Anodic dissolution characteristics

When dezincification occurs in service the brass dissolves anodically and this reaction is electrochemically balanced by the reduction of dissolved oxygen present in the water at the surface of the brass. Both the copper and zinc constituents of the brass dissolve, but the copper is not stable in solution at the potential of dezincifying brass and is rapidly reduced back to metallic copper. Once the attack becomes established, therefore, two cathodic sites exist —the first at the surface of the metal, at which dissolved oxygen is reduced, and a second situated close to the advancing front of the anodic attack where the copper ions produced during the anodic reaction are reduced to form the porous mass of copper which is characteristic of dezincification. The second cathodic reaction can only be sufficient to balance electrochemically the anodic dissolution of the copper of the brass, and without the support of the reduction of oxygen on the outer face (which balances dissolution of the zinc) the attack cannot continue. 

Contact with steel, though less harmful, may accelerate attack on aluminium, but in some natural waters and other special cases aluminium can be protected at the expense of ferrous materials. Stainless steels may increase attack on aluminium, notably in sea-water or marine atmospheres, but the high electrical resistance of the two surface oxide films minimises bimetallic effects in less aggressive environments. Titanium appears to behave in a similar manner to steel. Aluminium-zinc alloys are used as sacrificial anodes for steel structures, usually with trace additions of tin, indium or mercury to enhance dissolution characteristics and render the operating potential more electronegative. 

The reduction wave of peroxydisulphate at dme starts at the potential of the anodic dissolution of mercury. The current-potential curve exhibits certain anomalous characteristics under various conditions. At potentials more negative than the electrocapillary maximum, a current minimum can be observed this is due to the electrostatic repulsion of the peroxydisulphate ion by the negatively charged electrode surface. The current minimum depends on the concentration and nature of the supporting electrolyte, and can be eliminated by the adsorption of capillary active cations of the type NR4. ... 

Anodic dissolution of n-Si can also proceed at a polarization under illumination. The maximum current is limited by illumination intensity when the saturation photo current density is lower than the critical current, Ji. The characteristics of i-V curves of n-Si under a high illumination intensity, when the reaction is no longer limited by the availability of photo generated carriers, is identical to that for p-Si. Similar also to p-Si, formation of PS on n-Si occurs only below the critical current, Jx 24... 

The electrochemical reactions and processes involving the anodic dissolution of silicon in HF solutions have been extensively studied in the past. Table 5 provides a summary for the characteristics of the anodic processes that are relevant to the formation of PS (details are documented in Ref.1.)... 

Under -> open-circuit conditions a possible passivation depends seriously on the environment, i.e., the pH of the solution and the potential of the redox system which is present within the electrolyte and its kinetics. For electrochemical studies redox systems are replaced by a -> potentiostat. Thus one may study the passivating properties of the metal independently of the thermodynamic or kinetic properties of the redox system. However, if a metal is passivated in a solution at open-circuit conditions the cathodic current density of the redox system has to exceed the maximum anodic dissolution current density of the metal to shift the electrode potential into the passive range (Fig. 1 of the next entry (- passivation potential)). In the case of iron, concentrated nitric acid will passivate the metal surface whereas diluted nitric acid does not passivate. However, diluted nitric acid may sustain passivity if the metal has been passivated before by other means. Thus redox systems may induce or only maintain passivity depending on their electrode potential and the kinetics of their reduction. In consequence, it depends on the characteristics of metal disso-... 

It is suggested that the anodic dissolution will be inhibited if the adsorbed anion and the reaction intermediate are stable and hardly dissolve in aqueous solution. On the contrary, if the reaction intermediate is relatively unstable and readily dissolves into aqueous solution, the anion will function as an electrocatalyst accelerating the metal dissolution rate. It is now common knowledge that hydroxide ions, OH, catalyze the anodic dissolution of metallic iron and nickel in acid solution [81,82]. It is also known that chloride ions inhibit the anodic dissolution of iron in acidic solution [83]. No clear-cut understanding is however seen in literature on why hydroxide ions catalyze but chloride ions inhibit the anodic dissolution of iron, even though the two kinds of anions are in the same group of hard base. We assume that the hardness level in the Lewis base of adsorbed anions will be one of the most effective factors that determine the catalytic activity of the adsorbates. Further clarification on the catalytic characteristics will require a quantum chemical approach to the adsorption of these anions on the metal surface. 

With particular attention to excess interface electrons, an anodic procedure has been performed for n-type InP (100). The anodic voltammogram was similar to Figure 15. No specific characteristics have been observed and a stable overlayer of indium sulfide has been formed. Equations 4, 5 and 6 show the reaction mechanism of aqueous (NH S solution with the InP substrate during anodic sulfidation. Equation 5 is responsible for the anodic dissolution of InP in (NH S solution when the hydroxyl group is dominating the solution. Equation 6 is the one responsible for sulfidation and depositing sulfides. 

However, in natural environments, soil-specific characteristics means that fractions of Fe(j,q), liberated by anode dissolution, will be rendered unavailable to react... 

Anode potential and current density play major roles during ECM and affect the manner in which the metal is removed from the anode, which in turn controls the surface finish. Polarization curves can be studied to understand the role of potential and current density in anodic dissolution. Figure 2.6 shows different polarization curves and their characteristic behavior on the nature of metal dissolution. Curve... 

Although the chemical reactions at the sili-con/HF interface are the same for n- and p-type silicon, there is a basic asymmetry in their electronic properties. For the dissolution of silicon under an anodic bias , the p-type Si is forward-biased and the current is caused by thermally generated majority carriers. The n-type Si is reverse-biased and undergoes charge depletion. The current is characteristic of a minority carrier flow. For low-doped n-Si, anodic dissolution uses photogenerated minority carriers. The i-V curve for p-type Si has been discussed earher and comparison with the i-V curves for n-Si are presented here. For n-Si under reverse bias condition, a dark current and/or a photocurrent are observed, depending on the doping density and Ulumination level. 

The phenomenon of decreasing the surface coarseness of a metal upon anodic dissolution under certain conditions is defined as electropolishing. In cases when polishing occurs, the current-voltage curve was found to exhibit a plateau characteristic for diffusion cmitrol of the dissolution process. Some facts point to the complex nature of the phenomenon of electropolishing. 

It was found that electropolishing occurs in systems which, under anodic dissolution, exhibit a limiting current characteristic of diffusion control, i.e., currents are dependent on flow rate of the electrolyte past the anode, as shown in Figure 24. Over a certain range of potentials at the limiting current plateau, at which maximum brilliance is obtained, oscillations of potential occur at a virtually constant current density, with an amplitude of over 0.40 V, and without appreciable damping. Also, a significant photoelectrochemical effect is found. 

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