Anodic dissolution potential

An electroreductive Barbier-type allyla-tion of imines (434) with allyl bromide (429) also occurs inaTHF-PbBr2/Bu4NBr-(Al/Pt) system to give homoallyl amine (436) (Scheme 151) [533]. The combination of Pb(II)/Pb(0) redox and a sacrificial metal anode in the electrolysis system plays a role as a mediator for both cathodic and anodic electron-transfer processes. The metals used in the anode must have a less positive anodic dissolution potential than the oxidation potentials of the organic materials in order to be present or to be formed in situ. In addition, the metal ion plays the role of a Lewis acid to form the iminium ion (437) by associating with imine (435) (Scheme 151). 

Nickel. Most nickel is also refined by electrolysis. Both copper and nickel dissolve at the potential required for anodic dissolution. To prevent plating of the dissolved copper at the cathode, a diaphragm cell is used, and the anolyte is circulated through a purification circuit before entering the cathodic compartment (see Nickel and nickel alloys). 

The difference in oxygen concentration is now large and the potential of the metal within the crevice is now more negative than the freely exposed metal the predominant reaction in the crevice is anodic dissolution resulting in a high concentration of Fe and Cr ions. 

When dezincification occurs in service the brass dissolves anodically and this reaction is electrochemically balanced by the reduction of dissolved oxygen present in the water at the surface of the brass. Both the copper and zinc constituents of the brass dissolve, but the copper is not stable in solution at the potential of dezincifying brass and is rapidly reduced back to metallic copper. Once the attack becomes established, therefore, two cathodic sites exist —the first at the surface of the metal, at which dissolved oxygen is reduced, and a second situated close to the advancing front of the anodic attack where the copper ions produced during the anodic reaction are reduced to form the porous mass of copper which is characteristic of dezincification. The second cathodic reaction can only be sufficient to balance electrochemically the anodic dissolution of the copper of the brass, and without the support of the reduction of oxygen on the outer face (which balances dissolution of the zinc) the attack cannot continue. 

Pickering, H. W. and Byrne, P. J., Partial Currents During Anodic Dissolution of Cu-Zn Alloys at Constant Potential , J. Electrochem. Soc., 116, 1492 (1968)... 

The anodic dissolution of nickel is also dependent on the amount of cold work in the metal and in the active region the anodic current density of cold worked material at a given potential is up to one order of magnitude greater than that of annealed material. 

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

The thermodynamic and electrode-kinetic principles of cathodic protection have been discussed at some length in Section 10.1. It has been shown that, if electrons are supplied to the metal/electrolyte solution interface, the rate of the cathodic reaction is increased whilst the rate of the anodic reaction is decreased. Thus, corrosion is reduced. Concomitantly, the electrode potential of the metal becomes more negative. Corrosion may be prevented entirely if the rate of electron supply is such that the potential of the metal is lowered to the value where it is found that anodic dissolution does not occur. This may not necessarily be the potential at which dissolution is thermodynamically impossible. 

Similar considerations apply to oxidation. An anion which is considerably more stable than water will be unaffected in the neighbourhood of the anode. With a soluble anode, in principle, an anion only needs be more stable than the dissolution potential of the anode metal, but with an insoluble anode it must be stable at the potential for water oxidation (equation 12.4 or 12.5) plus any margin of polarisation. The metal salts, other than those of the metal being deposited, used for electroplating are chosen to combine solubility, cheapness and stability to anode oxidation and cathode reduction. The anions most widely used are SOj", Cl", F and complex fluorides BF4, SiFj , Br , CN and complex cyanides. The nitrate ion is usually avoided because it is too easily reduced at the cathode. Sulphite,... 

Fig. 19.15 Schematic representation of range of corrosion potentials expected from various chemical tests for sensitisation in relation to the anodic dissolution kinetics of the matrix (Fe-l8Cr-IONi stainless steel) and grain boundary alloy (assumed to be Fe-lOCr-lONi) owing to depletion of Cr by precipitation of Cr carbides of a sensitised steel in a hot reducing acid (after Cowan and Tedmon )...

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-... 

Upon an increase of the anodic reverse potential finally up to 8 V versus Li the cyclic voltammogran corresponding to Fig. 9 remains unchanged, showing that the passivating layer at the electrode also protects the solvents (PC and DME) from being oxidized. Subsequent deposition and dissolution of lithium at the passivated electrodes remains possible when the electrode is passivated but the cycling efficiency decreases. 

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... 

The same kind of analysis was performed for anodic dissolution of copper through ion-transfer film where a current or potential oscillation... 

Trigonal, metallic selenium has been investigated as photoelectrode for solar energy conversion, due to its semiconducting properties. The photoelectrochemistry of the element has been studied in some detail by Gissler [35], A photodecomposition reaction of Se into hydrogen selenide was observed in acidic solutions. Only redox couples with a relatively anodic standard potential could prevent dissolution of Se crystal. 

It was concluded from this and related works that suppression of the photodissolution of n-CdX anodes in aqueous systems by ions results primarily from specific adsorption of X at the electrode surface and concomitant shielding of the lattice ions from the solvent molecules, rather than from rapid annihilation of photogenerated holes. The prominent role of adsorbed species could be illustrated, by invoking thermodynamics, in the dramatic shift in CdX dissolution potentials for electrolytes containing sulfide ions. The standard potentials of the relevant reactions for CdS and CdSe, as well as of the sulfide oxidation, are compared as follows (vs. SCE) [68] ... 

Bard AJ, Wrighton MS (1977) Thermodynamic potential forthe anodic dissolution of n-type semiconductors - A crucial factor controlling durability and efficiency in photoelectrochem-ical cells and an important criterion in the selection of new electrode/electrolyte systems. J Electrochem Soc 124 1706-1710... 

The reduction wave of peroxydisulphate at dme starts at the potential of the anodic dissolution of mercury. The current-potential curve exhibits certain anomalous characteristics under various conditions. At potentials more negative than the electrocapillary maximum, a current minimum can be observed this is due to the electrostatic repulsion of the peroxydisulphate ion by the negatively charged electrode surface. The current minimum depends on the concentration and nature of the supporting electrolyte, and can be eliminated by the adsorption of capillary active cations of the type NR4. ... 

Surface studies are difficult in the case of many metal electrodes since their regions of ideal or perfect polarizability are very narrow that is, the potentials of anodic dissolution (or oxidation) of the metal and of cathodic hydrogen evolution are close... 

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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