Anodic oxidation sacrificial anodes

Contact with steel, though less harmful, may accelerate attack on aluminium, but in some natural waters and other special cases aluminium can be protected at the expense of ferrous materials. Stainless steels may increase attack on aluminium, notably in sea-water or marine atmospheres, but the high electrical resistance of the two surface oxide films minimises bimetallic effects in less aggressive environments. Titanium appears to behave in a similar manner to steel. Aluminium-zinc alloys are used as sacrificial anodes for steel structures, usually with trace additions of tin, indium or mercury to enhance dissolution characteristics and render the operating potential more electronegative. 

FIGURE 12.20 In the cathodic protection of a buried pipeline or other large metal construction, the artifact is connected to a number of buried blocks of metal, such as magnesium or zinc. The sacrificial anodes (the magnesium block in this illustration) supply electrons to the pipeline (the cathode of the cell), thereby preserving it from oxidation. 

The calculation shows that zinc is oxidized preferentially over iron. Later in this chapter we describe the use of zinc as a sacrificial anode to prevent corrosion of iron. 

Using electrons for the electrolytic reduction of metal salts, Reetz and coworkers have introduced a further variation to the tetraalkylammoniumhalide-stabilization mode [192-198]. The overall electrochemical process can be divided into the following steps (i) oxidative dissolution of the sacrificial Metbuik anode, (ii) migration of Met ions to the cathode, (iii) reductive formation of... 

Low-valent lanthanides represented by Sm(II) compounds induce one-electron reduction. Recycling of the Sm(II) species is first performed by electrochemical reduction of the Sm(III) species [32], In one-component cell electrolysis, the use of sacrificial anodes of Mg or A1 allows the samarium-catalyzed pinacol coupling. Samarium alkoxides are involved in the transmet-allation reaction of Sm(III)/Mg(II), liberating the Sm(III) species followed by further electrochemical reduction to re-enter the catalytic cycle. The Mg(II) ion is formed in situ by anodic oxidation. SmCl3 can be used in DMF or NMP as a catalyst precursor without the preparation of air- and water-sensitive Sm(II) derivatives such as Sml2 or Cp2Sm. 

Summary Until now, the only possible anode reaction was the oxidation of the sacrificial anode. We have now found an alternative anode reaction without formation of metal chlorides. New anode materials turned out to be completely stable against oxidation. 

Another study on the electrosynthesis of (alkyl) M compounds (M = Ge, Pb, Sn n = 2, 4) provides illustrative examples37. Sacrificial cathodes of Cd, Zn and Mg were used to produce the corresponding metal alkyls which are subsequently oxidized on sacrificial anodes of Ge, Sn and Pb. The cells are of very simple construction, with the proper metal electrodes. Diethylcadmium is utilized in this way for the manufacture of tetraethyllead from lead acetate and triethylaluminum in the following reaction sequence ... 

Synthesis with sacrificial electrodes is employed as a direct method in several other preparations of organometallic compounds and complexes. 3-Hydroxy-2-methyl-4-pyrone derivatives of Sn 1 (and of Zn, Cu, In and Cd as well) were prepared using the metal as an anode. The low oxidation state Sn(II) compound is obtained by direct electrolysis134. 

The introduction of such a layer can dramatically improve the fuel cell performance. For example, in the SOFC with bilayered anode shown in Figure 6.4, the area-specific polarization resistance for a full cell was reduced to 0.48 Hem2 at 800°C from a value of 1.07 Qcm2 with no anode functional layer [24], Use of an immiscible metal oxide phase (Sn()2) as a sacrificial pore former phase has also been demonstrated as a method to introduce different amounts of porosity in a bilayered anode support, and high electrochemical performance was reported for a cell produced from that anode support (0.54 W/cm2 at 650°C) [25], Use of a separate CFL and current collector layer to improve cathode performance has also been frequently reported (see for example reference [23]). 

In an undivided cell, the sacrificial anode is used so that it is oxidized in preference to the silyl and chloride ions. There is thus a requirement for the reduction potential of the sacrificial anode to be more negative than the reduction potential of the silyl chlorides, Eesich that is, E°m < E°sici (Equation (3) and Equation (4)) ... 

The previous examples used the three-electrode electrochemical system. An alternative was utilized by Ajayan et al. to prepare Ag NP coated SWCNTs [217]. An electrode was fabricated consisting of SWCNTs attached to a Ti cathode and a silver contact pad as a sacrificial anode (Fig. 5.16(a)). The electrode was submerged in an aqueous solution and a potential was applied resulting in oxidation of Ag metal to Ag2+ ions which then subsequently deposited onto the SWCNT cathode. Although experimentally complicated, silver NPs, wires and patterns were controllably deposited on the SWCNTs (Fig. 5.16(a), (b)) [217],... 

CO2 (Scheme 67) [277]. Recently, the com-hination of a new electrolyzer for organic solvents and oxidation of metal powder as an alternative to sacrificial anodes has heen developed [269]. 

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). 

The mechanism of the Zn chloride-assisted, palladium-catalyzed reaction of allyl acetate (456) with carbonyl compounds (457) has been proposed [434]. The reaction involves electroreduction of a Pd(II) complex to a Pd(0) complex, oxidative addition of the allyl acetate to the Pd(0) complex, and Zn(II)/Pd(II) transmetallation leading to an allylzinc reagent, which would react with (457) to give homoallyl alcohols (458) and (459) (Scheme 157). Substituted -lactones are electrosynthesized by the Reformatsky reaction of ketones and ethyl a-bromobutyrate, using a sacrificial Zn anode in 35 92% yield [542]. The effect of cathode materials involving Zn, C, Pt, Ni, and so on, has been investigated for the electrochemical allylation of acetone [543]. 

Since the catalytic effect of Cu ions with Fenton s reagent has been recognized, continuing interest has been focused on the role of the Cu species in oxidizing systems. The electrochemical alkyltransfer reaction of trialkylboranes to carbonyl compounds has been performed in an undivided cell by using the sacrificial Cu anode in a DMF-BU4NI system, giving the alkylated products in 62 77% yields [580]. 

Zinc is more easily oxidized than iron. Therefore, zinc, not iron, becomes the anode in the galvanic cell. The zinc metal is oxidized to zinc ions. In this situation, zinc is known as a sacrificial anode, because it is destroyed (sacrificed) to protect the iron. Iron acts as the cathode when zinc is present. Thus, iron does not undergo oxidation until all the zinc has reacted. 

Cathodic protection is another method of preventing rusting, as shown in Figure 11.29. As in galvanizing, a more reactive metal is attached to the iron object. This reactive metal acts as a sacrificial anode, and the iron becomes the cathode of a galvanic cell. Unlike galvanizing, the metal used in cathodic protection does not completely cover the iron. Because the sacrificial anode is slowly destroyed by oxidation, it must be replaced periodically. 

The Reformatsky reaction can also be performed electrochemically either directly or using a mediator. Ni-catalysis has proven to be an efficient way to prepare j3-hydroxy ester or nitrile from the corresponding a-chlorocompounds (Table 14) [94]. Here again the first step is the oxidative addition of the cathodically generated Ni°bpy to the halocompound. The nature of the sacrificial anode also plays a crucial role in this reaction, though the formation of an organozinc intermediate has not been fully demonstrated. 

The advantage of using a sacrificial anode has been clearly pointed out. Magnesium was found to be the most convenient, the oxidation of which produces Mg ions which can enter the catalytic cycle to cleave the nickela-cycle intermediate and liberate Ni for further catalytic cycles (Scheme 7). Such a mechanism has been substantiated on the basis of the formation of the nickelacycle and its characterization by cyclic voltammetry. In the absence of Mg (reactions conducted in a divided cell in the presence of ammonium ions) the nickelacycle does not transform and the reaction stops when all the starting nickel compound has been reacted. Upon addition of MgBr2 to an electrochemically prepared solution of the nickelacycle, Ni(II) is recovered [114]. 

Cathodic protection involves connecting a metal to be protected to another metal that is more easily oxidized. The more easily oxidized metal serves as the anode and the metal to be protected is the cathode in an electrochemical cell. The metal that is oxidized is called the sacrificial anode because it is sacrificed to protect another metal. Metals such as zinc and magnesium are often used to protect iron. Cathodic protection is demonstrated in this activity by using two steel nails. The nails are placed on a shallow dish. Using a white or light-colored dish displays the oxidation better [iron (III) oxide, Fe Oj, referred to commonly as rust is more visible on a light-... 

Saccharides carbohydrates Sacrificial Anode in cathodic protection, the metal connected to the structure to be protected that is more readily oxidized than the structure... 

TABLE 10. Organoelemental compounds produced by electro-oxidation of Grignard reagents 5 on sacrificial anodes... 

As concerns the mechanism of anodic oxidation of 5 at sacrificial anodes, it can be noted that the process occurs at potentials close to those of the oxidation of the corresponding diorganomagnesium compounds (1). For example, half-peak potentials for the oxidation of 5b and lb in THF containing 0.25 M TBAP at a lead electrode measured at a scan rate of 0.3 V s are equal to Jip/2 = —1.73 and —1.72 V vs. 0.01 M Ag /Ag, respectively. However, the oxidation mechanism for both compounds is different, as shown... 

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