Oxide film formation

According to Sato et al.,6,9 the barrier-layer thickness is about 1.5 to 1.8 nm V-1, and increases to 3 nm around the oxygen-evolution potential. In Fig. 5, the scale of the electrode potential, Vrhe, is that of the reversible hydrogen electrode (RHE) in the same solution. The electrode potentials extrapolated from the linear plots of the potentials against the film thickness suggested that the potential corresponding to the barrier thickness equal to zero is almost equal to 0.0 V on the RHE scale, independent of the pH of the solution, and approximately agrees with the equilibrium potential for the oxide film formation of Fe or Fe. Therefore it is concluded that the anodic overpotential AE applied from the equilibrium potential to form the oxide film is almost entirely loaded with the barrier portion. 

Tremiliosi-Eilho G, Jerkiewicz G, Conway BE. 1992. Characterization and significance of the sequence of stages of oxide film formation at platinum generated by strong anodic polarization. Langmuir 8 658-667. 

Birss VI, Chang M, Segal J. 1993. Platinum oxide film formation reduction— An in-situ mass measurement study. J Electroanal Chem 355 181-191. 

Conway BE. 1995. Electrochemical oxide film formation at noble metals as a surface-chemical process. Prog Surf Sci 49 331-452. 

Later work by several research workers, and most particularly Greef (1969), showed that the very early stages of oxide film formation could be detected using more sensitive instrumentation and the changes in A, 4 and the light intensity correlated very well with the charge passed both in the anodic and cathodic sweeps (see Figure 3.13). 

There are purely electrochemical methods for finding the amount of simple radicals such as H or O on noble metal electrodes. Basically, they rely upon die assumption that when some electrical variation in die state of die electrode is brought about, die only effect it has is to reduce or augment die H or the O on die electrode surface. Now of course this is not so if die substrate is, say, iron, or indeed all but the noble metals, for there may be a co-dissolution of die substrate, or competing oxide film formation, etc. Spectroscopic methods (e.g., FUR in a millisecond response version) or ellipsometiy are not affected by such difficulties. 

During the past few years four methods have been developed for the study of the kinetics of oxide film formation, all being semicontinuous in nature. The polarimetric method has been adapted by Lustman and Mehl (14) for the measurement of the oxidation of copper. A spectro-photometric method has been applied by Waber, Sturdy, Wise, and Tipton (15) to the study of the oxidation of tantalum while a differential pressure method has been developed by Campbell and Thomas (16) for a study of the oxidation of a series of metals and alloys at elevated temperatures. We have chosen to use a sensitive weight gain method (17,18) in which a quartz microbalance is placed directly in the vacuum system. 

Similar treatment of crystals of tantalum and germanium151) and of GaAs156) and InSb157) as anodes in NMA apparently also leads to oxide film formation. 

Another interesting aspect is the oxide film formation... 

In the case of U, two electrolytes were found to lead to oxide film formation. One has been developed by Picklesimer (24) (20 cc of concentrated NH4OH + 80 cc of absolute ethyl alcohol) the other electrolyte (23g ethyl glycol + 500 cc concentrated NH OH sp gr 0.925 + 500 cc H 0) is used after electroetching at low voltage in the Robillara et al electrolyte (23). 

K. HAUFFE From our knowledge of the semiconducting properties of NiO, in particular the very low mobility of the holes, which is not understandable because of the unoccupied 3d level of the Ni ions, we believe that nickel oxide in contrast to other oxides has not a covalent but an overwhelmingly ionic structure. Nevertheless, under general conditions the charge carriers are the holes and not the nickel ions. Only under strong electric fields, at the very beginning of die oxide film formation at low temperatures, the electron transport via holes can become slower than the nickel ions. 

Certain systems which behave reversibly in the equilibrium state exhibit considerable polarization in the course of electrolytic reduction examples are the conversion of 5-valent vanadium to the 4-valent state, and of the latter to the 3-valent condition, the reduction of 6- to 5- and of 5- to 3-valent molybdenum, and the reduction of 6-valent to 5-valent tungsten. There is reason to believe, however, that in all these cases the abnormal behavior is to be attributed to the presence of oxide films on the cathode by producing a partial blocking of the surface, these oxide films increase the effective c.d., so that the potential rises. Considerable polarization, accompanied by oxide-film formation, occurs in the reduction of chromate to chromic ions, but it is not certain how far this system is reversible. 

Several reviews addressing the polarization behavior, d ion adsorption, competition between Cr adsorption and OH codeposition, oxide film formation, and cr ion discharge, as well as the kinetic aspects of the reaction on various oxide-covered and oxide-free surfaces that have been investigated during the past 15 years, have been published (55/, 333-338). Of these, particular mention should be made of Refs. 555, 335, 336, and 439-441, where the basic aspects of the properties of oxide electrodes and the kinetic aspects of oxide film formation in relation to Cl adsorption and the kinetics of Cr ion discharge were addressed. Mechanistic aspects of chlorine evolution were critically analyzed recently in an excellent article by Trasatti (338). In this article, the focus is primarily on the nature and characterization of the adsorbed intermediates partipatingin the course of CI2 evolution and their role in the electrocatalysis of the chlorine evolution reaction. As with the OER, in aqueous solutions CI2 evolution takes place on an oxidized surface of metals or on bulk oxide films, so that their surface states often have to be considered in treating the electrocatalysis of the reaction. 

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