Critical potential, for crevice

Critical potential for crevice corrosion in 0.5 M NaCl, pH 8, at room temperature... 

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. 

Figure 11 Effect of the temperature on the critical potentials for crevice and pitting corrosion on 304 SS in 0.5 M NaCl solution [12].

Electrochemical tests, which can be divided into two classes (a) those intended to measure critical potentials for crevice initiation or protection and (b) those used to derive initiation criteria from the behavior of materials in conventional solutions supposed to simulate the crevice enviromnent... 

The test method ASTM F7464 covers the determination of the resistance to either pitting or crevice corrosion of passive metals and alloys from which surgical implants are produced. The resistance of surgical implants to localized corrosion is carried out in dilute sodium chloride solution under specific conditions of potentiodynamic test method. Typical transient decay curves under potentiostatic polarization should monitor susceptibility to localized corrosion. Alloys are ranked in terms of the critical potential for pitting, the higher (more noble) this potential, the more resistant is to passive film breakdown and to localized corrosion. (Sprowls)14... 

Crevice corrosion in stainless steels is also only observed in corrosives containing chloride. As with pitting corrosion, crevice corrosion is initiated after a critical potential is exceeded. This threshold potential for crevice corrosion is always at a level further into the negative range than the pitting potential for the same material in the same solution. Crevice corrosion can therefore also occur in a material that is resistant to pitting corrosion in a given medium. 

There is a close relationship between crevice corrosion and pitting corrosion. Crevice corrosion only occurs in waste waters that contain chloride ions, and is dependent on the conditions in the crevice. Crevice widths of more than 0.5 mm are generally not critical however, the crevice depth must also be taken into account [30]. Crevices between stainless steels and insulating materials, e.g. plastics, are particularly susceptible to crevice corrosion. Experience shows that the most important potential for crevice corrosion, the critical potential Uc, is clearly more negative than the pitting potential Up, as shown in Figure 4 [31]. Thus, for example, for a chloride concentration of 1 g/1, these potentials are Uc = -t 0.10 V and Up = -t 0.45 V. Crevice corrosion between stainless steel and stainless steel was also observed however the risk of corrosive attack is only to be expected at very high chloride concentrations [27]. For the use of the materials listed in Table 4, only a very low probability of corrosion is to be expected if there are crevices in the components [32]. 

Potentiostatic Techniques To determine the critical potential of crevice initiation, coupons in a crevice former device are exposed for a fixed period of time under potentiostatic control and monitoring of the anodic current is used to detect the onset of active corrosion. Several experiments are performed at different potentials and the crevice potential is the threshold potential that corresponds to an infinite initiation time (see Fig. 8 and 9 at the beginning of this chapter). 

It is also an accepted fact that the crevice corrosion ceases to grow at potentials less positive than a certain critical potential resulting in crevice protection as shown for austenitic stainless steel in Figure 22.30 [59,61]. The critical potential, Ecrev, is called crevice protection potential or the critical crevice corrosion potential. It was found for a cylindrical crevice in austenitic stainless steel that the crevice protection potential shifts in the less positive direction as a logarithmic function of solution chloride concentration [61] ... 

It is in fact the acidification of the occluded crevice solution that triggers the crevice corrosion. The critical acid concentration, < , , for crevice corrosion to occur corresponds to what we call the passivation-depassivation pH, beyond which the metal spontaneously passivates. This critical acidity determines the crevice passivation-depassivation potential, and hence the crevice protection potential Ecrev. The electrode potential actually measured consists of the crevice passivation-depassivation potential, E -ev, and the IR drop, A/iIR, due to the ion migration through the crevice. Assuming the diffusion current from the crevice bottom to the solution outside, we obtain AEm = icmv x h constant, where crcv is the diffusion-controlled metal dissolution current density at the crevice bottom and h is the crevice depth [62], Since anodic metal dissolution at the crevice bottom follows a Tafel relation, we obtain Eciev as a logarithmic function of the crevice depth ... 

A somewhat alternative analysis of pitting attributes pit initiation to the activation of defects in the passive film, defects such as those induced during film growth or those induced mechanically due to scratching or stress. The pit behavior is analyzed in terms of the product, xi, a parameter in which x is the pit or crevice depth (cm), and i is the corrosion current density (A/cm2) at the bottom of the pit (Ref 21). Experimental measurements confirm that, for many metal/environment systems, the active corrosion current density in a pit is of the order of 1 A/cm2. Therefore, numerical values for xi may be visualized as a pit depth in centimeters. A defect becomes a pit if the pH in the pit becomes sufficiently low to prevent maintaining the protective oxide film. Establishing the critical pH, for a specific oxide, will depend on the depth (metal ions trapped by diffiisional constraints), the current density (rate of generation of metal ions) and the external pH. In turn, the current density will be determined by the local electrochemical potential established by corrosion currents to the passive external cathodic surface or by a potentiostat. Once the critical condition for dissolution of the oxide has been reached, the pit becomes deeper and develops a still lower pH by further hydrolysis. 

The anodic polarization curve for a specimen with an active crevice will be in principle as shown in Figure 7.17. In this case a very small free external surface is assumed, and any internal hydrogen reduction is disregarded. is the potential as measured with the reference electrode positioned outside the crevice. As explained above, the real potential in the crevice, Ei , is more negative. The lower limit for corrosion in an active crevice is the protection (or repassivation) potential Epr. However, the critical potential that must be exceeded for initiation of the ereviee corrosion process, the crevice corrosion initiation potential, is higher than the protection potential. 

Electrochemical techniques have also been used to provide a rapid test in ASTM F 746, Test Method for Pitting or Crevice Corrosion of Metallic Surgical Implant Materials. A specimen is anodically p>olarized to stimulate pitting and then returned to a less positive preset potential to determine if repassivation will occur. This is repeated at successively higher preset potentials until the final, or criticcJ potential is determined. More resistcmt materitJs display higher critical potentials. This relatively brief test does not wait for pits to initiate tmd reduces variabiUty by maintaining the potential of the specimen. 

In the ensuing crevice corrosion process, chloride-rich corrosion products with high iron and chromium contents are produced. The pH level of the electrolyte solution in the crevice is reduced by hydrolysis of these salts, accelerating the corrosion process. The probability of corrosion for a surface with crevices is thus greater than for a surface with no crevices. The level of the critical potential is determined mainly by the crevice geometry and the material involved [20, 21]. 

However, the significance and the intrinsic nature of this potential are still under discussion. There are at least two possible causes for active corrosion inside crevices to stop below a critical potential ... 

First, the critical potential be a deactivation potential , i.e., a potential that corresponds to a cancellation of the overpotential required for dissolution in the crevice. According to Starr et al. [82], if active corrosion stops by deactivation with decreasing potential, a further increase of the corrosion potential would cause an immediate reactivation of the crevice. Indeed, Starr et al. [82] and Dunn and Sridhar [83] observed such a reactivation on low-grade stainless steels in acidic environments. 

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... 

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