Anodic dissolution, of metal

In the course of electrolysis there may occur either discharge of anion at the anode, for instance, 

A typical example of dissolution of the anode being associated merely with concentration polarization is encountered in the case of mercury the relation between anode potential and current density is graphically represented in Fig. 29. which illustrates the conditions prevailing in a 0.66 normal solution 

In the case of anodically poorly soluble metals (such as nickel) the structure of the material may be of some importance. Rolled or forged metals have greater dissolution pressure than cast ones. As the material is heterogeneous the mechanically treated parts are dissolved first while the soft ones are loosened from the anode and deposited at the bottom of the vessel in the form of slurry. 

While some metals, such as zinc, enter the solution as ions of always the same valency, other metals (e. g. Cu, Hg, Sn, Pb, Cr, An, Fe, Co, Ni, Tl) can form ions of different valencies in the anodic process. In order to solve the problem of ionization in stages Luther s rule is applied. 

In the case of metal forming two cations, M and Jf, there will coexist the equilibria of three systems MIM, MjM and M jM with corresponding oxidation potentials e , s,sr s , or reduction potentials 7t , 7t° and tc ,2. Lot us assume that tho ion Mz, has a higher valency than M (Z2 Zj). The standard free energies related to individual respective processes are thus  

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Pits seldom form in close proximity to one another and it would appear that the area of passivated metal, which acts as the cathode for the local cell, is protected by the anodic dissolution of metal within the pit—a phenomenon that is referred to as the mutually protective effect see Section 1.5). 

Adsorbed species may also accelerate the rate of anodic dissolution of metals, as indicated by a decrease in Tafel slope for the reaction. Thus the presence of hydrogen sulphide in acid solutions stimulates the corrosion of iron, and decreases the Tafel slope The reaction path through... 

The electrochemical machining (ECM) of metals rests on the selective local anodic dissolution of metal. It is used to give metal parts the required shape and size, to drill holes, create hollows, cut shaped slots, and fashion parts of a complex pattern (e.g., the blades of gas turbines). It is an advantage of this method that it can also be used for hard metals (high-alloy steels and other alloys, metals in the quenched state, etc.). 

Itaya, K., Atomic-scale aspects of anodic dissolution of metals studies by in situ scanning mnneling microscopy, in Interfacial Electrochemistry, A. J. Wieckowski, Ed., Marcel Dekker, New York, 1999, p. 187. 

The anodic dissolution of metals on surfaces without defects occurs in the half-crystal positions. Similarly to nucleation, the dissolution of metals involves the formation of empty nuclei (atomic vacancies). Screw dislocations have the same significance. Dissolution often leads to the formation of continuous crystal faces with lower Miller indices on the metal. This process, termed facetting, forms the basis of metallographic etching. 

Such a transfer reaction of metallic ions in a series connection of two elementary steps involving an intermediate of an adsorbed metallic ion complex may be illustrated by the anodic dissolution of metallic iron in acidic solution [Despic, 1983] as shown in Eqn. 9-21 ... 

The state in which the anodic dissolution of metals proceeds from the bare metal stirface at relatively low electrode potentials is called the active state the state in which metal dissolution is inhibited substantially by a superficial oxide film at higher electrode potentials is called the passive state-, the state in which the anodic dissolution of metals increases again at stiU higher (more anodic) potentials is called the transpassive state. 

In the stationary state of anodic dissolution of metals in the passive and transpassive states, the anodic transfer of metallic ions metal ion dissolution) takes place across the film/solution interface, but the anodic transfer of o Q en ions across the Qm/solution interface is in the equilibrium state. In other words, the rate of film formation (the anodic transfer oS metal ions across the metal lm interface combined with anodic transfer of osygen ions across the film/solution interface) equals the rate of film dissolution (the anodic transfer of metal ions across the film/solution interface combined with cathodic transfer of oitygen ions across the film/solution interface). 

Corrosion can be controlled by Isolation of the metal from the corrosive environment by suppression of the anodic dissolution of metal and by suppression of the corresponding cathodic reaction. Isolation of corrosion prone metals from corrosive environments is probably the most general mechanism of the corrosion protection afforded by paint films, sealers, and similar polymer-based materials. Effective isolation requires that polymeric materials have good barrier properties and remain adherent in the presence of water and the products of metallic corrosion. Barrier properties and adhesion aspects of corrosion control are discussed in detail in subsequent sections. 

Anodic dissolution of metals in the presence of ligands allows the syntheses of various coordination complexes, and these have included examples of the preparation of a limited number of titanium complexes. For example, anodic dissolution of Ti in the presence of CO and cyclopentadienyl monomer is reported to yield [Ti( 5.C5H5)2(CO)2] [1]. This field has been reviewed but has not been extensively developed since the early work of Tuck and coworkers [2, 3]. 

Oxygen absorption, then, is the usual complementary cathodic reaction for the anodic dissolution of metals other than the most reactive ones (Li, Na, K, Ca, etc.), in near-neutral aerated water. Regardless of whether the cathodic reaction is 16.3 or 16.5, however, the following observations regarding bimetallic corrosion apply ... 

If these conditions are not satisfied, some process will be involved to prevent accumulation of the intermediates at the interface. Two possibilities are at hand, viz. transport by diffusion into the solution or adsorption at the electrode surface. In the literature, one can find general theories for such mechanisms and theories focussed to a specific electrode reaction, e.g. the hydrogen evolution reaction [125], the reduction of oxygen [126] and the anodic dissolution of metals like iron and nickel [94]. In this work, we will confine ourselves to outline the principles of the subject, treating only the example of two consecutive charge transfer processes O + n e = Z and Z 4- n2e — R. 

The direct electrochemical synthesis of metal alkoxides by the anodic dissolution of metals into alcohols containing conducting electrolytes was initially demonstrated by Szilard in 1906 for the methoxides of copper and lead.19 More recently the method has received some attention particularly in the patent literature.29-25 The preparation of the ethoxides of silicon, titanium, germanium, zirconium and tantalum by electrolysis of ethanolic solutions of NH Cl has been patented, although the production of the ethoxides was found to cease after several hours.24,25... 

On anodic dissolution of metals, if it takes place at equilibrium potentials, ions of different valencies are also formed in the ratio expressed by the equilibrium oonstant. So, for instance, copper anode emits Cu++ and Cu+ ions into the solution in a ratio that, under the afore-men ioned assumption and at... 

The electrodeposition of tin, Sn, has been reported in both basic and acidic EMICI-AICI3 ionic liquid [26]. A divalent tin species, Sn(II), can be introduced by the anodic dissolution of metallic tin. The introduction of a tetravalent tin species, Sn(lV), is also possible by dissolving tin tetrachloride, SnCL,. However, the evaporation of SnCl4 occurs in the case of an acidic ionic liquid. The irreversible reduction of Sn(IV) to Sn(II) occurs at around 0.91 and —0.9 V in the acidic and basic ionic liquids, respectively. The electrodeposition of metallic Sn is possible by the reduction of Sn(II) ... 

The electrodeposition of lead, Pb, has been investigated in an acidic EMICl-AICI3 ionic liquid [27]. A divalent lead species, Pb(II), can be introduced by dissolving lead dichloride, PbCl2, or the anodic dissolution of metallic Pb. Metallic Pb is obtained by the reduction of Pb(II) ... 

EMICl- [22, 37, 38, 51-54], and BMICI-AICI3 [55] ionic liquids. In the case of acidic ionic liquids, it is possible to introduce monovalent and divalent copper species, Cu(l) and Cu(II), by dissolving copper chloride and dichloride, respectively, and by the anodic dissolution of metallic copper. Metallic Cu can be obtained by the reduction of Cu(I). The formal potentials of Cu(I)/Cu in an acidic MPCl-BPC1-, EMICI-AICI3 are reported as 0.777 [49], 0.784 [50], and 0.837 [52] V, respectively. Cu(I) can be also obtained by the reduction of Cu(II), of which the formal potentials in the acidic MPCl- and BPCI-AICI3 are reported as 1.851... 

The electrochemistry of the anodic etching of semiconductors is similar in most respects to the anodic dissolution of metals. The main difference in the electrolytic behavior of metals and semiconductors is in the electrode material itself. 

Theory of metal surfaces work function N. D. Lang and W. Kohn, Phys. Rev. B 3, 1215 (1970). Coupled partial ion-transfer steps in the anodic dissolution of metals. D. Vanmaekelbergh and B. H. Erne, J. Electrochem. Soc. 146, 2488 (1999). 

High-rate anodic dissolution of metals has been studied intensively (Reviews [9, 12, 14, 21]). Several types of anodic metal dissolution are recognized. Sometimes, the dissolution of metals in the active state is used in ECM. Deep, small-diameter holes drilled in acidic solutions are an example. An acid is used in order to avoid sludge formation, which hampers the drilling. However, highly aggressive... 

The effect of ultrasound on the rate of anodic dissolution of metals was studied by Karavainikov [118] in 1973. He found that the rate of dissolution of Fe in 10% HC1 was slightly increased with ultrasonic vibration in the current density range of 0.08-0.4 A/m2. The surface after dissolution under ultrasound had a uniform, fine-grained structure giving diffuse dispersion of light. 

Typical examples of processes involving two or more adsorbed species are reactions of corrosion or anodic dissolution of metals, oxygen evolntion, etc. In the case of two adsorbed species B and C, the electrochemical... 

The electrochemical formation of catalytic systems via the anodic dissolution of metals has been studied by Koval chuk et al.39-42, for the polymerization of various vinyl monomers. 

Statement of the problem. Now let us consider mass transfer from a solid wall to a liquid film at high Peclet numbers. Such a problem is of serious interest in dissolution, crystallization, corrosion, anodic dissolution of metals in some electrochemical processes, etc. In many practical cases, dissolution processes are rather rapid compared with diffusion. Therefore, we assume that the concentration on the plate surface is equal to the constant Cs and the incoming liquid is pure. As previously, we introduce dimensionless variables according to formulas (3.4.5). In this case, the convective mass transfer in the liquid film is described by Eq. (3.4.1), the boundary condition (3.4.2) imposed on the longitudinal variable x, and the following boundary conditions with respect to the transverse coordinate ... 

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