Anodic dissolution potential dependence

The character of the processes that take place on anodic dissolution is dependent on the nature of metal. Those that have high enough negative potentials to dissolve directly in alcohols (lanthanides, for example) need only some additional anodic potential to overcome the overvoltage. The electric current yields (i.e., the ratio of the amount of alkoxide formed and that calculated... 

This may suggest that the nature of the cationic species produced by the anodic dissolution may depend on the surface conditions of the alloy and on the local environment. In particular, the analyses of passive films on stainless steels showed [43] that the external layers are very hydrated and contain chloride. On the other hand, the number of hydroxyl groups and chloride ions adsorbed on active surfaces is likely to be dependent on the local pH, chloride content, and surfece potential. Thus, dissolution from a passive surface may produce metallic hydroxy-chlorides different from those resulting from active dissolution, and active dissolution may produce cations depending on the local conditions. 

The anodic dissolution of nickel is also dependent on the amount of cold work in the metal and in the active region the anodic current density of cold worked material at a given potential is up to one order of magnitude greater than that of annealed material. 

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

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. ... 

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). 

Mercury is quickly limited at positive potentials (+0.25 V with respect to SCE). Beyond this potential, anodic dissolution of mercury occurs. However, mercury can be used at up to —1.8 or —2.3 V depending on whether the supporting electrolyte is acidic or alkaline. This range offers several possibilities, especially for the determination of heavy metals. The mercury used must be extremely pure (six-time distilled, under nitrogen). Unfortunately, the use of mercury as an electrode is a disadvantage because of its toxicity the mercury must be recycled after each use. 

Catalyst deterioration due to gas poisoning is only avoided by careful gas cleaning. Anodic oxidation followed by dissolution of Pt and transfer to the cathode is a serious cause for Pt loss. It is potential dependent and accelerates as the cathode potential increases, for instance under partial load or in off-time, when the cathode potential drifts toward the oxygen equilibrium potential. Therefore it is of utmost importance that whenever the fuel cell is switched off, the oxygen in the cathode lumen is rapidly exchanged by inert nitrogen and that the cell voltage under operation does not surmount 0.8 V. 

Results. The presence of Pt reduces the corrosion rate of Ti by shifting the free corrosion potential to more noble values (Fig. 6) where the Ti dissolution rate is slower. This shift is produced by the catalytic effect of Pt on hydrogen recombination which alters the cathodic reactions at the alloy surface. At the corrosion potential, the cathodic and anodic currents are equal. Although the shift in corrosion potential reduces the anodic current, anodic dissolution of Ti still occurs. The long-term corrosion rate of a surface alloy depends upon what happens to the Pt as the Ti is being dissolved. If Pt is removed from the surface, the corrosion rate will increase as the implanted volume of the alloy is dissolved. If Pt builds up on the surface, the corrosion rate should remain low. 

Surface states on a semiconductor in a vacuum can sometimes be explained by means of the spare bonds that dangle from atoms on surfaces, or defects associated with dislocations. Neither of these mechanisms works at the semicon-ductor/solution interface. The dangling bonds will be expunged by adsorbed water, etc. Experiment shows that the concentration of surface states on semiconductors in solution is strongly potential dependent, and that defects in the crystal structure would not be potential dependent, at least until anodic dissolution of the substrate itself began. 

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-... 

Dissolution and Deposition Potentials.—If a metal is placed in a solution of its ions a reversible electrode represented by M, is set up suppose its potential is E, Imagine now that an external source of potential is applied to this electrode so as to make it an anode of an electrolytic cell (p. 8) this will have the effect of increasing the potential, and since the electrode is reversible it will immediately commence to dissolve (cf. p. 184). It follows, therefore, that when a metallic electrode is made an anode, it wnll begin to dissolve as soon as its potential exceeds the reversible value E by an infinitesimal amount. In other words, the electrolytic dissolution potential of a metal when made an anode should be equal to its reversible (oxidation) potential (cf. p. 243) in the given electrolyte. The actual value depends, of course, on the concentration, or activity, in the solution of the ions with respect to which the metal is reversible. On the other hand, if the particular electrode under consideration is made a cathode, so that its potential is reduced below the reversible value, the reverse process, viz., deposition... 

The mean residence time is potential-dependent and is related to the anodic (dissolution) current density of the adatoms. Multiplying eq. (2.32) with eq. (2.31) and replacing E - obtains... 

Since the reaction rate at potentials above Vp is limited by the dissolution of oxide and the dissolution rate depends on the nature of the oxides (see Chapter 4), the change of current with potential indicates that oxide composition/structure varies with the formation. The nonstoichiometric composition of an anodically formed oxide film can be expressed as SiO with a higher value of n close to the oxide/electrolyte interface and a lower value of n close to the Si/oxide interface.A thicker oxide film has a bulk composition closer to the stoichiometric SiOa. The dissolution rate of anodic oxide depends on the composition of the electrolyte as shown in Fig. 5.46. " (also Fig. 5.7 ). It depends little on the type of material and doping levels. ... 

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. 

Vodyanov, a year earlier, had examined the effect of an ultrasonic field on the anodic dissolution of iron in sulphate solutions [119]. The dependence of an ultrasonic field (23 kHz) on the rate of anodic dissolution of Fe was investigated at pH 0.45-2.0 and at a SO concentration ofO. 1-1N. The current versus time curves at controlled potential showed that the ultrasonic field increased anodic polarization. 

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