Critical protection potential

In this section a survey is given of the critical protection potentials as well as the critical potential ranges for a possible application of electrochemical protection. The compilation is divided into four groups for both cathodic and anodic protection with and without a limitation of the protection range to more negative or more positive potentials respectively. 

II. The protection range lies at more negative potentials than the protection potential and is limited by a critical potential (// ... 

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The critical pitting potential cpr lies between the breakdown potential and the protection potential and can be determined by the scratch repassivation method. In the scratch repassivation method for localized corrosion, the alloy surface is scratched and exposed to a constant potential. The current change is monitored as a function of time and this will show the influence of potential on the induction time and the repassivation time. A careful choice of the level of potential between the breakdown potential and the critical pitting potential can give the critical pitting potential for a chosen material in given conditions.42 (Scully)14... 

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] ... 

FIGURE 22.30 Schematic polarization curves of a cyhndrical crevice in an anode of stainless steel in neutral solutions of three different chloride concentrations [59] h = crevice depth, Icrev — anodic crevice dissolution current, ccr = chloride concentration in the solution bulk, crcv = crevice protection potential, and /Crev— minimum crevice dissolution current at the critical crevice (protection) potential crcv. 

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 ... 

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. 

As we saw in the foregoing section, pitting corrosion of passive metals occurs beyond the critical pitting potential, plt. In order to protect passive metals from pitting corrosion, therefore, it is advisable to hold the corrosion potential as far less positive from plt as possible in the passive potential range. The presence of p-type oxides, however, makes Econ more positive and hence enhances the breakout of pitting corrosion. In the same way, metals corroding in the active state will accelerate their corrosion rates when their electrode potential is made more positive (more anodic) by the presence of p-type oxides. 

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Fig. 7.1 Typical cyclic polarization plot for stainless steel that shows the corrosion potential, c, critical pitting potential, Epn, protection potential, Eprot, and metastable pitting region.

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. 

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