Anodic dissolution techniques

The largest group of elements comprises those isolated from solution in the elemental form as a result of reduction, usually electrochemical. In acid solution, the electrolytic deposition of metal on a solid cathode is limited to noble and semi-noble metals. Trace analysis of copper and its compounds may serve as an example [100]. An anodic dissolution technique may be applied for the isolation of macroscopic amounts of copper. A sample in the form of a bar, plate, or wire is the anode in the electrolytic system. When current is passed through the electrolyte (nitric acid + persulphate), Cu is deposited on the graphite cathode, while most trace elements accumulate in the solution. In the trace analysis of platinum, the matrix has been also separated on a cathode [101]. 

S. Barabas, S.G. Lea, Application of anodic dissolution technique to automated analysis of metals. Determination of phosphorus in copper, Anal. Chem. 37 (1965) 1132. 

Langenegger, E. E. and Robinson, F. P. A., Effect of the Polarisation Technique on Dezincihcation Rates and Physical Structure of Dezincihed Zones , Corrosion, 24, 411 (1968) Brooks, W. B., Discussion of the De-alloying Phenomenon , Corrosion, 24, 171 (1968) Pickering, H. W., Volume Diffusion During Anodic Dissolution of a Binary Alloy , J. Electrochem. Soc., 115, 143 (1968)... 

For this reason, the dissolution of hydrous oxides does not require a high energy of activation. If hydrous oxides are dehydrated, they become dry oxides, which therefore acquire higher resistance to anodic dissolution. The most straightforward way to obtain dry oxides is to subject hydrous oxides to thermal treatments or better to prepare them as thin surface films by a non-electrochemical technique (thermal decomposition, chemical vapor deposition, reactive sputtering, etc.). 

The study of the effect of the adsorptions of various additives on the anodic dissolution has been the subject of several studies. For instance, the influence of the adsorption of N species on the anodic dissolution of Ni was studied in [43]. The dissolution and passivation of Ni in nitrite-containing acid solutions were investigated by Auger spectroscopy, AFM, and conventional electrochemical techniques. It was found that the dis-solution/passivation of the Ni surface is consistent with a competition between adsorbed OH and nitrogen-containing... 

Phenanthroline inhibits the anodic dissolution of iron and the new technique of surface-enhanced Raman spectroscopy (SERS) has identified the presence of [Fe(phen)3]2+ on the electrode surface during dissolution. 

The electrochemical technique can be used also for direct synthesis of bimetallic alkoxides. For instance, the anodic dissolution of rhenium in the methanol-based electrolyte that already contained MoO(OMe)4, permitted to prepare with a good yield (60%) a bimetallic complex RevMov,02(OMe)7, with a single Re-Mo bond [904], Application of the same procedure permitted the preparation of complex alkoxide solutions with controlled composition for sol-gel processing of ferroelectric films [1777]. 

Habashi and Torres-Acuna (H6) described a process that involved th direct recovery of copper and elemental sulfur by anodic dissolution o copper (I) sulfide (white metal). The white metal produced from th (jopper matte analyzed 77.85% Cu and 19.20% S. A closed vessel wa used to prevent the evaporation of the copper (II) sulfate acidified wit sulfuric acid electrolyte. The electrolysis was carried out at room tempera ture. The sulfur originally present in the white metal went into the slim as pure elemental sulfur. The elimination of the converting and polin, steps in the conventional copper smelting and refining process can b realized by use of this technique. 

Fig. 1 shows one of the first electrolytically deposited alumimun coatings to be obtained from this type of electrolyte. Since electrolytic aluminum deposition from this system has no true smoothening effect, thick layers become even rougher, as illustrated by the thickly coated cathode plate shown in Fig. 1. The cathodic deposition and the anodic dissolution of aluminum corresponded to almost 100 <7o of the amount expected according to the Faraday rule, which is an important prerequisite for even considering using this electrolysis technique for technical applications. Independently of the layer thickness, the deposited aluminum layers are found to be ectraordinarily pure. Spectroscopic investigations have revealed purities of up to 99.999%. Even when relatively impure raw aluminum with purities of 99.7% functions as the anode, very pure aluminum can be deposited. Therefore, obviously not only a technique of electroplating aluminum was discovered, but also a method of...

Electrochemical grinding and electrochemical deburring are techniques related to electromachining in which excess metal is removed by a combination of mechanical abrasion and anodic dissolution. 

Kolb s group presented a jump-to-contact technique by which the surface can be decorated with small metal clusters provided there is sufficient interaction between a sample and a tip, the effect discussed earlier. By means of this technique, they showed the way to generate different patterns on a different scale. On the nanometer scale they presented the structure made of 12-Cu clusters of 0.8-mn height (see Fig. 29). They reported an interesting observation on the surprising stability against anodic dissolution of the fabricated clusters, and they proposed to explain this effect by quantum confinement of the electrons in the clusters that leads to an energy gap between the electronic states of the clusters. ... 

The competition between redox reaction and anodic dissolution became very important in the development of stable regenerative solar cells on the basis of semiconductor-liquid junctions. As shown in the previous section, it is determined by the thermodynamic and kinetic properties of the processes involved. Information on the competitions between these reactions cannot be obtained entirely from current-potential curves, because in many cases they do not look very different upon addition of a redox system, especially if the current is controlled by the light intensity. Therefore, a rotating ring disc electrode (RRDE) assembly consisting of a semiconductor disc and a Pt ring is usually applied, i.e. a technique which makes it possible to determine separately the current corresponding to the oxidation of a redox system [62, 63]. 

Tsuru et al. [129-131] applied the double channel electrode transient technique to the investigation of the anodic dissolution of iron in sulphate and chloride aqueous media, in the pH range 1-3. The upstream electrode served as the generator electrode at which the anodic dissolution reaction occurred. Overall this involves... 

Over a decade, Bandi and co-workers have employed EGA-DTA techniques to determine second-phase compounds isolated from steels by selective chemical or anodic dissolution of the matrix. This method has been used qualitatively to identify and quantitatively to determine approximately 35 carbides, carbonitrides, and nitrides, some of which could not be identified... 

A second important group of alloying elements beside the passivity promoters are the dissolution moderators or blockers. These elements slow down the anodic dissolution rate. A prominent example is Mo in iron. Molybdenum does not form stable passive films but increases the stability of alloys significantly. Indirect conclusions that Mo hinders the surface mobility of Fe were confirmed during the last years with scanning probe techniques. Marcus and also Strehblow [135,136] confirmed in their surface science approach that the mobility of Fe is slowed down significantly. This increases the activation energy for the ion-transfer into the electrolyte and thus stabilizes the alloy. 

The situation becomes more complex when any in situ surface pre-conditioning techniques are employed often cathodic or anodic pretreatments are used. These should be treated with caution. Cathodic treatments can reduce oxide films and introduce hydrogen into tite metal, both of which can effect any subsequent electrochemical measurement [7,8]. Anodic treatment may passivate the surface or roughen it by anodic dissolution. 

Instead of chemically generated electrons as reducers, the electrolytic reduction of metal salts represents a successful alternative in cormection with the fabrication of tetra-alkylammonium salt-stabilized nanopartides [159-165]. The first step comprises an oxidative anodic dissolution of the corresponding metal, followed by the formation of zerovalent metal atoms at the cathode. Nucleation and particle growth then follow, some of which is stopped by the addition of a tetra-alkylammonium salt. This technique not only prevents the formation of byproducts but also allows a rather good size-selectivity high current densities lead to small particles, while low current densities yield larger spedes. Ammonium salt-protected nanopartides of Ti, Fe, Co, Ni, Pd, Pt, Ag, and Au may also be prepared in this way. 

By taking advantage of the flexibility of electrochemical anodic dissolution, different EMM techniques have been developed over the years by scientists. EMM can also be successfully utilized for fabrication of cylindrical microtools [16-18]. By controlling different machining parameters, conditions and influencing factors, geometrical shape, size, and finish of the cylindrical microtools may also be varied. 

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