Electrochemical techniques, anodic dissolution

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... 

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]. 

The direct electrochemical measurement of such low corrosion rates is difficult and limited in accuracy. However, electrochemical techniques can be used to establish a database against which to validate rates determined by more conventional methods (such as weight change measurements) applied after long exposure times. Blackwood et al. (29) used a combination of anodic polarization scans and open circuit potential measurements to determine the dissolution rates of passive films on titanium in acidic and alkaline solutions. An oxide film was first grown by applying an anodic potential scan to a preset anodic limit (generally 3.0 V), Fig. 24, curve 1. Subsequently, the electrode was switched to open-circuit and a portion of the oxide allowed to chemically dissolve. Then a second anodic... 

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. 

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]. 

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. 

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. 

The scratch repassivation technique discussed above may be more useful for studying SCC than pitting [107]. This is because natural pitting is often initiated at defect sites in the passive film that are electronic or chemical in nature. Such sites are not necessarily probed by a mech inical scratch. Scratching can simulate SCC where mechanical film rupture processes are operative and anodic dissolution controls the SCC process. In general, once the passive film is mechanically disturbed at some potential, an electrochemical current can be measured that will decay back to a low level when repassivation has occurred. A measure of the crack growth rate, V, (cm/s) can be obtained from these data... 

The possibility of synthesizing metal alkoxides by the anodic dissolution of metals into alcohols containing conducting electrolytes was demonstrated for the first time by Szilard in 1906 for the methoxides of copper and lead. Since then this technique has proved to be most promising. For example, the electrochemical method for the preparation of ethoxides of Ti, Zr, Ta, Si, and Ge was patented by the Monsanto Corporation in 1972, and was later applied by Lehmkuhl et al for the synthesis of Fe(n), Co, and Ni alkoxides M(OR)2 (R = Me, Et, Bu , and Bu ). 

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