Active 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 

In solutions containing different anions, as seen in Fig. 17, the sudden rise in the anodic current density mentioned earlier [see Section 111(2)] and characteristic of initiation of active dissolution occurs at different potentials. It was shown108 that, at least with halides, this potential is a linear function of the crystalline radius of the ion. 

If a well-defined compact oxide layer is grown to a certain thickness in a barrier-forming electrolyte (so that the electrode potential increases to very high values in order to maintain a constant current), when chloride ions are added, a dramatic decay of the potential results within milliseconds, as shown in Fig. 18.77 

Further oxidation at the same current density takes place at a relatively low constant potential, indicating that the oxide has stopped growing, i.e., that the efficiency of oxide formation, rjox, has changed from virtually 100% to zero. 

The constancy of the potential with increasing current density could be explained in terms of an automatic adjustment of the number of pits while maintaining a constant current per pit. At potentials more positive than the pitting potential, Kaesche67 has found the total current to increase with time. This complied very well with a model in which the true current density at the pit (found to be of the order of 300 mA/cm2) and the number of pits, 

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Artificial crevice electrodes have been used to study the effect of dichromate on active dissolution of aluminum. In these experiments, 50 pm thick commercially pure A1 foils were placed between thin plastic sheets and mounted in epoxy. This assembly was fixed against a square cell that accommodated counter and reference electrodes and a trap that allowed for H2 gas collection. A schematic illustration of this cell and electrode is shown in Fig. 9 (36). Crevice corrosion growth experiments were conducted in aerated 0.1 M NaCl solution with additions of either 0.01 or 0.1 M Na2Cr207. Artificial crevice growth experiments were conducted under potentiostatic polarization at potentials ranging from 0 to... 

Finally, a large number of phenomena connected with active electrochemical dissolution of aluminum in the electrolyte, promoted by the presence of aggressive anions, are considered to deserve special attention, because understanding of these phenomena is far from complete, and it is hoped that a review of them will stimulate further research. 

In many respects the action of NaOH on aluminum and silicon is similar. Streicher (52) reported an activation energy of 13.7 kcal/mole for the dissolution of aluminum in 2% NaOH. Orem (53) studied the action of 15% NaOH on spheres of aluminum and on holes drilled into plates whose surfaces were oriented in the three main crystallographic planes. He found that the relationship between the etch rates was ill < 110 < 100. ... 

There are various concepts about the aluminum silicates dissolution mechanism. Relatively recently a low rate of their dissolution was explained by inner diffuse regime. Currently more substantiated appears hydrolysis with the formation of activated complexes. According to this theory, the dissolution begins with the exchange of alkaline, alkaline-earth and other metals on the mineral surface of H+ ions from the solution (see Figure 2.26). At that, metals in any conditions are removed in certain sequence. In case of the presence of iron and other metals with variable oxidation degree the process may be accompanied with redox reaction. Hydrolysis is a critical reaction in the dissolution of aluminum silicates. It results in the formation on the surface of a very thin layer of activated complexes in Na, K, Ca, Mg, Al and enriched with H+, H O or H O. The composition and thickness of this weakened layer depend on the solution pH. These activated complexes at disruption of weakened bonds with mineral are torn away and pass into solution. For some minerals (quartz, olivine, etc.) the disruption of one inner bond is sufficient, for some others, two and more. The very formation of activated complexes is reversible but their destruction and removal from the mineral are irreversible. 

As a result of migration and hydrolysis, both chloride and hydrogen ions are present in the active pits and stimulate the further dissolution of aluminum. Consequently the... 

It seems that neither the electrostatic effect nor the dissolution of aluminum oxide can be neglected as done in a previous article [15] in which the authors presumed the protonation and deprotonation of different types of active site on alumina surface at low and high pHs and introduced surface-charge formation on four different active sites among them, two are likely misinterpreted. Because the dissolution of alumina takes place at any pH, as showed in a recent... 

The measures of solid state reactivity to be described include experiments on solid-gas, solid-liquid, and solid-solid chemical reaction, solid-solid structural transitions, and hot pressing-sintering in the solid state. These conditions are achieved in catalytic activity measurements of rutile and zinc oxide, in studies of the dissolution of silicon nitride and rutile, the reaction of lead oxide and zirconia to form lead zirconate, the monoclinic to tetragonal transformation in zirconia, the theta-to-alpha transformation in alumina, and the hot pressing of aluminum nitride and aluminum oxide. 

The linear dependence of the pitting potential on ionic radius is likely a reflection of the similarly linear relationship between the latter and the free energy of formation of aluminum halides.108 It is reasonable to assume that the energy of adsorption of a halide on the oxide is also related to the latter. Hence, one could postulate that the potential at which active dissolution takes place is the potential at which the energy of adsorption overcomes the energy of coulombic repulsion so that the anions get adsorbed. 

Excision reactions are sometimes accompanied by redox chemistry. For example, dissolution of the 2D solid Na4Zr6BeCli6 in acetonitrile in the presence of an alkylammonium chloride salt results in simultaneous reduction of the cluster cores (144). Here, the oxidation product remains unidentified, but is presumably the solvent itself. As a means of preventing such redox activity, Hughbanks (6) developed the use of some room temperature molten salts as excision media, specifically with application to centered zirconium-halide cluster phases. A number of these solids have been shown to dissolve in l-ethyl-2-methylimidazolium chloride-aluminum chloride ionic liquids, providing an efficient route to molecular clusters with a full compliments of terminal chloride ligands. Such molten salts are also well suited for electrochemical studies. 

Activation energies less than 42 kJ mol-1 indicate diffusion-controlled reactions, whereas reactions with Ea values higher than 42 kJ mol-1 indicate chemical reactions or surface-controlled processes. The data in Figure 7.32 represent rate constants (k for the acid dissolution of octahedral aluminum in kaolinite plotted against the reciprocals of the respective temperatures. From the slope of the line, the apparen energy of activation for dissolution of octahedral aluminum was found to be 101.7 kJ mol-1. 

Severdenko and his co-workers studied the effect of ultrasound on the dissolution rate and chemical activity of aluminum metal [124], Al disks were subjected to a 20-kHz ultrasound field and then samples were cut from sites corresponding to the potential antinode or supersonic bias antinode of the wave. The electrochemical properties and the dissolution rate of the Al samples pretreated in ultrasound fields were compared with data for blank Al samples. Their results showed that samples cut from sites located in the potential antinode of the wave were more negative than the values inherent to untreated samples. This was due to the decrease of the thermodynamic stability of the metal as a result of the formation of microdefects, microcracks, etc. The dissolution rate of the ultrasound-treated samples cut from these sites in aqueous NaOH solutions was also enhanced. The reverse effect was observed with samples cut from supersonic bias antinode sites, i.e. electrode potentials shifted in the positive direction, dissolution rate in NaOH solutions decreased, and the overall increase of the thermodynamic stability of the metal was attributed by the authors to the redistribution under the effect of the ultrasound field of dislocations to energetically more stable configurations. 

The partial orders with respect to [OH ] observed for most silicate mineral dissolution reactions can be explained by the surface complexation model (Blum and Lasaga, 1988 Brady and Walther, 1989). Brady -and Walther (1989) showed that slope plots of log R vs. pH for quartz and other silicates at 25 °C is not inconsistent with a value of 0.3. Plots of the log of absorbed OH vs. pH also have slopes of about 0.3, suggesting a first-order dependence on negative charge sites created by OH adsorption. Because of the similarity of quartz with other silicates and difference with the dependence of aluminum oxides and hydroxide dissolution on solution [OH ], Brady and Walther (1989) concluded that at pH >8 the precursor site for development of the activated complex in the dissolution of silicates is Si. This conclusion is supported by the evidence that the rates (mol cm s ) at pH 8 are inversely correlated with the site potential for Si (Smyth, 1989). Thus it seems that at basic pH values, silicate dissolution is dependent on the rate of detachment of H3SiO4 from negative charge sites. 

Solubility Products. The activity expression of equation 1,36 can be applied in general to any chemical reaction. In particular, it can be used to describe dissolution reactions for sohds such as aluminum hydroxide ... 

Pitting corrosion is usually associated with active-passive-type alloys and occurs under conditions specific to each alloy and environment. This mode of localized attack is of major commercial significance since it can severely limit performance in circumstances where, otherwise, the corrosion rates are extremely low. Susceptible alloys include the stainless steels and related alloys, a wide series of alloys extending from iron-base to nickel-base, aluminum, and aluminum-base alloys, titanium alloys, and others of commercial importance but more limited in use. In all of these alloys, the polarization curves in most media show a rather sharp transition from active dissolution to a state of passivity characterized by low current density and, hence, low corrosion rate. As emphasized in Chapter 5, environments that maintain the corrosion potential in the passive potential range generally exhibit extremely low... 

However, during impregnation of the support, dissolution of alumina followed by surface precipitation has been evidenced [1]. This phenomenon is increased when solubilized aluminum reacts with species such as cobalt, to form hydrotalcite-like coprecipitates, or molybdenum to form Andemon-lype heteropolyanion AMo6024H3 [2,3]. After calcination, these species do not lead to easily sulfiding species and thus decrease the overall yield in CoMoS active phase [4,5], which is of course detrimental for catalytic activity. 

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