Crevice corrosion initiation potential

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. 

The sample shown in Fig. 3 was held several hundred millivolts below its pitting potential [28]. After an incubation period, crevice corrosion initiated. The area immediately inside the crevice was... 

Critical potentials axe important in determining the resistance of a mefed to localized corrosion because they have a mechanistic underpinning and they are rapid measurements. Two potentials have been used as a measure of the crevice corrosion resistance—crevice/pit initiation potential (Ee or Ep) and crevice/pit repassivation potential (Ei ev or E ). The E or Ep is the potential at which the current... 

As a consequence, the polarization curves drawn on a specimen equipped with a crevice former device exhibit (Fig. 7) features very similar to those typical of pitting corrosion. Dining the upward potential scan, crevice corrosion initiates quite abruptly at a critical potential that is generally lower than the pitting potential. Then, during a subsequent backward potential scan, corrosion stops at a critical potential significantly lower than... 

Evidence of localized corrosion can be obtained from polarization methods such as potentiodynamic polarization, EIS, and electrochemical noise measurements, which are particularly well suited to providing data on localized corrosion. When evidence of localized attack is obtained, the engineer needs to perform a careful analysis of the conditions that may lead to such attack. Correlation with process conditions can provide additional data about the susceptibility of the equipment to locaHzed attack and can potentially help prevent failures due to pitting or crevice corrosion. Since pitting may have a delayed initiation phase, careful consideration of the cause of the localized attack is critical. Laboratory testing and involvement of an... 

Electrochemical noise A variety of related techniques are now available to monitor localized corrosion. No external polarization of the corroding metal is required, but the electrical noise on the corrosion potential of the metal is monitored and analyzed. Signatures characteristic of pit initiation, crevice corrosion and some forms of stress corrosion cracking is obtained. 

Crevice corrosion is another form of localized corrosion, which is easier to initiate than pitting. Since aevice corrosion will occur at lower potentials and temperatures than pitting, it is necessary to avoid crevice corrosion completely when trying to characterize resistance to pitting. Over the years, several suggestions have been presented for solving this problem. 

Pickering and coworkers [31, 34, 35] have demonstrated both experimentally and computationally that for systems that meet the criteria of the IR theory, lA is predicted. The amount of potential drop increases as one moves into the crevice because of the current leaving the crevice. If the geometry, solution conductivity, and passive current density of the material in the environment conspire to create sufficient ohmic drop, then the potential of some portion of the material within the crevice falls to the primary passive potential. Under these circumstances, the passive film is not stable and active dissolution occurs. The potential difference between the applied potential and the primary passivation potential is referred to as IR. Deeper still into the crevice the ohmic drop leads to decreased dissolution as the overpotential for the anodic reaction decreases. Thus, ohmic drop is responsible for the initiation and stabihzation of crevice corrosion according to this model. 

Potential control can be effective in suppressing the initiation and propagation of crevice corrosion. The electrochemical... 

In one of the tests, the corrosion potential of the sample in 9 g/1 sodium chloride is noted. The pH, initial, and final potential are noted. The current at -1-0.8 V versus SCE is noted. If localized corrosion is not simulated in the first 20 s, the polarizing currents will be small and decrease with time. If localized corrosion is not stimulated in 15 min, the test is terminated, and the sample is resistant to localized corrosion. Localized corrosion is indicated by increasing polarizing current with time (>500 pA/cm ). The potential is then returned to corrosion potential to determine if the sample will repassivate or the localized corrosion will continue to occur. Evidence of pitting and crevice corrosion should be noted in ASTM F 746 (30). 

Identical metals in contact with different concentrations In this case, the metal immersed in a dilute solution is dissolved from the electrode and deposited on the electrode immersed in a more concentrated solution. The other type of electrochemical concentration ceU is known as a differential aeration cell. In this case, the electrode potential difference occurs when the electrode is immersed in the same electrolyte with different oxygen partial pressures. Differential aeration initiates crevice corrosion in aluminum or stainless steel when exposed to a chloride environment. 

Cychc potentiodynamic polarization and constant-potential tests were used to study crevice corrosion for a Fe-based bulk metaUic glass alloy (SAM1651) and a Ni-Cr-Mo crystalline alloy (C-22) in a 4 M NaCl solution at 100 °C [105]. The results indicated SAM 1651 has a more positive breakdown potential and repassivation potential than C-22 and requires a more positive oxidizing potential to initiate crevice corrosion in constant-potential tests. C-22 corrosion propagation was more localized near the crevice... 

Figure 7.16 Potential-log current diagram for crevice corrosion, a) Initial and b) developed to a late stage. 1) Anodic and 2) cathodic curve (oxygen reduction) in neutral water. 3) Anodic curve in HCl solution, pH = 0, corresponding to a crevice solution. 4) Curve for the hydrogen reaction in the crevice (pH = 0-4). 5) Curve for the reduction of oxygen outside the crevice (pH = 8-10).

Critical initiation potentials can be defined by means of potentiostatic experiments with crevice specimens, e.g. as shown in Figure 7.24 [7.19, 7.20]. Possible initiation of crevice corrosion (at a given set potential) is indicated by a marked increase in the... 

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