Liquid metals dissolution process

While the subject of this chapter may seem counter to the title of the book, metal dissolution is vital in numerous aspects of metal deposition, counter electrode processes, pre-treatment protocols and electropolishing. This chapter outlines the current state of understanding of metal dissolution processes and discusses in some detail an electropolishing process that has now been commercialised using a Type III ionic liquid. 

This subchapter has shown that metal dissolution processes are important to numerous aspects of metal plating, however, very few concerted studies have been made in this area. An understanding of dissolution rates and processes, together with information on the stability of oxide films in ionic liquids, is essential for the development of successful metal finishing processes. 

The first stage of nitride synthesis from its constituents is the dissolution of nitrogen in liquid metal. This process was analysed [9,10] by ab initio quantum mechanical calculations using the density functional theory (DFT) approximation [11-13],... 

Nikitin, V. L, Intercrystalline Corrosion by Liquid Metals by Dissolution Process , Izv. Akad. 

The dissolution of sulfur in ammonia has been known for more than 100 years [17]. The identification of the chemical species in these solutions was a matter of confusion until the identification of S4N and 83 , by Chivers and Lau [18] and Bernard et al. [19], using Raman spectroscopy. When considering the species formed in the dissolution process, it is quite remarkable that this dissolution is reversible sulfur is recovered after evaporation of ammonia. These solutions are strongly colored (blue), mainly due to the electronic absorption band of S4N at 580 nm. It must be mentioned that this dissolution is moderately fast at room temperature (but much slower than the dissolution of alkali metals) and that the rate is much slower when temperature decreases. It should also be mentioned that concentrated solutions of sulfur in liquid ammonia can be used as the solution at the positive electrode of a secondary battery. The solution at the negative electrode can be a solution of alkali metal in liquid ammonia [20], the electrodes being... 

Anode material In aqueous solutions the anodic processes are either breakdown of the electrolyte solution (with oxygen evolution at an inert anode being favored) or the use of soluble anodes. The use of soluble anodes is limited by the passivation of many metals in aqueous solutions. In ionic liquids, however, the first option is not viable due to the cost and the nature of the anodic breakdown products. New strategies will therefore have to be developed to use soluble anodes where possible or add a sacrificial species that is oxidized to give a benign gaseous product. Preliminary data have shown that for some metals the anodic dissolution process is rate limiting and this affects the current distribution around the cathode and the current density that can be applied. 

For the electrodeposition of metals or alloys from air- and water-stable ionic liquids, it is necessary first to dissolve the corresponding metal ions in the ionic liquid. Such a dissolution process is made possible by introducing excess amounts of halide ions (such as Cl ) to form soluble metal-halide complex anions. Alternatively, the metal is dectrochemically oxidized in the ionic liquid to form the soluble salt such as Sn(Tf2N) in the trimethyl-n-hexylammonium [bis(trifluoromethyl)sulfonyl]amide ([TMHAj TfiN ) ionic liquid. 

Therefore it can be seen that metal dissolution is easier in Lewis basic melts. The zinc and aluminum deposition processes, which are by far the most frequently studied, are almost totally reversible. Since these metals have no other stable oxidation states the deposition and dissolution processes are very efficient [3-6]. This has the distinct advantage that the composition of the ionic liquid remains constant and the process becomes the removal of metal from one electrode and its deposition on the other electrode. 

As explained previously, electrodissolution in ionic liquids is a simple and efficient process, particularly in chloride-based eutectics. Type III eutectics based on hydrogen bond donors are particularly suitable for this purpose. However, it has been noted that the polishing process only occurs in very specific liquids and even structurally related compounds are often not effective. It has been shown that 316 series stainless steels can be electropolished in choline chloride ethylene glycol eutectics [19] and extensive electrochemical studies have been carried out. The dissolution process in aqueous solutions has been described by two main models the duplex salt model, which describes a compact and porous layer at the iron surface [20], and an adsorbate-acceptor mechanism, which looks at the role of adsorbed metallic species and the transport of the acceptor which solubilises... 

In the case of liquid Al, the solubility of oxygen is too low for a dissolution process to be effective. However, for other metals like Sn, it may be the principal mechanism. The variation in contact angle of Sn on AI2O3 shown in Figure 6.26 as a function of temperature displays three ranges. At low temperatures, high apparent... 

The transport steps may be controlling either in bringing reactant material to the reaction site (the metal-solution interface) from either phase, or in removing products from the site into the liquid phase,. Accepting the fact that metal reactions in conducting solutions are electrochemical in nature, it follows that control may reside in transport with respect to the cathodic process, or with respect to the anodic process, or with respect to both simultaneously a much less likely possibility. In metal dissolution reactions, the steps can be described still less equivocally for the first case, the transfer of reducible species from the solution to the electrode is involved for the second case, the removal of oxidized species from the electrode is involved. In the latter instance complications are usually caused by the formation of solid reaction products. 

The application of air-electrolyte mixtures as a working medium for ECM enables one to raise the localization of metal dissolution in places with the smallest gaps and, thus, to enhance the accuracy of electrochemical reproduction of TE on the WP. To achieve the highest efficiency of this method, several conditions should be fulfilled. The air-electrolyte mixture should be formed in the immediate vicinity of the gap inlet and the flow rate should be adequately high. A ratio between gas and liquid amounts from 3.0 to 3.5 is considered to be most preferable. Rigid stabilization of all the process parameters is required. The design and size of all parts of the mixing device and the values of inlet and outlet pressures are important. To avoid a considerable decline in productivity, the main metal stock should be removed in the air-free... 

Statement of the problem. Now let us consider mass transfer from a solid wall to a liquid film at high Peclet numbers. Such a problem is of serious interest in dissolution, crystallization, corrosion, anodic dissolution of metals in some electrochemical processes, etc. In many practical cases, dissolution processes are rather rapid compared with diffusion. Therefore, we assume that the concentration on the plate surface is equal to the constant Cs and the incoming liquid is pure. As previously, we introduce dimensionless variables according to formulas (3.4.5). In this case, the convective mass transfer in the liquid film is described by Eq. (3.4.1), the boundary condition (3.4.2) imposed on the longitudinal variable x, and the following boundary conditions with respect to the transverse coordinate ... 

The simplified hole model was shown to describe the data on C02 solubility in the alkali metal halide melts with good accuracy. The entropy changes in the process of dissolution are close to — 1 J mol-1 K-1, which agrees with the data of Novozhilov [311], and the solubility data obtained for the molten chlorides are in good agreement with Ref. [311]. An interesting fact was revealed— the solubility of C02 increased by four times upon the addition of a small concentration of Ni2+ ( 10-3 mol kg-1), introduced into molten NaCl as an admixture. A study of the kinetics of the dissolution process showed that the rate of C02 dissolution in alkali metal halide melts was defined by the rate of transfer of C02 from the gaseous phase into the liquid, but not by the diffusion and convection of the dissolved molecules in the melt. 

The main type of corrosion damages in liquid Pb, Bi and Pb-Bi is the dissolution of structural materials (steels) and their components in these coolants. The kinetics of dissolution processes can be of different nature. For example, in some cases the dissolution is localized on boundaries of grain, causing interstructure infiltration of liquid metal (Pb, Pb-Bi) into steel. 

Birch conducted the first electrochem-icaUy driven reduction process in liquid ammonia and demonstrated that the same type of products are obtained compared with the alkali metal dissolution method [33]. However, a common problem when carrying out electrochemical studies in liquid ammonia is the passivation of the working electrode by trace water present in the system [34]. This problem can be overcome by applying ultrasound. Asymmetric cavitation and collapse at the electrode surface casts microjets of solution towards it, thus depassivating by removing blocking material. In a typical procedure applied to 3-methylanisole in... 

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