in the critical pit solution, /sR = pit repassivation potential, /pit = pitting dissolution current, /a = anodic metal dissolution current in the active state in the bulk solution, and / = anodic metal dissolution current in the critical pit solution. 

The Ti02 film, being an n-type semiconductor, is electronically conductive. As a cathode, titanium permits electrochemical reduction of ions in an aqueous electrolyte. On the other hand, very high resistance to anodic current flow through the passive oxide film (i.e., significant anodic polarization) can be expected in most aqueous solutions. Elevated anodic pitting (breakdown and repassivation) potentials can also be expected with many titanium alloys. 

After 4 days, the risk of localized corrosion increased. At this time, the repassivation potential and the potential of the change from anodic to cathodic current were equal to the corrosion potential. The pitting potential was only about 0.1 V more noble than the corrosion potenti and the hysteresis still negative. The risk of pitting had increased enough to become a concern. 

Repassivation potentials are readily determined by using the galvanostatic method (Ref 59) or the constant potential-surface scratch test (Ref 59, 60). The galvanostatic method involves impressing an anodic current density of approximately 200 mA/cm (1290 mA/in. ) on the specimen for at least several minutes before measuring the repassivation potential of the sample. Reproducible, anambi-guous repassivation potentials are more difficult to derive by using reverse scan potentiodynamic techniques. 

Cao et al. [21] analyzed the PSD for some AISI 304 (MnS-containing) and 321 (Ti-bearing, then MnS-free) steels. No reference is made to the difference in inclusions between the two steels. For 321 steel, the elementary prepitting event is found to consist of a linear increase in anodic current, up to some pA in the tested conditions, followed by an exponential decrease. The frequency dependence of the PSD depends on the time characteristics of these two processes (growth rate of the micropit and repassivation time constant), which are potential dependent. Following the values of these time characteristics, and then the electrode potential, the PSD varies asat the high-frequency limit, with = 2 to 4. A white noise (no frequency dependence) is found at very low frequency (some 0.1 Hz). From this work, it is also inferred that the solution chloride content affects the nucleation frequency but not the growth or the repassivation kinetics of the micropit. 

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. 

Pitting and repassivation potentials. Two potentials that are often thought to characterize an alloy in terms of localized corrosion are the repassivation potential and the pitting potential and their values relative to the corrosion potential. A common interpretation is that pitting would occur if the hysteresis between the forward and reverse scans appeared as in Fig. 7.19 and the corrosion potential were equal to or anodic with respect to the pitting potential. The specimen imder test would be expected to resist localized corrosion if the corrosion potential lay cathodic with respect to the repassivation potential or if the polarization scan appeared as in Fig. 7.20.22 There are several ways to choose the repassivation potential. It can be chosen as the potential at which the anodic forward and reverse scans cross each other. Alternatively, it can be chosen as that potential at which the current density reaches its lowest readable value on the reverse portion of the polarization scan. One reason to choose the latter is that for some polarization scans, such as that in Fig. 7.20, the forward and reverse portions of the polarization scan do not cross each other. In any case, the choice should be consistent for all scans in any particular study. 

Formally,if / represents the anodic current and X the concentration in corrosion products in the pit, one writes dX/dt = KJ(V, X) - DX, where V is the metal-electrolyte potential difference (/ increases with V) and parameters K and D represent the production in corrosion products by the anodic dissolution and their dilution into the electrolyte by a diffusion process, respectively. A steady state is reached when dX/dt = 0 and d/dX) dX/dt) < 0, which also writes (d/dX)/(V,X) < (D/K). This condition is fulfilled when V is smaller than a critical potential Vp (the "pitting potential") which depends on the bulk electrolyte concentration and on the nature of the pitting site. Finally, the pitting potential acts as a bifurcation parameter beyond which a pit grows irreversibly and below which it repassivates (as far as the diffusion parameter D does not decrease). 

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