Passive alloys, crevice corrosion repassivation

ASTM F 746 Standard Test Method for Pitting or Crevice Corrosion of Metallic Surgical Implant Materials Measurement of pitting or crevice tendency by measuring repassivation tendency after polarization at noble potential. Applicable only to passive alloys. 

For a given crevice geometry, the critical potentials for crevice initiation and repassivation decrease with increasing chloride content (Fig. 10) and increasing temperature (Fig. 11) of the bulk solution. This means that the susceptibility to crevice corrosion of passivated alloys increases with the chloride content and the temperature. For example, titanium alloys become sensitive to crevice corrosion only in hot concentrated chloride solutions around 100/150°C [9,10]. Propagation rates also increase with temperature. 

The propagation of crevice corrosion can also be arrested by decreasing the potential of the outside surfaces below a critical value (see earlier). The existence of a repassivation or protection potential was recognized very early, in particular by Pourbaix et al. [81] for pitting corrosion. From a practical point of view, the existence of a protection potential below which no crevice corrosion is possible is of major importance because it guarantees the immunity of passivated alloys in near-neutral chloride solutions in the absence of oxidizing species and because it makes possible the cathodic protection of stmctuies. 

Chlorine gas, chlorine chemicals, and chlorine solutions. Titanium is widely used to handle moist or wet chlorine gas and has earned a reputation for outstanding performance in this service. The strongly oxidizing nature of moist chlorine passivates titanium, resulting in low corrosion rates. The selection of a resistant titanium alloy offers a solution to the possibility of crevice corrosion when wet chlorine surface temperatures exceed 70°C (Table 8.42). Dry chlorine can cause rapid attack of titanium and may even cause ignition if moisture content is sufficiently low. However, as httle as 1% water is generally sufficient for passivation or repassivation after mechanical damage to titanium in chlorine gas under static conditions at room temperature. 

Second, the critical potential may be a repassivation potential. It has been shown in artificial active crevices that lowering the potential of the free surfaces causes the local environment to become less aggressive (see, for example, Pourbaix [36]). Thus, at some point, the environment becomes not aggressive enough for active dissolution to be sustained and the metal surface in the crevice becomes passive. In this case, a subsequent increase of the corrosion potential does not produce immediate reactivation inside the crevice. Starr et al. [82] observed this situation on 12% Cr stainless steels in near-neutral environments Dunn and Sridhar [83] observed the same behavior on alloy 825. However, the repassivation may be attributed to different environment changes an increase of local pH in the crevice [82,84], a destabilization of the salt film that controls the... 

As already discussed, the main difficulty of this techruque, beside the problem of the crevice former geometry and reproducibility, is that the results are strongly dependent on the potential scan rate both because of the time-dependent stability of the passive films and because of the time-dependent evolution of the environment inside a crevice. In particular, the repassivation potential may be overestimated if corrosion is not well developed in the crevice and it can be underestimated if the potential backscan is too fest to allow the evolution of the local environment to be in quasi-steady conditions. It is generally admitted that the scan rate has to be very low, which causes the two critical potentials to become closer. But the appropriate scan rate must be determined on each system because it may depend on the alloy and on the environment. 

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