Dissolution current

The equation assumes that for a given AE (usually 10 mV) shift, the corresponding change Ai is solely attributable to an increase in metal dissolution current. However, in solutions containing high redox systems, this may be very far from the case. 

Fig. 3. Evans-diagram for the cementation of Cu2+ and Pb2 with zinc amalgam of different zinc content. If the zinc concentration in the mercury employed for this special extraction technique is low, the anodic zinc-dissolution current density may be diffusion controlled and below the limiting cathodic current density for the copper reduction. The resulting mixed potential will lie near the halfwave potential for the reaction Cu2+ + 2e j Cu°(Hg) and only Cu2 ions are cemented into the mercury.

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. 

In the polarization curve for anodic dissolution of iron in a phosphoric acid solution without CP ions, as shown in Fig. 3, we can see three different states of metal dissolution. The first is the active state at the potential region of the less noble metal where the metal dissolves actively, and the second is the passive state at the more noble region where metal dissolution barely proceeds. In the passive state, an extremely thin oxide film called a passive film is formed on the metal surface, so that metal dissolution is restricted. In the active state, on the contrary, the absence of the passive film leads to the dissolution from the bare metal surface. The difference of the dissolution current between the active and passive states is quite large for a system of an iron electrode in 1 mol m"3 sulfuric acid, the latter value is about 1/10,000 of the former value.6... 

In the third case, the transpassive state appears at a more noble potential than the passive state, where the dissolution current that was suppressed at the passive region again increases. The boundary potential... 

Figure 19. Effects of chloride ion and proton on anodic dissolution current of passive metal.20 pCl - og[CT], pH -log[H+]. p.s. pit and a.s. pit indicate polishing-state pit and active-state pit, respectively.

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.)...

Inside a pit in electrolytic solution, anodic dissolution (the critical dissolution current density, and diffusion of dissolved metal hydrates to the bulk solution outside the pit take place simultaneously, so that the mass transfer is kept in a steady state. According to the theory of mass transport at an electrode surface for anodic dissolution of a metal electrode,32 the total increase of the hydrates inside a pit, AC(0) = AZC,<0),is given by the following equation33,34 ... 

From the first reaction, assuming that the anion adsorption process is rate determining, and neglecting the potential difference as shown in Eq. (75a), the average dissolution current density is written as a function of the surface concentration of the aggressive ions, (CJx, y, 0, /)), i.e.,... 

The mechanism of this type of instability can be elucidated as follows First, at the portion where the anion concentration happens to be higher than other portions of the surface, according to the first reaction equation [Eq. (76a)], the dissolution current density also becomes higher. From Eq. (78b), the current density fluctuation is expressed by the following simplified equation,... 

Determination of Local Corrosion States by Measuring Dissolution Current... 

Figure 38. Classification of nonequilibrium fluctuations. (Reprinted from M. Asanuma and R. Aogaki, Non-equilibrium fluctuation theory on pitting dissolution. I. Derivation of dissolution current equations." J. Chem. Phys. 106,9938,1997. Copyright 1997, American Institute of Physics.)...

In addition, assuming that the rate-determining step is the bulk diffusion (i.e.,ka/(DJt) lholds), we can derive the minimum dissolution current observed after the fluctuation-diffusion current, that is,... 

According to Eq. (110), the dissolution current initially increases with time however, it is gradually suppressed by a subsequent decrease in the double-layer overpotential as follows The overpotential V applied to the electrode has the following relationship with other overpotentials,... 

After passing the minimum state, the dissolution current starts to grow owing to the unstable growth of the asymmetrical fluctuations, which is expressed by Eq. (110). Taking logarithms of both sides ofEq. (110), it follows that... 

Dissolution, fluctuations during, 252 Dissolution current, in passivation, 292 fluctuations, 292 minimal, 285... 

Figure 1T2 shows anodic d cathodic polarization curves for the partial CD of dissolution 4 and deposition 4 of the metal and for the partial CD of ionization 4 and evolution 4 of hydrogen, as well as curves for the overall reaction current densities involving the metal (4) and the hydrogen (4). The spontaneous dissolution current density 4 evidently is determined by the point of intersection. A, of these combined curves. 

One should note that j02 is the oxide-forming current while jw3+ is the dissolution current. Hence, their ratio determines rjox. 

The overpotential A( A< 0/s) could not be experimentally determined. However, taking only the first term in Eq. (24) (which is a reasonable assumption at any real anodic dissolution current density), one could derive the ratio of the Tafel slopes of the two currents as... 

Fig. 9-16. Polarization curves of anodic oxidative dissolution and cathodic reductive dissolution of semiconductor electrodes of an ionic compound MX iiixcp) (iMxh )== anodic oxidative (cathodic reductive) dissolution current solid curve = band edge level pinning at the electrode interface, dashed curve = Fermi level pinning.

Fig. 10-11. Anodic photoexcited dissolution current of an n-type semiconductor electrode of gallium arsenide as a function of electrode potential in a 0.6 M sulfuric add solution lo - photon intensity = diotocurrent. [From Memming-Kelly, 1981.]...

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