Pitting corrosion repassivation potential

The enhanced corrosion resistance in reducing environments from molybdenum is achieved, however, at the expense of corrosion resistance in oxidizing conditions. Therefore, pitting and repassivation potential are expected to be lower them most other titanium alloys, eilthough no data are available. 

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. 

Cathodic inhibitors slow the rate of the oxygen reduction reaction, and hence the companion oxidation reaction must also slow. This results in an overall decrease in the corrosion rate, as well as a decrease in the free corrosion potential. For the best inhibitors, the decrease in the corrosion potential is usually to a value well below the alloy s pitting or repassivation potential. Cathodic inhibitors have the advantage of being able to improve corrosion resistance at low concentrations. For example, chromate added at micromolar concentrations to an aerated dilute chloride solution is enough to significantly reduce the rate of oxygen reduction. 

Pitting and Crevice Corrosion The general literature for pre-dic ting pitting tendency with the slow scan reviews the use of the reverse scan if a hysteresis loop develops that comes back to the repassivation potential below the FCP (E ) the alloy will pit at... 

Figure 11 shows idealized polarization curves for the cases where the temperature is above the CPT (pitting) and below the CPT (transpassive corrosion). These polarization curves show the pitting potential ( ),), transpassive potential (E,), and repassivation potential (E ). Ep and E, are defined as the potentials at which the current density unambiguously... 

Figure 25 Current versus time behavior for Type 302 stainless steel in 1,000 ppm NaCl at (a) a potential between its repassivation and breakdown potentials, and (b) at a potential below its repassivation potential. Note the existence of an incubation time before stable localized corrosion occurs in (a). The small, short-lived current spikes during the first 400 s are due to the formation and repassivation of metastable pits, which can also be observed in (b), although they are of a smaller magnitude.

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 44b Effect of charge density on the repassivation potential for pitting and crevice corrosion.

Figure 26 Illustration of the period of propagation of localized corrosion (pitting) as defined by the relative values of EC0RR and the breakdown (EB) and repassivation potentials (Er). The shaded areas associated with EB and ER illustrate the uncertainties in the values of these two parameters.

These tests focused on the determination of a materials resistance to localized (pitting) corrosion. To accomplish this goal, three types of electrochemical experiments were conducted (cyclic polarization, electrochemical scratch, and potenti-ostatic holds) to measure several key parameters associated with pitting corrosion. These parameters were the breakdown potential, EM, the repassivation potential, Etp, and the passive current density, tpass. 

FIGURE 22.31 Schematic potential-dimension diagrams for localized corrosion of stainless steel in aqueous solution [63] Epit — pitting potential, ER = pit repassivation potential, Ep = passivation potential in the critical pit solution, Emv — crevice protection potential, rj]13 = critical pit radius for pit repassivation, a = pit repassivation, and b = transition from the polishing mode to the active mode of localized corrosion. 

If the passive film cannot be reestablished and active corrosion occurs, a potential drop is established in the occluded region equal to IR where R is the electrical resistance of the electrolyte and any salt film in the restricted region. The IR drop lowers the electrochemical potential at the metal interface in the pit relative to that of the passivated surface. Fluctuations in corrosion current and corrosion potential (electrochemical noise) prior to stable pit initiation indicates that critical local conditions determine whether a flaw in the film will propagate as a pit or repassivate. For stable pit propagation, conditions must be established at the local environment/metal interface that prevents passive film formation. That is, the potential at the metal interface must be forced lower than the passivating potential for the metal in the environment within the pit. Mechanisms of pit initiation and propagation based on these concepts are developed in more detail in the following section. 

Fig. 25 The repassivation potential as a function of prior crevice corrosion and pit depth showing a bounding value independent of penetration depth [5 ].

The development of pits starts with a crack or a hole of atomic dimension in the passive film caused, e.g., by tensions or by local chemical dissolution of the fihn. Permanent pitting corrosion can start above a critical potential and a critical concentration of the chloride ions. Above these critical values repassivation is prevented by the adsorption of the aggressive anions in the crack or the hole. The small dimensions of the crack or hole stabilize the large potential drop between active and passive surface. 

Tested alloys in physiological solutions have the ability to repassivation. The repassivation ability of the material determined from the curve back after exceeding pits nucleation potential. Bidirectional potentioki-netic curves for sample I is shown in Fig. 6 (arrows indicated the direction of the potential change). The repassivation potential for sample I (the most resistant in the environment) is about 0.09 V vs. SCE (Fig. 6). Below this potential passive layer is stable, and the material should not undergo the pitting corrosion. 

---