Exchange current density metal dissolution

Little work has been done on bare lithium metal that is well defined and free of surface film [15-24], Odziemkowski and Irish [15] showed that for carefully purified LiAsF6 tetrahydrofuran (THF) and 2-methyltetrahydrofuran 2Me-THF electrolytes the exchange-current density and corrosion potential on the lithium surface immediately after cutting in situ, are primarily determined by two reactions anodic dissolution of lithium, and cathodic reduc-... 

Figure 15.7 Influence of exchange current density for different surfaces on the rate of metal dissolution by hydrogen evolution.

From Eqs. (1222) and (12.23), it is clear that the corrosion current depends upon the exchange currents (i.e., available areas and exchange-current densities), Tafel slopes, and equilibrium potentials for both the metal-dissolution and electronation reactions. To obtain an explicit expression for the corrosion current [cf. Eq. (12.22)], one has first to solve Eqs. (12.22) and (12.23) for A0corr. If, however, simplifying assumptions are not made, the algebra becomes unwieldy and leads to highly cumbersome equations. 

Fig. 12.18. Changing the exchange current densities produces a shift of the corrosion potential from a medium value toward the equilibrium potential of (a) the metal dissolution or (b) the electronation reaction.

Once the crack is initiated, the metal surface inside the crack may be quite different from the normal surface of the metal. Thus, in the course of plastic deformation, the metal could have developed slip steps [see Fig. 12.77(c)] which contain crystallographic planes of high Miller index at which the specific dissolution rate (or exchange current density) may be larger than that at the normal metal surface. Anodic current densities of some 104 times those at a passive surface have been shown to appear at a metal surface that is yielding under stress (Despic and Raicheff, 1978). 

When the potential of the metal is more negative than the equilibrium potential, metal deposition is more rapid than metal dissolution. The entering positive charge from the metal ions shifts the potential to more positive values until the equilibrium potential is reached. At this point both processes occur at equal rates. Each process involves an exchange current density which is equal in magnitude and opposite in sign to that of the other, so that the net current is zero. In the absence of an external current, the equilibrium potential is the stable limiting value. 

When there is a net current through the electrolytic cell, the rates of deposition and dissolution are not equal. As a result, the potential drop at the electrode surface is different from the equilibrium potential. The difference is called the overpotential. If the magnitude of the external current density is small compared to the exchange current density, the departure from equilibrium is also small. In this case, the electrode potential is close to the equilibrium value given by the Nernst equation and for practical purposes, the overpotential can be neglected. It should be noted that two or more electrode processes that have different equilibrium potentials may occur independently of each other at the same metal surface (Newman 1991). 

Anodic Inhibitors Inhibitors that directly affect the anodic reaction, that is, the metal dissolution process, are termed anodic inhibitors. Addition of an anodic inhibitor to the corrosion system (dashed and dashed-dotted lines in Fig. 1) can either lower the rate (i.e. the exchange current density) of the anodic process... 

Hydrogen evolution rate on the tin surface increases when tin is coupled with inert platinum. The observed increase in Fig. 6.6 results from the exchange current density difference of the coupled metals. The intersection between the tin dissolution polarization curve and the polarization curve for hydrogen evolution on tin results in fcorr.Sn- When equal surface area of tin (1 cm and platinum (1 cm are coupled, the sum of the rates of hydrogen evolution reactions on both metals is equal to the total rate of hydrogen evolution. 

Figure 6.15 Tafel plots for a metal ion transfer reaction, dissolution and deposition of Cd/Cd ". From the slopes the charge transfer coefficient is determined, a z = 1.09 and (1 — a )z = 0.91. With z = 2 one obtains = 0.55 and (1 — aj = 0.45. The exchange current density is Ig = —2.8.

The second holds that metals passive by Definition 1 are covered by a chemisorbed film—for example, of oxygen. Such a layer displaces the normally adsorbed H2O molecules and decreases the anodic dissolution rate involving hydration of metal ions. Expressed another way, adsorbed oxygen decreases the exchange current density (increases anodic overvoltage) corresponding to the overall reaction M -1- ze. Even less than a monolayer on the surface is... 

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