Pitting corrosion breakdown potential

The polarization resistance (Rp) of a metal/electrolyte sj m and the pitting or breakdown potential Eb) can be determined using at least two-electrode system. Subsequently, the rate of metal dissolution or corrosion rate is calculated using a function of the form icorr = / (i > Rp) > <> Th methods are... 

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 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. 

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. 

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