Aluminum anodic dissolution

TLC is used to determine copper in aluminum alloys. The process involves the sampling of the investigated material by anodic dissolution, development of TLC plate with acetone -f HCl -f HjO (70 15 15), and the identification of analyte by 1-(2-pyridylazo)-2-naphthol [70]. A TLC system comprising silica gel as stationary... 

Inasmuch as this was denied as the possible cause of pit initiation (cf. Vetter and Strehblow44), there should be no doubt that, not only in pits, but wherever anodic dissolution of aluminum... 

Active anodic dissolution occurs when all the electrochemically oxidized aluminum passes into the aqueous phase and the oxide layer does not grow, i.e., the current efficiency of oxide formation... 

Nonactive/slightly reactive electrode materials include metals whose reactivity toward the solution components is much lower compared with active metals, and thus there are no spontaneous reactions between them and the solution species. On the other hand, they are not noble, and hence their anodic dissolution may be the positive limit of the electrochemical windows of many nonaqueous solutions. Typical examples are mercury, silver, nickel, copper, etc. It is possible to add to this list both aluminum and iron, which by themselves may react spontaneously with nonaqueous solvent molecules or salt anions containing atoms of high oxidation states. However, they are not reactive due to passivation of the metal which, indeed, results from the formation of stable, thin anodic films that protect the metal at a wide range of potentials, and thus the electrochemical window is determined by the electroreactions of the solution components [51,52],... 

It appears that in several nonaqueous systems, the difference between the potentials of the A13+/A1 couple and that of other couples, such as Zn2+/Zn, Sn2+/ Sn, Cu2+/Cu, and others is much lower than in aqueous solutions [456-458], Consequently, electroplating of Al alloys from nonaqueous solutions is feasible and may even be advantageous over metallurgical processes of alloy formation [456,459,460], Anodic dissolution of aluminum electrodes has also been investigated [461-465], It appears that reversible behavior of the A13+/A1 couple may be obtained in a number of nonaqueous systems (see next section). 

The processes that take place in the pit and in its vicinity are shown in Fig. 20M(a). At the concentration of chloride ions found in seawater, the passive layer on aluminum breaks down, and anodic dissolution of the metal can occur. This happens mostly inside the pit, where the supply of oxygen is slow. On the other hand, oxygen reduction can readily take place on the surface of the metal outside the pits, where its diffusion path is short. Thus, the cathodic area is typically... 

Fig. 1 shows one of the first electrolytically deposited alumimun coatings to be obtained from this type of electrolyte. Since electrolytic aluminum deposition from this system has no true smoothening effect, thick layers become even rougher, as illustrated by the thickly coated cathode plate shown in Fig. 1. The cathodic deposition and the anodic dissolution of aluminum corresponded to almost 100 <7o of the amount expected according to the Faraday rule, which is an important prerequisite for even considering using this electrolysis technique for technical applications. Independently of the layer thickness, the deposited aluminum layers are found to be ectraordinarily pure. Spectroscopic investigations have revealed purities of up to 99.999%. Even when relatively impure raw aluminum with purities of 99.7% functions as the anode, very pure aluminum can be deposited. Therefore, obviously not only a technique of electroplating aluminum was discovered, but also a method of...

In 1954, Lehmkuhl in his dissertation discussed two alternative mechanisms for the cathodic deposition and anodic dissolution of aluminum from the new organo-aluminum electrolytes [118]. One possibility is the preliminary deposition of alkali metals as a result of electrolytic dissociation of the 1 2 complex, resulting in alkali metal cations, and subsequent chemical reaction of this alkali metal with free or coordinated aluminum trialkyl, yielding aluminum metal and alkali metal tetraalkyl aluminate (see Scheme 2). 

This phenomenon degrades the Coulombic efficiency of the anode, then it has to be minimized for practical applications where high capacity is required. This could be overcome by particular aluminum alloys more resistant to corrosion, but the competing requirement of fast anodic dissolution makes very difficult the research of the suitable material [36, 37]. Another possibility to improve the anode performance is to modify the electrolyte composition by adding corrosion inhibitors [38]. The difficulties met up today in this field leave the possibility to use the aluminum-air batteries only as mechanically rechargeable systems, with practical performance (300-500 Wh/kg) very far from the theoretical values (Table 1.8 Fig. 5.14). 

Electrolytic dissolution in nitric acid has been used at the Savannah River [B22] and Idaho Qiemical Processing plants [AlO, All] to dissolve a wide variety of fuels and cladding materials, including uranium alloys, stainless steel, aluminum, zircaloy, and nichrome. The electrolytic dissolver developed by du Pont [B22], pictured in Fig. 10.4, uses niobium anodes and cathodes, with the former coated with 0.25 mm of platinum to prevent anodic corrosion. Metallic fuel to be dissolved is held in an alundum insulating frame supported by a niobium basket placed between anode and cathode and electrically insulated from them. Fuel surfaces facing the cathode undergo anodic dissolution in a reaction such as... 

Similar to the formation of porous aluminum oxide a passivation - dissolution mechanism can be used to form nanopo-rous structures on InP. If (OOl)n-InP is polarized anodically under illumination in HCl solutions, nanoscaled pores are formed [117]. For potentials up to 1.2 V vs. SGE the main reaction is uniform anodic dissolution. Above this potential porous InP with a surface oxide is formed. The overpotential and anodizing time influence pore diameter (110-250 nm), wall thickness (16-50 nm) and pore length... 

For aluminum, the outer surface of the oxide layer in humid environments is considered to be a mixture of aluminum oxide and aluminum hydroxide. After the adsorption of chloride ions, an ion exchange can occur leading to the substitution of hydroxyl ions by chloride ions [179, 180]. After the chemical attack of the oxide, aluminum is electrochemicaUy dissolved. The chloride ions are regenerated after the dissolution of the transitory hydrox-ychloride compounds. Thus, a relatively small amount of chloride ions can result in a progressive attack of the protective layer. Within the head of the filiform filament, the anodic dissolution of aluminum leads to a local acidification of the anolyte due to the hydration of aluminum ions. It has been observed that a secondary cathodic reaction, the reduction of hydrogen ions, can occur. Hydrogen evolution has been observed within the head [166]. 

Galvanized steel is a common example of galvanic coupling where steel (Fe), with a standard electrode potential of —0.440 V vs. SHE, is cathodicaUy protected by zinc, which has a more active standard electrode potential of —0.763 V. Obviously, zinc is not a corrosion-resistant metal and cannot be classified as a barrier coating. It protects steel from corrosion through its sacrificial properties. Because zinc is less noble than iron in terms of the standard electrode potentials, it acts as an anode. The sacrificial anode (zinc) is continuously consumed by anodic dissolution reaction and protects the underlying metal (iron in steel) from corrosion. In practice, sacrificial anodes are comprised of zinc, magnesium alloys, or aluminum. 

The strategy of metal alkoxides synthesis is entirely related to the electronegativity of the element concerned. Some electropositive metals, such as alkali metals, alkaline earth metals, and lanthanides, react directly with alcohols. But some less electropositive metals such as magnesium and aluminum require a catalyst (I or HgCy for successful reaction with alcohols. Use of electrochemical synthesis by anodic dissolution of some metals or metalloids (Sc, Y, Ti, Zr, Nb, Ta, Fe, Co, Ni, Cu, Pb, Si, Ge, etc.) in dry alcohol performs a promising procedure because it does not produce any by-products except hydrogen gas. Another applicable method for the synthesis of some alkoxides (B, Si, Ti, Zr, Hf, Nb, Ta, Fe, etc.) is the reaction of their chlorides with alcohols which require a base such as... 

Iron is similar to aluminum in that a protective oxide forms in nearly neutral solutions. However, for iron the field of oxide stability is substantially greater at elevated pH, and iron is far more resistant to alkaline solutions compared with aluminum. Contributing to the overall resistance of iron, is the generally more noble half-cell electrode potential for the anodic dissolution reactions which lower the driving force for corrosion reactions. It is, however, apparent from Fig. 2b that this resistance disappears in more acidic solutions [2]. 

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