Pitting breakdown potential

Figure 45 Cumulative distributions of pitting breakdown potentials for the commercial purity (CP), high sulphur (HiS), and high purity (HiP) 304L steels. (From Ref. 50.)...

FIG. 5—Cumulative probability distributions of pitting breakdown potentials for three Type 304 stainless steels In 1000 ppm NaCl with different sulfur contents high sulfur ( ), commercial purity (A), and high purity (O) [22]. 

Cyclic anodic polarization procedures based upon ASTM G5 (Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements) have been used to evaluate the localized corrosion resistance of stainless steel alloys in paper machine white waters. The difference between the open circuit or naturally occurring corrosion potential and the pitting breakdown potential of various materials has been reported by many investigators [lO-Id]. Bowers [14] called the difference between the breakdown potential and the potential of the cathodic/anodic current reversal the margin of safety. He also noted the effect of the sulfate to chloride concentration ratio on localized corrosion of Types 304 and 316L stainless steels, These results permitted alternative materials of construction to be ranked and their limits of resistance to be defined. 

Increasing concentrations of bicarbonate tended to raise the breakdown potentials but also increased the corrosion potentials. This, in combination with a high chloride concentration, high bicarbonate concentrations may raise the corrosion potentials such that they border on passivation breakdown. The increase in hysteresis loop size on potentiodynamic cycles with increasing bicarbonate concentration shows a lowered resistance to pitting attack and crevice corrosion. 

The critical breakdown potential, which is the positive potential limit of stability of the oxide film. At this potential and more positive potentials, the oxide film is unstable with respect to the action of anions, especially halide ions, in causing localised rupture and initiating pitting corrosion. 

Figure 11. Schematic diagram of anodic polarization curve of passive-metal electrode when sweeping electrode potential in the noble direction. The dotted line indicates the polarization curve in the absence of Cl-ions, whereas the solid line is the polarization curve in the presence of Cl ions.7 Ep, passivation potential Eb, breakdown potential Epit> the critical pitting potential ETP, transpassive potential. (From N. Sato, J, Electrochem. Soc. 129, 255, 1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

At the area between the breakdown potential Eb and the critical pitting potential pit local film breakdown occurs, which leads to the creation of pit nuclei. However, these nuclei are immediately repassivated. Consequently, in this potential region it is concluded that breakdown and repair are continuously repeated without creating pit growth. 

From these treatments, it can be said that there is a potential region from the passivation potential to the lowest film-breakdown potential within which the passive film is stable against electrocapillary breakdown. At the potential beyond the critical pitting potential, not only passive film... 

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.

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. 

Potentiostatic methods. Once the breakdown potential is determined by cyclic potentiodynamic polarization methods, polarizing individual samples at potentials above and below this value will indicate the validity of the chosen scan rate and give some kinetic data on the initiation and propagation of pits at different levels. Another possibility is to initiate pits above the pitting or breakdown potential and then shift to lower values above or below the protection potential. It is assumed that at imposed values below the protection potential, one should observe current decrease until complete repassivation. 

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... 

FIGURE 22.26 Schematic polarization curves for anodic metal dissolution, passivation, passivity breakdown, pitting corrosion, and transpassivation Eb = film-breakdown potential and Ep]l — pitting potential. 

Fig. 7.14 Effect of chloride-ion concentration on the anodic polarization of type 304 stainless steel. Dashed lines indicate breakdown potentials, Eb pit. Curves A and B are schematic representations of polarization of cathodic reactions of relatively (A) high and (B) lower oxidizing strength. Based on Ref 27...

The presence of chloride ions in concrete leads to variations in the anodic behaviour of steel, modifying the anodic polarization curve as shown in Figure 7.8. The passivity range is reduced because its upper limit, Efiu known as the breakdown potential or pitting potential decreases as the chloride content increases [8] it passes from values of about -1-600 mV SCE in uncontaminated concrete to values below —500 mV in concrete with a high content of chloride. 

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