Anodic dissolution solution composition

As with most other metals, the anodic behaviour of nickel is influenced by the composition of the solution in which measurements are made, particularly if the solution is acidic. Acidic solutions containing d ions or certain sulphur compounds in particular have a pronounced influence both in increasing the rate of anodic dissolution in the active range and in preventing passivation, and in stimulating localised corrosion . Thiourea and some of its derivatives have a complex effect, acting either as anodic stimulators or inhibitors, depending on their concentration . 

Anodic dissolution reactions of metals typically have rates that depend strongly on solution composition, particularly on the anion type and concentration (Kolotyrkin, 1959). The rates increase upon addition of surface-active anions. It follows that the first step in anodic metal dissolution reactions is that of adsorption of an anion and chemical bond formation with a metal atom. This bonding facilitates subsequent steps in which the metal atom (ion) is tom from the lattice and solvated. The adsorption step may be associated with simultaneous surface migration of the dissolving atom to a more favorable position (e.g., from position 3 to position 1 in Fig. 14.1 la), where the formation of adsorption and solvation bonds is facilitated. 

The potential-decay method can be included in this group. Either a current is passed through the electrode for a certain period of time or the electrode is simply immersed in the solution and the dependence of the electrode potential on time is recorded in the currentless state. At a given electrolyte composition, various cathodic and anodic processes (e.g. anodic dissolution of the electrode) can proceed at the electrode simultaneously. The sum of their partial currents plus the charging current is equal to zero. As concentration changes thus occur in the electrolyte, the rates of the partial electrode reactions change along with the value of the electrode potential. The electrode potential has the character of a mixed potential (see Section 5.8.4). 

Fig. 3. Current-potential curves for anodic oxidation of H2CO on different metals. Dotted lines current attributable to the anodic dissolution of Cu and Co electrodes. Solution composition 0.1 mol dm-3, 0.175 mol dm 3 EDTA, pH = 12.5, T — 298 °K. Adapted from ref. 38.

The electrochemical behavior of the cadmium electrodes in alkaline solutions was intensively studied [313-318]. It was suggested [314-318] that during anodic dissolution of the Cd electrode in alkaline solutions, a passive layer consisting of Cd(OH)2 and CdO is formed, and Cd(II) soluble species are also generated. The composition of the anodically formed layer on cadmium in alkaline solution was dependent on the electrolyte cation [319]. In 1 M NaOH and KOH solutions, both / -Cd(OH)2 and y-Cd(OH)2 were formed, while in 1 M LiOH, /J-Cd(OH)2 was the only product. 

The electrochemical technique can be used also for direct synthesis of bimetallic alkoxides. For instance, the anodic dissolution of rhenium in the methanol-based electrolyte that already contained MoO(OMe)4, permitted to prepare with a good yield (60%) a bimetallic complex RevMov,02(OMe)7, with a single Re-Mo bond [904], Application of the same procedure permitted the preparation of complex alkoxide solutions with controlled composition for sol-gel processing of ferroelectric films [1777]. 

Even the application of Ti/Pb02 anodes to wastewater treatment may be limited by the possible release of toxic lead ions, due to their dissolution under specific anodic polarization and solution composition. 

Deposition of metals on a silicon surface can be either a conduction band process or a valence band process depending on the redox potential of the metal and solution composition. Deposition of Au on p-Si in alkaline solution occurs only under illumination indicating that it is a conduction band process due to the unfavorable position of the redox couple for hole injection. " On the other hand, deposition of platinum on p-Si can occur in the dark by hole injection into the valence band. For Cu, although the deposition proceeds via the conduction band as shown in Fig. 6.9, it can also proceed via the valence band because a large anodic current of n-Si occurs in the dark in copper-containing HF solution as shown in Fig. 6.10. The reduction of copper under this condition is via hole injection. The holes are consumed by silicon dissolution and the silicon reaction intermediates then inject electrons into the conduction band, resulting in the anodic current on n-Si in the dark. 

The effective dissolution valence of silicon, ra, that is, the average number of electrons flowing through the external circuit per dissolved silicon atom, has been found to vary with silicon material, solution composition, anodic polarization,... 

As an example, the experimental results for the anodic dissolution of alloys of the nickel-chromium system are presented in Figs 5 and 6. This composition is the basis of several superalloys, which are machined using the ECM. Alloy components - chromium and nickel - exhibit different tendencies to passivate. Nickel is weakly passivated in the NaCl solution its tendency to passivate is much stronger... 

Chemical analysis of the solutions after anodic dissolution have shown that the oxidation state of chromium in the dissolution products depends on the alloy composition and, correspondingly, on the potential of alloy dissolution. At potentials less positive than the potential of the onset of pure-chromium passivity breakdown, chromium dissolves from the nickel-based alloys as Cr(III). The alloys with chromium contents of not more than 15% dissolve in this manner in NaCl solution. At higher Ea, chromium from the alloy dissolves, for the most part (about 90%), in the form of Cr(VI). This is true for all alloys in Na2SC>4 (or NaNC>3) solution and for the alloys containing more than 25% chromium in NaCl solution. 

The nature of copper dissolution from CuAu alloys has also been studied. CuAu alloys have been shown to have a surface Au enrichment that actually forms a protective Au layer on the surface. The anodic polarization curve for CuAu alloys is characterized by a critical potential, E, above which extensive Cu dissolution is observed [10]. Below E, a smaller dissolution current arises that is approximately potential-independent. This critical potential depends not only on the alloy composition, but also on the solution composition. STM was used to investigate the mechanism by which copper is selectively dissoluted from a CuAu electrode in solution [11], both above and below the critical potential. At potentials below E, it was found that, as copper dissolutes, vacancies agglomerate on the surface to form voids one atom deep. These voids grow two-dimensionally with increasing Cu dissolution while the second atomic layer remains undisturbed. The fact that the second atomic layer is unchanged suggests that Au atoms from the first layer are filling... 

The synthesis of disordered LADH-Cl was performed according to different procedures (1) the interaction of mechanically activated Al(OH)3 with the aqueous solution of lithium chloride [48] (2) the precipitation from the solutions of lithium chloride and aluminium by adding an alkali [17,18] (3) the anode dissolution of metal aluminium in the solution of Li Cl [49] (4) the treatment of the mixture composed of crystalline Al(OH)3 and LiCl (LiCl H20) in the mill-activators [50]. One can see (Table 2) that all the samples of the disordered LADH-Cl (No. 1 - 4) obtained under different synthesis conditions are close to each other in chemical composition and are distinguished by the nonstoichiometry of lithium, compared to the well crystalline sample (5), the... 

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