Pitting potentials critical

The last example presented in this section deals with the pitting corrosion of Fe in CIO solutions. Perchlorate is less known as an aggressive ion but reveals some unique and remarkable characteristics with regard to pitting corrosion. For example, the critical pitting potential (1.46 V against a standard hydrogen electrode (SHE) for Fe/1 M NaClO ) can be measured with an accuracy of less than 4 mV [61] which is very unexpected if compared to... 

Table 1.19 gives some critical pitting potentials collated by Uhlig for different metals in 0 -1 N NaCl at 25 °C, in which in most cases the potential was first increased incrementally and held constant at each potential for 5 min in order to obtain an approximate value of the critical pitting potential. This may be followed by a separate series of experiments in which fresh specimens are held at potentials close to b for 12 h or more and the surface... 

Table 1.21 Effect of molybdenum additions to Fe-15Cr-13Ni stainless steel on the critical pitting potential J S.H.E.)...

The critical pitting potential will, in general, decrease with the concentration of chloride ions, but will increase with the concentration of inhibiting oxy-... 

It is apparent that the critical pitting potential for a given alloy depends on the concentration of chloride ions, on the concentration of inhibiting anions in the solution and on the temperature of the solution. Unfortunately, the situation is complicated further by the fact that there is an induction period for the onset of pitting, which means that the pitting propensity... 

Vermilyea" has adopted a thermodynamic approach to pitting, and considers that the critical pitting potential is the potential at which the metal salt of the aggressive ion (e.g. AICI3) is in equilibrium with metal oxide (e.g. AljOj). On the basis of this theory the critical pitting potential should decrease by 0-059V per decade increase in chloride ion concentration. Vermilyea s theory successfully predicts the values of the critical potentials for Al, Mg, Fe and Ni, but in the case of Zr, Ti and Ta there are large discrepancies. 

Bohni, H. and Uhlig, H. H. Environmental Factors Affecting the Critical Pitting Potential of Aluminium , J. Electrochem. Soc., 116, 906 (1969)... 

Laboratory tests in which the specimen is immersed in a solution conducive to pitting such as an acidified FeClj solution (redox potential above the critical pitting potential E, ). 

More details of other factors that affect the critical pitting potential have been discussed by Uhlig and his co-workers" . They indicated that for stainless steel the critical pitting potential decreased with increasing concentration of chloride ion. At a fixed chloride level, passivating ions in solution, such as sulphate and nitrate, etc., cause the pitting potential to become more positive at a sufficient concentration these ions totally inhibited pitting, as shown in Fig. 19.40 for SO and CIO . 

Critical Pitting Potential the most negative potential required to initiate pits in the surface of a metal held within the passive region of potentials (it varies with the nature of solution, temperature, time, etc.). 

Considering the similarity between Figs. 1 and 2, the electrode potential E and the anodic dissolution current J in Fig. 2 correspond to the control parameter ft and the physical variable x in Fig. 1, respectively. Then it can be said that the equilibrium solution of J changes the value from J - 0 to J > 0 at the critical pitting potential pit. Therefore the critical pitting potential corresponds to the bifurcation point. From these points of view, corrosion should be classified as one of the nonequilibrium and nonlinear phenomena in complex systems, similar to other phenomena such as chaos. 

Figure 3. Current vs. potential curve for iron dissolution in phosphoric acid solution at pH 1,85. Ep, Flade potential Ep, passivation potential Epii- critical pitting potential EiP, transpassivation potential. Solid and broken lines correspond to the cases without and with CF ions, respectively.

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... 

As mentioned in Section II.3, in the presence of film-destructive anions such as chloride ions, beyond the critical pitting potential Epiti pitting dissolution proceeds, creating semispherical pits (polishing-state pits), which are different in shape from the irregular pits that develop at the active region that is less noble than the activation potential Ea, where the corrosive reaction moves from the passive state to the active state (usually the activation potential Ea is different and less noble than the passivation potential Ep). 

Generally, for ideally polarized electrodes, the plots of the electrode potential against either the chemical potential of the component in question or its activity are referred to as the Esin and Markov plots the slope of the plot is called the Esin and Markov coefficient.82 Aogaki etal.19 first established the expression of the critical pitting potential with respect to the composition of the solution (i.e., the Esin and Markov relations corresponding to the critical condition of the instability obtained in the preceding sections) and also verified them experimentally in the case of Ni dissolution in NaCl solution. 

As shown in Fig. 25, an example of the extrapolation of the current transient obtained from the potential sweep yields the critical potential after ascertaining that the data obtained are independent of the sweep rate. Figure 26 exhibits the results of the critical pitting potential measurement for the majority salt of NaCl and the minority ion of Ni2+when the concentration of NaCl is varied under the condition of constant Ni2+ionic concentration. From the plot in Fig. 26, it follows that... 

At the potential beyond the critical pitting potential, the passive metal electrode system turns unstable. As mentioned before, the asymmetrical fluctuations arise from the electrostatic interaction between the electrode surface and solution particles in the double layer, so that the pitting current develops rapidly, and pits grow simultaneously. 

Therefore, from the analyses of the asymmetrical and symmetrical fluctuations in Sections HL 1 and m.4, it is concluded that the polishing-state pits discussed here, which appear beyond the critical pitting potential, have only one representative length (i.e., the autocorrelation distance of the asymmetrical fluctuations), which suggests that the morphology of the... 

After the electrode potential is changed beyond the critical pitting potential, the fluctuations turn unstable through the critical state. At the same time, the reactions occurring at the surface yield new asymmetrical fluctuations in accordance with the potential difference. 

The autocorrelation distance is determined by the total overpotential (0Q) of the double layer, which is measured from the critical pitting potential and the coverage 0 of the passive film. From the experimental results which will be discussed later, the actual function form is determined as... 

---