Metal 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. 11-10. Anodic polarization curves observed for metallic iron, nickel, and chromium electrodes in a sulfuric acid solution (0.5 M H 2SO 4) at 25°C solid curve = anodic metal dissolution current dot-dash curve s anodic oxygen evolution current [Sato-Okamoto, 1981.]...

The rate of corrosion of the metal is obviously given directly by the rate of metal dissolution hence, the corrosion current /corr is equal to the metal-dissolution current... 

Since the metal-dissolution current is equal to the product of the corresponding current density [Pg.142]

Fig. 12.39. The change in potential from Atpcorr oA(f>e U results in a decrease of the metal dissolution current to zero, but also in an increase in the electronation current, and this increase in current has to be supplied from an external source.

The anodic metal dissolution current thus increases with the electrode potential up to a certain potential, called the passivation potential. As shown in Figure 22.7 for iron in acid solution [9,10], in the potential region more positive than the passivation potential, the metal passivates into almost no dissolution current due to the formation of a surface oxide film several nanometers thick, which we call the passive film [11], In contrast to the passive state of the metal, the active state refers to the metal undergoing anodic dissolution at significant rates below the passivation potential. 

FIGURE 22.17 Metallic passivation schematically illustrated by anodic and cathodic polarization curves of corroding metals (a) active corrosion, (b) unstable passivity, and (c) stable passivity i+ = anodic metal dissolution current and i = cathodic oxidant reduction current. 

FIGURE 22.27 Schematic polarization diagrams for pitting dissolution of metals (a) polishing and active modes of metal dissolution and (b) change in dissolution modes /= metal dissolution current, Evit — pitting potential, and EP — passivation potential. 

FIGURE 22.28 Schematic polarization diagrams for the repassivation of pitting dissolution of metals Er = repassivation potential, /-pit — pit radius, /pit = pitting dissolution current, and /a — metal dissolution current in the active sate. 

FIGURE 22.29 Schematic polarization diagrams (a) for the repassivation of pitting dissolution of metals and (b) for transformation from the electropolishing mode of pitting to the active mode of localized dissolution EP = passivation potential in the solution bulk, p = passivation potential in the critical pit solution, /sR = pit repassivation potential, /pit = pitting dissolution current, /a = anodic metal dissolution current in the active state in the bulk solution, and / = anodic metal dissolution current in the critical pit solution. 

It is in fact the acidification of the occluded crevice solution that triggers the crevice corrosion. The critical acid concentration, < , , for crevice corrosion to occur corresponds to what we call the passivation-depassivation pH, beyond which the metal spontaneously passivates. This critical acidity determines the crevice passivation-depassivation potential, and hence the crevice protection potential Ecrev. The electrode potential actually measured consists of the crevice passivation-depassivation potential, E -ev, and the IR drop, A/iIR, due to the ion migration through the crevice. Assuming the diffusion current from the crevice bottom to the solution outside, we obtain AEm = icmv x h constant, where crcv is the diffusion-controlled metal dissolution current density at the crevice bottom and h is the crevice depth [62], Since anodic metal dissolution at the crevice bottom follows a Tafel relation, we obtain Eciev as a logarithmic function of the crevice depth ... 

FIGURE 22.37 Schematic polarization diagrams for the corrosion of an active metal electrode (a) alone and (b) in contact with an n-type metal oxide under photoexcitation /a(M) = anodic metal dissolution current, /C(M) = cathodic current at metal, and / (ox) = anodic oxygen current at photoexcited oxide. 

The total current equals the anodic metal dissolution current for high anodic polarizations, and the cathodic hydrogen evolution current for high cathodic polarizations, leading to the semilogarithmic Tafel presentation (Equations 1.160a and b) ... 

This, of course, assumes a 100% current efficiency regarding metal dissolution, i.e. no other competitive electrochemical reactions occur. 

At a the net anodic reaction rate is zero (there is no metal dissolution) and a cathodic current equal to I" must be available from the external source to maintain the metal at this potential. It may also be apparent from Fig. 10.4 that, if the potential is maintained below E, the metal dissolution rate remains zero = 0), but a cathodic current greater than /"must be supplied more current is supplied without achieving a benefit in terms of metal loss. There will, however, be a higher interfacial hydroxyl ion concentration. 

Referring to Fig. 11.5b, the initial rise in current corresponds to simple metal dissolution, expressed quantitatively through the Tafel equation relating potential and current logarithmically, and for multi-grained metals... 

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... 

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.

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 ... 

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. 

When such a polyfunctional electrode is polarized, the net current, i, will be given by ii - 4. When the potential is made more negative, the rate of cathodic hydrogen evolution will increase (Fig. 13.2b, point B), and the rate of anodic metal dissolution will decrease (point B ). This effect is known as cathodic protection of the metal. At potentials more negative than the metaTs equilibrium potential, its dissolution ceases completely. When the potential is made more positive, the rate of anodic dissolution will increase (point D). However, at the same time the rate of cathodic hydrogen evolution will decrease (point D ), and the rate of spontaneous metal dissolution (the share of anodic dissolution not associated with the net current but with hydrogen evolution) will also decrease. This phenomenon is known as the difference effect. 

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