Metal dissolution surface-bulk transfer

The limiting current density is an important parameter for the analysis of mass transfer controlled electrochemical processes and represents the maximum possible reaction rate for a given bulk reactant concentration and fluid flow pattern. During anodic metal dissolution, a mass transfer limiting current does not exist because the surface concentration of the dissolving ion (e.g., Cu + when the anode is composed of copper metal) increases with increasing current density, eventually leading to salt precipitation that blocks the electrode surface. 

The same discussion with the same equations holds for anodic reactions with diffusion of the species Red as the rate-determining step. In the case of intense metal corrosion, the diffusion of cations from the electrode surface to the bulk may become the rate-determining step, with their aecumula-tion at the electrode surface and the final precipitation of a salt film. These processes are important for intense active metal dissolution and localized corrosion, as will be discussed in Sec. 1.5.4. At small current densities, a superposition of charge transfer and diffusion control is obtained. Therefore the current density increases exponentially in the vicinity of the Nernst potential ac-... 

Inside a pit in electrolytic solution, anodic dissolution (the critical dissolution current density, and diffusion of dissolved metal hydrates to the bulk solution outside the pit take place simultaneously, so that the mass transfer is kept in a steady state. According to the theory of mass transport at an electrode surface for anodic dissolution of a metal electrode,32 the total increase of the hydrates inside a pit, AC(0) = AZC,<0),is given by the following equation33,34 ... 

Electron transfer between metal centers can alter the course of reaction in several ways (46). Thermal excitation may create especially reactive electron holes on the oxide surface, causing reductant molecules to be consumed at the surface at a higher rate. More importantly, electrons deposited on surface sites by organic reductants may be transferred to metal centers within the bulk oxide (47). This returns the surface site to its original oxidation state, allowing further reaction with reductant molecules to occur without release of reduced metal ions. Electron transfer between metal centers may therefore cause changes in bulk oxide composition and delay the onset of dissolution. 

In this equation, aua represents the product of the coefficient of electron transfer (a) by the number of electrons (na) involved in the rate-determining step, n the total number of electrons involved in the electrochemical reaction, k the heterogeneous electrochemical rate constant at the zero potential, D the coefficient of diffusion of the electroactive species, and c the concentration of the same in the bulk of the solution. The initial potential is E/ and G represents a numerical constant. This equation predicts a linear variation of the logarithm of the current. In/, on the applied potential, E, which can easily be compared with experimental current-potential curves in linear potential scan and cyclic voltammetries. This type of dependence between current and potential does not apply to electron transfer processes with coupled chemical reactions [186]. In several cases, however, linear In/ vs. E plots can be approached in the rising portion of voltammetric curves for the solid-state electron transfer processes involving species immobilized on the electrode surface [131, 187-191], reductive/oxidative dissolution of metallic deposits [79], and reductive/oxidative dissolution of insulating compounds [147,148]. Thus, linear potential scan voltammograms for surface-confined electroactive species verify [79]... 

The diversity of reactions that are considered to be surface mediated has also increased over the past decade. It is not only strict sorption/desorption and precipitation/dissolution processes that are important but also the surface mediation of reactions such as electron transfer (eg. 14-17), hydrolysis 18) and various photochemical transformations. In addition certain solid phases, in particular metallic iron, iron oxides and smectitic clays, are capable of transferring electrons in and out of their bulk structure (eg. 19-23), When viewed in this context, minerals should not be considered as passive solids, or even as simple sources of a reactive surface but must be considered as bulk reactants. 

The anodic partial reaction also involves a charge transfer at the interface because a metal atom loses electrons. It then dissolves in the solution as a hydrated or complexed ion and diffuses towards the bulk. In the vicinity of the metal surface, the concentration generated by dissolution therefore often exceeds that of the bulk electrolyte. Once the solubility threshold is reached, solid reaction products begin to precipitate and form a porous film. Alternatively, under certain conditions, metal ions do not dissolve at all but form a thin compact oxide layer, called passive film. The properties of the passive film then determine the rate of corrosion of the underlying metal (Chapter 6). 

The effects of pulsed waveforms are extremely complex and poorly understood, but the following effects are generally accepted. During the off period of a pulse, no net electron transfer can take place and the cathode surface is refreshed with metal cations as a result of convective diffusion. During the on period, the surface metal ion concentration will initially approach the bulk solution value but will decay with time, i.e. the technique involves non-steady state diffusion. A limiting case is a surface metal ion concentration of zero, i.e. complete mass transport control. The reverse (anodic) current may lead to selective dissolution of high points on the deposit due to their enhanced current density, producing a more compact or smooth surface. 

For more than a century, a number of different aluminum alloys have been commonly used in the aircraft industry These substrates mainly contain several alloying elements, such as copper, chromium, iron, nickel, cobalt, magnesium, manganese, silicon, titanium and zinc. It is known that these metals and alloys can be dissolved as oxides or other compounds in an aqueous medium due to the chemical or electrochemical reactions between their metal surfaces and the environment (solution). The rate of the dissolution from anode to cathode phases at the metal surfaces can be influenced by the electrical conductivity of electrolytic solutions. Thus, anodic and cathodic electron transfer reactions readily exist with bulk electrolytes in water and, hence, produce corrosive products and ions. It is known that pure water has poor electrical conductivity, which in turn lowers the corrosion rate of materials however, natural environmental solutions (e g. sea water, acid rains, emissions or pollutants, chemical products and industrial waste) are highly corrosive and the environment s temperature, humidity, UV light and pressure continuously vary depending on time and the type of process involved. ... 

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