Aluminum cathodic reaction

Aluminum is directly applied in its metallic form when it serves as battery anode. The battery concepts considered are in general single-use types (primary batteries). The most developed systems belong to the metal-air batteries, using the reduction of atmospheric oxygen as the cathode reaction, e.g., (-) A1 / KOH / 02 (+) or (-) A1 / seawater / 02 (+). The main discharge reactions are ... 

Nearly pure cadmium sponge is precipitated by the addition of high-purity. lead-free zinc dust. The cadmium sponge then is redigested in spent cadmium electrolyte, alter which the cadmium is deposited by electrolysis onto aluminum cathodes. The metal is then stopped from the electrodes, melted, and cast into various shapes. Reactions which occur during the electrolytic process are (Roasting) ZnS +1,0 — — ZnO +... 

At aluminum cathode, hydrogen gas is released according to the following reaction ... 

Some of these compounds can be oxidized by the anodic products and the current efficiency decreases. A very complicated case is the reduction of A1—O—F anions to produce aluminum (see Section II.A). Usually, the charge transfer overvoltage is low in the case of the cathodic reactions. 

The most obvious way to proceed would seem to be with the use of an appropriate molten salt. Magnesium can be deposited from anhydrous molten MgCl, and aluminum can be deposited from a cryolite bath, since in these baths metal deposition is the only cathodic reaction that can take place. The quality of the deposits in these baths is usually poor, however, and they are used for metal winning rather than electroplating. 

Note that the two cathodic reactions are given—the first tends to be more important in acids and for reactive metals such as aluminum, while the second tends to be more important in neutral solutions and for less-reactive metals such as steel and copper alloys. 

The electrolytic production of aluminum is carried out in Hall-Heroult cells that have changed little in nearly 100 years [39], The Hall-Heroult process operates at a high temperature (about 1250 K) and utilizes a molten salt electrolyte of alumina (AI2O3) and cryolite (Na3A102), with additives such as calcium fluoride and aluminum trifluoride. The cathode reaction is the reduction of AP+, with a consumable carbon anode. The overall reaction in the Hall-Heroult cell (shown schematically in Figure 26.15) is... 

A major improvement was realized with the use of indium, a metal with a very low first ionization potential (5.8 eV) which works without ultrasonic radiation even at room temperature [87]. As the zero-valent indium species is regenerated by either zinc, aluminum, or tin, a catalytic amount of indium trichloride together with zinc, aluminum [88], or tin [89] could be utilized in the allylation of carbonyl compounds in aqueous medium. The regeneration of indium after its use in an allylation process could be readily carried out by electrodeposition of the metal on an aluminum cathode [90], Compared with tin-mediated allylation in ethanol-water mixtures, the indium procedure is superior in terms of reactivity and selectivity. Indium-mediated allylation of pentoses and hexoses, which were however facilitated in dilute hydrochloric acid, produced fewer by-products and were more dia-stereoselective. The reactivity and the diastereoselectivity are compatible with a chelation-controlled reaction [84, 91]. Indeed, the methodology was used to prepare 3-deoxy-D-galacto-nonulosonic acid (KDN) [92, 93], N-acetylneuraminic acid [93, 94], and analogs [95],... 

Interface Potential and Pit Initiation. It is generally accepted that pit initiation occurs when the corrosion potential or potentiostatically imposed potential is above a critical value that depends on the alloy and environment. However, there is incomplete understanding as to how these factors (potential, material, and environment) relate to a mechanism, or more probably, several mechanisms, of pit initiation and, in particular, how preexisting flaws of the type previously described in the passive film on aluminum may become activated and/or when potential-driven transport processes may bring aggressive species in the environment to the flaw where they initiate local penetration. In the former case, the time for pit initiation tends to be very short compared with the initiation time on alloys such as stainless steels. Pit initiation is immediately associated with a localized anodic current passing from the metal to the environment driven by a potential difference between the metal/pit environment interface and sites supporting cathodic reactions. The latter may be either the external passive surface if it is a reasonable electron conductor or cathodic sites within the pit. 

Selective defluorination of 1,3-difluorobenzene to fluorobenzene has been successfully carried out by use of cathodic reduction at mercury in diglyme containing Bu4NBp4and a catalytic amount of a dimethylpyrrolidinium (DMP ) salt. In this reaction, DMP is first reduced to form an amalgam, which reduces difluorobenzene catalyti-cally as shown in Scheme 1. Also, cathodic reduction of perfluoroben-zene at an aluminum cathode in aqueous DMF provides benzene in moderate yield (Eq. 4). 

Chloride ions attack oxide layers on iron, aluminum, and magnesium. Subsequently, the metal is electrochemically dissolved. The hydration of Fe " ", AP" ", or Mg " " releases protons and thereby leads to an acidification of the tip of the filament. At the cathodic site, the primary cathodic reaction, the reduction of oxygen to hydroxyl ions takes place. In between the anode and the cathode a potential gradient is estahhshed, which forces anions to migrate to the front and cations to the back. As the distance from the anode increases, the pH also increases on the basis of the dilution of hydronium ions and the migration of hydroxyl ions from the cathodic site. When favorable conditions are reached, the corresponding hydroxides of the cations are formed as gels. As the head advances, these hydrated corrosion products lose their water and convert to the dry corrosion products that fill the tail see Ref. [168] and references therein. 

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]. 

Negatively biased aluminum metallizations can corrode in the presence of moisture because of the high pH (basic) produced by the cathodic reaction of water reduction. The high pH can dissolve the passive oxide on aluminum along with the corresponding increase in conductor resistance possibly up to open-circuit value. 

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