Potentials repassivation

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 20. Pit-dissolution current density pit radius and ion concentration buildup AC in the pit electrolyte corresponding to the critical condition for growing pits on 18Cr-8Ni stainless steel to passivate at different repassivation potentials, EK, in 0.5 kmol m 3 H2S04 + 0.5 kmol m-3 NaCl during cathodic potential sweep at different sweep rates.7 (From N. Sato, J. Electrochem. Soc. 129,261,1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

Figure II. Schematic anodic polarization curves at a fixed temperature. Determination of either transpassive potential ( ,) or pitting potential ( p and repassivation potential ( ,) at the critical current density (/ ). t rcvis the current density at which the scan is reversed. ...

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... 

Plotting the repassivation potentials (or the pitting potentials and the transpassive potentials) as a function of the specimen temperature evaluates the CPT. An example of an evaluation is shown in Fig. 20. 

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 42 Cyclic polarization curve for Type 302 stainless steel in 1,000 ppm NaCl. Note the definition of the breakdown and repassivation potentials, the vertex current density, and the appearance of metastable pits.

Figure 44a A compendium of repassivation potential versus charge density data from the literature for various Ni-Fe-Cr-Mo alloys in Cl" solutions. Some of the charge densities were calculated from the data provided in the original references. (From Ref. 41.)...

Important evidence supporting the application of Ew is the long-term po-tentiostatic data of Dunn and Sridhar (47), in which potentiostatic holds of alloy 825 in 1000 ppm Cl" at 95°C have been performed for up to 18 months. They found that the repassivation potential determined for deep pits at short times corresponded well to the potential below which localized attack did not occur over long times. Localized attack did occur only 10 mV above the highest observed repassivation potential. 

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

Figure 46 Schematic of applied current density vs. time observed for mechanical scratching of a surface exposed to a solution above and below its repassivation potential.

Figure 48 Plot of breakdown and repassivation potentials vs. temperature for different steel grades in 1 M NaCl. Filled symbols for breakdown and open symbols for repassivation potentials. (From Ref. 30.)...

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.

M Na2Cr2Ov to aerated 1.0 M NaCl solution has virtually no effect on the pit polarization curve compared to a dichromate-free solution. Additions of 0.5 M Na2Cr2Ov increase the repassivation potential by about 90 mV, while a 2.0 M Na2Cr2Ov addition nearly stops pit propagation. The concentrations of dichromate required to inhibit thin film pit growth are consistent with those proposed by Kaesche (23) but are much greater than those required to slow metastable pitting in bulk A1 samples. 

From an engineering perspective, the repassivation potential is a more important parameter than the potential for pit nucleation. We want to know the potential below which pits will not grow. This is analogous in theory to measuring KiC or Kiscc in mechanical and SCC testing. One way to test this is to produce a completely bare surface that is dissolving rapidly, and determine at what potential it can repassivate. An easy way to do this is what may be termed an electrochemical scratch. ... 

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. 

Two main points should be remembered from this lab. First, the breakdown potential is not necessarily the best measurement of pitting resistance. This is because pitting can occur at potentials below EM, as was demonstrated by metastable pitting in test 4. Ebi corresponds to the potential for stable pit growth and propagation only. Pits can nucleate, however, at any potential above the repassivation potential. Secondly, the effects that additional anions have on the pitting behavior is concentration dependent and not mass dependent. 

Figure 6.23 Schematic of a polarization curve showing Ep (pitting potential) R (repassivation potential) relative to Ecorr, critical potentials and metastable region (Frankelf...

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