Pitting potentials densities

Because of possible errors in determining pitting potentials from I(t/) curves, it is safest to take them from pit density-potential diagrams which can be determined by chronopotentiostatic experiments. Figure 2-16 shows the results of experiments on 1.4031 CrNi stainless steel (AISI304) in neutral waters [52]. 

The ease with which stainless steels can passivate then increases with the level of chromium within the alloy and so materials with higher chromium content are more passive (i.e. conduct a lower passive current density) and passivate more readily (i.e. the critical current density is lower and the active/passive transition is lower in potential). They are also passive in more aggressive solutions the pitting potential is higher. 

Stress-corrosion cracking based on active-path corrosion of amorphous alloys has so far only been found when alloys of very low corrosion resistance are corroded under very high applied stresses . However, when the corrosion resistance is sufficiently high, plastic deformation does not affect the passive current density or the pitting potential , and hence amorphous alloys are immune from stress-corrosion cracking. 

The constancy of the potential with increasing current density could be explained in terms of an automatic adjustment of the number of pits while maintaining a constant current per pit. At potentials more positive than the pitting potential, Kaesche67 has found the total current to increase with time. This complied very well with a model in which the true current density at the pit (found to be of the order of 300 mA/cm2) and the number of pits,... 

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... 

It should be mentioned that passive layers are not protective in all environments. In the presence of so-called aggressive anions, passive layers may break down locally, which leads to the formation of corrosion pits. They grow with a high local dissolution current density into the metal substrate with a serious damage of the metal within very short time. In this sense halides and some pseudo halides like SCN are effective. Chloride is of particular interest due to its presence in many environments. Pitting corrosion starts usually above a critical potential, the so-called pitting potential /i]>j. In the presence of inhibitors an upper limit, the inhibition potential Ej is observed for some metals. Both critical potentials define the potential range in which passivity may break down due to localized corrosion as indicated in Fig. 1. 

In near-neutral dilute chloride solutions, concentrations of chromate, less than those suggested by Kaesche, have been observed to increase the pitting potential. Figure 6a shows anodic polarization curves from high purity A1 wire loop electrodes in deaerated 1.0 mM chloride solutions (25). Additions of 25 to 50 pM of sodium chromate were shown to elevate the pitting potential by hundreds of millivolts and reduce the passive current density by about a factor of 2. 

In the range of electrode potential more positive (more anodic) than the pitting potential, the pitting corrosion occurs in the presence of chloride ions and the metal dissolution at a pit, initially hemispherical, proceeds through the mode of electropolishing, in which concentrated chloride salts in an occluded pit solution will control the pit dissolution. It is likely that the polishing mode of metal dissolution proceeds in the presence of a metal salt layer on the pit surface in the salt-saturated pit solution. It was experimentally found with stainless steels in acid solution [54] that the pit dissolution current density, pit, is an exponential function of the electrode potential, E (Tafel equation) ... 

Mass transport in the pitting dissolution determines the localized chloride concentration, car, which is proportional to the product, plt x rpit, of the pitting current density, pit, and the pit radius, rpjt. Since pit is an exponential function of E, the electrode potential will have to be a logarithmic function of the pit radius, rpit, in order for car to hold its critical concentration, e er- The critical electrode potential, Er, for pit repassivation, therefore, depends logarithmically on the radius of the pit ... 

Fig. 7.20 Temperature dependence of pitting potential defined as potential at which current density reaches 100 mA/m2. Same alloy as Fig. 7.19. Dashed curve approached as potential scan rate used in Fig. 7.19 is decreased. Based on Ref 32...

Fig. 7.24 Relation between the pitting potential of 17 wt% Cr, 16 wt% Ni steels with elements shown in 0.1 N NaCl + 0.25 N Na2S04 and the critical current density for passivation in 1 N H2S04 + 0.05 N NaCl at 40 °C. Source Ref 36...

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