Oxidative dissolution

The protective quality of the passive film is detennined by the ion transfer tlirough the film as well as the stability of the film with respect to dissolution. The dissolution of passive oxide films can occur either chemically or electrochemically. The latter case takes place if an oxidized or reduced component of the passive film is more soluble in the electrolyte than the original component. An example of this is the oxidative dissolution of CrjO ... 

Bacterial leaching is another example of oxidizing dissolution whereby specific bacteria either directiy attack the sulfide mineral or indirectiy enhance the regeneration of the oxidant. 

The mechanisms of oxide dissolution and scale removal have been widely studied in recent years. This work has been thoroughly reviewed by Frenier and Growcock who concluded, in agreement with others", that oxide removal from the surface of steel occurs predominantly by a process of reductive dissolution, rather than by chemical dissolution, which is slow in mineral acids. 

In general there does not appear to be any direct correlation between the rate of the chemical dissolution of oxides and the rate of scale removal, although most work on oxide dissolution has concentrated on magnetite. For example, Gorichev and co-workers have studied the kinetics and mechanisms of dissolution of magnetite in acids and found that it is faster in phosphoric acid than in hydrochloric, whereas scale removal is slower. Also, ferrous ions accelerate the dissolution of magnetite in sulphuric, phosphoric and hydrochloric acid , whereas the scale removal rate is reduced by the addition of ferrous ions. These observations appear to emphasise the importance of reductive dissolution and undermining in scale removal, as opposed to direct chemical dissolution. 

Ta and Nb containing minerals, 4 Ta and Nb oxides dissolution, 258 Tantalum fluoride properties, 25 Tantalum extraction, 285-288... 

Pathway temperatures must be strictly controlled (especially in single-phase systems) to create a balance between low-temperature oxide dissolution and high-temperature mass transfer limitations. 

C) for cast iron and up to 140 °F for marstenitic SS (60 °C). It is widely used where silicates are present with the iron oxides. Typically, 5 to 7.5% HC1 is employed. The ammonium bifluoride normally is present at 0.5%, but it may be increased to a maximum of 1.5% for a boiler that has not been cleaned for many years. The presence of hydrofluoric acid (HF), which is formed by the reaction of ammonium bifluoride with HC1 (see equation), tends to increase the rate of iron oxide dissolution and reduce the corrosion rate of exposed steel, when compared to using HC1 alone. This is due to the stability of the hexafluoroferric ion (FeFg3 ), which prevents the ferric ion from corroding exposed steel. 

As the overall concentration of copper and copper oxides in the boiler deposit increases, however, less thiourea is required. This is because, as ferric ions are generated during the iron oxide dissolution process, they oxidize the plated copper, which can then be removed from the boiler by forming a complex with thiourea. Conversely, if ferric ions are not generated, the plated copper remains and no complexing can take place. 

Plutonium Oxide Dissolution. All four sites dissolve impure PuO, residues in concentrated HND3 (10 to 14M) containing HF (<0.3M). Whereas material calcined at temperatures of... 

Spent anode residues from electrorefining (which contain approximately 20-30 percent of the plutonium fed to the process) are either recycled back to electrorefining, or, if high enough in impurities, are oxidized and sent to oxide dissolution. The spent salt is sent to aqueous dissolution (see Figure 1). 

Nelson, M. B., Davis, J. A., Benjamin, M. M. and Leckie, J. O. (1977). The Role of Iron Sulfides in Controlling Trace Heavy Metals in Anaerobic Sediments Oxidative Dissolution of Ferrous Monosulfides and the Behavior of Associated Trace Metals." Air Force Weapons Laboratory, Technical Report 425. 

Hence, in the absence of a redox system in solution the anodic reaction of FeS2 yields iron oxide/hydroxide and water-soluble sulfate ions. The compound does not undergo non-oxidative dissolution. 

Using electrons for the electrolytic reduction of metal salts, Reetz and coworkers have introduced a further variation to the tetraalkylammoniumhalide-stabilization mode [192-198]. The overall electrochemical process can be divided into the following steps (i) oxidative dissolution of the sacrificial Metbuik anode, (ii) migration of Met ions to the cathode, (iii) reductive formation of... 

The present technique enables light-induced redox reaction UV light-induced oxidative dissolution and visible light-induced reductive deposition of silver nanoparticles. Reversible control of the particle size is therefore possible in principle. The reversible redox process can be applied to surface patterning and a photoelectrochemical actuator, besides the multicolor photochromism. 

The remaining methods sketched in Figure 5.15 either deal with spatially confined oxidation/dissolution of the substrate or describe means of studying electrochemical reactions on a nanometer scale. 

At the O/S interface, for each molecule of alumina formed inside the oxide layer, i.e., three O2- ions transferred across the O/S interface, six hydrogen ions are formed. Thus, the acidity at the interface tends to rise to an extent which depends on the rate removal of these ions by some mechanism. In view of Eqs. (13) to (15), this should lead to oxide dissolution and a further decrease... 

Satisfactory agreement of experiments with kinetic laws, described by Eqs. (44) and (45), are observed only for tantalum and niobium, when the current efficiency approaches 100%. Even for these metals, certain deviations occur which could be attributed to space charge effects,82 electronic leakage currents,83 or other factors. In the case of aluminum, these deviations are relatively large, as, even in barrier-forming electrolytes, some oxide dissolution takes place from the very beginning of voltage supply to an anodized sample.32... 

When the results for oxide growth and anion incorporation172,160 are compared with the kinetics of space charge accumulation in barrier and porous alumina films [see Section IV(1)], it can be concluded that anion incorporation modifies the electrostatics of the external oxide interface, thus influencing oxide dissolution and pore formation.172... 

Analysis of experimental data shows that the dependence of the geometrical parameters of oxides on the temperature and concentration of electrolyte is different for galvanostatic and potentio-static conditions (Fig. 35).221 It appears that potentiostatic anodization is limited mainly by processes in the bulk of the oxide and thus is not influenced by temperature (Fig. 35b), whereas the galvanostatic anodization regime involves oxide dissolution processes at the O/S interface depending both on Tel and Cel. 

Therefore, other factors that have not yet been studied and are not easily quantifiable, such as the absorption properties of the C.-T. adduct at the surface of the metal powder and the solubility of the formed species should be important in determining the oxidation properties of C.-T. adducts towards metal powders. Furthermore, some extrinsic factors inherent to the experimental conditions, such as reaction temperature, reagent concentration, and nature of the solvent have been reported to affect the overall yield or the course of the reaction, and led to separation of different products in some cases.55 59 In any case, it appears that the simultaneous presence of the donor molecule and the di-/inter-halogen lowers the oxidation potentials of the metals, allowing their oxidation, dissolution, and complexation. 

Effluents emerging from sulfide-rich waste-dumps have special characteristics, such as very low pH (< 4), high metal solubility and presence of iron colloids, which provokes water turbidity and precipitation of ochre-products. These effluents are generically named acid mine drainage (AMD), since they result, primarily, from mineral-water interactions involving some sulfide minerals that typically produce acidity upon oxidative dissolution. 

Oil rigs are made of steel. The sea in which they stand contains vast quantities of dissolved salts such as sodium chloride, which is particularly aggressive to ferrous metals. The corrosion reaction generally involves oxidative dissolution of the iron, to yield ferric salts, which dissolve in the sea ... 

In both reactions, electron transfer induces the dissolution of the solid phase i.e., reductive and oxidative dissolution, respectively. Although no kinetic implications follow directly from the thermodynamic considerations, there are cases where the redox rate is related to the redox equilibrium (see e.g., Eq. 9.12). 

Except for phthalic acid, all other carboxylic acids studied induce considerable increases in the light compared to the dark values (the relatively high rate of iron oxide dissolution induced by oxalic acid has been extensively studied (5,8). Phthalic acid actually appears to stabilize the iron oxide against photodissolution despite the solution phase complex exhibiting some photoactivity. 

Equation 9—49 is the anodic transfer of surface cation into aqueous solution (cation dissolution) and Eqn. 9-60 is the anodic oxidation (hole capture) of surface anion producing molecules ofX2, i (e.g. gaseous oxygen molecules irom oxide ions). Electric neutrality requires that the rate of cation dissolution equals the rate of anion oxidation hence, the rate of the oxidative dissolution of semiconductor electrode can be represented by the anodic hole current for the oxidation of surface anions. 

In the state of band edge level pinning where all the change in electrode potential occurs in the space diarge layer, Mec, the anodic polarization curve of the oxidative dissolution follows Eqn. 9-53. As anodic polarization increases, the electrode interface enters a state of Fermi level pinning, in which all the change in electrode potential occurs in the compact layer, A ir, and the concentration of surface cations in Eqns. 9-54 then decreases with increasrng anodic polarization. 

Figure 9-16 illustrates the polarization curves for the anodic oxidative and the cathodic reductive dissolution of ionic compound semiconductors. The anodic oxidative dissolution proceeds readily at p-type semiconductor electrodes in which the mqjority charge carriers are holes whereas, the cathodic reductive dissolution proceeds readily at n-type semiconductor electrodes in which the majority charge carriers are electrons. 

Fig. 9-16. Polarization curves of anodic oxidative dissolution and cathodic reductive dissolution of semiconductor electrodes of an ionic compound MX iiixcp) (iMxh )== anodic oxidative (cathodic reductive) dissolution current solid curve = band edge level pinning at the electrode interface, dashed curve = Fermi level pinning.

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