Mechanism of metal dissolution

The first equation describes the reproduction of the kink site position in the process of separation of an atom from the kink site position and formation of an ad-atom. The second step represents desorption of the ad-atom. The charge transfer is split. Following the description of partial charge transfer in Section 4.3 with the introduction of a partial charge transfer coefficient A the charge transfer in the first step is = Xz, and the charge transfer in the second step Az j = (1 — A )z. The intermediate ad-atom can be a lower valent oxidation state stabilized as an anion complex. 

If the first step, the separation, is rate deteimining (usually at higher potentials) the rate equation is 

The part of the potential drop across the double layer acting on the transition state of separation is described by a p. If the density of kink site positions in the first approximation is independent of the potential, the Tafel equation would be 

If the second step, the desorption, is rate determining (for equilibrium conditions between atoms in kink positions and ad-atoms) a rate equation for ad-atom desorption. 

Taking into account the equilibrium between the ad-atoms and the kink site positions a rather complex expression follows for the charge transfer coefficient The following Tafel equation 

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The same two-step mechanism of metal dissolution has also been delivered for the anodic dissolution of nickel in acid solutions [Sato-Okamoto, 1964]. The mechanistic concepts for iron dissolution other than the two-step mechanism have also been presented in the literature [Heusler, 1958], 6ind the mechanism of metal dissolution is still a subject of research [Plonski, 1996]. 

Similar electropolishing experiments were carried out using different grades of stainless steel (410, 302, 304, 316 or 347) and it was found that the mechanism of metal dissolution and the oxidation potentials for the metals were very similar. The slight exception was the 410 series steel (which has no Ni, unlike the 300 series steels which have 8-14%). The 410 steel required a more positive oxidation potential to break down the oxide in the ionic liquid whereas once the oxide was removed the... 

Figure 10.4 Mechanism of metal dissolution (A) via ad-atom intermediates and (B) direct dissolution of kink atoms.

The existence of the chemical mechanism of metal dissolution was later proved in the case of several metals in solid state also (see section 6,5). 

On the other hand, pit initiation which is the necessary precursor to propagation, is less well understood but is probably far more dependent on metallurgical structure. A detailed discussion of pit initiation is beyond the scope of this section. The two most widely accepted models are, however, as follows. Heine, etal. suggest that pit initiation on aluminium alloys occurs when chloride ions penetrate the passive oxide film by diffusion via lattice defects. McBee and Kruger indicate that this mechanism may also be applicable to pit initiation on iron. On the other hand, Evans has suggested that a pit initiates at a point on the surface where the rate of metal dissolution is momentarily high, with the result that more aggressive anions... 

With regard to the anodic dissolution under film-free conditions in which the metal does not exhibit passivity, and neglecting the accompanying cathodic process, it is now generally accepted that the mechanism of active dissolution for many metals results from hydroxyl ion adsorption " , and the sequence of steps for iron are as follows ... 

Valverde, N., and C. Wagner (1976), "Considerations on the Kinetics and the Mechanism of the Dissolution of Metal Oxides in Acidic Solutions", Ber. Bunsenges. Physik. Chem. 80, 330-333. 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

The Electron Transfer Step. Inner-sphere and outer-sphere mechanisms of reductive dissolution are, in practice, difficult to distinguish. Rates of ligand substitution at tervalent and tetravalent metal oxide surface sites, which could be used to estimate upward limits on rates of inner-sphere reaction, are not known to any level of certainty. 

Metal corrosion is a superposition of metal dissolution or the formation of solid corrosion products and a compensating cathodic reaction. Both processes have their own thermodynamic data and kinetics including a possible transport control. Furthermore, metals are generally not chemically and physically homogeneous so that localized corrosion phenomena, local elements, mechanical stress, surface layers, etc. may play a decisive role. Therefore, one approach is the detailed analysis of all contributing reactions and their mechanisms, which however does not always give a conclusive answer for an existing corrosion in practice. 

THE BASIC ELECTROCHEMICAL concepts and ideas underlying, the phenomena of metal dissolution are reviewed. The emphasis is on the electrochemistry of metallic corrosion in aqueous solutions. Hie role of oxidation potentials as a measure of the "driving force" is discussed and the energetic factors which determine the relative electrode potential are described. It is shown that a consideration of electrochemical kinetics, in terms of current-voltage characteristics, allows an electrochemical classification of metals and leads to the modern views of the electrochemical mechanism of corrosion and passivity. 

The principles of electrochemistry are useful in explaining many of the chemical mechanisms of metal CMP. Surface layer formation, metal solubility, and metal dissolution are all explained by electrochemistry.Surface films which are generally oxides or hy-... 

The corrosive behavior of a few metals is essentially determined by the kinetics of the dissolution of the corrosion products. This seems to be the case for Zn in HCOs" solutions, for passive iron in acids, and for passive A1 in alkaline solutions. The mechanism of the dissolution of iron and of the passivation of... 

In this group, the method of control of current efficiency distribution is of prime importance (see Sect. 12.2.1), but not just the current density, as in the above group of methods. The mechanism of enhancing a degree of localization of metal dissolution via a proper choice of electrolyte composition and conditions of ECM was considered in detail in Refs 9 and 23. 

The kinetics of several well-known electrochemical reactions have been studied in the presence of an ultrasonic field by Altukhov et al. [142], The anodic polarization curves of Ag, Cu, Fe, Cd, and Zn in various solutions of HC1 and H2S04 and their salts were measured in an ultrasonic field at various intensities. The effect of the ultrasonic field on the reaction kinetics was found to be dependent on the mechanism of metal anodic dissolution, especially on the effect of this field on the rate-determining step of the reaction. The results showed that the limiting factor of the anodic dissolving of Cu and Ag is the diffusion of reaction products, while in the case of Fe it is the desorption of anions of solution from the anode surface, and at Cd the limiting factor is the rate of destruction of the crystal lattice. Similar results were obtained by Elliot et al. [ 143] who studied reaction geometry in the oxidation and reduction of an alkaline silver electrode. 

It has been stated by several noted authors in electrochemistry that, in the case of metal deposition, charge is carried across the interface by ions rather than by electrons. " Unfortunately, the above authors did not implement the consequence of this difference in the analysis of the mechanism of metal deposition and dissolution. In one instance/ the author went as far as to state that... 

It can be seen that rapid dissolution occurs within the first hom, followed by a more gradual dissolution that proceeds continuously (i.e. about 30 hours). A mechanism of metal oxide dissolution is reported in literature [24] (equations 1 and 2). 

The treatment of surface titration data within the framework of transition-state theory (TST) thus appears to be one of the most promising tracks for elucidating and unifying the mechanisms of mineral dissolution. The goals of this chapter are to further explore this approach and to show that it can be applied to model the dissolution of complex oxides having several types of surface metal cation sites. 

Minerals with Kinetic Dissolution Condition Minerals of this group are considered in everyday life insoluble. Ihey include mostly metal oxides, hydroxides, sulphides and aluminum sihcates. The mechanism of their dissolution is dominated by hydrolysis whose nature depends on the structure and composition of minerals. Their dissolution under any conditions has kinetic condition, i.e., it is controlled by extremely slow chemical reactions of surface complexation. The rate of their dissolution is noticeably lower than 10 ° mole m s and the solubility does not exceed 10" mole l Besides, both their dissolution rate and solubility depend on pH values. These minerals are most common in the Earth crust and often play a leading role in the formation of imderground water composition. It is convenient to subdivide minerals with kinetic dissolution regime into three groups 1- silica, 2 - oxides, hydroxides and sulphides of metals, 3-aluminum silicates. 

Anodic processes of metal dissolution that change the dimensional characteristics of the material with consequent reduction of the mechanical strength... 

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